Transactions of the Geological Society, 1st series, vol. 3/On the Geological Features of the North-eastern Counties of Ireland
Extracted from the Notes of J. F. Berger, M.D. M.G.S.
Read April 15th, 1814.
With an Introduction and Remark,
By the Rev. W. Conybeare, Member of the Geological Society.
Read April 5th, 1816.
To collect in one point of view the general results deducible from the ensuing detached observations, and enable those who may be unacquainted with Ireland to follow their course with greater facility, it seems desirable to introduce them by a rapid survey of the general features which distinguish the district they refer to.
That district may be described as limited by Dundalk bay on the S.E. and by Lough Foyle on the N.W. including towards the south the counties of Down, Armagh, and the N.E. angle of Lowth lying between Dundalk and Carlingford bay; and on the north, Antrim and Derry: that portion of Tyrone which extends along the S.W. shore of Lough Neagh, between Derry and Armagh, being also comprehended as falling within the same general outline.
This district is marked by three distinct systems or groups of mountains, one of which occupies the more southern counties; while the more northem are divided between the two others.
The Mourne Mountains form a well defined group extending from Dundrum bay to Carlingford bay in the southern extremity of Down.
Slieve Donard is the highest summit of this group; it has been said to rise 3150 feet above the level of the sea, but the estimate of Mr. Templeton, who assigns only 2590 as the elevation of the highest points of the Mourne Mountains, appears more correct, and nearly agrees with that of Dr. Berger, who calculates its height at 2654 feet.
To the west of the main group the Fathom hills, Slieve Girkin or the Newry Mountains, and Slieve Gullen, all situated in the south-east of Armagh, and the Ravensdale and Carlingford Mountains in the north of Lowth, may be considered as its appendages.
Granite is the prevailing constituent of all these ranges.
To the north of the Mourne Mountains Slieve Croob composed of syenite, and Slieve Anisky of hornblende rock, both situated in the county of Down and barony of Lower Iveagh, constitute an elevated tract dependent upon but placed at some distance from the main group.
Hornblende rock and primitive greenstone are abundant on the skirts of the granitic district. Mica slate has been noticed only in one instance. Exterior chains of transition rocks advance far to the west and north of this primitive tract, extending westwards across Monaghan into Cavan and on the north-east to the southern cape of Belfast Lough and the peninsula of Ardes.
The primitive nucleus bears but a very small proportion in superficial extent to these exterior chains, which are principally occupied by greywacke and greywacke slate.
It is highly worthy of observation that the points of the coast of Scotland immediately opposite the Peninsula of Ardes, where the greywacke terminates abruptly on the Irish side of the North Channel, present in the neighbourhood of Port Patrick and through the greater part of the Mull of Galloway, a resumption of the same formation; nor is the analogy in the structure of the two countries confined to this one point. The hills of the Mull of Galloway are a branch of the great chain of mountains which under the name of the Lead hills and other local titles, traverses the whole of Scotland on the south reaching from sea to sea, and the composition of this chain agrees through its entire extent with that of the mountainous tract just described, the transition rocks forming its predominating constituent, enveloped by which several small districts of granite occur, as in the hills of Cairnsmuir and Criffel; while the whole is distinguished from the great northern chain of mountains or Grampians, by the rare occurrence or total absence of mica slate, which in the latter is remarkably prevalent.
On the north of the Mourne Mountains rise the Bann one of the tributary streams of Lough Neagh, and the Lagan which flows into the estuary of Belfast; and on the south several small rivulets which run immediately into the sea, the base of the mountains being in that direction washed by the Irish Channel.
The mountain group which I have thus designated, rises at the distance of about 30 miles to the N.N.W. of the external chains of the first system.
It forms a large mountainous tract comprehended between the river Roe and the Strabane, partly situated in Derry and partly in Tyrone.
From its southern boundary it emits two water courses; the Mayowla which takes an eastern direction and flows into Lough Neagh; and the Moyle which passes to the west and joins the Strabane river.
On the north the Fanghan waters a beautiful valley, at length discharging itself into Lough Poyle.
Sawell is the highest point of this group: Dr. Berger has fixed its altitude at 2257 feet above the sea. The whole of this extensive and lofty tract is primitive to the very front towards Lough Foyle. Mica slate is the predominating rock, constituting exclusively almost nine-tenths of the district: it is accompanied by primitive limestone in the lower part of the country.
On the eastern bank of the Roe this system of mountains is succeeded by a range of secondary heights, covered by an enormous platform of basalt and forming a part of that system which will in the next place be described. These secondary masses repose upon and conceal the mica slate in the eastern part of Derry, but the mica slate again emerges from beneath this covering, after an interval of near 30 miles, in the N.E. of Antrim, and swells into mountains which break down abruptly towards the coast between Tor Point and Cushenden bay.
The exact correspondence in structure of the opposite points of Ireland and Scotland here again demands our observation; the Mull of Cantire which faces Tor Point resuming the chain of mica slate which was there broken off. The Cantire hills are connected with the Grampians, a chain strikingly similar to that which has been above described in all the circumstances of its composition; and it may be added, as compleating this analogy, that the mica slate of Ireland is succeeded on the south (where the Antrim coast exhibits it in section) by a conglomerate, perfectly resembling that which is so well known as skirting the Grampians on their southern border.
We have therefore strong grounds for believing, without incurring the charge of generalizing too hastily, that the two mountain systems described, should be regarded as prolongations of the great northern and southern chains of Scotland, the former distinguished by the prevalence of mica slate, the latter by that of greywacké.
In the eastern part of Tyrone which intervenes between these two systems of mountainous ground, a coal formation occurs associated with that variety of limestone which is usually found underlying or alternating with the coal measures in Great Britain.
The position of this coal field offers a new analogy with Scotland, where the interval between the southern and northern mountains is principally occupied by a broad zone of rocks connected with the coal formation, upon which are placed those vast overlying masses of trap formation which constitute the Campsie hill, the Ochills, &c. and which, again, correspond both in their constitution and position, with the basaltic group which forms our third system. Before describing that system, it is necessary to observe that another small coal field occurs at Ballycastle, near the edge of the mica slate district in the north-east of Antrim.
This group may be more accurately described as separated into two chains, bounding on the east and west the trough or valley through which the river Bann flows from Lough Neagh to the Ocean.
The eastern chain lies in the county of Antrim, being comprehended between the valley of the Bann and the Northern Channel. It presents an abrupt declivity towards the east, falling with a gentle slope towards the west, in which direction the beds composing its mass dip.
The hills of which it is composed are generally detached and distinct, but yet so closely grouped together that there can be no impropriety in considering them as parts of a single chain.
According to Dr. Berger's measurement Kock-lead in the northern extremity of the chain is the highest summit, it rises 1820 feet above the level of the sea; but the basis of this mountain is occupied to the height of 500 feet by primitive rocks, (connected with that district of mica slate which has been before mentioned as appearing in this part of the country) leaving only 1320 feet for the thickness of the secondary strata peculiar to this system. Diris hill, near the southern extremity of the chain, is wholly composed of those strata and attains an elevation of 1475 feet above the sea: it is situated about two miles to the west of Belfast.
On the S.E. this chain pours several small streams into Belfast Lough, and on the N.E. into the North Channel; all these have a very short course. On the west three rivers of rather more importance take their rise; the six mile water and the river Main which flow into the bay of Antrim in the N.E. angle of Lough Neagh; and the Bush which empties itself into the ocean a little west of the Giants Causeway.
The western chain included between the Roe and the Bann forms the exact counterpart of the former, but the strata here dip in a nearly contrary direction, namely, towards the north-east; the fall of the hills being gradual in this direction, while they front the west and south with abrupt and precipitous escarpments. Cragnashoack at the southern extremity, is as might be expected from this general of the line, the highest summit, it rises 1864 feet above the sea, exceeding by 44 feet the loftiest point of the eastern chain. Slieve Gallion, an insulated hill which stands in an advanced position at some distance from the south bank of Cragnashoack is less elevated by 240 feet.
Benyavenagh, the extreme mountain on the north, is one of the lowest in the chain, rising only 1114 feet.
The Roe, the Clady, and Aghivey, are the principal water courses this chain sends forth; the two latter are feeders of the Baun, the first empties itself into Lough Foyle.
The geological constitution of this third system is highly important and interesting, it is as has been already observed, wholly secondary and uniformly covered by enormous stratified masses of basalt; this covering appears to acquire its greatest thickness on the north. The basaltic cap of Benyavenagh, the most northern summit of the western chain measuring more than 900 feet; and that of Knock-lead, similarly situated in the eastern chain 980 feet: the average depth of this superstratum may therefore be safely estimated at 545 feet, and its superficial extent at 800 square miles, a solid mass of extraordinary and imposing dimensions.
In the strata underlying the basalt. the English geologist is agreeably surprised to recognise many of the most important of those formations, which reposing upon the coal measures, occupy such an extensive tract in the south and eastern counties of his own island, but which in Ireland are entirely confined to the comparatively small district now under consideration, newer extending far beyond the circumference of the great basaltic area; a circumstance which almost naturally leds to the conjecture that they may have been originally much more extensive, but have been elsewhere removed by the agency of some destroying and denuding force, to which in this quarter alone an effectual resistance was opposed by the firm and massive suprestratun of basalt which covered and protected them.
The beds alluded to occur in the following order, proceeding from beneath the basalt downwards.
1. Chalk.─This formation, which in England cannot be estimated at less than 800 feet in thickness, does not in Ireland average more than 200. It agrees exactly with the lower lads of the English chalk, as seen in the Isle of Wight and Isle of Purbeck, which are distinguished from the higher beds of the same formation by their very superior consolidation. It is impossible for any two portions of the same formation to be more entirely identified, by every external character and by the fossils and organic remains contained in them, than are these Irish beds, which have been frequently called white limestone, with the English chalk in the above places: there as in England the lowest beds are destitute of flints which the upper contain in abundance.
2. Mulattoe, an arenaceous stone, with a calcareous cement, of a speckled appearance (whence its name) derived from numerous disseminated spots of green earth. It agrees altogether in its character and fossils with the green sandstone, which occurs in a similar geological position underlying the chalk in England: the thickness of this deposit appears to vary very considerably, and has not in any instance been precisely ascertained.
The numerous beds of coarse calcareous oolites, which in England succeed this green sandstone, are entirely wanting in Ireland, and the mulattoe reposes immediately on the lias limestone. Analogous circumstances are not however wanting in England. In the neighbourhood of Lyme Regis, in Dorsetshire, the sandstone has extended itself over the cut goings of the oolites, rests upon the lias in a manner similar to that just described; and as it is covered with beds of chalk, the whole section affords an exact counterpart to that presented by the Irish series.
3. Lias Limestone, a blue argillaceous limestone disposed in thin beds, alternating with slate clay, and distinguished by the ammonites, gryphites and the remains of the pentacrinus, which it abundantly contains. It may be worthy of remark that the sections given by Guetard and Monnet in the Atlas Mineralogique de la France, and the accompanying description, clearly prove the existence of this formation near Metz. Its thickness in Ireland has not been ascertained.
4. The lias in Ireland reposes, as in England, on beds of red and variegated marle, containing gypsum and further distinguished by numerous salt springs; the marle is underlaid by a thick deposit of red and variegated sandstone containing clay galls. These four formations which, together with the basalt, constitute the whole mass of the mountains belonging to the third system, cannot be estimated as possessing a less average thickness than from 800 to 1000 feet: the whole system appears at the north-eastern and south-western extremities to repose upon the coal formation, and its associated limestone, and this again on transition or primitive rocks.
The four formations, more particularly specified, vary considerably in thickness in different places. The mulattoe and lias are often entirely wanting, so that the chalk rests on the sandstone No. 4: this position seems to arise from the superior stratum in such cases extending beyond the outgoings of one or more of the inferior strata, and being thus brought into contact with beds which, when the series is full, occupy a yet lower place.
The chalk and sandstone are however remarkably constant; it may indeed be said to be almost universal within this district; one exception, however, is afforded by Cross hill a little to the west of Fairhead, where the basalt overlies even these, and reposes immediately on the regular coal measures of the Ballycastle district.
The section presented at this point is also worthy of mention, on account of its general resemblance with that of a part of the Campsie hills, published by Col. Imrie in the Wernerian Soc. Trans. Vol. 2. The whole series may be examined with the greatest advantage in the neighbourhood of Belfast, where all its members occur.
The proofs which the deep vallies, separating the detached eminences characteristic of this system, afford of their formation by an agent which has excavated and scooped out as it were portions of the solid strata, have been most ably and clearly detailed by Dr. Richardson in the appendix to the Statistical Survey of Antrims The general appearances he describes are common (it should be remarked) to the vallies of all countries composed of nearly horizontal strata; but one phenomenon, as stated by him, seems almost peculiar to Antrim, namely, that the materials so removed have been entirely carried off, leaving no traces behind them. This circumstance seems to incline Dr. Richardson to consider the agent which has acted in the manner described, as some unknown and undiscoverable cause, and to hesitate in receiving the common and surely probable opinion which regards diluvial currents as presenting a satisfactory solution; but it must be remembered that the solitary instances he adduces can never weigh against the great majority of cases in which the fragments of the rocks so destroyed not only occur in abundance where they might be expected, but exhibit the most unequivocal marks of their having experienced the action of agitated waters. He who has examined the valley of the Thames from its source downwards can be at no loss for illustrations of these positions, the vast deposits of waterworn debris of oolite which this valley where it traverses the range of hills occupied by this rock, and those of flint pebbles where it crosses the chalky hills. or the Chilterns, will instantly occur to his mind. It is surely more philosophical to suppose that the violence of the currents has swept away the debris of the Antrim excavations into Lough Neagh on the one side, and into the sea on the other, than that these excavations owe their origin to some unknown cause, distinct from that which appears to have produced all others.
It should be added, that in the county of Londonderry waterworn fragments of basalt really occur in considerable quantity, particularly near the valley of the Roe.
It is impossible to dismiss the subject of denudation without remarking, that appearances analogous to those so forcibly described by Saussure as occurring in the Jura mountains, (where scattered fragments of the alpine rocks attest the subsequent excavation of the great valley now separating the two chains) are also presented, though on a much smaller scale, among the hills we have just examined, where rolled fragments of primitive rocks are often found on the summit of high basaltic ridges, which are at present cut off from all communication with the primitive districts by numerous intervening valleys. (See Sampson's Statistical Account of the County of Londonderry.)
The arrangement adopted in the following observations is, that suggested by the geological position and relations of the rocks described.
|I. Primitive Rocks||A. Granite.|
|B. Gneiss and mica slate.|
|C. Primitive limestone.|
|D. Primitive trap.|
In the Mourne mountains and adjoining districts an extensive formation of granite occurs, but neither here nor elsewhere in the north of Ireland can we recognize the same varieties of that rock which the mountains of Wicklow present.
The granite now to be described seems rather to agree in its characters with the newer granite of the Wernerians, a rock supposed to be closely allied to syenite. It appears to constitute almost the whole mass of the Mourne mountains, whence it passes across Carlingford-bay into the county of Louth, extending to the summit of the Ravensdale mountains. It soon crosses the limits of Louth to enter into Armagh, composing part of the Faughel hills, whence it expands itself into a flat and elongated ridge, well known under the name of Slieve Gullen, forming likewise the Slieve Girkin or Newry mountains, and the lower and upper Fathom hills, of which the latter keep close to the right bank of the Newry river.
On the north-west side of the Mourne mountains, where they slope gradually into the plain, the same rock reaches Rathfriland, a table-land of inconsiderable elevation.
Within the geographical boundaries just assigned, the granite is spread over a surface that measures 324 square English miles, comprehending the highest ground in the north of Ireland.
A few masses of other unstratified rocks, of which notice shall be taken elsewhere, occur in this formation.
Some primitive but stratified rocks also rest upon it in many places, generally arriving at their greatest elevations on their south-western boundaries.
The texture of this granite is either porphyritic or finely granular. The felspar always appears the prevailing ingredient, usually grey, more rarely milk white and earthy; the quartz has a smoky tinge, and in the granular variety it generally occurs crystallized in double six-sided pyramids; the mica is of a brown-black colour, and bears but a very small proportion to the other two constituents.
Amongst the accidental ingredients I remarked but two, viz. crystallized hornblende, chiefly abundant in the porphyritic variety, and small redish garnets in the granular. The two varieties are, I believe, mingled both together, so at least they occur on the top of Slieve Donard. I more particularly noticed the granular on Slieve Muck, Slieve Birna, Ravensdale, and Slieve Gullen; the porphyritic on Newry and Fathom mountains. The imbedded crystals of hornblende cause this rock to act on the magnet.
Water-worn pebbles of porphyritic syenite, occasionally containing flesh red crystals of felspar and iron pyrites, are very frequent at the base of the Mourne mountains on the road from Ross Trevor to New Castle. It is probable that these have been derived from the disintegration of neighbouring masses of that rock, occurring as Subordinate beds in this granitic formation; Since on the shore at Glass Drummond, a ledge of porphyritic syenite, evidently connected with the granitic mass of the adjoining mountain, is seen running out into the sea.
If the granitic formation, above described, be identified with the newer granite of the Wernerians. it may be conjectured that it reposes on mica slate.
Has not been noticed as occurring within the district now described, although the mica slate in the north—cast of Antrim sometimes assumes a character which it is not easy to distinguish from that of this rock.
Of all the primitive rocks mica slate appears to be the most widely distributed over the north of Ireland.
In Armagh, the sides of that narrow valley which separates Slieve Gullen from the Slieve Girkin moutains, and contains the small lake of Cum Lough, are principally composed of mica slate.
In the north-eastern angle of Antrim, mica slate forms the prevailing substratum through a district comprising about 40 square English miles. and extending from north-west to South-east, between the mountain of Knocklead (mouth of Ballycastle) to the valley of the little river of Glendum, which empties itself into the sea at Cushendon, on the north side of Red Bay.
Within this district, however extensive deposits of very recent formation (principally chalk and basalt) occur towards the summits of the hills, capping the primitive rock.
The valley of Glendun is formed exclusively of mica slate as far as Done, a village more than five miles inland, here the secondary deposits commence, at the elevation of 590 feet above the sea.
From the mouth of the Glendun river the mica slate extends along the coast, in a northerly direction, as far Murlock Bay, near Fairhead, a distance of about seven English miles: it is here associated with a few other primitive rocks, hereafter to be described. The local name of Cushleak is given to this part of the coast.
Vestiges of mica slate occur on the road from Cushendon to Ballycastle. I have seen it in situ at Ballyvarleys in the bed of a rivulet, skirting the base of Knocklead on the north-east, and have traced it in several other points on the slope of that mountain, particularly at Kileseg, and the low Market hill near Ballycastle. Passing into the county of Londonderry, that great and central mass of mica slate, which from the parallel of the Mayowla river reaches Lough Foyle, extending to the east and west between the Roe and the Moyle, claims our principal attention; it may be computed to cover a surface of at least 476 square English miles of mountainous ground, over which several distinct summits are scattered, including the following, of which I have determined the elevation:—Sawell 2257; Feen Glen 2097; Mullaghash 1677; Moneynieny 1477; Sphell Covagh 1867; and Dunlogan mountain 1467 feet.
The river Roe, from the neighbourhood of Newtown Limavaddy to its source, may be assigned as the general line of demarcation between this primitive group and the red sandstone which forms the base of the secondary mountains of the third system, (see Introduction): the mica slate is however occasionally seen on both sides of the channel of the Roe, as is the case at the romantic waterfall called the Dogsleap, near Newton, where a fine natural section of that rock is displayed, and lower down the river in the deer park of Mr. M'Causland; from the district of Tamna Arran near the head of the valley of the Roe, the boundary passes along the Douglas river, a branch of the Mayowla to its confluence: thence it sweeps to the Cast of the Cairns of Slieve Gallion, and near the confines of Moneymore and Lissane, towards the sources of the Ballinderry river, in the north-east of Tyrone.
A little to the north-east of the source of the Roe, and almost surrounded by the secondary and basaltic ridges of Benbradagh and Cragnashoack, we are surprised at meeting with a small insulated district of mica slate; it forms the entire mass of the mountain of Coolcoscrahan, which rises nearly 1300 feet above the level of the sea.
The characters of the mica slate vary much less than might be expected, considering the extent it occupies in the north of Ireland; upwards of two-thirds of it belong to the talcky variety, the remainder to the common, or that which contains the least quantity of mica and the greatest of quartz.
A circumstance rather remarkable is, that, amongst the multiplicity of specimens I have examined, I do not remember one that contained garnets. How far that extensive formation of mica slate may be metalliferous, it is impossible to say in a country hitherto so little explored.
The subordinate rocks which not infrequently occur in the mica slate, shall be noticed separately.
Primitive limestone exists in several parts of the counties of Antrim and of Londonderry as a subordinate member of the mica slate formation with which it sometimes alternates.
Granular and blue micaceous limestone, with veins of coloured spar, quartz, and green chlorite, occurs on the north-west side of Cairntogher, in the county of Londonderry, at the height of about 800 feet above the level of the sea.
The same granular and micaceous limestone exists in Bennady Glen, and at the old church near Dungiven, at the latter place in large lamellar concretions, passing into compact with a greenish grey colour: at Banagher church it occurs blue, micaceous, and in small granular concretions. Near Clady, on the road from Dungiven to Londonderry, it is extremely talcky with some quartz nodules; and in the deer park of Mr. M'Causland, near Newtown Limavaddy, it possesses, the same character, containing some thin layers of quartz and a few iron pyrites.
Lastly, on the north-east side of Slieve Gallion, there is a primitive limestone which contains crystallized hornblende in abundance: it breaks spontaneously into large rhomboids, incrusted over the natural joints with calcareous spar, of a green-yellowish colour.
At the point of Taur in Antrim, the colour of the limestone varies from grey to reddish grey, and greenish grey; the concretions are rather large, and the texture passes sometimes from granular into compact; it contains only a small quantity of magnetic iron pyrites.
The strata are alternately coarsely and finely slaty: the thickness of the whole mass may be rated at fifty feet. Veins of calcareous spar intervene as seams between the individual strata.
At Toberbilly, on the north-west side of Knocklead, near Ballycastle, at an elevation of four hundred and ninety-four feet above the level of the sea, I traced a bed of granular and micaceous limestone through a considerable extent towards the town-land of Cloughamany.
The concretions are rather small, and there are linings of talc along the seams of stratification.
The remark of Werner, that primitive trap contains no iron clay, but is wholly or almost entirely composed of hornblende, appears to be extremely appropriate, and to draw an excellent line of demarcation between primitive and the secondary trap.
More than two-thirds of the Foy mountain, near Carlingford, are composed of a succession of stairs formed of primitive trap.
Proceeding from the bottom towards the summit of that mountain, which is 1850 feet high, the trap is seen passing from an homogeneous to a porphyritic texture. The latter imparted by lamellar crystals of felspar of a white colour, and of rather considerable size: gradually the accidental ingredient becomes the, essential one; the colour of the felspar becomes greenish, and in that state the rock approaches more nearly perhaps to the character of sienite than to that of trap.
In its genuine state, in the lower part of the mountain, the rock puts on the usual coated appearance which originates from decay.
On the eastern slope of Slieve Birna, one of the Mourne mountains, at one half nearly of its height, I observed a bed of hornblende rock, apparently interposed in the granite.
At Slieve Anisky, a hamlet on the road from Castlewellan to Dromore, I noticed another bed of hornblende rock, but I am doubtful whether it belongs to this class or to that of transition.
This subordinate member of the primitive trap series frequently occurs, forming distinct beds, in the mica slate of Antrim and Londonderry.
In the former county it is found in the valley of Glendun, and along the coast from Cushendon to Tor-point.
In the latter it occurs in Bennady-glen, in Aglish-glen, and in the bed of the Roe river near Dungiven. The bed of hornblende slate in the latter place occupies an extent not less than four hundred yards, ending by the old church, where it runs parallel to a bed of primitive limestone, before mentioned. It has there a tendency to hornblende rock, while in Bennady-glen it displays a granular texture and a few plates of mica.
I have found greenstone in Ravensdale park, at the foot of the mountain of that name, and on the west side of the Faughel hills. I suspect that in those two instances, the latter particularly, it forms beds in the newest granite.
rests against the acclivities of the Mourne mountain, but the strata never rise very high, seldom exceeding 5 or 600 feet; one instance only was observed of a hill exclusively formed of this rock, and that did not attain a greater elevation than about 800 feet.
Attempts have been made to quarry it for roofing slate, and were the works conducted with spirit they might perhaps supply Ireland with as good slate as that now imported from Wales, which appears to belong to the same formation.
The greenstone slate of the Mourne mountains contains apparently no crystallized hornblende in the basis, though it is disseminated through the latter, as is shewn by the manner in which it fuses before the blowpipe. I have remarked in it some crystals of glassy actynolite passing into hornblende, veins of quartz, magnetic and common iron pyrites. The basis sometimes approaches in its nature to clay-slate.
This aggregate rock composes the top of Clark's hill or Slieve-sleet, five hundred and forty-eight feet above Castlewellan. I also noticed it on Slieve Croob, between Slieve Nasky and Bakaderry town. I saw it again in loose blocks about Castlewellan.
A Druidical monument at Coagh near Cookstown, is partly made of an aggregate of crystals of hornblende. The same compound I observed, but not in situ, at the basis of the mountain of Coolcoserahan.
Felspar porphyry occurs in the county of Down, in the bed of the Finish, on the north-west side of Slieve Croob, near Drummara in the lower Iveagh; and in a decomposing state at Ballyroany, four or five English miles north-east of Rathfriland. In the first of these localities it is interposed in a compound rock of granular quartz and mica.
In the mica slate district of Antrim, several beds of felspar porphyry are found; they may be traced along the coast from Torpoint to Cushendon, and thence inland along the old road to Ballycastle.
In Londonderry, the fundamental rock on the east side of Slieve Gallion is a variety of felspar porphyry strongly resembling sienite, with which rock it probably alternates in this mountain.
In the same neighbourhood felspar porphyry may be traced without interruption from the top of the glen of Latterane to the bed of the Knockadoo river on the road from Lissane to Moneymore.
I have traced this rock in the low country adjoining the town of Newry for three or four miles on the road to Dundalk, for the same space on that to Armagh, and towards Bainbridge as far nearly as the four mile house. It is perhaps more extensive.
The sienite is unstratified, cropping out in independent masses, but rent into pieces mostly of a rhombic figure, the natural joints of which are generally rusty.
It is finely granular, composed of felspar of a blood red, flesh red, or greyish colour, with some quartz, hornblende, and black mica, besides two ingredients, accidental but not uncommon, namely, iron pyrites and small garnets.
The sienite that crowns the summit of Foy mountain near Carlingford, contains only a few broad plates of black mica, with greenish felspar and hornblende in large concretions.
Slieve Croob in the lower Iveagh, the most conspicuous hill of a small group that lies nine miles in advance to the north of the Mourne mountains, seems formed on its north-east and south-east sides of different varieties of sienite, some of which are porphyritic and very beautiful: the crystals of hornblende are extremely well defined, and the compact felspar constituting the basis has a brownish or smoky colour.
This sienite crops out at intervals from Bakaderry town to the top of Slieve Croob, occupying an elevation of about 900 feet.
Slieve Gallion, in Derry, exhibits sienite in connection with porphyry, and either of a porphyritic texture or in large concretions; I met with an elegant variety on the road from Lissane to Lough Finca; the felspar is either slightly green or flesh-red with hornblende, quartz, and some pyrites. This rock acts strongly on the magnet.
In the bed of the Black-water near the valley of the Mayowla, on the north-west side of Slieve Gallion, the sienite is mostly composed of crystallized hornblende with some felspar and iron pyrites, verging therefore into greenstone.
|B. Transition limestone.|
|C. Transition trap.|
|D. Old red sandstone.|
occupies a great part of the baronies of Ardes, Castlereagh, and the two Iveaghs in the north of the county of Down, whence they extend through Armagh and Monaghan into Cavan. The accompanying map (Pl. 8.) will convey a sufficiently accurate idea of their extent. Their characters do not differ from those which they exhibit in other countries already frequently described.
The greywacke slate is worked extensively for rooting slate at Ballyalwood in the center of the peninsula of Ardes: a variety however, still better adapted to this purpose, remains yet neglected at Cairn Garva, on the west-south-west of Conbigg hill.
At Cultra, on the north-east of Belfast lough, Binty slate and drawing slate occur as subordinate formations in the greywacke.
Lead and copper ores have been found in this formation at Conbigg hill, between Newtown Ards, and Bangor, and I believe at some other places. A mine was formerly worked at Conbigg, but is now abandoned.
The entrance of the noble bay of Carlingford is bounded on the south-west side by an obtuse point of land very low, and almost on a dead level, whence the Foy mountain boldly rises, falling away to the south towards Cooley, where it forms the ridge named Golding mountain.
The greater part of this flat point of land, ending on the north at Carlingford Castle, is occupied by a limestone formation which I am disposed to refer to that of transition.
It covers a square surface of about nine English miles, reappearing at the Block-house island in the middle of the bay, but I am not aware that there are any vestiges of it on the shore in the county of Down, which limits the north—east side of the bay.
The strata lean on the east side of the Golding mountain dipping to the south east: they however rise to a very inconsiderable height upon its abrupt declivity.
There are quarries opened in a continual line to the extent nearly of one English mile, at the foot of the Golding mountain, the depth of the quarries varying from fifty to eighty feet: limestone is raised besides in other places nearer to the shore, at Cooley, Mullaghtre, and the Gan rocks.
This limestone is remarkable, because it alternates both with transition trap and with greywacke slate; evidently so with the former at Cooley, and with the latter at the Gan rocks.
The solid strata are frequently traversed by veins and thin layers of calcareous spar: between them intervene many thinner beds passing into slaty marl.
The limestone itself somewhat varies in its characters at the different places where it occurs. In point of colour it passes from bluish-black to smoke-grey: the first variety is more compact, the lamellar concretions less evident, and it is sometimes traversed by veins of flinty slate; small quartz concretions are occasionally though rarely interspersed in it. I have seen in it crystallized iron pyrites; it contains superficial impressions of cornua ammonis, covered with iron pyrites; madrepores, both ramose and columnar; nautili figured by Walcott, (Bath fossils fig. 44.)
Two small limestone districts occur near the shores of Strangford Lough, one at Lisbawn near Down Patrick on the south-west, and the other near Cumber on the north-west; the latter appears to be of the magnesia variety; orthoceratites have been noticed in it.
It has not been ascertained to what formation these limestones belong, but since they occur in a district where greywacke prevails, they may with greater propriety be noticed here than under any other article.
The formation of greenstone which alternates with the limestone at Carlingford has invariably a crystalline texture, but differs in its other characters in presenting the following varieties.
1. Common greenstone.
2. Close grained and porphyritic.
3. Approaching to greystone.
- The beds are three or four feet thick.
- The beds are three or four feet thick.
On the east side of Slieve Gallion at the head of a narrow glen called Tintugb Glen, about 1300 feet above the level of the sea, crop out several unconnected masses of greenstone.
They are totally unconnected with the flat stratified trap which overlies the chalk, and crowns the summit of the mountain. The fissures are lined with calcareous spar, and veins of crystallized heavy spar and of red compact iron stone are contained in this rock.
This formation has been observed only in the two following districts, and even there its extent appeared to be very limited.
1. In the county of Down, on the sides of Strangford lough.
2. On the N.E. coast of Antrim, between the bays of Cushendall and Cushendon; and again in Murloch bay.
Vestiges indicative of the old sandstone may also be traced along part of the shore of the Isle of Rathlin opposite Fairhead.
The tract of sandstone first specified extends from the E.N.E. side of Strangford Iough to Scabro hill on the opposite side of that lough, appearing likewise at Ballymasca, Dunlady, and Kirkdonnel.
Scabro hill rises 483 feet above Strangford lough, its summit is composed of a cap of greenstone about 150 feet in thickness; the remaining 330 consist principally of the sandstone, which may be observed in one of the quarries opened on that hill (to procure this material for architectural purposes) in distinct beds of very variable thickness alternating with greywacke.
The colour of this sandstone is reddish or greyish; its texture is either conglomerate, including fragments of greywacke slate, or finely granular, composed of quartzose grains imbedded in a cement, sometimes calcareous and sometimes siliceous. The greenstone which caps this hill differs very slightly from that associated with the floetz trap.
Lord Londonderry has caused this formation to be bored to the depth of 500 feet in the fruitless search for coal on the east side of Strangford lough near Mount Stewart; if to this depth the height of the sandstone on Scabro hill be added, it will give from 800 to 900 feet as the known thickness of this formation. The greatest length of this district of sandstone does not exceed six or seven English miles. it appears to rest upon greywacke.
The tract of this formation between the bays of Cushendall and Cushendon, is yet more limited than the preceding. On the coast it occupies a line of between three and four English miles, and extends about the same distance in an inland direction. The highest point of the cliffs on the coast in this range is only 124 feet; but the hill of which they form the escarpment rises at Jeaveragh near Cushendall church to the height of 522 feet: this is the greatest elevation which the sandstone of this district attains.
The strata dip into the sea towards the E.S.E. under an angle of about 32°. In the bay of Cushendon several caverns of considerable magnitude occur in this rock.
The general character of this formation is that of a conglomerate; it passes however into a coarsely granular texture, and in one place (the Red bay of Cushendall) into a finely granular: its colours vary from red to grey.
The conglomerate contains in great abundance large pebbles of quartz, and more rarely of hornstone porphyry; also of a rock which appears to have been a greenstone porphyry, but is much altered by decay; and lastly of mica slate. The coarsely granular variety consists of quartzose concretions imbedded in an argillo-calcareous cement.
This sandstone formation appears to rest on the mica slate which succeeds it on the north side of Cushendon bay, and occupies the district of Cushleak described in a former article. At the opposite or N.W. extremity of that district we may again trace the sandstone in Murloch bay: it there appears very distinctly on the beach near the great Whin dyke, in its conglomerate form.
To the westward of Church bay in the Isle of Rathlin, and near a spot called the Black rock, I found fragments of the old sandstone associated with blocks of syenite in such abundance as to impress me with a strong belief of the former existence of both these rocks in that direction. Were the sea to retire a few fathoms and disclose the foundations of Rathlin, we should very probably discover the trap which constitutes the present surface of that island, resting on the old sandstone; and that rock in its turn reposing upon syenite.
A. Limestone underlying the Coal formation.
B. Coal formations.
C. Sandstone formations.
E. Green Sandstone or Mulattoe.
At the entrance of Cookstown on the road from Coagh, there are quarries of a shell limestone formation supposed to extend itself to the south as far as to Steward's town, and nearly for one mile in the other directions: it is disposed in strata alternately solid and compact, earthy and marly which vary from one to three feet in thickness, and dip north-west at an angle of about 27°. The texture of this limestone is compact, though composed of distinct lamellar concretions: it is traversed by veins of calc spar of a reddish colour, and is soluble in acids without residuum: the specific gravity is 2.84. It is used for building; and the mortar made from it is said to be considerably stronger than that of the newer white limestone. It contains organic remains.
At Desartmartin in the county of Derry, ten miles to the north by east of Cookstown, there is a similar shell limestone of a smoke grey colour, which is also quarried. The strata dip north west at an angle of eight degrees. They contain two sorts of terebratulites, namely, terabratulo giganteo and terabratulo producto, and imbedded nodules of a glassy quartz sometimes of the size of an hazel nut, a circumstance that renders the blasting of this limestone with gunpowder attended with danger. I was informed that at the depth of seven yards, the bed of limestone is exhausted and reposes on a stratum of clay, but I had no opportunity of ascertaining the fact myself.
The shell limestone of Desartmartin re-appears two miles farther to the north-east at Dromore, the strata preserving the same dip, direction and characters: the quarries of Dromore are more extensive than those of Desartmartin.
At Gore Tarminey in the parish of Kilcronaghan not far from Dromore, the shell limestone shows itself again, but under the character of swine stone: it is variegated, and contains terebratula gigantea. The strata dip at an angle of 12°.
At the coal works of Ballycastle two English miles north of the town, strata of shell limestone are to be seen on the shore, and may be traced at low water running out to sea in their line of bearing, which is N.E. 17°. they dip therefore to S.E. 73°.
The strata are alternately solid and marly, the former constituting beds of three or four feet in thickness, the latter not exceeding half a foot. The solid strata are of a compact limestone with lamellar and rhombic concretions, the colour smoke grey.
I have examined several specimens of a variety of this limestone containing ramose madrepores, said to have been found on the west side of Lough Neagh in Tyrone.
I have to notice four partial coal formations; they occur in the following counties.
- I. Tyrone: at Coal-island and Dungannon.
- II. Antrim: near Ballycastle.
1. Two Coal fields, as coal formations are usually styled in England, exist in the county of Tyrone, at no great distance the one from the other, viz. at Coal island and Dungannon; with the latter I am totally unacquainted: it is I believe, by far the most extensive of the two: whether they may be considered as connected and thus constituting but one single formation, I do not pretend to say.
The coal formation of Coal island is in an open part of the country, though with a gently waved surface. The whole extent of the coal district or Pound, as it is called, does not exceed as I was informed, four hundred yards square.
The works in this district appear to have been prosecuted formerly with more activity than at the present period, some of the pits being now abandoned; several however still remain, but they seem to be conducted with little of capital or of spirit. The steam engine has not yet been introduced; the power of horses only is employed to raise both coals and water, which last is unfortunately very abundant. In one pit called the Mary Anne, which I visited, 150 barrels were computed to be drawn out every day. None of the pits (so far as I was informed) exceed 75 yards in depth: the quantity of coals raised daily in the Mary Ann pit amounted to thirteen tons, though no more than thirty colliers were employed. The coals are said not to cake, they are applicable to all domestic purposes, and are I believe mostly consumed by the inhabitants of the adjacent part of the country, notwithstanding a bounty allowed by government to send them either to Newry, Belfast, or Dublin.
Several seams of coal occur, the main bed is nearly six feet in thickness; its general direction is north-west and south-east: the dip of the strata is here towards south-west. At Dungannon, as I was informed, this direction becomes reversed.
2. The collieries of Ballycastle occupy an extent of less than one English mile along the coast. They have been long wrought, and were once in a more flourishing state than they are now: they formerly used to send from ten to fifteen thousand tons of coal to the market yearly, whereas the ground bailiff with whom I conversed several times, assured me that the quantity now exported did not amount to more than fifteen hundred or two thousand tons.
Owing to prejudice, I believe, rather than to greater expence, the country people prefer burning turf rather than coals, and even the inhabitants of the Isle of Rathlin, who have but a very spare quantity of that combustible, come to the Main to carry it over instead of coals, though ultimately it must be more expensive and surely more troublesome.
The Ballycastle coals are therefore sent to Dublin, chiefly on board the numerous trading vessels bound to that port from Londonderry, which thus instead of going thither in ballast, take a freight of coal on their passage. The Irish coals in Dublin receive a bounty equal to the duty laid upon the English coals.
There are but four coal works now wrought out of twelve which were formerly opened near Ballycastle: Gob colliery is the most extensive and advantageous.
As the beds of coal crop out a few feet above the level of the sea, there is no occasion for sinking shafts, but some of the horizontal galleries are of great extent; that of Gob colliery into which I went, is not less than eight hundred and twenty yards: they have a considerable quantity of water, and, which is worse, they suffer so much from foul air that the colliers cannot stay more than eight hours out of twenty-four in the mine. The beds dip to the south-east about one foot in nine.
I should apprehend that this partial and broken formation is mostly exhausted.
It may not be improper to subjoin here the series of the rock-measures at two or three of the collieries which I visited in the north of Ireland. I shall first give the provincial names, adding the scientific terms which appear to correspond to them.
IN THE COUNTY OF TYRONE,
|from the surface downwards..|
|provincial terms.||geognostic names||feet.||inch.|
|2||white-metal||Grey-white clay Ironstone||4|
|6||Grey-metal||Reddish brown clay iron-st.||21|
|14||Main Seam of Coal||5 to 6|
This account of rock-measures falls short of the depth of the pit itself, by seventy six feet, and we must either suppose that some of them recur once or twice more in the series, or rather that it is not perfectly correct: my authorities in this and the following table were the miners who attended me as guides.
COLLIERY AT BALLY-CASTLE IN THE COUNTY OF ANTRIM,
|from below upwards..|
|provincial terms.||geognostic names||feet.||inch.|
|4||Main Till||Main bed of Slate-clay||18|
The preceding tables evince the general resemblance which the members of these coal formations bear to those of other countries. Iris necessary to add very few remarks.
The coal of these districts is almost entirely slate coal; it may perhaps be suggested as a general observation concerning the Irish coal formations, that supposing a line drawn across the Island in the parallel of Lough Allen from east to west, the coal found to the north of that line is principally slate-coal, and that on the south cannel coal. In one of the works, however, in Coal Island, a bed of cannel coal six feet thick is said to have been wrought.
The slate clay at Mary Ann Pit contains impressions of aquatic plants, vegetable impressions similar to those of the Derbyshire collieries have also been observed in the Ballycastle district.
Clay ironstone occurs both in nodules and beds, and brown spar passing into sparry iron ore has been observed at the coal works of Ballycastle, where it forms a bed of some thickness traversing the cliff at a little depth below the principal seam of coal.
A bed of trap lying in a conformable position between the strata of sand, occurs a little to the eastward of the coal works of Ballycastle, between Fairhead and the little Glen of Port-na-crea, along the Crag of Sron Gal or of the “ White-nose.”
This geognostic fact, the only one of the kind I have seen or heard of in the north of Ireland, was pointed out to me by the ground bailiff of the collieries, who called it a horizontal dyke.
Its thickness is about two feet. It has a slaty structure, and is micaceous where it lies immediately in contact with the sandstone, the usual characters of which appear altogether unaltered. It breaks spontaneously into forms resembling triangular pyramids; the texture is remarkably close, it is almost entirely formed of finely crystallized hornblende, with some thin scaly parts of white felspar: it acts on the magnet, and yields a black enamel before the blowpipe.
* Several vertical dykes also occur traversing the coal measures near Ballycastle, these will be particularly described in the account of sections on the coast.
On the skirts of that elevated district in the county of Antrim and in the eastern division of Londonderry, which is occupied by the more recent flœtz rocks (namely lias, green sand and chalk, surmounted by a vast cap of basalt) extensive deposits of sandstone occur, forming the basis on which this lofty platform appears to repose.
These deposits of sandstone appear therefore to be in every instance of greater antiquity than the lias which they support, they seem also to be partly coeval with and partly more recent than the formations already described, as they are seen either alternating with or overlying the members of those formations where they can be traced in connexion with them.
They may be considered as identified partly with the coal sandstones of England, and partly with the formation here known by the name of red rock marle, the lower beds of these deposits being referable to the former class of rocks and the higher which contain gypsum to the latter; to ascertain however the precise demarcation of these subdivisions is a task for which materials are as yet wanting.
A further difficulty occurs in two instances, namely, at Cushendall bay and at Murloch bay, where these deposits are brought apparently into contact with the old red sandstone, without the intervention of the coal measures or their associated limestone, so that it is hardly possible to assign the points at which the old sandstone terminates, and is succeeded by these younger members of the same family.
The sandstones intended to be described in the following article, occupy the following localities.
1. The valley of the river Lagan from Moira to Belfast, and the north-west shore of Belfast Lough as far as the Peninsula of Magee.
2. Several points on the north-east coast of Antrim.
3. A line in the eastern division of Derry and north east of
Tyrone, extending from the mouth of the river Roe to the south west extremity of Lough Neagh.
1. Sandstone of the valley of the Lagan and Belfast Lough.
The sandstone formation may be traced from the south west extremity of the county of Antrim, where it joins that of Down, occurring between Moira and Lisburn, at Maherameslt, Spencer's bridge and Maheragall; it proceeds along the valley of the Lagan, extending into that of the Forth, a small river which becomes tributary to the Lagan near Belfast; from that town it occupies the road to Carrickfergus, whence it ascends the valley of the Woodburn, another brook discharging itself into Belfast Lough; it afterwards extends as far as Castle Chichester, in the island of Magee.
The prevailing colours of this sandstone are various shades of red and reddish brown, but it is variegated by many coloured stripes; it contains clay-galls; its texture is friable; the cement is calcareous; spangles of mica constantly occur.
At Carrickfergus, where the rock is more than usually adherent, it is raised for flags.
The upper members of this formation become argillaceous, consisting of beds of reddish brown marle, alternating with a greenish slaty marle, and an indurated grey calcareous marle; these may be seen at Maheramesk near Moira, in the valley of the Forth near Belfast, in the valley of the Woodburn, near Carrickfergus, and at Castle Chichester in the island of Magee. In all these localities, veins of a delicately white fibrous gypsum occur: salt springs, the usual accompaniment of similar strata, are found near Carrickfergus, and in other places.
The sandstone has been sunk into near Lisburn nearly 200 feet without reaching its inferior extremity, and the red marl which overlies it near Belfast is estimated as varying from 40 to 100 feet in thickness.
The outgoings of the sandstone strata are low, they dip towards the hills on the north west.
2. Sandstone on the Cast and north east coast of Antrim.
The same sandstone which has thus been traced as far as the head of Lame Lough, may thence be pursued in various points on this coast, but since its superior edge is often depressed below the level of the sea, and on the other hand its inferior edge is in one instance elevated far above that level resigning the coast to formations of greater antiquity, it presents in this part of its course an irregular and broken line, of which the description will be the more readily followed if postponed to the explanatory notes on the section of that part of the coast subjoined to this paper.
3. Sandstone in the east of Derry.
The variegated marle which covers the sandstone, first makes its appearance, rising from the level of the sea, a little to the west of Downhill near the mouth of the Roe, thence it passes to the south west, bassetting along the base of the bold headland of Macgilligan, and of Benyavenagh the most northerly mountain of the secondary chain in Londonderry. At Kedy hill the next of the groupe, the sandstone, which continues ascending towards the south west, attains the elevation of 450 feet above the sea; in the adjoining mountain called Donald's hill, its upper limit rises to 900 feet; it occupies nearly the same level in Benbradagh, and at Cragnashoack the southern point of the chain gains its greatest height, being there 1589 feet above the sea. In this mountain the sandstone and basalt are in contact, the chalk being deficient. Cragnashoack and Fairhead are, it is believed, the only instances in this district where the trap is seen decidedly reposing on any other rock than the chalk.
The sandstone of Benyavenagh, of Kedy and of Donald hill, is red, slightly variegated with yellowish stripes of a rather loose texture; it has a calcareous cement, and contains specks of mica.
The sandstone of Benbradagh has the character of sandstone slate, the texture coarsely granular; colour greenish white; with a calcareous cement, and containing spangles of mica. It extends to Dungiven, and probably reposes on the mica slate, which occupies the left bank of the Roe; its thickness in this hill cannot be less than five hundred feet. About a quarter of a mile from Dungiven, at the opening of Benady glen a sandstone conglomerate occurs, probably connected with this formation,
The insulated primitive district of Coolcroscrahan, which is situated behind Benbradagh on the north east, appears to be encircled with a ring of sandstone and marle.
In Cragnashoack, two varieties of sandstone may be traced, the upper beds consisting of a coarse grey sandstone with a calcareous cement, while the lower strata are more distinctly slaty and micaceous, and have a yellowish colour and a siliceous cement.
From Cragnashoack the sandstone extends to the south, abutting on the north Cast slope of Slieve Gallion. Near this point it appears to repose upon the shell limestone of Desert Martin, ranging from thence by Moneymore towards Lissane; on the Canesee river, a small brook which is crossed in passing between the two last mentioned villages, strata of sandstone conglomerate are found.
Near Cookstown, grey sandstone passing into white occurs on the road to Dungannon, it extends beyond the bridge over the Blackwater river, the outgoings of the strata rise south-west 43° under an angle of 8°. There are other sandstone quarries one mile and a half from Cookstown towards Moneymore; a red variety is here found, it is finely granular, with a calcareous cement, the strata crop out south-west 53°, at an angle which varies from 13° to 25° as they increase in depth.
At Coal island, in the county of Tyrone, sandstone constitutes the superficial stratum, two varieties may there be distinguished, both however agreeing in having a siliceous cement; the one is hard, coherent, and finely granular, of a yellowish grey tinge, with spangles of bright mica abundantly disseminated; the other is characterised by a looser and coarser texture, an inferior specific gravity, and fewer spangles of mica.
[This article is supplied by the Editor from the joint observations of Mr. Buckland and himself, the route pursued by Dr. Berger, not having allowed him to examine this formation in the points which exhibit it to the greatest advantage.]
This formation consists of beds of slate clay alternating with thin seams of argillaceous limestone; the limestone is compact but not crystalline, of a bluish colour passing into smoke grey; the fossils which characterise it are cornua ammonia, gryph's, and the columnar joints of the pentacrinus; this formation may be studied most advantageously along the line of coast between Gerron point and Lough Larne, on the east side of Antrim, where several sections of it are exhibited. On the south of Gerron point the extensive bay of Glenarm opens; the village of Glenarm itself is situated near the south cape of the bay, between two lofty hills of chalk covered with basalt; the range with which these are connected sweeps in a bold semicircle round the bay, receding considerably from the line of coast which generally presents only a flat beach; towards the centre of the bay, however, a low crag of red sandstone occurs, distinguished by the name of the red braes of Carnallock; half way between these and Glenarm, the clay of the lias formation may be traced, interposed between the red sand and the green sand which underlies the chalk.
But this formation is far better displayed after doubling the Cape of Glenarm in the cliffs beneath the deer park, about two miles south-east from that village; here it emerges from under the chalk and green sand, and exhibits a thickness exceeding 100 feet. Near the outlet of Lough Larne lias has been traced in Port Muck, at the north-east extremity of the island of Magee, and borings, engaged in from the hopeless speculation of discovering coal, have proved its existence among the substrata in other parts of that island, where it rests on red marle containing gypsum. On the western shore of Lough Larne, lias occurs about one mile south from the town whence the lough derives its name; it skirts the margin of the lough for some distance towards the south, but does not appear to rise many feet above high water mark. Only one thin stratum of the limestone is here visible; this contains gryphites and pentacrinites.
We understood that lias had been traced in several places between Larne and Belfast. In Colin Glen, about four miles south-south-west of that town, the mulattoe rests on a slate clay probably of this formation.
The localities already cited range along the south eastern border of the great basaltic and secondary area; the same formation appears also to exist towards the northern boundary. From specimens which we examined in the collection of the Dublin Society, we had reason to believe that this is the case in the neighbourhood of Ballycastle, but we did not ourselves observe this formation in situ in that point. About half way between Ballycastle and Bushmills near Ballintoy, the chalk formation rises sufficiently high to disclose its substrata; a valley opening towards the sea, near White Park, shews that they here consist of the slate clay of the lias formation, with gryphites and ammonites. Farther east the chalk cliffs again emerge from the level of the sea immediately beyond Dunluce Castle, and continue to rise till they are broken of at the commencement of Portrush Strand. As they here exhibit nearly the same thickness which they possess near White Park, We are naturally led to expect the recurrence of similar substrata near this point; and accordingly, in the peninsula of Portrush, a singular rock is seen, divided by interposed masses of greenstone, but containing ammonites and gryphites, and possessing exactly that character which would be assumed by the slate clay before described, if indurated by the action of heat. In the explanatory notes accompanying the sections of the coast a more detailed account of this peninsula will be found.
The cabinet of the Dublin Society contains specimens of lias from the neighbourhood of Magilligan, about eight miles westward from Portrush, and close to the north-west angle of the basaltic area.
To the general account of this rock given in the Introduction, it may be added, that on treating different specimens with acids they were found to contain about nine-tenths of calcareous matter, the residuum consisting of green chloritic grains, mixed with quartzose sand or gravel.
The mulattoe is seen in Colin glen, a valley on the south-west of Divis hill near Belfast, and in several other places on the slope of that mountain; it is evidently interposed between the slate clay of the lias formation and the chalk. It occurs also at Cermoney, about five miles north-east from Divis, The chloritic grains in the mulattoe of Divis are small and very numerous, the calcareous base greyish and compact; it is traversed by slender veins of calcareous spar: besides the smaller grains of quartz, it contains a few larger pebbles with a reddish tinge.
Since it is not quarried for any economical purpose, we have no opportunity of ascertaining accurately the thickness or extent of the bed.
It is seen however near Larne, and considerably further north near Gerron point, occupying a position corresponding to that in which it occurs near Belfast, and it seems highly probable that it extends beneath the chalk throughout the intervals separating these points; indeed I am inclined to believe that where the series approaches to completeness, this member will seldom be found wanting.
In Murloch bay, where the line of chalk commences on the cast of the greenstone mass of Fairhead, a thin seam of quartzose pebbles cemented by green sand, affords traces of this formation; it separates the chalk from a thick bed of red sand.
Along the western escarpment of the basaltic mountains in Londonderry, the mulattoe is seen underlying the chalk in Kedy Donald and Ballyness hills. Between Ballyness and Benbradagh, at the head of the vale of the Kelvin river, a wild circus opens, barred on the north by basaltic ridges, and having the insulated primitive district of Coolcroscrahan on the south: the Glen Ullin water and Donavenny brook rise also within the area, and flowing in a direction contrary to that of the Kelvin, join the Aghivey river itself, tributary to the Bann. Within this area a remarkable bed of marle occurs, which is mentioned here, on account of its containing numerous green particles; but it differs so essentially from the Mulattoe in its chemical composition, containing only one-fiftieth of calcareous matter, that it cannot with propriety, though resembling it in external appearance, be considered as the same rock: the base is a soft clay, greyish, with a slight red tinge; the structure is thin slaty; it contains some small worn quartz pebbles: the chalk has not yet been traced in this district, but along the banks of Donavenny brook, a breccia formed of chalk flints, imbedded in a calcareous cement, and underlying the basalt, seems to indicate its proximity. This breccia is probably connected with the aggregation of flints, usually interposed between the basalt and chalk, as described in the next article.
The subjoined list of the organic remains occurring in the Mulattoe of Ireland, will be found to agree closely with those presented by the green sand in England.
|Belemnite||Colin glen and
|Venus? Lin.||fragment of||Colin glen|
|Ditto||Gryphus, two varieties||Belfast Mountains|
|Mytilus||Cristal Galli||Colin glen|
Lin. Tr. v. 8. p. 6. f. 1
The chalk formation in Ireland has hitherto been frequently distinguished by the name of white limestone, its compact texture having, caused those who were acquainted only with the superior beds of the English chalk formation to hesitate in admitting their identity: the occurrence of the cornu ammonis among the fossils of the Irish chalk was also supposed to furnish another distinctive character; in both these circumstances, however, it agrees with the lower beds of the English chalk; further, its geological position, (reposing as it does on the same bed of calcareous sandstone, with chloritic grains, which supports the English chalk) the analogy of its fossils, and the absolute sameness of its general features, will no longer permit us to describe by a distinct name, this important formation.
It need not excite any surprize to find the floetz trap incumbent on a bed of such recent formation, since in the north of Italy it may be seen to rest upon strata still less ancient, and analogous to those which occur in the basins of London and of Paris.
The Irish chalk is seldom of a texture sufficiently loose to soil the hand, and in the few instances where this does take place, it is in a very slight degree: its general colour is either perfect white, or white with a very slight tinge of yellow; towards the lower beds it passes into an uniform ash colour, the texture then becomes still more compact. It is sometimes traversed by slender veins of calcareous spar, these are more frequent in the lower beds: it contains (though in very small quantity) kidney shaped nodules of iron pyrites; but the most striking of its imbedded contents are the flinty nodules which traverse the mass in regular horizontal strata, distant from each other, by an average interval, about two feet and a half; the flints cease in the lowest beds. Among the flints some of remarkable size, and of a regular and apparently organic figure occur; these are known by the name of Paramoudra, and are also found in the chalk of Norfolk. It is superfluous to describe farther the characters of these beds of flinty nodules than by saying, that they are precisely the same with those presented in every English chalk pit.
The average thickness of the chalk formation in Ireland may be estimated at between two and three hundred feet; the upper beds seem to have been partially redissolved before the basaltic mass was deposited upon them, since along the line of junction a confused aggregation of chalk flints exists, imbedded in the lowest member of the trap deposit, which is usually a bed of ochreous bole: the flints so imbedded have usually themselves acquired a red tinge, apparently by percolation, from the oxidated iron of the stratum in which they lie. This aggregation of flints, bedded in red ochreous bole, forms at Macgilligan, a stratum thirteen feet thick. It may be observed, also, near Lame and near Belfast, and seems indeed of almost universal occurrence.
List of organic remains found in the chalk of Ireland.
|Echinus||cidaris mamillata||Kenbaan Antrim; Slievgallion Derry.|
|scutatus||Pollen quarry near Agenloo Deny.|
The foregoing specimens closely agree with the fossils of the English chalk.
Belemnites are common in most of the quarries opened on this bed in Ireland: many of the bodies in the English chalk, once supposed to be belemnites, are now regarded as the pallisadoe spines of the echinus. Some few, however, are undoubted belemnites; the Irish specimens appear to be generally true belemnites.
Specimens of cornua ammonis, from five inches to one in diameter, occur in the Pollen quarry; but not having myself examined these, I cannot say how far they agree with some of the very few varieties which are found in the lower strata of chalk in England. The flinty nodules often contain fossils similar to those of the surrounding chalk, and present, in addition to these, traces of alcyonia.
The chalk is frequently traversed by basaltic dykes, and often undergoes a remarkable alteration near the point of contact; where this is the case the change sometimes extends eight or ten feet from the wall of the dyke, being at that point greatest, and thence gradually decreasing till it becomes evanescent. The extreme effect presents a dark-brown crystalline limestone, the crystals running in flakes as large as those of coarse primitive limestone; the next state is saccharine, then fine grained and arenaceous; a compact variety having a porcellaneous aspect and a bluish-grey colour succeeds: this towards the outer edge becomes yellowish white, and insensibly graduates into the unaltered chalk. The flints in the altered chalk usually assume a grey yellowish colour; the altered chalk is highly phosphorescent when subjected to heat.
Examples of this conversion of the chalk into granular marble may be seen on the east slope of Divis mountain, near Belfast, in a ravine to which Dr. Macdonald has given the name of Allan's Ravine, in honour of a mineralogical friend.
They are again exhibited in the neighbourhood of Glenarm, where a singular compound dyke, consisting of three branches, traverses the chalk: the included masses of which are altered in the manner above described.
A dyke, very similar to the preceding, occurs in the isle of Rathlin, near Church bay, and produces the same effect on the chalk. This dyke appears to be resumed on the opposite point of the Antrim coast at Kenbaan head, where the granular marble is likewise found. (See section, plate 10. and the explanatory notes.) A dyke near Ballintoy affects the chalk in the same manner.
In the south-west extremity of Antrim the same marble is said to occur near a whin dyke; the spot assigned is Bamersglen, near Trummery, about one mile north-east from Moira.
Many more examples of this very interesting fact would, there is little doubt, be found on further examination; but those cited are sufficient to prove that it is not unfrequent.
On the shore, a little to the west of the pier of Ballycastle, a singular vein occurs in the chalk, which there forms the inferior portion of a chalk capped with basalt; the basalt immediately incumbent on the chalk approaches to the character of wacke. The vein in question is calcareous, but contains imbedded balls of wacke; to the presence of which the difference of its characters from those of the chalk that it traverses, may perhaps be attributed. The limestone forming the vein is compact, breaking spontaneously into parallelepipeds, the greater side of which is perpendicular to the direction of the vein; it contains about nine-tenths of calcareous matter, the residuum appearing to be clay, with some specks of bright mica. The width of the vein is 17 feet; its direction N.E. S.W. 33°.
Near the top of the stratum of chalk which crowns the cliffs of Murloch bay, a bed five or six feet thick, of wacke approaching to basalt, occurs interstratified with the chalk in a conformable position.
I shall now proceed to trace the outgoing of the chalk round the basaltic area, beginning in the south-eastern quarter, and pursuing it to the south-western, inserting in parallel columns the names of places observed in the order of their succession along this route, and the observations I had an opportunity of making concerning the dip of the strata, the elevation of the formation, &c.
|places.||direction of the dip.||angle of dip.||height of the lower limit of the formation.||height of the upper limit of the formation.||general remarks.|
|Magheralin, near Moira, county of Devon||North west 8°||from 30 to 32||The chalk proceeds from hence towards the White mountains near Lisburn, by Soldierstown and Brookhill; it forms a low table lands. At Magheralin the bluish-grey variety occurs.|
|White mountain, near Lisburn Antrim||North-west 28||from 38 to 40|
|Colin Glen, south-west of Divis, near Belfast||about 450 feet above the level of sea.||The line of junction, with the basalt, is not strictly horizontal but very irregular.|
|East slope of Divis||about 450 feet|
|Cave hill||768 feet|
|Carmoney moutains & Carrickfergus hills||considerably lower than in Cave hill|
|Port Muck, northeast of the isle of Magee||emerging from the sea||The east coast of Magee is faced with a bold range of precipices, called tje Gollins, which are entirely basaltic, the chalk being sunk below the level of the sea|
|Shor of Lough Larue, about 1 mile south from the town||but little elevated above the sea||about 300||From hence the line of the chalk is distinctly traced in the section of the coast|
|Lurgethan mountain, near Newton Glens||Hence the mountain mass of red sand chalk and basalt, sweeps inland in a curved line towards Tesbulliagh and the head of Glendun|
|Mountain of Teabuliagh, 3 miles notrh-west of Newton Glens, forming a ridge between the rivers Glenaan and Ballyeemin||735|
|Eastern slope of Slieve Norry, towards the head of Gledun||885|
|Holster hill, middle Cushleak||695||Here the mica slate of Cushleak, receives a cap of red sandstone, chalk and basalt
|Murloch bay, on the east of Fairheader||560||652 the summit of the cliff|
|Benvaan, on the south of Murloch||885||This form a platform of chalk, covered with grass|
|Ballypatrick, 3 miles south from Fairhead||622|
|Knocklead, an insulated mountain, 2 miles south from Ballycastle||S.W||very small||895||The intersection of the conical section surface of Knocklead with the plane of the chalk stratum, forms a zone completely encircling its middle region. In the exact direction of that plane the stratum of chalk (here interrupted by a deep valley) is resumed on the slope of the opposite mountain to the south. Towards the west it catches the plain near Ardmoy, where pits are opened. At Corky, 6 miles south from Ardmoy, and higher up the same valley, there are also chalk pits. See Dr. Richardson's papers.|
|Ballycastle, west of the pier||S.W. 17||from 15 to 18||emerging from the sea||The upper limit of the chalk stratum through the line of coast between these points, about 30 miles distant, keeps nearly the level of the sea, vacillating slightly on either side of that level, but seldom rising very high, and probably seldom submerged very deeply beneath it: the same description applies to the opposite island of Rathlin. It is unnecessary here to insert detailed particulars, which will be found represented in the sections of the coast.|
|Down hill, on the west of the river Bann, in the county of Derry||N.E||emerging from the sea|
|Benyavenagh||on the north side but little elevated above the sea, on the south about 200 feet|
|Kedy||N.E||10||550?||600||The deposit here hardly exceeds forty feet|
|Donald hill||800||900||The deposit here increase in thickness to 100 feet: the upper beds are soft enough to soil the fingers, the lower quite compact.|
|Benbradagah||The thickness of the deposit here is greatly increased, amounting to no less than 363 feet: it is less regular stratified than usual. Towards its upper extremity it wears the character of considerable detached masses; in Kedy Donald and Ballyness mulattoe is interspersed between the chalk and sandstone, but in Bendradah the appear to be in contact.|
In Carintogher and Cragnashoack the chalk appears to be wanting, and the basalt to repose immediately on the sandstone; but in the insulated mass of secondary strata which cap the primitive mass of Slieve Gallion (on the south of the great valley of denudation, through which the Mayola flows) the chalk again appears underneath the basalt, and there attains its greatest elevation, being quarried at the considerable height of 1460 feet above the level of the sea, the deposit is however thin and the strata much split.
On the east of Slieve Gallion is a valley about three miles in breadth, occupied chiefly by red sand and marle, and succeeded near Moneymore by a ridge of elevated ground, exhibiting chalk, rising over the sandstone and surmounted by basalt; this ridge rises from Lough Neagh. At Ruskey it is but little raised above the level of that lake; it ranges in a direction towards the north, passing a little to the west of Magherafelt, a continuation of it may be traced to the east of the insulated primitive district of Coolcoscrahan, thence bending round the source of the Kelvin, and joining the lofty chain before described near Ballyness. The western escarpment of this ridge may be considered as limiting in that direction the great basaltic area. Benbradagh, Cragnashoack and Slieve Gallion, which lie without the boundary so assumed, forming insulated and outlying masses, separated from the principal and continuous basaltic region by vallies of denudation.
The elevation of this ridge does not appear to be very considerable; it seems probable that the mulattoe will be found underlying the chalk in this ridge as it does in the hills with which it is connected from Ballyness to Benyavenagh.
The south western limit of the area of chalk would be determined by a line from Moneymore to Magheralin, this passes diagonally across Lough Neagh, the strata which occur along it are of course concealed but it is said that chalk makes its appearance at Temple Patrick, near the north-east angle of the lake, being there exposed by the excavation of the deep valley through which the Six mile water flows.
This formation in Ulster comprises the following members.
- 1. Tabular basalt,
- 2. Columnar basalt,
- 3. Greenstone,
- 4. Greystone,
- 5. Clinkstone porphyry,
- 6. Bole or red ochre,
- 7. Wacke,
- 8. Amygdaloidal wacke,
- 9. Wood coal.
The section given by Dr. Richardson of the cliffs near the Causeway well illustrates the arrangement of these substances in constituting the aggregate mass of flœtz trap.
The following simple minerals occur imbedded in the rocks of this series; they are enumerated in the order of their more frequent occurrence.
|1. Granular olivine.||5. Zeolite, comprizing analcime, mesotype, stilbite & chabasite.|
|2. Augite.||6. Iron pyrites.|
|3. Calcareous spar.||7. Glassy felspar.|
|4. Steatite.||8. Chalcedony; passing sometimes into semi-opal.|
Granular olivine in disseminated grains seldom fails in the tabular basalt; and sometimes occurs also in greystone. It has a remarkable mode of decay, becoming tender, and iridescent.
Augite usually accompanies greenstone and sometimes greystone. Steatite is almost exclusively limited to the prismatic basalt; and it is likewise the only heterogeneous ingredient I have observed in it. It exists in small round specks, of a bottle-green colour so dark, that it is not easily discernible from the mass itself: it has a dull and waxy appearance; is soft, the streak greyish.
Glassy felspar characterizes the clinkstone porphyry. Calcareous spar, zeolite, iron pyrites, and chalcedony usually occur in association with the preceding substances.
Since the flœtz trap forms the superficial rock of the area we are now to describe, it may be proper to insert in this place a list of the most remarkable hills by which that surface is diversified, and some other remarks on its general forms. In the list of hills, the same order will be followed as before in tracing the chalk, beginning with the south-east angle and ending with the south-west. The thickness of the trap formation where it has been observed will be added.
|Eastern chain from South to North.|
|Height of the hill
above the level of
|Thickness of the
cap of trap.
|On the Northern Coast.|
|Cliff above Dunluce||157|
|Slieve Ard, the highest point of,
the Island of Rathlin
|Western chain from North to South.|
The ridge above Moneymore has not been determined.
The above table will convey an accurate idea of the general configuration of the circumference of the basaltic area.
The aggregate mass of all the formations constituting the third system of mountains, appears to form a species of basin of which the lowest point is situated near the center of the valley of the Ban; hence they rise, towards the south-east, where they lean against the exterior chains of the Down mountains; towards the north-east, where the primitive rocks of Cushleak emerge; and towards the south-west where they abut on the great central chain of Londonderry. In this last direction, all the strata attain their greatest height, the chalk standing in Slieve Gallion at 1460 feet and upwards, and the basalt in Cragnashoack at 1864 feet. It is rather remarkable that the cap of basalt grows gradually thinner in proceeding towards the same point. On the north-east edge of this basin, the inferior limit of the basalt, or its junction with the chalk, is 500 feet lower, but the thickness of the cap of trap, increasing in the inverse ratio of this diminution of level, raises the summit of Knocklead till it nearly rivals Cragnashoack. In the south-east border, at Cave hill, the line of junction is about 200 feet lower than in Knocklead; the cap of trap is there very thin, but in the adjacent summit of Divis nearly equals in thickness that which covers Knocklead. At Divis the line of junction is depressed 200 feet below the level which it occupies at Cave hill.
The high chains which form the borders of this basin on the east and west, present a far less rapid slope towards its interior than towards its circumference.
Thus between the towns of Belfast and Antrim, the watershed or highest point of the road which there crosses the eastern chain, is situated three miles and a half from Belfast, and eight miles and a half from Antrim., at the elevation of 997 feet above the level of the sea; the ascent therefore on the side of Belfast, which is that of the outgoings of the strata, is 285 feet per mile, and 117 only on the Antrim side, which is that of the dip of the strata collectively taken.
The Western chain, between Newton Limavaddy and Colerain, was found by the same method to have a fall of 177 feet per mile on the side of its outgoing, and of 110 feet in that of its dip.
The detached summits of the principal basaltic mountains form ridges which exhibit a general tendency to arrange themselves in a direction pointing north and south.
This is the prevailing rock of the trap district under consideration, occupying at least nine-tenths of its whole area.
It is disposed in strata, or rather beds, of considerable thickness.
The characters of this rock are too well known to need description.
Some of the strata contain imbedded specimens of most of the simple minerals already enumerated as subordinate to the flœtz trap formation; others are vesicular; others of an homogeneous texture. The strata also vary materially in their degree of induration, and they are occasionally separated by beds of bole, amygdaloidal wacke, &c.
The strata of columnar basalt seem to occur almost exclusively towards the northern boundary of the basaltic area.
The pillars are composed of the most compact and homogeneous variety of basalt, containing a small quantity of steatite occasionally imbedded in its mass, and possessing the property of being more or less sonorous when struck by the hammer.
Besides the well known columnar strata exhibited by the Giant's Causeway and adjoining cliffs, of which the principal is 54 feet in thickness, and a second 44 feet, similar strata are exhibited in the following places.
In Glen Ravel, at the distance of about four miles from Cushendall, I observed in the bed of a stream flowing from the mountain of Slieveance, an abrupt façade of tabular basalt, approaching the columnar form, fronting the north-east quarter.
In the isle of Rathlin, there are several systems of pillars along I the northern coast: at Kenrammer, “ the thick Head,” I counted no less than seven in succession all nearly vertical, but none very I regular; some were matted amongst themselves.
At Thivigh, “ the side-point,” there is a sort of headland sloping down into the sea; it is covered with grass, but the section sideways exhibits two assemblages of square pillars, not unlike those of Fair-head; the lower system comprehends those of the greatest dimensions; the upper one, those that are the best defined.
Rhue-na-Scarse or Roanscarave, in the town-land of Craigmacagan, presents another projecting point of land, with a real causeway, in neatness hardly inferior to the Giant's causeway itself, the pillars being almost vertical; the pavement is nearly flat or horizontal.
At Doon-point, tabular basalt alone occurs; whence it appears that the late Dr. Hamilton has mistaken it either for Thivigh or Rhua-na-Scarse.
Near Ushet-haven, on the south-east side of a hill, named in Irish Broagh-mor-na-Hoosid, there is another very elegant causeway: it runs to the extent of four hundred and sixty yards in a direction from north-east to south-west, coming up to the top of the hill towards the latter point, and falling down the opposite way. The pillars are five and six sided; a few have seven sides. I measured two of the largest size; one of them was three feet, the other two feet eight inches in diameter.
Along the slope of the hill, the pillars are so completely disengaged from the soil, that I could easily determine their angle with the horizon; I found it 22°.
On the main at Knocksoghey, the highest ridge of land between Ballycastle-bay and White-Park-bay, some flœtz-trap pillars, most of which are neatly defined, are quarried for building. Another quarry of the same rock has been opened at Ballynastrade near Ballintoy.
Croaghmore, one of the hummocks that lies at the greatest distance from the coast, is completely formed of an assemblage of pillars almost vertical, and jointed as is usually the case.
In the town-land of Craigahulliar, one of the most beautiful colonnades that can be seen lies under a mass of tabular basalt. Its extent is only one hundred and ninety feet, in a direction from east-north-east to west-south-west, presenting its façade towards the north-north-west: the pillars are from eighteen to fifteen feet in height, and the individual joints of which they are composed, one foot and a half; most of them are five-sided, others have four and six sides; they are remarkably sonorous.
I have remarked some rude attempts of the flœtz trap to assume a prismatic configuration at the bridge of Bushmills, at Magilligan, and on the north-west side of Donald-hill, but not distinct enough to be more particularly noticed.
It was natural to suppose that basaltic pillars, containing so much iron and standing in a situation approaching to perpendicular in many instances, might in time become natural magnets, with I the south pole uppermost and the north pole lowermost.
This however, I have never found to be the case; but I remarked a complete polarity in a rounded block of greenstone that was lying rather deep in the ground on the top of Donald-hill in the county of Londonderry: the needle of a pocket compass that had been laid upon it went half way round and stood there permanently, the north pole of the needle pointing to the south and the south pole to the north.
The pillars of Fairhead, and the adjacent summit of Fairhead, are composed of greenstone containing augite.
Basalt passes into greenstone in the most imperceptible manner: in the first stage, the felspar exists under the form of short, scaly parts either white or greenish; the texture becomes more finely granular and crystalline; the fracture in the great is not into flat conchoidal pieces, but more usually into irregular and blunted fragments. This finely granular and almost compact greenstone is also more tough than common flœtz-trap.
It is difficult to determine the precise geognostic relations of the basalt and greenstone in this district; but since the latter here occupies an extent comparatively small, it must be considered as a formation subordinate to the former; whether however they form distinct beds, or pass insensibly into each other by a gradual transition, is as yet undecided.
The greenstone columns of Fairhead and Cross hill are destitute of the regular articulations and neatness of form which distinguish the basaltic pillars of the Causeway; they form enormous prismatic masses, often quadrilateral, and these latter appear to be formed of a congeries of smaller prisms, aggregated in a manner which brings to the mind the clustered assemblage of shafts forming a Gothic column: the greatest length of these columns is not less than 250 feet; the greenstone is highly crystallized, the concretions being distinct and large, and contains augite.
Slievemish, remarkable mountain, which lies like a colossal landmark in the middle of the county, is from its basis to its summit composed entirely of greenstone, thus forming a mass of nine hundred feet in thickness.
Notwithstanding Slievemish has at a distance the appearance of at cone, yet it is, like all the other mountains in Antrim, much more extended in the direction from north to south than in a transverse section; the ascent is steep and almost impracticable on the west side, where we rise to the top by a succession of short terraces similar to a flight of stairs.
The greenstone is here remarkably beautiful, being of a tender mountain green, interspersed with crystals of augite and granular olivine; the fracture is in flat or scaly concretions: it lies in distinct tabular masses two or three inches thick, perpendicular to the horizon, or sometimes with a slight dip to the westward.
The mountain of Teabuliagh near Newton Glens, has a cap of finely granular greenstone five hundred feet in thickness.
The rock which overlies the chalk at Magheralin, may perhaps with greater propriety be arranged as greenstone than basalt.
The top of Squires hill and of Cave hill, near Belfast, are both composed of this rock.
The little table-land forming the summit of Divis mountain consists of a beautiful clinkstone porphyry of a reddish brown colour, containing elongated lamellar crystals of glassy felspar, and concretions of bluish white chalcedony: the rock is very sonorous. A variety of clinkstone porphyry also occurs in the neighbourhood of the old red sandstone, near Newton Glens, so situated that it is difficult to pronounce what is its position or geognostic relations with regard to that rock: it will be more particularly described in the explanatory notes on the section.
The porphyries of Sandy-brae, &c. in the interior of the basaltic aræ, of which the geognostic situation is likewise uncertain, are referred to a separate article at the end of these extracts.
Occurs in beds of various thickness, alternating with and underlying the basaltic strata in the cliffs near the Giant's Causeway; in Cave hill, on Macgilligan, along the east side of Ushet lough in the isle of Rathlin, and in several other places. Although the specific gravity of the bole is low it contains a great proportion of iron, (acting powerfully on the magnet) and might with propriety be classed among the earthy and ochrey iron stones.
Near the Giant's Causeway it is confined to the lower portion of the cliffs, where beds of it in various states are seen, sometimes assuming the character of a decided red ochre, sometimes variegated red and cream yellow, containing imbedded heterogeneous portions, wearing at a small distance the external appearance of agates, but when examined found to be as tender as the softest steatite. Specific gravity 1,92.
Similar characters apply to the bole in other places. The lowest bed of the trap formation where it comes in contact with the upper surface of the chalk, and contains chalk flints imbedded in its mass, is most generally a red ochrey bole.
This rock appears to be of extremely rare occurrence in the Ulster trap series; it is found however in Portnoffer near the Causeway, underlying the upper stratum of columnar basalt, and alternating with the wood coal described in article 9; the thickness of the whole amounting to eight feet: it forms vesicular concretions of a trapezoidal or rounded figure; without coherence; texture dull and earthy, with a few specks of mica; the colour varies from yellowish grey to brownish grey, apparently derived from iron ochre. The bed or horizontal dyke of wacke traversing the chalk cliff at Murlough bay has before been noticed. A seam of wacke, not more than three or four inches thick, is interposed between the greenstone and sandstone of Cross hill.
Colour variegated reddish or greyish brown; fracture dull and scaly; soft, owing to seatite which occurs both in grains and in larger disseminated concretions.
It also contains radiated zeolite in considerable abundance, in concretions of all sizes: and cubic zeolite has been found at Benyavenagh, but is very rare.
Chalcedony is likewise found imbedded in this rock, but is not very common.
Has been found in seams varying from two inches to four or five feet in thickness, alternating with trap rocks near Ballintoy; associated with the bed of wacke, underlying the upper columnar stratum in the cliffs of Portnoffer on the east of the Giant's Causeway, at Killymorris near the center of the basaltic area, and at Portmore and other places along the eastern shore of Lough Neagh. At Portmore the beds are said to exceed considerably in thickness the dimensions above given. The texture of the wood is often remarkably distinct, and indicates that it is a species of fir. It has even been asserted that the roots and branches of the trees could be traced.
The wood coal at Portnoffer has the exterior surface of some of its fragments penetrated to a certain depth by small nests of augite imperfectly crystallized.
1. Account of certain porphyritic rocks of doubtful formation.
2. Alluvial formations.
Near the centre of the basaltic area, and about seven miles north north-east from Antrim, a very remarkable district occurs, in which a reddish variety of clay porphyry prevails.
This district is situated between Templepatrick on the south and Kells and Connor on the north, including an area of about four English miles; proceeding from Connor in a westerly direction, you first meet the porphyry formation at Camecome, scarcely a mile distant from that village, and continue upon it for about three miles, as far as the source of a small brook called Loonburn, which empties itself into the Six mile water above Templepatrick.
Numerous small hills are scattered over this tract; Sandy-brae, Brown Dodd, Tardree, Forthill, and Carnearny, are the names of the most considerable. These are all low hummocks, exhibiting in their outline a striking approach to regular segments of circles, and, as Dr. Richardson has well remarked, strongly contrasted with the basaltic ridges which every where surround the porphyritic area, and uniformly present an abrupt escarpment on the one side, and a gentle slope on the other. The hill of Sandy-brae rises 537 feet above Doagh, which, from its situation relatively to Lough Neagh, cannot be estimated at less than 200 feet above the level of the sea.
The clay porphyry of this district has a reddish brown basis, containing imbedded in it concretions of smoky quartz, earthy and glassy crystals of felspar, and olivine. Specific gravity 2,43. It is usually much weathered, its decomposition giving rise to a red sandy soil, whence the district derives its name.
At Carnecome is a large standing block of clay porphyry, of an ash-grey colour, containing the same ingredients as that above described.
Pitchstone porphyry and pearlstone porphyry occur apparently as subordinate members in this formation.
Two large masses of each variety may be seen at the bridge across the Loonburn, on the road from Connor to Doagh.
The most sound and interior part is bluish black, possessing a splendent and vitreous lustre: its specific gravity is 2,52; a little nearer to the surface it passes to olive green, with a waxy texture, and has a specific gravity of 2,50; at the surface it appears yellowish green and rather earthy, with a specific gravity of 2,40; all these different shades sometimes occur in the same specimen, and appear evidently to result from decay.
Engaged in the basis round concretions of smoky and vitreous quartz, with lamellar crystals of glassy felspar possessing a yellowish tinge occur. Common opal, nearly approaching to precious opal, accompanies this pitchstone porphyry; it is either disseminated in plates or in small strings.
The texture is formed of vesicular and distinct concretions loosely coherent, of an elongated or irregular shape, the angles being smooth and rounded. Their colour is smoke-grey or bluish, with a pearly lustre. They seem formed of concentric and very thin coats. The fracture of this mineral is imperfectly conchoidal; it cuts glass but faintly, and emits a faint argillaceous smell when breathed upon. Fragments, exposed to the blowpipe, intumesce to four or five times their first volume, fusing into a foamy and light glass, not unlike pumice stone. Radiated zeolite is the only fossil I am acquainted with that resembles pearlstone in the characters of fusion. The specific gravity of two different specimens, I found 2,38.
About 76 miles to the north of this district, at Ballycloghan, two miles north-west from the village of Broughshane, there is a bed of clay porphyry extending towards Slieve Mish on the south-east; it is quarried as a freestone, and when raised in thick slabs, is used for window seats.
The basis is compact and sometimes earthy, of a greyish white colour; it contains imbedded concretions of smoky quartz, lamellar crystals of white felspar, and a few interspersed plates of brown mica; it adheres to the tongue slightly, and fuses into a transparent but frothy enamel; the specific gravity is 2,43.
The occurrence of a porphyritic district, surrounded on all sides by a vast area of basalt, must be considered as one of the most singular facts which the country we have examined presents. The question to what formation do these porphyritic rocks belong, immediately suggests itself, but the materials which observation has hithereo afforded cannot be considered as authorizing any decided answer. Many geologists, among whom it will be sufficient to mention Dr. Macdonnel and Dr. Richardson, consider them as referable to the class of transition or primitive rocks, and regard their appearance in this situation as the result of a vast denudation which has stripped away the basaltic masses which once covered them. This opinion seems principally grounded on the resemblance between these porphyries and those occurring in more ancient countries.
Another opinion represents these rocks as subordinate members in the flœtz trap formation. In favor of this it is urged that the general dip of the strata constituting the great basin of the basaltic area must (unless we suppose them to have been affected by a great dislocation and elevation) have carried the substrata on which the trap reposes to a much lower level than that which is actually occupied by the hillocks in question; and it has been asserted as a corroborating fact that the porphyry is actually seen to rest upon the basalt in one of the ravines which traverses the district, while still lower at Templepatrick, the stratum of chalk on which the basalt really reposes makes its appearance.
The occurrence of similar rocks subordinate to flœtz trap is by no means unexampled in other countries. Near Newry a narrow dyke of pitchstone porphyry extends for half a mile to the west, and in a continuation of the same line clay porphyry occurs. This dyke traverses sienitic rocks: an account of it is given in Dr. Fitton's notes on the mineralogy of the vicinity of Dublin, p. 53.
A singular formation of clay porphyry approaching to clinkstone porphyry occurs near Newtonglens in Antrim, seemingly associated with the old red sandstone; it is particularly described in the notes on the sections of the coast, appended to the present paper.
At B. Macrevan, in the county of Antrim, half way between Glenevey and Lough Neagh, bituminous wood in disseminated pieces loosely imbedded in a loamy soil has been found.
Two shafts, each sixty feet deep, were formerly sunk to obtain it, but are now abandoned, the stock being exhausted. I however procured some specimens. The wood has evidently retained its fibrous texture, and burns with a vivid flame; specific gravity 1,124.
Not far from B. Macrevan, at B. Vorally, in Sandy Bay, there are still remaining on the shore, a few stumps of bituminized wood, the fissures of which are penetrated by silex, and sometimes even lined by quartz crystals. It does not flame, and its specific gravity, from the quartz it contains, amounts to 2,267.
It was once the general opinion that the waters of the lough had the property of petrifying, and that the quartz contained in the bituminous wood of Sandy Bay had been deposited from them. The experiments however, made by Mr. Tennant during his stay at Belfast, appear to discountenance that belief, since he found no traces of silica whatsoever in the water of the lough.
The penetration of the wood by the siliceous matter is sometimes compleat; sometimes one extremity of the same fragment is thus petrified while the other remains in a ligneous state. The oak, the holly, and the hazel appear to have been the trees thus affected.
A phenomenon occurring in some caverns near the Black Rock, on the south of Church Bay in the isle of Rathlin, may be properly referred to this article.
These caverns (four in number) although excavated in the basaltic rock and at a point remote from any calcareous formation, are yet invested with calcareous stalactites, depending from their roofs, and by their droppings upon the floor depositing a crust of about an inch in thickness.
This circumstance appears worthy of attention, since the calcareous matter seems evidently, from the situation of the caverns, to have been derived from that which enters as a chemical ingredient into the composition of the basaltic rock, separated from the mass and deposited in its present situation by the percolation of water which the rain or springs must have furnished.
It proves therefore the permeability of basalt which has sometimes been denied, and gives countenance to the opinions of those who consider the nodules of calcareous spar and zeolite, occurring in the amygdaloidal varieties, as the results of an infiltration which has gradually filled up what were once vesicular cavities; it is remarkable that the substances so occurring are such as the chemical constitution of the matrix would qualify it to afford by a similar process; and in the instances above described, that very process may be detected passing under our immediate observation.
A vesicular variety of basalt, of which the pores contain water, occurs at Ballylaglan on the north of Coleraine. Dr. Richardson has mentioned it as a proof of the aqueous origin of basalt, believing it impossible for the water contained to have insinuated itself since the original consolidation of the rock; but it has been found that by heating this basalt, the water may readily be driven ofl; and doubtless the same pores which allow its escape would with equal facility permit its entrance.
Collected by the Rev. W. Conybeare, M.G.S.
From the joint Observations of
The Rev W. Buckland, M.G.S. Reader in Mineralogy to the University of Oxford,
The Section, Plates 10. 10*. accompanying these notes, exhibits a line of coast extending rather more than fifty miles from the promontory on the south of Glenarm in Antrim, to the strand of Macgilligan in Londonderry, where the basaltic mountains receding to the south finally quit the vicinity of the sea.
Throughout this space the cliffs present a series of highly interesting and instructive sections, which indeed leave us little to desire in elucidating the structure of this important district, since they traverse and expose in succession all its constituent formations from the mica slate to the floetz trap.
In endeavouring to lay before the Society a transcript from these records of nature, the general principle adopted has been that of a combined series of elevations, projected upon vertical planes of which the direction is continually shifting so as to be always parallel to the greater flexures of the coast: but, since it appeared in many instances that the introduction of perspective would convey a clearer idea of the phenomena to be represented, the principle above laid down has often been departed from; and the delineations in their present state cannot be considered either as elevations or perspective views, in any strict sense. Two principles which cannot in truth be combined have certainly thus been forced together, but it is hoped usefully so with reference to the information to be conveyed.
To those who consider the varying aspects assumed by the same points when viewed under different angles in sailing past them, it must be obvious that much of incorrectness will of necessity intrude into the attempt of persons not practically experienced in surveying, to construct such projections as have been described from their detached sketches; and to such objections it is felt that the present example lies particularly open: it is indeed offered merely as an approximation to a correct representation. Such as it is, however, it is presented to the Society, in the belief that the errors are not of a nature to interfere with fidelity in laying down the great geological features which distinguish this interesting coast, and that these features are in themselves so important as to render any delineation which might in this respect (however imperfect in others) pretend to some degree of accuracy, desirable.
In describing these sections the very ample information collected by Dr. Berger concerning this district, will render it unnecessary to exceed the limits of a rapid survey; this I shall commence at the south-east extremity of the line represented, proceeding towards the north-west.
In this direction the lias formation is seen at the extremity of the section occupying the lower regions of the promontory over which the Deer park of Glenarm extends, near its southern point; the ground here ascends with a rapid slope, above which cliffs of chalk covered by basalt rise to a considerable height: advancing towards the north the lias dips beneath the level of the sea, and the beach is skirted by a chalk cliff.
On doubling the promontory, Glenarm presents itself situated in a narrow valley bounded on the east by the Deer park hill, and on the west by Bellaire hill, both of which in their sections towards the sea exhibit cliffs of chalk covered by basaltic platforms. The distant hill seen rising above the opening of this valley, is Slieve Mish, it consists entirely of greenstone. Near Glenarm is the remarkable compound dyke mentioned in page 172.
The headlands of Glenarm on the south, land the bold promontory of Gerron on the north, include a bay which extends for six miles between them. Towards the centre of this bay the coast is generally flat, but a low bank of trifling extent called the braes of Carnalloch, here displays the red marle and sandstone (No. 4. of the Introduction); and the lias and green sand (No. 3 and 2.) may be traced between the chalk of Bellaire hill and this point; these formations occupy the base of the mountains which are seen skirting this bay at a small distance inland, and of which the superior regions are exclusively composed of rocks of the floetz trap formation. The summits of these mountains considerably exceed 1000 feet in height. The cliffs of Gerron point are formed by the abrupt termination against the coast of a ridge connected with them.
These cliffs exhibit basalt towards the summit, and chalk in their central regions; the beds on which the chalk reposes are concealed along the base of the promontory by enormous masses of the superior strata, which have subsided in this direction and form a range of advanced terraces, causing the headland when viewed at a small distance, to appear as if composed of four strata alternately chalk and basalt. Those who have visited the southern coast of the Isle of Wight will on seeing Gerron point be strongly reminded of the similar subsidence there distinguished by the name of the under-cliff.
After doubling Gerron point a scene of the greatest magnificence gradually discloses itself; the ridge running westward from Gerron is seen extending far inland on the south of the valley of Glenarif and crowned by the conical summit of Cruach-a-Crue; while on the opposite side of that valley a similar but more lofty ridge terminates in the singular mountain of Lurgethan, which appears as the frustum of an enormous cone of great height and comparatively narrow base.
Both Cruach-a-Crue and Lurgethan present thick basaltic masses on their summits, resting at considerable elevations on strata of chalk; in the former mountain the lower beds are concealed by grassy slopes, in the latter red sandstone is exhibited in several points towards its central region: here probably a more full examination might detect and ascertain the thickness of the green sand and lias, which from the general structure of the district might be expected to intervene between the chalk and red sand; the spot certainly appears very favorable for such an enquiry.
At the foot of Lurgethan the coast presents two low cliffs divided by a valley which affords a passage towards the sea to a small rivulet; these cliffs are composed of red sandstone and a conglomerate containing rounded fragments of quartz, the rock being altogether similar to that which Dr. Berger has described as the old red sandstone in the neighbouring hill on which Cushendon church is built: since, between these cliffs and the sections visible in the precipices which occur near the summit of Lurgethan, a considerable space intervenes through which the substrata are concealed by grassy or cultivated slopes, it is not easy to determine the geological relations existing between the sandstone in the higher region and that on the level of the sea.
On the most southern of these cliffs near the strand at the mouth of the Glenarif river stand the remains of an old fortification known by the name of Red bay Castle; close to this spot several basaltic dykes traverse the conglomerate, one of these, remarkable for its great thickness, having resisted the action of the waves which have encroached considerably on the adjacent cliffs, presents some bold detached crags projecting from the beach. The conglomerate forming the wall of the dyke has undergone a great degree of induration, its cement assuming the appearance of a compact hornstone; thus it has been enabled to oppose to the sea a resistance almost equal to that of the basalt itself, and is still seen adhering on the sides of the advanced crags above mentioned.
From the most northerly of the two cliffs a ridge extends towards the ascent of Lurgethan on the south-west, running through the townland of Killnadore; in several points along this ridge, and particularly at Nockans and Tully, a very remarkable formation of porphyry may be traced. Dr. Berger considers it as clinkstone porphyry and describes two varieties here noticed by him; one of them distinguished by a reddish brown and the other by a bluish grey colour, both containing concretions of glassy quartz and of calcareous spar, the latter of which in the first variety occurs in the form of detached crystals, but forms veins in the second. He adds that it crops out in independent masses. He mentions also Court Martin, an old entrenchment near Cushendahl, on the road to Cushendon, as another locality of the porphyry.
From the observations made by Mr. Buckland and myself on these rocks we were induced to believe that they were associated with and subordinate to the old red sandstone. The position of the ridge of Killnadore is distinctly indicated in the section.
On the north of this ridge is the mouth of the river Balyeemin on which the little village known by the double name of Newton Glens or Cushendahl is situated; the loftier ridge of Lurgethan also runs to the west forming a bold amphitheatre of beds skirting the southern bank of that river, and at length joining the still more elevated mountain of Teabuliagh which extends between the confluence of the Ballyeemin and the Glenaan a tributary streamlet. The summit of Teabuliagh exhibits basalt, chalk, and red sandstone; and from hence these formations continue to crown the mountain chain which runs at the distance of about six miles from the sea, while the lower hills which occur nearer the coast are occupied by rocks of much greater antiquity.
Returning to Cushendall and tracing these as exhibited in the section of the cliffs, the old red sandstone and conglomerate (fully described page 149) lines the coast as far as the bay of Cushendon; near its termination at this point it is worn into many grotesque forms, and presents considerable caverns: here Mr. Buckland observed that the cement of the conglomerate often passed from a mechanical to a chemical state, and assumed all the characters of regular clay porphyry of a reddish colour, which appears to indicate the true relations of the porphyry at Killnadore.
On the north of Cushendon point the valley of Glendun is seen penetrating far into the country; through this valley the mica slate makes its appearance underlying the old red sandstone. The mountains in the back ground are a continuation of the secondary range from Teabuliah and are of similar constitution.
The cliffs of Cushleak ranging from the mouth of the Glendun river to Murloch bay present mica slate, containing subordinate beds of primitive limestone, syenite, and felspar porphyry.
The hills rising above these cliffs, as they approach Murloch bay, become covered with beds of red sandstone and chalk towards their summits, and some of the highest points exhibit caps of basalt. It is highly desirable that the slopes of these hills should be carefully explored; by examining the channels furrowed in their sides by the wintry torrents, we might hope to ascertain the whole series of rocks here intervening between the primitive formations and chalk.
Murloch bay is bounded on the south by a promontory, which exhibits in the cliffs at its base beds of mica slate dipping towards the north-west under an angle of 45°; but immense slopes of debris (consisting mostly of primitive rocks) interfere to prevent the possibility of determining the structure of the lower regions of the cliff as soon as the bay is entered. Above these slopes red sandstone is seen supporting chalk; the former rock attains the elevation of 560 feet above the beach, the latter extends to the summit of the cliff about one hundred feet higher: between the red sand and the chalk a thin bed of green sand, cementing quartzose pebbles, may be traced. The bed of wacke here occurring in the chalk, has been noticed, page 173, and several whin dykes are observable traversing the red sand. The most remarkable objects on the beach are a large basaltic dyke, which rises in a projecting crag; and still further to the north, near the centre of the bay, the detached conical mount of Drimnakill, which is formed by a vast subsided mass of columnar greenstone, the columns being thrown confusedly together in every possible direction. Near the whin dyke a conglomerate, resembling that of Cushendon, may be seen; so that this rock appears on either side to skirt the primitive formations of Cushleak.
Beyond the mount of Drimnakill, the lower parts of the cliffs are still concealed by slopes of debris, among which subsided masses of columnar greenstone form the prevailing feature; and near this point bold and lofty precipices of that rock take place of the chalk and red sandstone in the higher region, and, stretching to the north, constitute the well-known promontory of Fairhead, which bounds Murloch bay on that side.
It is greatly to be regretted, that the convulsions of which this bay appears to have been the theatre, having covered the regular strata with shattered fragments and piles of ruin, heaped together in the wildest confusion, have thrown an impenetrable obscurity over its structure, which, from the narrow space in which so many formations are successively exhibited, must be regarded as more important than that of any other point on this coast.
From this cause it is impossible to ascertain the exact relations of the greenstone at this point with the chalk and red sand. The general appearance is, that the mass of greenstone abuts abruptly against that of the sandstone, both being placed at the same level. It has been said that the greenstone and sandstone are here to be observed, alternating with each other. (See notes to the poem of the Giant's Causeway, by Mr. Drummond.) But it may be questioned whether this representation is not founded on a hasty view of some of the subsided masses above mentioned. The greenstone, where it first appears, exhibits two columnar strata, separated by a bed of amorphous greenstone.
Near the point where the greenstone is lost, we observed the traces of adits formerly driven into the sandstone in search of coal, beds of which appear to occur among the lower members of the sandstone formation. The position of one of these adits is such as to afford some countenance to the supposition, that the greenstone and sandstone here alternate; and, since a thin horizontal bed of trap certainly does occur interstratified with the coal measures on the other side of Fairhead, such an opinion cannot be rejected without further examination.
It may be conjectured that the coal measures underlie the greenstone through the entire range of Fairhead, since they again make their appearance beneath it on the east side of the promontory; but the whole of its base is too much encumbered with debris to allow of our ascertaining this point. On the west side, at a lower level than the coal measures, we noticed a ledge of clay porphyry running out to sea; this rock is probably analogous to the porphyries of Killnadore.
The columnar greenstone of Fairhead, and the coal measures which underlie that rock in Gobb cliff, have been already described, and I have to mention but very few additional circumstances, principally relating to the whin dykes which traverse that cliff`; of these dykes, the first in advancing from the east, is Carrick Mawr, the “ great crag,” a name well deserved by its dimensions: it forms a broad causeway, traversing the beach and terminating in a nearly insulated mass of rocks rising about thirty feet; of this mass only the central line consists of the dyke itself, the sides being evidently composed of portions of the strata traversed by it, but much altered in their character and degree of induration by its contact. These beds appear to have been chiefly derived from the slate clay of the coal measures, which has become so compact as to assume the character of flinty slate. In one point this rock maybe seen on one side of the dyke, and on the other the sandstone grit, which usually accompanies the coal beds, also in a highly indurated state: its colour changed from red to white, and its mass penetrated by minute grains of iron pyrites. At fifteen yards distance from the dyke the alteration ceases, and the sandstone resumes its usual character, becoming reddish and destitute of pyrites. Where the dyke traverses the great insulated mass of slate, it is very irregular both in thickness and direction. The works of Gob colliery have reached, this dyke 500 yards inland from the face of the cliff: the coal is altered by it to a considerable distance from its point of contact, being reduced to the state of a cinder, which can be employed only for burning lime. This dyke throws out the measures of Gob colliery, which are not recovered on its eastern side: its breadth is about 12 feet where it comes to the surface of the cliff, but varies considerably in different parts of its course.
Immediately behind Carrick Mawr, an opening may be seen in the cliff which forms the channel of a torrent; pursuing this inland for the distance of about a mile it is found to issue from a small lake surrounded with basaltic cliffs. We were ourselves prevented from visiting this spot, but from the descriptions we received of it were led to wonder, that none of those who consider this country as volcanic had selected it, as a crater,
At some distance east from Carrick Mawr dyke is the Saltpans dyke, which also throws out the coal of Gob mine on the west; its breadth is eight yards. Still further is a thin dyke; both these alter the coal. North Star dyke follows. This is often cut through in the collieries, its breadth is eight yards: it does not shift the coal, but reduces it to cinder for nine feet on each side of its contact.
Where the cliff declines towards Ballycastle, the coal measures terminate abruptly, and are succeeded by trap rocks. It seems that this position must have been produced by a fault and subsidence.
Near Ballycastle a broad valley opens into the interior towards wards the south and east. This valley is principally occupied by the red sandstone, which, in those directions terminates against the mica slate, forming the base of Knocklead (the round-backed mountain seen in the distance) and extending thus far in continuation of the primitive district already described in Cushleak. Between 4 and 500 feet on the ascent of this mountain, a bed of primitive limestone may be traced. Above this, through an interval of about 400 feet, the strata have not been ascertained; but probably the red sandstone, and perhaps also the lias and mulattoe, may be found in this space, since the chalk appears at the elevation of between 8 and 900 feet; and the whole is crowned by a cap of basalt, 980 feet in thickness.
On the eastern side of Ballycastle bay, the basalt rises into low cliffs, and the substratum of chalk emerges from the level of the sea, but soon sinks again. In this interval the vein of wacke and compact limestone, described by Dr. Berger, page 172, occurs; from hence to Kenbaan there is nothing that requires particular attention.
Of this interesting spot it is hoped that the general section, and the views in Plate 11. fig. 2. will convey an accurate conception.
A mass of chalk, extending about a furlong in the face of the cliffs, is here seen, terminated abruptly, and with every appearance of violent convulsion at both extremities. Towards the east it is underlain as well as overlain by basalt, and loses itself, forming a narrow tongue surrounded by that rock. In this quarter a portion of the chalky strata, elsewhere horizontal, exhibits a remarkable curvature. Towards the west the chalk runs far out to sea, forming a sharp and narrow point of land, of which the greatest height is about 70 feet. The isthmus which joins this point with the main land, is completely broken through. The ruins of an ancient tower rise above this chasm, on the sides of which the course of an enormous whin dyke may be traced; and the whole promontory is shattered in every direction, masses of basalt (sometimes mingled with chalky debris and flints) protruding through numerous fissures. The chalk, where it comes into contact with these dykes, is often converted into a compact and crystalline marble.
The mass of chalk just described is covered by thick beds of basalt; but near the top of the cliffs, which here rise about 300 feet, are seen two other beds, which viewed from a small distance appear to be chalk, one rising to the surface above the western, and the other above the eastern extremity of the inferior chalky mass. These, on nearer examination, appeared to be a breccia composed of fragments of chalk, of various sizes, intermixed with flints and basaltic concretions. The outer surfaces of these fragments are much altered, and they are penetrated by small nests of a greenish substance appearing to be steatite, the interior presenting the chalk in its usual state.
The opposite coast of the island of Rathlin exhibits, as will be seen by the section of it in Plate 10. an exact analogy to that of the main land; and on that point of it lying directly over against Kenbaan head, a singular combination of dykes occurs, seeming to be continuations of those which at the latter place appear to have been attended with such extraordinary disturbances. Here, within the distance of 90 feet, three dykes may be seen traversing the chalk, which is converted into a finely granular marble, where contiguous to the two outer dykes, and through the whole of the masses included between these and the central one: these dykes are situated a little to the west of Church bay, they are marked in the section Plate 10; and a ground plan, on a larger scale, is added in Pl. 11. fig. 2. representing their appearance as traced upon the beach.
Carrick-a-rede is the next remarkable headland to Kenbaan. Between these promontories the chalk twice rises above the level of the sea, and as often sinks beneath it; the strata exhibiting evident marks of dislocation, and either of elevation or subsidence.
Carrick-a-rede is an insulated crag of rudely prismatic basalt; the dangerous rope bridge thrown across to connect it with the mainland by those engaged in the salmon fishery, has rendered it celebrated.
Beyond Carrick-a-rede the limestone again rises, is traversed by some whin dykes and near Sheep island forms a cliff about 100 feet in height. Here a large detached basaltic rock rises close to it on the beach, appearing to have been brought by subsidence to the same level, and to a parallel position. The same remark may be extended to Sheep island, itself a basaltic mass.
Above these cliffs are seen the lofty basaltic hills of Knocksoghey and Croaghmore, where are the columnar strata mentioned in page 183; the wood coal described in page 188 also occurs in this neighbourhood, close to the village of Ballintoy, of which the spire is seen forming a conspicuous land mark.
The chalk suffers a partial interruption, attended as usual with dislocation of the strata, near Ballintoy, but again rises to considerable height on the sloping ground which skirts White Park bay. In a valley near Ballintoy, the inferior limit of the chalk is exposed, and a substratum of a bluish slate clay, containing gryphites and ammonites, (apparently the same which alternates with the lias near Glenarm) is laid open. The chalk is abruptly broken off on the east of White Park bay, in a little cove called Port Braddin. Here the basalt abuts directly against the chalk, and that arrangement of strata, so well known as constituting the magnificent range of promontories in the neighbourhood of the Giant's Causeway, commences. Since the chalk, which is here placed on the same level with the highest of these strata, must certainly in its regular place occur beneath the lowest, and since the whole series has been ascertained considerably to exceed 400 feet in thickness, the subsidence of the basaltic mass at this point must have been very great.
The neighbourhood of the causeway, with all its remarkable features, its superb storied façades, distinguished by a double order of columns, its whin dykes and its caverns, have been so fully and ably described by Dr. Richardson in the Philosophical Transactions, that they are already familiar to every reader interested in such subjects. His list of the strata here ascertained has been already given, page 177. On the strand near the mouth of the Bush, are two parallel ledges of chalk and basalt.
The appearances of the cliffs between Bushfoot Strand and Portrush Strand, are so similar to those which we have already found frequent occasion to notice, that this part of the section does not appear to require further elucidation. Near the termination of the chalk cliff on the west, a very inexplicable phenomenon presents itself; a large spherical mass of basalt, appearing to be completely enveloped in the chalk.
The singular peninsula of Portrush demands more particular attention; a long line of strand separates it on either side from the cliffs which rise at about the distance of a mile to the east and west.
The peninsula itself, which may be about a mile in circumference, is fenced with low cliffs on the west, north, and east; those on the west present a rudely prismatic greenstone; those on the north and east tabular masses of greenstone, overlying, and in some places appearing to alternate with, a very remarkable rock which has been the subject of much discussion among the supporters of opposite theories.
It is a flinty slate, exactly similar to the indurated slate clay which forms the wall of the Carrick Mawr dyke, in the Ballycastle collieries; and the analogy is rendered the more striking, from the further resemblance of the greenstone of that dyke to the greenstone of these cliffs. In this flinty slate are contained numerous impressions of cornua ammonis invested with pyrites, the shells being similar to those found in the slate clay underlying the chalk near Ballintoy.
The advocates of the Neptunian theory consider this rock as a variety of basalt, and refer triumphantly to its included fossils, as affording a conclusive argument against the Volcanists.
Professor Playfair, on the other hand, expresses his belief that “ the rock containing the shells is the schistus or stratified stone, which serves as the base of the basaltes, and which has acquired an high degree of induration by the vicinity of the great ignited mass of whinstone.” See illustrations of the Huttonian theory.
The resemblance of this rock to the indurated slate-clay of Carrickmawr, and the identity of its fossils with those of the slate-clay underlying the chalk near Ballintoy, together with the relative position of Portrush and of the chalky cliffs on the east, appeared to us to give the greatest weight to this very ingenious conjecture; and we felt convinced while examining the spot, that the rock was no other than the slate-clay of the lias formation in an indurated state.
The remaining portion of the section will sufficiently explain itself: it terminates where the secondary formations turning to the south finally quit the coast. The headland of Macgilligan, the most northern point of the mountain range formed by their escarpment in the west of Derry, is seen at the distance of three miles inland; it is remarkable for the grandeur of the scenery presented by its shattered precipices.
Of the island of Rathlin or Raghery, the section represents only the cliffs extending from its most westerly promontory to the centre of its southern line of coast. From the point where the section terminates, the precipices round the southern cape and the eastern and northern shores are exclusively composed of basalt, and frequently present columnar groups. (See page 182.)
To the conjecture of Dr. Berger, however, that sandstone and older rocks exist at no great depth in the south-eastern quarter of the island. (See page 151.) the positive assertion of Mr. Hamilton must be considered as adding great weight. “ An heterogeneous mass of freestone, coals, iron ore, &c. which forms the east side of Ballycastle Bay, and appears quite different from the common fossils of the country, may be traced also directly opposite running under Raghery, with circumstances which almost demonstrably ascertain them to be the same veins.” Letters on the coast of Antrim, page 8.
From the exact correspondence in structure between the opposite points of coast upon this island and the main, Mr. H. infers that “ Raghery standing as it were in the midst between this and the Scottish coast may be the surviving fragment of a large tract of country, which at some period of time has been buried in the deep.”
The pebbles of sienite which are scattered over various parts this island, and must have travelled hither from Cushleak, concur to prove the later formation of the channel now separating it from the mainland. Dr. Mac Culloch observed appearances analogous to these in the island of Staffa, and deduces from them the same conclusion. See Transactions of the Geological Society, Vol. II. page 207.
The facts stated in the introduction to this paper (see pages 123, 125.) are of such a nature as leave no doubt in the mind of the writer, of the former continuity of the Irish and Scotch coasts.
The greatest depth of the channel between Rathlin and Antrim is 53 fathoms, and between the north-east of Ireland and south-west of Scotland 90 fathoms.
It now only remains to explain the principles followed in the construction of the geological maps accompanying this paper.
The larger of these comprehends the whole of the district which has been described, as it is limited, in the Introduction, page 121.
Much attention has been bestowed on the delineation of the mountains chains; the heights of the principal summits are inserted from Dr. Berger's measurements.
The more important soundings on the coast are added.
In a map on so small a scale it was not found practicable to distinguish every minute formation by different colours; it became therefore necessary to assume as the basis of the colouring, the prevailing rocks which, in association with other subordinate formations, constitute districts of considerable extent.
Thus the districts to which separate colours have been assigned, are
1. The granitic district of the Mourne mountains, &c. The hornblende or primitive trap rocks, on the border of this district, are distinguished by an appropriate mark.
2. The mica slate districts of Londonderry and Antrim, including primitive limestone, felspar, porphyry, and sienite, each distinguished by peculiar signs.
3. The transition district, encircling the Mourne mountains. The lead mines in this district are marked.
4. The shell limestone underlying the coal.
5. The sandstone district, of which the lower beds are associated with the coal, and the upper with gypsum. The collieries are distinguished by black circles.
6. The zone formed by the basset of the chalk round the escarpments of the mountain groupe, which forms our third system. It was not possible to insert the lias and greensand intervening between the sandstone and chalk; but their localities are fully indicated in the paper and in the sections.
7. The great district of floetz trap.
The smaller map, Pl. 9, exhibits a general outline of the opposite points of Scotland and Ireland, intended to shew the connection between the principal formations in each country, as pointed out in the Introduction. The colours represent─1. Granite. 2. The mica slate district of the Grampians, and the northern chain in Ireland. 3. The transition district of the Lead hills in Scotland, and of Down in Ireland. 4. The secondary rocks associated in the coal district.5. The floetz trap districts.
in the counties of
LOUTH, ARMAGH, DOWN, ANTRIM, AND LONDONDERRY,
Calculated from the Level of the Sea by Barometrieal Measurement,
|County of Louth.|
|1||Highest part of the road between Dundalk and Newry, by Jonesborough||283|
|2||Barensdale mountains, highest part of Lower Dundalk Barony||1644|
|3||Foy mountain; the highest of the Carlingford mountains||1850|
|4||Golding mountain; Mountain town? lower ridge of Carlingford mountains, (Cooley mountain of M'Kenzie ?)||1055|
|County of Armagh.|
|1||Fathom mountains; the highest part of along the Newry Rive||820|
|2||Cum-Lough; between Slieve-Gullen and Slieve-Girkin, or Newry mountains||305|
|3||Killevy or Kilsleve-Church, at the base of Slieve-Gullen, on the south-side||503|
|5||Slieve-Girkin or Newry mountains||1340|
|6||Faughell or Foughall mountains, a little to the north-east of Jonesborough||822|
|County of Down.|
|1||Rosstrerve-hill, at a large block of granite not quite to the top of the hill||865|
|2||Lough-Sally? at the entrance of a peaty circus (Deer's Meadow?) situated at the foot of the Monroe mountains to the south||421|
|Lough Shannagh; on the slope of Slieve Muck||1265|
|4||Slieve Snaven (Slieve Birna?)||2370|
|5||Bingan mountain (Brem Buncin?)||2396|
|7||Newcastle Slate quarry; on the acclivity and towards the base of Slieve Donard||191|
|10||Clark-hill, Slieve Slut? within (referred to Castlewellan) the demesne of the Earl of Annesley||548|
|12||Bakaderry town, near a cross bearing the date 1675||369|
|14||Temple of the winds; Mount Stewart||113|
|15||Captain Allen's fort, (Black Abbey P) one of the highest spots across the 'peninsula. of Ards, near B. Atwood||140|
|16||Windmill of B. Neboly||167|
|19||The watershed between Strangford-Lough or L. Cone and Belfast Lough, not far from Kirk Donnel||263|
|County of Antrim.|
|1||Dirris mountain, Belfast||1475|
|2||Doon ravine, or Allan's Ravine, upper part, Belfast||531|
|3||Cave hill, Belfast, upper line of the white limestone||768|
|4||Mc Arth Forth: Cave-hill top||1064|
|Thickness of the cap of trap||295|
|6||Rambling-hole, upper part of, Collin's Glen||546|
|7||Black Mountains; Belfast||1040|
|8||Watershed of the road between Belfast and Antrim||997|
|9||Ditto, referred to Lough Neagh||355|
|10||Hence elevation of Lough Neagh||132|
|11||Glenery Church, referred to Lough Neagh||170|
|11||The same referred to the sea; (more problematical)||302|
|12||Drumadaragh, or Ballyroy-hill, above Dough||660|
|13||Sandy Brae hill, above Doagh||527|
|14||Slemish Mountain, above Broughshane||1130|
|15||Carnybuy? highest part of the road between Coleraine and Bushmills||527|
|16||Cliff by Dunluce-castle||157|
|17||Orbilt? (Dunmull top)||463|
|18||Cape Pleaskin: from two observations||354,8|
|19||Bengore-bead, from two observations||328,3|
|20||Clogber; alittle to the north-east of Bushmills||130|
|21||Craigppark, to the south east of Bushmills||304|
|22||Ballychra-crafey: townland of Glenstiaghey||530|
|23||Cliff (of chalk) facing Sheep island||130|
|24||Croaghmore, near Ballintoy||471|
|25||Cross-hill, (B. Castle Collieries) upper line of the freestone||363|
|Thickness of the cap of trap||144,4|
|28||Murloch, upper line of the red freestone||560|
|29||Murlocb, limestone cliff||652|
|30||Benvaan top, near Murloch||855|
|31||Coolnagopag, or Camanmore, near Murloch, from two observations||1130|
|33||Cloughamurry town-land, an acclivity of Knock lead, upper apparent limit of the granular and micaceous limestone||493|
|34||Cape Castle: on an acclivity of Knocklead: upper apparent limit of the white limestone||840|
|County of Antrim.|
|35||Ballydummn: on the acclivity of Knocklead: upper apparent limit of the white limestone||895|
|36||Munnanacloick, on the acclivity of Knocklead; upper apparent limit of the white limestone||920|
|37||B. Patrick, upper line of the white limestone||622|
|38||B. Patrick, top||955|
|39||Breen, a swampy ground, main feeder of the bush R.||762|
|40||Ardagh: level of the Glenchesk R.||797|
|41||Highest part of the old road between B. Castle and Cushendun||945|
|42||Middle Cushleak: lower line of the white limestone||695|
|44||Oona, habitations in the valley of Glendun||590|
|45||Beagh habitations, Glendun valley||367|
|47||Glendun: upper apparent limit of the white limestone||885|
|49||Upper limit of the white limestone at Aghan, Oona mountain||882|
|50||Teabulliagh: upper Hue of the white limestone||735|
|51||Teabulliagh mountain top||1235|
|52||Cliff above the caves of Cushendnn, called Crap nagh||123|
|53||Teavuagh; by Cushendall Church||522|
|54||Slieve-Cross; highest ground between Cushendun and Cushendall, on the right of the road||742|
|55||Highest part of the Shore-road, between Cushendun and Cushendall||610|
|56||Craig Rammer mountain; a little to the north-west of Cushendall||559|
|Isle of Rathlin, or Raghery|
|68||Slieve-na.-varagin (the rocky-hill?)||93|
|69||Slack-na Calye Cliff||240|
|COUNTY OF LONDONDERRY.|
|2||Mayola R. bridge; nearly half way between Cookstown and Dungiven||414|
|3||Highest part of the road from the bridge on the Mayola R. to Dungnven||1177|
|4||Cragnashoack-hill (the Hawk's-hill); the upper line of the sandstone||1589|
|5||Cragnashoack mountain: top||1864|
|The three following heights were directly referred to Dungiven, and thence calculated above the level of the sea.|
|7||Benbradagh-hill; upper line of the sandstone||903|
|8||Benbradagh-hill; upper line of the white limestone||1266|
|10||The Glebe-house of Garvagh||440|
|The eight following heights have been referred directly to this spot, and thence calculated above the sea.|
|11||Broka-bhoy: a swell of gravel-rag on the slope of Coolcuscrehan mountain||710|
|12||Coolcoscrehan mountain top||1292|
|13||Dunavenny brook; a feeder of Allen water, being the first appearance of the primitive formation on the slope of Cairntogher||718|
|14||Cairntogher mountain top||1600|
|15||A swampy ground on the south side of Cairntogher; the head of several rivers||1090|
|16||Donald hill: upper line of the sandstone||900|
|17||Donald hill top||1399|
|18||Ballyness limestone quarry: upper line of the limestone 717|
|19||Down-hill: mausolæum of the late Lord Bristol||255|
|20||Benyavenagh mountain towards Magilligan point., a complete mass of stratified trap. (rating the limestone at 80 feet.)||1145|
|21||Cady-hill; upper line of the white limestone||604|
|24||Watershead of the road between Coleraine and Newtown-Limavadd||885|
|COUNTY OF LONDONDERRY.|
|Many of the following mountains belong to the county of Tyrone, as well as to that of Londonderry; since they limit the two counties.|
|25||Mullaghash mountain top||1677|
|26||Sawell mountain top||2257|
|27||Knocken Bunn; Monterloney: the highest cultivated ground I have seen in the North of Ireland. Crop of oats||918.|
|28||Fin-Glen or Fion-Glen mountain top||2097|
|29||Moneynieny; the mountain of wonders||1477|
|30||Sphell-Cooagh: the Cuckoo's mountain||1867|
|31||Dunlogan-hill; an appendage to Sphell-Cooagh||1467|
|33||Ch. o'Hagan's Inn: from Mayola bridge to Dungiven||338|
|34||Lough Fai or Finea; on the west side of Slieve Gallion mountain||767|
|35||Highest ground on the road between Cookstown and the Mayola bridge||841|
|36||Upper line of the limestone on Slieve Gallion||1459|
|37||Tamach; highest part of; Slieve Gallion||1624|
|The thickness of the limestone formation is but very inconsiderable|
|38||Cumber-Clady 2 Mr. James Ross' house: Fanghan valley: from two observations||260|
|39||Lesstress-hill; by the waterfall of Ness||660|
- Mem. R. I. Academy, vol. 8.
- The assertion that the basaltic group of Ireland corresponds in constitution and position with that of the Campsie hills, Ochills, &c. requires perhaps to be stated more distinctly, and more in detail; and in the first place with regard to the correspondence of these districts in the internal constitution of the component rocks. From the description of the Ochills given by Mr. Mackenzie in the 2d volume of the Wernerian Transactions, it appears that the prevailing rock is a variety of lloeta trap, possessing characters intermediate between basalt and clinks tons, occasionally exhibiting columns, passing into greenstone, and in places associated with amygdaloid. Beds of claystone porphyry, of felspar porphyry and of compact felspar cap some of these eminences. The Campsie hills have been ably but more generally described by Col. Imrie in the same publication; be represents them as universally covered by a thick mass of flœtz trap, divided into beds, partly amorphous and partly columnar. The trap formation of the north of Ireland consists principally of basalt divided into beds, partly amorphous and partly columnar; the latter variety resembles clinkstone in the property whence that rock derives its name; the basalt passes into greenstone, as may be seen in the mountain of Slievemish and other places; and it alternates with amygdaloid. A formation of clay porphyry also occurs in the center of the basaltic area.
In the second place, with regard to the position of the Scotch and the Irish trap districts, there are points of agreement and points of differences the great features of agreement are those mentioned in the text, namely, that both occur in the vicinity of coal fields, and in a position intermediate between the northern and southern mountain chains. The points of difference are, that the Campsie hills seem, at least along their southern border, to repose immediately on the coal measures; and the same observation maybe applied to the junction of the Ochills with the coal held of Clackmananshire at Westertonn. Whereas the basalt of Ireland is generally separated from the coal measures by several intervening beds of considerable importance and of much more recent formation, which appear to be altogether wanting in Scotland. It may be even objected to the instance of Cross hill near Fairhead, which is cited towards the end of this introduction as an example of the occurrence of trap in Antrim under the same conditions with that of Scotland, namely, in contact with the coal measures, that the trap in this instance and through the whole range of Fairhead, assumes a character so widely different from the compact and small grained basalt of the neighbourhood, (being a highly crystalline greenstone) as almost to warrant the suspicion of its being a distinct formation, not withstanding its close geographical proximity to the great basaltic mass. I confess myself however to be fully persuaded that this suspicion will on further examination be found groundless. The rock in question closely agrees in its texture with that of Slievemish, which it is quite impossible to detach from the general mass of trap. The island of Arran appears to form an important link in the connection I have endeavoured to trace, between the opposite coasts of Ireland and Scotland; the center of this island is occupied by a primitive district, comprising granite, mica slate and syenite; the mica slate extends to and skirts the N.W. coast. The N.E. coast presents a small coal formation precisely analogous to that of Ballycastle, with beds of breccia and of red sandstone resembling those which occur in the Antrim coast near New own Glens; this sandstone forms the prevailing substratum through the whole south of the island, where it is covered by a singular columnar clay porphyry, and by greenstone, both of which rocks are generally referred to the flœtz trap series. It seems probable that the porphyry which occurs on the opposite coast of Cantire at Devar near Campbelltown, and the syenitic rock which forms the crag of Ailsa may be considered as connected with these formations. The whole island is traversed by numerous dykes of basalt and pitchstone. A similar structure is continued through the corresponding portions of Bute. And an exact resemblance prevails between the northern and southern districts of these islands and the opposite coasts of the estuary of the Clyde in which they are situated; thus the primitive rocks, forming the north of those islands, are resumed on the north of the estuary in Cowal and Cantire; while the sandstone, breccia, and trap rocks of their southern district are continued in that direction through the Cumbray isles, and at Largs and the whole southern bank of the Clyde: the line of junction between these secondary and primitive formations seems to pass near the southern extremity of the Peninsula of Cantire, where a small secondary district, containing, like that of Arran, some indications of coal occurs near Campbelltown. See Jameson Mineral of Scottish isles, vol. 1. p. 134.
Whether any connection can be traced between the flœtz trap of Ulster and the extensive deposits of similar rocks in the islands of Sky, Egg and Mull, is a question which will be examined with greater advantage when the geology of these islands has been further elucidated by the able descriptions of Dr. Mac Culloch. It should appear that in these islands strata of shell limestone containing belemnites and ammonites, and appearing to be of more recent formation than those associated with the coal fields of the main land, occur; hence we may perhaps be led to expect that they will be found to exhibit a still nearer approach to the structure of the hills which constitute our third system. Towards the west, the limestones and sandstones associated with the coal formation extend from the banks of Lough Neagh to those of Loch Corrib in Connaught, passing by Lough Erne and Lough Allen; but the overlying trap is confined to the district described in this paper, with one trifling exception which is presented by the Corliew mountains in Roscommon, two miles to the north of Boyle, where the sandstone is covered by a cap of greystone: the structure of these hills, therefore, agrees still more nearly with the trap ranges of Scotland than does that of the great Ulster group.
- the article on some porphyries of doubtful formation at the end of these extracts.
- The term transition is here employed merely as the name of a class of rocks intervening in their position and intermediate in their character between the primitive and secondary classes, no other or hypothetical meaning is attached to it. The old red sandstone here described, has been associated with these rather than with the secondary rocks, because it appears in one instance at least to alternate with greywacke.
- The name of this quarry it the White quarry.
- The arrangement of the sandstone formations in the north-east counties of Ireland, forms the most difficult problem presented by their geological relations. The sandstone of Lough Strangford with its cap of greenstone presents so obvious an analogy to the structure of Cragnashoack at the southern extremity of the floetz trap chain in Londonderry, that we might be almost tempted to infer the identity of the sandstone in both instances, and, since that of Cragnashoack is certainly the newer variety, to question the propriety of assigning to that of Scabro hill the antiquity which has been claimed for it in the text: its apparent connection with the sandstone of Belfast lough, also seems to favor the idea of its belonging to the newer variety, for in travelling between Belfast and Newtown Ards, the road is said not to exhibit any rock but sandstone, in situ, although hills of greywacke rise within a small distance on either side: yet the fact mentioned by Dr. Berger of the alternation of this sandstone with greywacke, seems decisive as to its age.
The sandstone of Cushendon also appears to require further examination: one of the, most interesting facts concerning it appears to have escaped Dr. Berger's notice, namely, its connection with a formation of reddish clay porphyry. The observations made by Mr. Buckland and myself on this formation will be found in the account of the sections presented by the coast, appended to these extracts. It is only mentioned at present as affording an analogy between the sandstone which skirts the mica slate of Cushleak, and occurs in a similar situation in the island of Arran.
- The limestone here described appears to constitute a portion of that great limestone formation which may be traced through the counties of Kilkenny, Kildare, Dublin, the Meaths, Roscommon, the soutlr-east of Mayo, Sligo and Fermanagh. The great coal districts of Kilkenny in the south, and of Lough Allen in the north, repose upon it; as also do those of Dungannon and Cool Island described in the next article.
The points described in the text are principally situated on the north-west of the coal formations last mentioned, and near the line separating the sandstone associated with the coal, from the primitive mountains connected with the great chain of Londonderry.
On the south of these coal districts the limestone is yet more extensively displayed, intervening between them and the northern boundary of the greywacke district in Armagh: it is here exhibited skirting the banks of the river Blackwater above Charlemont for several miles; and the country on the south and south-east between Charlemont, Loughgall, Kilmore and Armagh, is principally occupied by this limestone. On the east of this district near Hillsborough and Lisburn, it seems probable that the sandstone and greywacke come into contact, the limestone being wanting: but still farther in the same direction, at Cultra and Holywood, on the southern shore near the middle of Belfast Lough, the limestone again appears in a position intermediate between the sandstone and greywacke, but probably not in immediate contact with the latter, a red sandstone of older formation being said to occur in the interval. This limestone is of the magnesia variety, (which also occurs in the same formation Dublin) its texture compact, but not crystalline, its fracture granular, its colour ochre yellow; it contains cavities lined with calc spar, and presents organic remains: the thickness of the beds varies from one to six feet.
- The above section relates only to the beds contiguous to the coal now worked. Sections of the entire cliff have been given by Mr. Whitehurst in his Theory of the Earth, page 260, and by Mr. Dubordieu in the Statistical Survey of Antrim; but it is impossible to reconcile the one to the other. From the hasty survey which I was myself able to take of this spot, I can only furnish the following particulars:─
The total height of Cross Hill, in the north face of which the collieries are worked, is about 500 feet, of this elevation about 150 feet is formed by a cap of columnar green-stone, which reposes on alternating strata of sandstone and slate-clay, these extend about 150 feet in depth, and cover the bed of coal in which the workings are situated, and which occurs at an elevation of about 200 feet above the beach; below it are other strata of slate-clay, with perhaps some seams of imperfect coal and sandstone, and we particularly observed beds of the latter rock, of an intensely red colour and of enormous thickness: lastly, strata of limestone are seen emerging from the sea towards the west of the collieries; the interval between the coal and the limestone is much concealed by slopes of debris, covered with an imperfect vegetation, which rest against the base of the cliff.
An opinion is prevalent among some of the miners, that a thicker stratum of coal than that at present in working would be found by driving adits beneath the level of the sea.
- On the opposite side of Belfast Lough this sandstone extends towards Holwood, where it rests upon the limestone before described, and we traced it for some distance on the road from Belfast to Newton onwards.
Feet 1. Basalt rudely columnar 60 2. Red ochre or bole 9 3. Basalt irregularly prismatic 60 4. Columnar basalt 7 5. Intermediate between bole and basalt 8 6. Coarsely columnar basalt 10 7. Columnar basalt, the upper range of pillars at Bengore had 54 8. Irregularly prismatic basalt. In this bed the wacke and wood coal of Port Noffer are situated 54 9. Columnar basalt, the stratum which forms the Causeway by its intersection with the plane of the sea 44 10. Bole or red ochre 22 11. 12 Tabular basalt divided by thin seams of bole 80 13. 14. 15. Tabular basalt occasionally containing zeolite 80 16
- On the shore of Belfast Lough, traces of an ancient forest have been discovered beneath a pest bog. Here a singular phenomenon (first communicated to the Geological Society by Dr. Mac Donnel) occurs, hazel nuts being found, the kernels of which are often converted into calcareous spar, while the shells remain unaltered. I am not aware that the remains of the elk, so common in the alluvial districts of other parts of Ireland, have yet been found in this quarter.
- As this is the only part of Dr. Berger's notes, in which Lough Neagh is mentioned, I have taken the opportunity of stating some facts concerning it.
Its height above the level of the sea is 132 feet.
Its greatest depth between Arboe and Gartrea points nearly in the contre, 45feet
Its greatest length from north-west to south-east, 19 miles 6 furlongs.
Its superficial contents 97,775 acres.
A constant tradition has prevailed in Ireland, at last since the days of Giraldus Cambrensis, that it owes its origin to some violent convulsion.
The northern and eastern shores of the laugh presents several sandy beaches, in which fine chalcedonic pebbles are found.
- The first point in which the cliffs of the Antrim coast expose sections of the basaltic rocks, is Blackhead, on the south-east of the peninsula of Magee; this point is (following the indented line of the coast) more than twenty miles to the south of that at which the delineations accompanying this paper commence: our information concerning that interval not being sufficiently precise to admit its being exhibited in such a form.
The following short notice will however contribute in some measure to supply the deficiency, and being prefixed to the descriptions in the text will render them a continuous survey of all that part of the basaltic area which presents a precipitous face towards the sea.
From Blackhead the eastern coast of the peninsula of Magee exhibits a long and lofty range of basaltic cliffs called the Gobbins, extending nearly eight miles towards Portmuck at the north-east extremity of the peninsula; near this point the chalk emerges from beneath the basalt, and the lias from beneath the chalk. Hence to the mouth of Lame tough the cliffs cease, the hills rising with a gradual acclivity from the beach; the same character is applicable to the opposite side of the lough near Larne: the chalk and lias, continue to occupy the level of the sea through this space, but about two miles to the north of Lame the chalk again sinks beneath the low basaltic cliff of Black-cave-head; a ridge of the same rock extends from hence skirting the beach for three miles to Ballygelly head, a promontory exhibiting rude and irregular columns; near this point the inferior strata emerge, but from the flatness of the coast being flat do not render themselves distinctly visible. About three miles beyond Ballygelly-head however the red mule (No. 4 of the Introduction) may be traced, and about two miles further the lias begins to show itself in the southern extremity of the Deer park hill of Glenarm: this is the point at which the engraved section commences.
- In an earlier part of the paper it was erroneously stated that this porphyry had escaped Dr. Berger's notice, and the editor did not discover his mistake till the state of the press tendered it necessary to substitute acknowledgment for correction. His oversight arose from the circumstance of its being described among the floetz trap series, though its connection with the old red sandstone and its position near the foot of Lurgethan, while the floetz trap is confined to the summit of that mountain, certainly renders its introduction in rush a place ah act of very questionable propriety.
- It should be noticed, that in the section all the coal measures are coloured with a dark tint, though this in fact belongs only to the coal itself and to the slate clay; the thick beds of red sandstone which alternate with these strata form the predominating features of the cliff.
This system of colouring has been adopted, because it seemed necessary to distinguish the coal measures from the red sandstone not containing coal.
- I cannot more clearly describe the general arrangement of the chalk and basalt, as displayed on the Antrim coast, than by transcribing the following paragraph from the late Mr. Hamilton's Letters, a work which may be classed with Mr. White's well-known letters on the Natural History of Selborne, as one of the most elegant models which our language possesses of writing upon such subjects; and as affording one of the few examples which prove that they are capable of being treated in such a manner as shall render them not only interesting to the enquirer into the detail of science, but engaging to the man of general information and cultivated taste.
“ The northern coast of Antrim seems to have been originally a compact body of limestone, (chalk) considerably higher than the present level of the sea; over which, at some later period, extensive bodies of vitrifiable stone have been superinduced in a state of softness. The original calcareous stratum appears to be very much deranged and interrupted by these incumbent masses. In some places it is depressed greatly below its ancient level. Shortly after it is home down to the water's edge, and can be traced under its surface: by and by it dips entirely, and seems irrecoverably lost under the superior mass. In a short space, however, it begins to emerge, and after a similar variation recovers its original height.”—Letters on the Coast of Antrim, page 6.
- Desiring to keep that description of facts which must serve as the ground-work of theory, and which seems, in the present state of science, the most useful employment of the geologist, distinct from conclusions merely speculative, I have hitherto studiously refrained from expressing the views which I have been led to form on the origin of basalt, and of the other rocks usually associated under the general name of flœtz trap.
But while describing the striking appearances presented by Kenbaan cliff, I cannot forbear to declare the conviction which this spot first impressed upon my mind, and to express my full assent to the arguments of those who maintain the igneous origin of such formations.
I would observe then that this formation is distinguished by characters so directly opposed to those which all rocks undoubtedly of aqueous origin possess, that no hypothesis which ascribes both to a common origin, can be otherwise than contradictory, and at variance with itself. For
I. Of all other formations, the least ancient are the least elevated; but this, the most recent of all, yet rivals the primitive mountains in height.2. Of all other formations, the degree of consolidation decreases together with its age, their texture passing from crystalline through the several gradations of sub-crystalline, compact, coarse, and lastly earthy; while in this formation, even where it rests on chalk, the crystalline texture of the oldest rocks frequently recurs.
3. Whin dykes, which are indisputably connected with this formation, differ from all other mineral veins, in the circumstance of their traversing all rocks indifferently; while of other veins, particular classes are exclusively associated with particular rocks.
Such being the negative evidence against the Neptunian hypothesis, I proceed to that which is positive in favor of the volcanists; as
1. The identity of chemical composition in basalt and lava.
2. The constant occurrence of tr-ap rocks in volcanic districts.
3. The confession of the Wernerians themselves, that the basalt of Auvergne is of igneous origin.
4. The testimony of those best acquainted with districts still exhibiting active volcanoes. Such persons, as Dolomieu and Spallanzani, have uniformly maintained the igneous origin of basalt, while those who have contended against it have generally been unacquainted with countries of this description.
Having thus alluded to, rather than stated, some of the general arguments on which this question appears to me to depend, I return to Kenbaan, where the basalt is seen extending from beneath as well as overlying the great mass of chalk, which has at one extremity assumed such a curvature as would naturally result from lateral pressure; and at the other is rent, and shattered in the most extraordinary manner, the basaltic matter insinuating itself into the fissures, and often converting by its contact chalk into granular marble, while fragments of the chalk, of all sizes, appear to have been forced upwards and imbedded in the basaltic rock, having suffered in their superficial parts where the basalt touches them a most remarkable change.
It seems impossible to conceive appearances more utterly irreconcilable with the hypothesis, that the basalt was deposited regularly above the chalk from a state of aqueous solution. On the other hand, were we to imagine a. priori, the phenomena which would probably result from the eruption of a current of ignited lava from beneath the chalk, and its subsequent diffusion over the upper surface of the chalk, while the whole was submerged beneath the sea, and under a considerable pressure, they would exactly accord with those which may actually be observed at Kenbaam.
To the same purpose the changes effected by the whin dykes of this district on the rocks they traverse might be cited. Thus we have instances:
1. Of the conversion of old red sandstone to hornstone. See page 201.
2. Of the conversion of the slate clay of the coal measures to flinty slate, and of the reduction of the coal itself to cinders. See pages 205, 206.
3. Probably, also, of the conversion of the slate clay of the lias formation into flinty slate. See page 213.
4. Of the conversion of chalk in several places into granular marble. See pages 172. 173.
Hence, if it be allowable to speculate on subjects so remote from actual observation, I would infer that the hypothesis which ascribes the formation of the theta trap rocks to submarine volcanoes, which were active at a very remote period before the seas and continents had assumed their present relative level, is both in itself more consistent, and in its application to the actual phenomena more satisfactory than any other.
It is evident that the basaltic mass of Ulster was accumulated antecedently to the last great convulsion which has modified the surface of our globe, excavating its rallies, and constituting its alluvial deposits.
- They were ripe, and some had already been cut on the 20th September 1811.
- This and the five following heights were referred, as before, directly to Dungiven.