Transactions of the Geological Society, 1st series, vol. 4/Supplement to the Mineralogy of Sky
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|On the Strata in the Neighbourhood of Bristol→|
By J. Mac Culloch, M.D. F.L.S. President of the Geological Society, Chemist to the ordnance, Lecturer on Chemistry at the Royal Military Academy, and Geologist to the Trigonometrical Survey.
When the Sketch of the Mineralogy of Sky was drawn up for the Society's volume in 1813, I had no prospect of again visiting that remote country. Circumstances having led me to traverse the same ground in much greater detail, I have been enabled to supply the deficiencies which were acknowledged in that paper, and to correct the errors into which I had unwarily fallen. I hold it my duty to supply the one and to correct the other; and consider that an acknowledgment of the latter is the greatest mark of respect I can pay to that body under whose protection they were laid before the readers of its Transactions.
No apology can be offered for want of industry; but the extent of this island, the difficulty of traversing it, and the intricate disposition of its rocks, offer some excuse for deficiencies, where want of time was further superadded to all other obstacles. For errors there is no excuse, but in correcting them it will not be useless to point out the causes from which they arose, since other observers may take warning from them, and learn to mistrust all observations which are not founded on rigid investigation, free from conjecture, and free from system.
To conclude respecting what is, from that which ought to be, will lead, as it has already led, to greater errors than those which I shall have to record. Equal hazard arises from judging of the structure of a district by the examination of specimens only. In rocks the specimen is not always an abstract of the geological nature of the series in which it occurs, and the mistakes which have here arisen from this cause will be equally apparent with those which have resulted from the preceding one. The last source of error which I shall notice was the imperfection of the outline of the island as it is given in Mackenzies chart by which I was guided. Here, among other similar errors, the distance between the head of Loch in daal and that of Loch Eishort, which scarcely exceeds a mile and a half, is marked at five miles. Hence, finding my observations to disagree with the map on which I attempted to record them, I abandoned altogether a pursuit which, had I continued it, would have led me at that time, as it has since done, to determine the sandstone series to a much greater extent than I then imagined it to occupy.
The most leading error is that which states the promontory of Sleat as composed of micaceous schistus. This substance occurs in several places, and often in distinct beds of considerable thickness, while in others it consists of mere laminæ interposed among the other rocks. These rocks are of very various composition, but as gneiss occurs in considerable quantity among them, exceeding greatly the space occupied by the micaceous schist, the latter will probably be considered by most geologists as subordinate to the former, and the gneiss as being the lowest and fundamental rock of Sky. But the truth is, that this series is not amenable to any systematic rules, and it will be better to state the fact as it exists, than to incur the risk of a similar error by transferring to gneiss that rank which I had before conferred on micaceous schist. It will be seen that the series presents anomalies which cannot be removed by any theory of subordination, and that the history of the district of Sleat is not the least interesting part of the unexpected appearances presented by this very instructive island.
The characters of the gneiss are so strongly marked in one part of the space which it occupies, that there can be no difference of opinion respecting it. Besides mica or hornblende it contains a conspicuous proportion of high red felspar and quartz, being at the same time distinctly laminated, and from the contrast of its colours, very remarkable.
From this regularity of structure and composition it passes into a substance for which there is no name in the present nomenclature of rocks, and which can only be ranked with gneiss by assuming a considerable latitude of character. This rock is a compound of felspar and quartz with chlorite schist, these substances being still interlaminated as before, so that each mineral generally occupies a distinct portion of the specimen, the latter becoming substituted for the mica that characterizes the regular varieties. By degrees the chlorite schist becomes predominant, and at length the felspar is excluded, so that all appearance of gneiss ceases and a simple series of chlorite schist remains. I suppress a detail of the endless varieties found through this series, as such substances can rarely be rendered intelligible in description. But I may add that hornblende schist, so generally found to accompany gneiss, alternates here also with it under many different aspects.
With respect to the position and boundaries of this series, it is found occupying beds of which the elevated edges present a rectilinear direction towards the north-east, dipping to the eastward in an angle which varies between 30 and 50 degrees. Its boundary towards the west lies near the small island Oransa, where it is succeeded immediately by the graywacké schist and the accompanying quartz rock or hard sandstone which were described in the original paper, but which I shall presently describe again in greater detail, having had an opportunity of verifying much of that which was only conjectural, and of extending its limits to a much greater distance than I had foreseen.
Although the boundary of this series, in which gneiss and chlorite slate form the principal parts, is thus defined at the northern end of its western side, no such decided change is perceived at the southern end of the same line, which, if protracted from the place first mentioned near Isle Oransa, would cut a point on the western side of Sleat. The interior of the country is too much encumbered with peat and with vegetation to permit of any decision on a subject so obscure as is the point of change between the gneiss series and the rocks which follow it, and I must therefore limit myself to the appearances which occur on the sea shore, where every change can be traced in the most minute manner. Here there will be found a transition, unexpected and improbable as it may at first seem, between the two series, that of the gneiss, and that of the graywacké and sandstone which follows it; but at what point between the two extremities of the gneiss boundary a decided change takes the place of a gradual transition it will be for ever impossible satisfactorily to determine. I must add that the limit of the gneiss series on the eastern side of Sleat is the sea shore itself.
I must now proceed, before entering further on the transition of the gneiss series, to describe that which in the original paper I called the series of blue quartz rock and schist, in which there is somewhat to amend and somewhat to supply. With this I must here include the red sandstone, formerly separated from the other two rocks on the same systematical views which led into the other errors already mentioned. The conclusions then drawn were sufficiently justified by the partial view of the country which I had at that time obtained, but they were founded on observations too limited. A more complete investigation, with a greater disregard of theoretic views, would not only have led to sounder conclusions, but have removed many difficulties which I encountered both in the examination and in the attempt to reconcile discordant phenomena.
Although on reviewing the places I examined before, I find the description formerly given of these substances locally correct, and the conjecture I had formed of the nature of the rocks toward the Kyle ri'ch equally so, yet an examination of additional parts of this series renders it necessary to remodel the whole description, as it possesses a degree of intricacy which it was impossible to suspect, and which nothing but a very accurate examination could ever have induced me to credit, since it is at variance with the usual phenomena that attend these rocks.
The whole series presents from one end to the other a repetition of the same parts, although the several substances are in different places differently proportioned, the one exceeding in one place, while in another a different member of the series will be found predominant. One exception to this rule will afterwards be noticed.
The rocks which compose the series are the following.
Red sandstone, more or less indurated, of which the general characters were formerly described.
Quartz rock, or, as some may prefer to name it, indurated sandstone, passing from lead blue to grey and brown, sometimes pure, at others containing felspar.
Schist, which is sometimes not to be distinguished from ordinary clay slate, and at other times contains particles of quartz and mica. If one term is to be used for the whole it must probably be called graywacké schist.
White compact quartz rock: this substance is found only in one part of the series.
In the original paper I described the red sandstone as following the blue rock and schist in conformable order, which it in fact does throughout a considerable tract without any repetition of the two latter. But on pursuing these beds further than I then did, whether backwards or forwards, according to their relative inferiority or superiority, repeated alternations of all those substances occur.
The dip which I also described as constant and westerly, is only thus regular from that part of Loch Eishort whence my examination at that time commenced, to its upper portions, ascending according to the order of the beds. In tracing from that point towards the last, or downwards according to the order of the beds, through those parts of the country respecting which I formerly offered only conjectures on the similarity of the series, I do not find those conjectures verified; the position of the beds becoming first vertical and then reversed and irregular; ultimately settling in a dip towards the east, the reverse of that which predominates on the western or upper side of the series. But whatever irregularities are found in the dip, there are none in the direction, which with a slight local disturbance near Ord is invariably rectilinear, and on the north-east line or nearly so.
On the north-eastern end of this series, where it forms the mountains of the Kyle, the rocks can be traced perfectly from the gneiss at Isle Oransa to the commencement of the limestone near Broadford, this space comprising the collective thickness of the strata; but through this tract the quartz rock or indurated sandstone is predominant. If these strata are prolonged toward the south-west their characters change, or they are discontinuous in composition according to the line of their direction, since the schist and quartz rock are most abundant toward the north-eastern end, while red sandstone prevails at the opposite one.
The space which they have been represented to occupy on the original map must also be extended, and to a certain degree this may be done by prolonging the line of direction from that point near Isle Oransa where the junction of the gneiss is found.
I have already related the error committed by allowing too much space to the micaceous schist, which occurs only as one of the members of a series principally formed of gneiss and chlorite schist. Another of the sources of that error will now appear when I describe the last enumerated member of the red sandstone series; and it will no less excite surprise than operate as a caution in the present state of geological science, against judging of rocks by analogies, or by any other evidence than that of actual and careful examination.
Meeting near Loch Eishort with the white compact quartz rock which was described in the original paper, I concluded that here, as elsewhere, it was a member of the primary rocks, and therefore without hesitation laid it down among the series which I considered to be micaceous schist, concluding also that I had in reaching it arrived at the bottom of that series which contained the blue quartz rock and schist. That white quartz is however a member of this very series, holding a parallel course with it, and being preceded as it is followed by red sandstone, blue quartz rock, and graywacké schist. This circumstance forms a material addition to the history of quartz rock, and an alteration no less material in that of the series to which the red sandstone belongs.
I must here remark that in calling so complicated a series of rocks by the title of only one of its members, the red sandstone, I have done so only on account of the necessity of using one term for the temporary purpose of a geological description, and that this one was chosen from the predominance of that substance. I can only add that this predominant member corresponds in mineral character with the other red sandstones which in Scotland occupy the intermediate place between the primary rocks and the secondary strata containing organic remains, and that it is connected by visible indications with many similar portions of the same rock to be found not only in Sky but in the neighbouring islands. As it is also followed in a conformable order by the limestone series formerly described, it possesses another common character with the usual red sandstone of other districts. If any person shall think that the whole of this series should be ranked under graywacké and called a transition series, I can have no objection, but am at the same time unable to see what advantage is gained by the substitution, or in what respect the science is aided by it; while at the same time the red sandstone will be as much transferred from its usual place in the one case as the graywacké is in the other. But the island of Sky is very often unsystematical: that which follows perhaps still more so than that which has preceded.
In attempting to trace the red sandstone below, or rather beyond the white quartz rock, it is not found to terminate on the north-western shore of Sleat, at the place where, according to the line of direction prolonged from Isle Oransa, it should end. Instead of that, the alternations of sandstone and schist continue. Gradually however they increase in frequency, and becoming at length undulated and contorted, they cannot at a distance be distinguished from gneiss in their general aspect and disposition. In examining the substances, the first alteration perceived is the gradual induration of the sandstone, which becomes first a compact quartz containing grains of red felspar. At length the felspar acquires a laminar tendency, the schist still remaining unchanged, the rock thus becoming an irregular gneiss (if it may be so called) consisting of laminæ of quartz, felspar and graywacké schist. Approaching the point of Sleat the schist gradually becomes green and glossy, thus passing into chlorite slate, and here we arrive by an insensible gradation to the variety of gneiss which I formerly described as found there. In thus pursuing the red sandstone on the western side of Sleat, the reason appears why I could not when speaking of that rock, define the boundary of the gneiss on this side of the island, and I need now scarcely repeat, that any attempt to examine rocks so constituted, in the interior country, would leave nothing but doubt and uncertainty; for which reason I have limited the description to the sea coast, where every foot of the rock through all its transitions admits of free examination.
I have no commentary to offer on these facts, which seem calculated, if not to excite disbelief, at least to set our present systems at some defiance. It is possible to reconcile them only by supposing that the red sandstone series belongs to the primary rocks, and the whole of these strata which extend from the point of Sleat to the limestone of Strath, will thus form a succession of rocks alternating with and graduating into each other. That the gneiss is a primary rock can, I imagine, admit of no dispute.
In formerly describing the limestone district, I expressed my expectation that the strata of Kilbride and those at the entrance of Loch Eishort would be found identical with those at Broadford, and that expectation has been realized by a complete examination of the district in question.
On the Broadford shore the commencement of the limestone beds is found near a small farm called Lucy, but their actual contact with the sandstone cannot be seen, on account of a sandy beach which covers the junction. It is probable that some conglomerate exists in this interval, as detached masses of such a rock are found in different places in the hilly grounds between Strath and Loch Eishort; but I need not dwell on a circumstance so common every where that it can scarcely fail to be present here. As the inclinations of the sandstone and limestone are in the same direction near this junction, the latter however dipping only five degrees to the north-west while the former dips ten, there can be no doubt but that they are connected in the usual regular order of succession.
From this, which I shall call the lowest line of the limestone, it can be traced under various interruptions along a high ridge of hills to Loch Eishort, where it coincides with the beds formerly described as found there. Independently of this connection, the identity of the whole is proved by the correspondence of the organic remains, which at Broadford however are more abundant than on the opposite side. The principal difference at the two extremities consists in the numerous beds of shale and sandstone that alternate with the limestone strata on the Broadford side, and in the inferior solidity and thickness of the calcareous beds; while at the same time the harder schist, which divides them on the south-western shore, is absent, the one appearing to be a substitute for the other. The shale is a mixture of black clay, sand, and mica, thickly and imperfectly fissile, and the sandstone which is of different colours, but generally brownish, contains much clay and calcareous earth, the organic remains being found in each of these beds just as they are in the limestone.
The interruptions, to which I have here alluded, that prevent us from tracing the limestone over the hills that bound the southern side of Strath, arise partly from the boggy and covered nature of the ground, and partly from the intrusion of a hill of syenite, which extends far from the portion formerly noticed, towards Broadford, and which can in many places be distinctly traced overlying the limestone, shale, or sandstone, as either of these happens to be present at the point where the contact is exposed. There is no satisfactory evidence to be procured here of that change from the stratified to the unstratified limestone which I have described in the original paper, since there is no situation where the contact of the two can be precisely traced. Yet there is even here sufficient evidence to give rise to such a suspicion, and more than enough to confirm the observations formerly recorded, and to justify the conclusions deduced from them. To enter into further details on this subject would now be superfluous, as the feebler evidence is of little value where the stronger has preceded. I shall only add, that beds of ordinary quartz are in one place found regularly interstratified with the marble limestone, as if the power which had converted the common limestone into this one, had also changed the sandstone into quartz: and that many gradations by which the ordinary limestone appears to pass into the marble, can also be traced, although in consequence of the irregular nature of the ground they are widely dispersed. I ought also to add that in one of these intermediate portions I found layers and scattered specimens of bodies having the general aspect of those obscurely organized fossils which have been all confounded under the name of alcyonia, consisting of a calcareous carbonate whiter than the surrounding rock, their surfaces being covered with minute but irregular crystals of the same substance, and being so much more durable than the surrounding materials as to remain protruding after these have been washed away.
To the topographic detail as given in the original paper, I must now also make an addition, the nature of which will be better understood by inspecting the improved map.
The strata on the Broadford shore may be traced to a place opposite Scalpa, but without a name, where they terminate in a succession of beds consisting of the shale only. Alter some interruption, in consequence of the intrusion of a mass of syenite and trap, a small patch of irregular limestone is seen, which soon ceases in consequence of the renewal of the syenite, not to appear again till we arrive at Loch Sligachan.
On this shore the overlying position of the syenite can be easily traced at the places of contact, demonstrating that it here combines the same double relation to the stratified secondary rocks which it is found to bear elsewhere; cutting through them at the same time that it covers them.
The boundary between the upper portions, or the north-western line of the Strath limestone and the syenite, is extremely irregular, although it is not often possible to procure a sight of the actual contact, or even of the probable junction of these different rocks. It is however a sufficient proof of that irregularity, that as marble is found at the foot of the ascent of Ben-na-caillich, so syenite abounds on the opposite side of the valley, while every where throughout it patches of the latter rock, often of very small extent, are found surrounded on all sides by limestone.
I must further add to the description of the limestone that pectines of considerable size are found among its upper beds, together with terebratulæ, and numerous fragments of shells of which some resemble portions of mytili, but the whole in too imperfect a state to admit of accurate examination.
The description of the limestone of Sligachan having in the original paper been left imperfect, I may now add to it the following particulars. Beds of the stratified limestone and shale, succeeding a narrow portion of the red sandstone, and precisely resembling the strata of Broadford, extend from the portions of irregular limestone before mentioned to the shore of the loch. These also dip to the north-west, but at a considerable angle, and the direction of their elevated edges, like that of all the regular rocks of Sky, is to the north-east, or thereabouts. It is evident that this limestone is a portion of the same series which occupies Strath, the intermediate parts having been either displaced or overwhelmed by the syenite.
The circumstance of greatest difficulty in comparing these two portions of limestone, is the intervention of the red sandstone, in conformable position, and therefore apparently alternating with them, since the angle of inclination has in all the same tendency. It is not easy to admit of this alternation consistently with what we know of the relative positions of the red sandstone with limestone of this character in other places. Unfortunately Sky itself offers no clue by which we can trace this connection more intimately, or on which we could found some theory of it less at variance with ordinary experience. Whatever the nature of this difficulty may be, I must for the present suffer it to remain unexplained, since without detailing the history of all the islands in the vicinity which partake of and elucidate the structure of Sky, no adequate conjecture can be offered respecting it. There is here no room for such a description, but I hope on some future occasion to give a collective view of the whole group, and thus to render the geological history of the principal island less incomplete than I am still compelled to leave it. The connections of the western islands with each other and with the main land are so intimate, and the light obtained from one portion is so necessary for the elucidation of others, that the separate description of any individual of the group must always be imperfect.
The account of the limestone which is found near Ord on the southern shore of Loch Eishort was in the original paper imperfect, as well in respect to its topography as its mineralogical description. It occupies a small hill which includes the house of Ord, and is singularly irregular in its position, as well with respect to its own arrangement, as to its connection with the neighbouring rocks, among which, as I have already shown, there occurs a great degree of confusion. Notwithstanding this irregularity, a careful and close investigation of it will leave no doubt respecting the superiority of its position to the sandstone with which it is associated, and however widely separated from the more regular beds on the opposed shore, there is no want of indications to prove that it forms a portion of the limestone of Strath; its present confusion appearing, like that of the neighbouring sandstone, to have arisen from some common cause acting on both, to which also we may perhaps attribute the peculiarities which its structure and composition present. Its stratification is in general sufficiently apparent on the great scale, although in the more detached portions often invisible, in which respect it possesses a resemblance to the marble limestone formerly described. But I need not detail those peculiarities of structure which can scarcely be rendered intelligible by words. That which is most remarkable is the large quantity of siliceous matter it contains. This is found dispersed through it in irregular nodules, often scarcely differing from common flint, or rather resembling that variety of chert which in other situations is found in limestones. These nodules are white, grey and mottled, in some places of an obscure pale red, and they are so predominant in a few situations as nearly to exclude altogether the calcareous matter.
In the original paper I represented the sandstone of Strathaird as a portion of a series superior to the limestone of Strath, and a subsequent and more extensive examination of the country enables me to confirm this view. But I may add to it the following remark, which is not unworthy of notice. The trap veins which form so conspicuous a feature on the eastern side of this promontory are crowded together in the manner already described only along a certain, though by far the greatest, portion of the shore. At the extremity of the promontory they are rare, and are scarcely found on the western side. They appear indeed to be connected with the body of the trap which was described as covering the stratified rocks, and to be ramifications or processes from that mass. On the western side, and at the point of Aird, where they are rare or altogether wanting, the strata consist of a soft white calcareous sandstone, and are nevertheless apparently continuous with the hard ones formerly described as occurring where the trap veins predominate; while the identity is still further marked by the correspondence of the same complicated schistose structure, that structure being even more apparent in the softer rocks, as more readily yielding to the action of the elements. I remarked formerly that these strata, however separated in position, were analogous to the white sandstone which occurs at Portree and elsewhere in the north-eastern portion of Sky, and is accompanied by limestone, shale, and coal. Having then but little acquaintance with that part of the island, the description of these strata was avowedly left imperfect, and it is now necessary to supply the deficiency.
As I remarked not long ago that a description of the neighbouring islands was requisite to throw light on the obscure connection of the red sandstone with the limestones of Strath and of Sligachan, so I may here repeat that the history of the uppermost strata of Sky, which I now propose to sketch, would be materially elucidated by that of Rasay. But as the description of this island would be here inadmissible, I can only say generally, that the deficiencies of connection which occur in Sky, and which compel me on many occasions to have recourse to inference and analogy, are in a great measure supplied by the structure of that island, which, while it is more continuous and accessible, is at the same time such as to leave no doubt respecting the identity of strata separated at present by a narrow arm of the sea.
It has been seen that as the gryphite limestone immediately follows the red sandstone, so it is succeeded by the calcareous white and grey sandstone of Strathaird. The same succession may be traced in a more circuitous manner by comparing the strata of Sligachan, Scalpa, and Rasay. But it is apparent on considering the map of Sky, that the portion of these uppermost strata which occupies the district of Trotternish is separated from the gryphite limestone by an interval, partly the result of the direction of the shores, and partly produced by the intervening mass of trap and syenite. There is no actual contact of the two to be seen, but the nearest indication of a connection between them is to be found at Loch Sligachan. I have already described the limestone which occurs on the southern shore of this inlet. On its northern side there are seen a few beds of white, brown, and black sandstone, separated from that limestone by the breadth of the loch only, but lying in a regular order conformable to it, and doubtless connected with it under the depths of the sea. These are immediately cut off by a mass of trap, which extends without interruption for nearly two miles along the shore, thus depriving us of all means of tracing any connection between them and the next stratified rock. That rock appears at Conurdan, occupying a low situation on the sea shore, in a thin series of nearly horizontal but somewhat irregular beds surrounded on all sides by trap. These beds consist of a brown calcareo-argillaceous sandstone, similar to one of the beds at Loch Sligachan, and characterized by the spheroidal concretion which prevail through the greater part of the sandstone of this district. After an interval of trap the same sandstone re-appears as we approach Portree, but still scarcely visible except in the natural sections of the shore, since the whole interior surface of the land is covered by the superincumbent trap, which conceals the structure of this country from the most watchful eye. Here it immediately presents a collection of beds of enormous thickness rising into lofty cliffs, which, although inaccessible, may be approached in favourable weather so near from the sea as to leave no doubt respecting their nature. This is the hill of Camiskianevig which forms the southern side of Portree harbour.
The mass of trap which overlies these strata cuts through them in the interior of the harbour, and thus forms another interruption between them and the corresponding ones, which again appear similar dimensions on the northern side of the harbour. From this place they continue to form high cliffs, covered in a similar manner by trap, and extending along the coast for a considerable space towards Holme. Limestone occurs together with the sandstone in the interval last described, but the shore is so difficult of access on account of its rocky boundary, the want of creeks or harbours where a boat may land, and the general prevalence of a heavy sea, that it is not possible to trace every point, nor, consequently, to perceive where the changes of the strata take place; while, from the mural front and excessive height of the cliffs, they are themselves absolutely inaccessible. To add to the difficulty, the limestone when weathered puts on the grey colour and aspect of the sandstone so perfectly, that it is often difficult to distinguish them, when even within reach, without the assistance of a recent fracture.
I must here premise that the whole of the strata hereafter to be described, as well as those new mentioned, have a regular and even dip towards the north-west, which is at a small angle, although no opportunity is offered of ascertaining its quantity. In this respect they are conformable, if they are not absolutely consecutive, to the Strath limestone, and I may add that this dip is apparent over the interior country wherever they can be seen, while at the same time it is indicated generally by the gradual disappearance of the lower beds on the west side of the promontory, their thickness amounting on that shore to a few feet only, while on the east side it reaches to many hundreds. I need scarcely say that cases of obvious disturbance connected with the interference of trap must be excepted from this general rule. I
Passing Holme the shore becomes occasionally more easy of access, although the mural line of cliff continues, and here limestone strata are found to have succeeded to the sandstone. These strata contain spheroidal concretions similar to those which attend the sandstone, and which increase the difficulty of distinguishing between the two substances at that distance from which alone they are visible. Hence it is with some doubt that I must speak of the absolute nature of the whole strata between Portree and Holme; a matter fortunately of no serious moment, as geologists are well aware of the intimate connection subsisting between these strata, which have been fully examined and described in many parts of the British islands.
Alternations of micaceous shale and of brown sandstone are found in the calcareous beds, of which the colour and composition vary materially, although the predominant colour is smoke-grey, the aspect earthy, and the composition argillaceous. The only organic remains which I could find among them were a large ammonite and a belemnite often exceeding a foot in length. As far as I can discover, our conchologists have not yet ascertained these species or distinguished them by specific names.
But it is unnecessary to enter into minute details respecting this limestone, since it must be already seen that it belongs to the lias, a rock well known to geologists, and already often described under all its varieties of aspect.
I shall take some future opportunity of describing this important series as it occurs throughout the western islands, since it is so dispersed as to involve the history of many of them, and to render it impossible to give an adequate account of it in a paper so local as this.
Proceeding northwards along this shore, it appears that the beds which follow are superior in position to the preceding. This should result from their general dip, but it cannot be distinctly ascertained. Here, common shale begins to appear in alternation with the other substances, and the quantity of siliceous schistus strewed on the shore proves that this substance also exists somewhere in the cliffs; doubtless under the same circumstances which I formerly described at Duntulm. I must add that the specimens sometimes contain shells, and that, resembling basalt in appearance and texture, they confirm the truth of those suspicions respecting the asserted existence of organic substances in that rock, which it is here sufficient to have mentioned.
Together with these detached blocks of siliceous schist are found similar fragments of a cherry substance, extremely hard and brittle, and breaking into acute conchoidal fragments, but possessing an earthy aspect. Its colours vary from greyish white to dark smoke grey, and I may add that its degrees of induration are also various. Occasionally, portions of the siliceous schist are attached to it, the separation being marked by well defined planes, and, from the contrast of colour, very conspicuous. If there were any doubt that this chert was originally a portion of the lias indurated by the same process that has converted the shale into siliceous schist, it would be removed by the fact that on the western shore of this district. the two substances are found in situ, associated in the same manner and in various states of transition from common lias and shale to chert and siliceous schist.
The last portions of limestone to be seen on this shore occur at the island of Fladda, occupying a very low position, and at length disappearing gradually below the trap, which beyond this point forms the whole coast as far as Duntulm, constituting also the islands of Trodda and Fladdahuna, as well as the various picturesque rocks which are scattered to the north of the point of Hunish. This bed of limestone abounds in organic remains, but so condensed together, and so broken, as to present no specimens capable of being ascertained: they resemble fragments of some sort of cockle and of anomiæ, or perhaps ostreæ.
If we proceed to the western side of this promontory for the purpose of recovering these strata, we find them at Duntulm, from which place they extend interruptedly for a few miles along this shore, when they finally disappear. The same organic remains, the same shale, the same limestone, calcareous sandstone, and siliceous schist, mark the identity of these with the strata on the eastern side, an identity still further confirmed by the prevailing correspondence of their inclinations. I may at the same time add that a greater facility of access to the upper beds, the only ones here to be found, assists us in obtaining a more correct notion of those beds which from their elevation above the shore are inaccessible on the east side, and that we thus become acquainted with those numerous varieties of the lias limestone, which having often been described by geologists, serve to confirm the nature of these last and uppermost of the stratified rocks of Sky. The nature and origin of siliceous schistus can here also be traced in many other places besides that most conspicuous one at Duntulm which I formerly described; and so many gradations between that rock and shale are to be observed that the most satisfactory evidence of their connection can be obtained.
From a comparison of these several facts, the details of which I have from the nature of this supplementary paper thought it necessary to condense, it is apparent that the fundamental rocks of the district of Trottemish, are those secondary and stratified substances which are connected with the lias formation, and that these are both surmounted and intersected by trap. If but little additional evidence of this view can be obtained from an examination of the interior country, that little is at least satisfactory. The same substances occur in numerous places, where precipitous faces or the sections formed by rivers expose the rocks that lie beneath the trap. If they are disjointed in position, or if they appear promiscuously scattered, they still retain their natural connection, while the identity of their mineral structure is every where consistent. In one place only some strata of a quartz rock are to be seen, which might lead us to hesitate did we not recollect that in other instances the same causes which have converted shale into siliceous schist have also been found to change sandstone into quartz.
The same causes which formerly prevented me from examining the strata of Trotternish, the deficiency of which I have now supplied, also impeded the investigation of the coal which is connected with them. Although I have since followed and traced the appearances of this mineral in those places where it has been observed, there is but little satisfactory information to be obtained respecting it. The cause of this obscurity is easily understood. It has been remarked that although the basis of this promontory consists of the stratified rocks which have been just described, the whole is surmounted and intersected by trap. The decomposition of this rock, and that of the softer strata which lie beneath, have moreover covered the whole country with a deep soil, which from its fertility tends further to conceal the nature of the rocks on which it reposes. Hence it is only in the casual exposure of some jutting rock or broken face, some denuded acclivity or bed of a stream, that any access can be procured to the stratified substances, and from this cause it is rarely, if ever, possible to trace the relations of the particular stratum which comes into view. It is among such dispersed portions of strata that the appearances of coal are observed. They are not infrequent, but are always extremely scanty, both in their thickness and in their apparent horizontal extent, since the strata which contain them are every where cut off by veins or by masses of trap. They are interposed, as we might expect, among the shale and sandstone, and, as we may conclude from the general bearings of the strata already described, occupy the upper beds of this formation. It is impossible to say that they do not exist at a greater depth, since the inferior strata, as I have already shown, can scarcely be considered sufficiently accessible to enable us to determine on the absence of a substance of which the thickness does not exceed a very few inches. Nevertheless, no indications of coal can be perceived along the eastern line of cliffs, where the deeper strata are exposed, and we may therefore for the present conclude that they lie above the lias and its associated sandstone, or at least among its uppermost beds. It can serve no purpose to enumerate the places where these indications of coal have been observed, since they cannot be verified on the map, and are indeed generally nameless. As the strata rarely exceed an inch in thickness, it is equally evident that those which are visible are worthless in an economical view, while the certainty of a speedy interruption from the intrusion of trap removes all temptation to penetrate to greater depths, or to expend capital in a more effectual research.
While on the subject of coal, I may add to the former account, that I have observed portions of wood coal in more places than those originally enumerated, but they are no where of sufficient importance, or marked by any such peculiarities as to require further description.
In describing the several trap rocks of Sky, I am sensible of having often spoken generally, when the circumstances might perhaps have admitted of more accurate details. The cause of this however is principally to be sought in the imperfect acquaintance which geologists still possess with this infinitely varied and obscure class of rocks, an obscurity which increased experience is daily tending to remove. Repeated and careful examination of them as they occur in the western islands, have, since the time at which the original paper on Sky was drawn up, enabled me considerably to amend their history, and to dispose of them in a more exact and connected manner; but as the detail would here be inadmissible, from the length of discussion to which it would lead, I shall make no attempt to improve the former imperfect remarks, but reserve that which might be here added, for some future communication. I shall however attempt to amend one or two of the descriptions contained in the former paper, where I had been obliged to rely on a distant view, and was therefore compelled to speak only in the most general terms.
The first of these portions of trap is that which occupies the district of Trotternish, of which, as well as of the stratified substances but just described, I had formerly an opportunity of forming only a very superficial notion.
As I have just shewn, it both intersects and surmounts the secondary strata, while in many places it appears also to be horizontally or conformably interstratified with them. These interferences are very remarkable, and exhibited on a scale of such extent as to include every circumstance which has yet been described on the subject of their junctions. But without numerous drawings no adequate idea of them can be conveyed, and as there is little to be said respecting them which would not be a repetition of the remarks which have on numerous occasions been made on similar appearances, I shall forbear to enter into details respecting them. I shall only observe, that all these irregularities occur in a stratified, which taken in a general view, has the character of a stratified trap, since notwithstanding them it bears a strong parallelism to the already parallel strata with which it is associated. It is abundantly plain that the appearance of stratification in the trap is here the result of the form of the rocks on which it is placed, or among which it has intruded, in the former case surmounting them, and in the latter appearing to alternate with them. The instances of this apparent alternation are highly interesting, from their great extent, as well as from the perfect conviction which they present of the fallacious nature of this supposed connection. In many cases the alternations of the trap are as regular, as decided, and as evenly parallel, as those of the stratified rocks themselves, the sandstone and limestone among which it lies. Yet in no instance does it not happen, but that at some point or other the alternating bed of trap will detach an intersecting vein, unite itself to the superincumbent mass, or, quitting the interval between two given beds of limestone or sandstone, make its way across the one immediately above or below, and then proceed with a regularity as great, for another long space, between some other pair of proximate strata. In one or more instances I have observed this to happen after more than a mile in extent, throughout all which space not the minutest irregularity had appeared to indicate any thing else than a perfectly conformable and alternating stratification. I have no doubt that, could such extensive exposure be oftener procured, all the instances of supposed alternation between the trap rocks and the stratified ones would prove similar to these.
With respect to the trap itself it is most generally amorphous. As we approach however towards the northern end of the promontory it becomes columnar, and this character prevails round the points of Aird and Hunish beyond Duntulm, where it at length terminates. Although the columns are formed on a large scale, and are individually rude and imperfectly defined, yet their picturesque effect, when seen from a point of view where they can be properly comprehended as a whole, is not less symmetrical than that of the faces of Staffa, while at the same time they far exceed it in grandeur as well as in absolute magnitude. Their height reaches from 200 to 300 feet and upwards, a dimension, however large, not sufficient to overpower the due proportion which should exist between the aggregate structure and the parts of which it is composed, since the magnitude of the columns is proportioned to their height, and the total effect therefore similar to that of Staffs, where the proportions are so nicely adapted for beauty.
With respect to the composition of this variety of trap, there is necessarily some uncertainty, since the great extent of it, as well as the inaccessible nature of most parts, renders it utterly impossible to examine it throughout. We also know that the various members of this family are often found irregularly intermixed, so that to have ascertained the composition of one portion of a mass, gives us no assurance that we have made ourselves acquainted with that of the whole. Yet I am inclined to think that the greater part will be found to consist of a substance analogous to greenstone, in which augit occupies the place of hornblende, a rock of great frequency in Scotland, and often, perhaps generally hitherto confounded with common greenstone, unless in a few such remarkable cases as that of Rum, where the substances are too distinct to admit of mistake. It may be called augit rock, without introducing any confusion into mineralogical nomenclature.
For the sake of topography I must here mention a small mass of trap, lying on a part of the coast of Sleat not easily visited, and omitted in the original paper. It occupies a projecting point south of Talivil, where its place has been marked in the amended map. It covers a space of about a mile in extent, lying over the red sandstone. It is rudely columnar and slightly porphyritic, and is also remarkable for a schistose fracture parallel to the axis of the columns. It is accompanied by a small and very unintelligible fragment of limestone breccia, which appears here totally out of its place, and unconnected with the surrounding rocks.
I formerly represented the difficulties which impede the examination of the Cuchullin hills. Since that period I have obtained access to a larger portion of them, but still there is much unseen, probably inaccessible to human footsteps. That portion however is important, and I shall here describe it, although much remains to be done before the history of this division of Sky can be considered complete.
I remarked in the former paper that hypersthene was found united to felspar and hornblende in the rocks which surround Coruisk, but in the same place I also stated that a large portion of these rocks consisted of common greenstone. I have now reason to think this observation incorrect, and that the only greenstones (formed of felspar and hornblende) are found in veins. The difficulty of distinguishing between hornblende and hypersthene when the parts are very minute, was another cause of error, which a more intimate acquaintance with the place and a far more extended examination of specimens have enabled me to correct. In thus correcting my own errors I shall also correct those of other observers, since I may point out a well known district, Airdnamurchan, where the same rock as that of the Cuchullin hills has been hitherto mistaken for greenstone.
Although the hills themselves which encircle the romantic valley and water of Coruisk are utterly inaccessible on this side, yet it is easy every where to examine their bases, while the continuity of the beds or sheets of rock, from the foot to the very summit of the ridge, and its remarkable external characters, leave no doubt respecting the identity of their composition throughout. This is the rock on which the elements seem to make no impression, and on which no soil accumulates, causes which equally determine the permanent and rugged nature of their spiry outline. I have no hesitation in saying that the whole of the group as far as it is visible from Coruisk, as well as the opposite side of Garsven far on towards Loch Brittle seaward, and the smaller mountains which separate this valley from Blaven, consist of the same rock, but to what extent it may reach northwards cannot be determined until the whole shall have been traversed, unless the rugged outline and external general characters are admitted as a proof of identity of composition. In this case the whole of the Cuchullin is a mass of hypersthene rock, with the exception only of the veins which it contains, which consist of basalt, compact felspar, augit rock, syenite, felspar-porphyry, and lead-blue claystone. I have chosen the term hypersthene rock to designate this new and important member of the trap family, since like that of augit rock it is explicit, and introduces no confusion into the existing nomenclature.
There appear but two prevalent varieties of composition. In the first the mixture consists of hypersthene with greenish compact felspar, and in the second with crystallized white felspar possessing generally a slight glassy lustre. This latter variety seems to predominate, and is much more easily recognized than the former, which from its greenish hue and the minuteness and intimacy of the admixture, is often difficult to be distinguished from common greenstone. The principal variations of appearance are produced by the greater or less proportion of the hypersthene, by the varieties of its colour, and by the unequal magnitude of the crystals. While on the subject of Coruisk I ought to add, that the sonorous rock formerly described is either a compact felspar or a compound of that substance and augit in minute admixture, and not a greenstone; a term which has been too indiscriminately lavished on many of the obscurer members of the trap family.
The last circumstance respecting trap which requires correction is the account of a vein passing through the marble quarry in Strath, and supposed to terminate in a mass of syenite. I did not in the original paper lay much stress on the conclusions which might be drawn from it, but I can now however say that no instance has occurred to me in Sky of a trap vein being cut of by the syenite. The half opened state of the quarry at the time I saw it, and the rubbish with which it was encumbered, misled me into the report which I gave; a report which a moment's view of its present exposed state was sufficient to rectify. The trap veins (for there are two,) enter it on one side together, appearing at first like one; and being cut deeply through on that side of the excavation, while they were not to be seen on the other, I readily concluded them to be terminated. By a very singular coincidence they diverge from each other immediately at the place of entrance, branching away in an angle greater than a right one, and in this interval the excavation was effected, without exposing the separated veins, which I afterwards traced through the soil on the opposite side, after the rubbish was in some measure removed. The remainder of the error consisted in mistaking an irregular lump of a very anomalous kind of sandstone which is entangled among the marble and the trap veins now described, for the syenite which is in the immediate vicinity similarly interfering with the limestone, and of which, pieces detached by the workmen were lying upon this sandstone as if they had been recently separated from it. If care and caution are required in examining the most simple appearances among the regular rocks, a tenfold portion is necessary when we are engaged in investigating the irregular ones. The whole of the original remarks on this quarry must therefore be obliterated from the record.
There yet remain with respect to the trap rocks of Sky many facts which have resulted from the later more extended examination I bestowed on them. But as these would lead into details inconsistent with the purposes of this paper, and as they are important rather in a general view than as illustrating the history of that island, I shall reserve them for some future communication.
Having thus made the additions and corrections which appeared of most importance in the geological history of Sky, I shall proceed to enumerate some minerals which were either entirely omitted or but imperfectly seen.
In the small island of Oransa, and still more conspicuously in an islet adjoining to it, there is to be found a mass of actinolite rock, which can also be traced to the adjoining shore of Sky near to Camiscross. It lies among the gneiss, holding an uniform and parallel course with it, and as the beds of gneiss are here nearly vertical, it presents the appearance of a vein, its edge alone being visible. Nevertheless, its conformity with the gneiss, the analogy which it bears to common hornblende-schist, and its actual gradation into that substance, leave no doubt respecting its true character. The edge of this bed is very irregular, as the bed itself is interrupted and split in various places by intruding laminæ of gneiss, thus contracting in some places to the breadth of two or three inches, and again enlarging to that of as many feet. It is formed of a confused crystallization of actinolite of a pale green colour, the crystals being almost always very minute, and so entangled that the fracture often appears as much granular as it does schistose. It does not present those varieties which occur in the well known rock of Glen Elg, where the fine fibrous, granularly schistose, and distinctly crystallized, occur together, nor does it, like that one, contain talc. Its analogy to hornblende slate in a geological view is as obvious as is the resemblance of the two minerals, and its passage into that rock is here effected by the addition of crystals of black or greenish hornblende, which gradually increase in number till the actinolite is entirely excluded.
I have in another place mentioned the regular north-easterly direction of the gneiss, and I think it interesting to remark that the actinolite rock of Glen Elg corresponds with this one, as well in direction as in quality. If a north-east bearing be taken from it on the map so as to intersect Glen Elg, it will be found to touch a point near Eilan reo'ch, as near to the place of the actinolite rock there situated as it is reasonable to expect from the nature of the map, and I may add that the direction of the gneiss on both sides is correspondent. There is therefore every probability of its being a prolongation of the same bed, but to what further extent it may be traced is beyond the bounds of this investigation to enquire. The total distance included between the two points is about seven miles.
In formerly enumerating the members of the zeolite family which are to be found in Sky I mentioned laumonite on the authority of others, although I do not know the name of the individual to whom the discovery is attributed. Since that period I have myself found it in the same place between Loch Eynort and Loch Brittle in which the decomposing stilbite formerly described is to be seen. It is occasionally mixed with the stilbite, but is also found in very large masses, either alone or intermixed with crystals of calcareous spar. These masses have fallen from the cliffs above, and lie detached on the shore. They consist principally of a confused crystallization, but cavities are also found in them in which the mineral has crystallized at liberty and in its regular form. These crystals exceed a quarter of an inch in length, and the substance is here invariably of a white colour. There is nothing more remarkable in this mineral than the contrast between its present and its original state with respect to hardness. The lumps which I have described sometimes exceed twenty pounds in weight, yet they remain entire on the beach notwithstanding they must have fallen, together with the other rocks which are here found, from an elevation of many hundred feet. At the present time the slightest contact causes them to crumble into atoms.
To the varieties of analcime which were formerly enumerated I may add another which is also to be found at this spot. It is of a brick-red colour, but not crystallized, and is largely mixed with the amygdaloidal rock that predominates at this place.
I formerly mentioned that epidote was found crystallized in cavities of the different trap rocks both in Garsven and in Glamicb. In addition to that, I may here say that fragments of the same rocks are to be found at the foot of the former hill, in which this mineral appears to form a constituent part of the trap, being uniformly mixed with the other ingredients throughout the whole mass.
It has been said in some of the popular accounts of Scotland, that agate pebbles were found near Dunvegan, but having never seen specimens from Sky, and doubting the authority on which the report was founded, I took no notice of this circumstance in the former paper. I have now however found them, although in no great abundance in the same place where the laumonite occurs, a part of the coast so very rarely accessible, and under the most favourable circumstances so hazardous to attempt, that it will not fall to the lot of many to follow me to the same spot. They are of a grey colour, zoned in the usual manner, and sometimes contain cavities lined with quartz crystals. Similar geodes of quartz, of considerable size, are found in the same rocks, without the investing coat of agate; and it is further not unusual to find crystals of stilbite, of chabasite, and of filamentous mesotype, sprinkled over their interior surfaces.
Since the former account was drawn up I have also found olivin, a mineral which, however common among trap rocks, must be rare in the Western islands, as this is the only instance in which I have ever observed it. A single block detached from the cliffs above, in the place last mentioned, contains it in great abundance. It is imbedded in a rock the basis of which is a black indurated clay, the same as that which here constitutes the greater part of the amygdaloids. It forms an equable mixture with this substance, being in the shape of small irregular crystals, which, when after exposure to weather the clay has decayed, appear so conspicuous, that the whole seems a solid mass of granular olivin.
The last mineral to be added to the former list is manganese. This is found, but in very small quantity, in an unexpected situation, being mixed in the form of its red oxide with the white marble, and accompanying the steatite of Strath already described.
I shall conclude this supplement with an account of an alluvium which I lately found in a part of the island not formerly visited, and which is deserving of notice on account of its independent nature, and the difficulty which will be found in giving an adequate explanation of its origin.
It is to be observed near Kylehaken, occupying a space of about a mile on the shore, but not exceeding a few hundred yards in breadth, terminating in one side on the elevated ground, as it does in the sea on the other. It seems to be the remains of a plain once much more extensive, since its boundary towards the sea consists of a series of straight lines, the loose materials assuming the usual angle and exhibiting precisely the same appearances which characterize the terraces that line the alluvial vallies through which active rivers have cut their way. The bar of Kylehaken harbour, and the gravelly soundings of this shore, which render it an insecure anchorage, equally indicate an extent once more considerable, and confirm the supposition produced by its straight edge and the angle of its declivity. Its surface is about 60 or 70 feet above the level of the sea.
No rivers at present flow in the vicinity of this plain, nor is there, from the form of the ground, any reason to suppose that they have ever flowed so as to enable us to account for this deposit of loose materials. The substances are nevertheless rounded, and consist of those rocks which are seen in the neighbourhood, presenting a large proportion of the various hard sandstones, with some occasional pebbles of gneiss and of hornblende slate. It might perhaps be imagined that the ordinary fragments of the mountains which back this little plain, descending to the sea and there rolled, might have been rejected by the tides so as to form these banks, but this supposition is invalidated partly by the presence of gneiss and hornblende slate, which do not occur among these mountains, and partly by the altitude of the banks above the present high-water mark. It must doubtless be granted that if at some more ancient period the strait of Kylehaken was narrower than it now is, the same tide-wave which now passes through it would cause a much more considerable elevation of its tides. But it is already very narrow, and no possible contraction that can be imagined would be sufficient to produce a difference of elevation so great as would be required for this purpose. It must be added to this difficulty that the uniformly level surface of the plain is an insurmountable obstacle to this supposition.
In defect of any other solution it can only be supposed that this is a fragment of some ancient diluvian deposit, instances of which, although very rare in the islands, are sufficiently abundant upon every part of the continent of Scotland. No estimate can be formed of its original extent, nor can any valid conjecture be offered of the mode in which it has been so abruptly cut down. It is however likely that although the present direction of the tides is such as not materially to exert any action on it, that direction may have varied in the progress of time, from alterations in the shape of the bottom of this very narrow channel, subjected four times in every day to the alternating action of a most rapid stream, as well as from the probable removal of a similar alluvium from the opposite shore of the main land. As we find analogous causes producing daily and visible changes of the same nature in the courses of rivers, the supposition is not incompatible with facts, since the narrowness of the Kylehaken channel and the rapidity of its tide, give it in this respect all the characters of an inland river as far as the contraction extends. We may perhaps indulge our conjectures still further in supposing that Sky was once united to the main land by means of this alluvium, and that the gradual effect of the tides circulating through the bay on each side had at length produced the effect in question; an effect not at all inadequate to its powers, and of which parallel examples occur in the lateral action of rivers on the alluvia of vallies; on the banks of the Tay and in many others of the principal rivers of Scotland. Greater effects have often been attributed to the corrosive powers of the sea, and in indulging this speculation I have kept far within the range commonly occupied by geologists. I may remark that the narrowness of the channel, which in one part does not exceed a quarter of a mile, and the shallowness of the soundings compared with the depth of those which separate the other parts of Sky from the main land, are friendly to this supposition. These scarcely exceed ten fathoms in the middle, although there are some deeper holes on each side ranging to thirteen, the bottom being every where gravelly, as if, like the banks, it was the remains of some former alluvium.
- The differences between the map of Sky which accompanies this supplementary paper, and that formerly given, require some explanation, that the readers of the Geological Transactions may be able to appreciate the reasons which have led to the alterations, and account for those which, in a geological point of view, will be shown to be more apparent than real, and to see what degree of confidence they may repose in the present one.
In laying down the places of the different rocks, I formerly made use of the documents from which Mr. Faden's travelling map was constructed, and I was guided in the examination by Mackenzie's chart. The incorrectness of these has been the leading cause of the apparent differences in the relative extent of the rocks as they have been delineated, while their insufficiency for the purposes of a reticulum, arising from the great distortions of the outline of the country and their deficiencies in its internal features, prevented me from laying down many of the places where the existence or boundaries of particular rocks had been accurately ascertained. Hence the geological delineation bore no proportion to the accuracy of the observations. To these defects if there be added the blanks which the partial nature of the former survey necessarily left, the discrepancies of the two will not appear so considerable as they seem at first sight. I may also add, that it has been found expedient to change the colours by which the different rocks are denoted; another apparent cause of difference.
For the present observations I have made use of Mr. Arrowsmith's map, with certain variations which I think it necessary to explain, that the degree of confidence to be reposed, whether in the original or in the alterations, may be understood.
A line drawn from the head of Loch Snizort to that of Loch Slapin will, under slight exceptions, separate the estate of Lord Macdonald from those which lie to the westward of it. It is not too much to say of the latter division that the outline of the coast is incorrect, and the internal features something worse than conjectural. But the division which constitutes Lord Macdonald's estate, having been taken from a survey of that district executed by an intelligent provincial surveyor for his Lordship, is deserving of considerable confidence, and will be found over most parts sufficiently accurate in its internal features for the purposes of the geological record. Even in the latter I have found it necessary to make some additions and alterations; in the former part of the island these are considerable. I must however say, that even in this case I have limited the corrections to those parts of the map where it was necessary to denote the boundaries of different rocks; the northern parts, presenting only one substance, were not in immediate need of it, and I do not pretend to arrange the geography of the island. It is necessary to point out the alterations which have been made.
The region lying between Loch Eishort and the foot of the syenite hills, which includes the valley of Strath, has been redrawn and reduced as far as was possible to the present size, the intricacy of the position in the rocks which constitute it absolutely requiring a more accurate detail of the leading features of the ground. Slight alterations have been made in the shades which indicate the relative altitudes of the hills, wherever that was called for, and the result will be apparent to those who shall compare the present map with Arrowsmith's. At the point of Sleat and in the Kyle ri'ch two or three alterations have been made in the line of coast, these being absolutely required; others which might have been made have been omitted, as they were not wanted for the particular objects of this work.
The changes made on the eastern side are much more important. The estate of Strathaird which includes Blaven and Coruisk, has been corrected from a private survey in possession of the proprietor, Mr. Macalister. The Cuchullin hills are in the original map entirely misplaced, while the remarkable lake above mentioned has been omitted. These I have rectified as far as was in my power, since both their form and extent were important. The tract to the north of this including Mr. Macleod's property, remains with very little alteration, since its uniformity of composition did not call for any particular care.
The directions of the strata are marked by prolonged lines extending to the neighbouring shores or islands wherever they presented the same substances, and on these the tendency as well as the quantity of the dip is occasionally pointed out by an appropriate index and numbers, wherever it seemed necessary to specify them.
Finally, whatever corrections may be found in this map are merely intended to serve the purpose of this paper; the formation of a correct map of Sky must yet be considered a work far distant.
- Allowing 24° for the variation of the needle, the bearing of this bed appears to be north-east by east, but as the variation on this coast is not at present accurately known, as no examination of local irregularities has been instituted, and as the maps themselves are erroneously laid down, I have in this as in other instances held it fruitless to state their bearings with rigid accuracy.
- This mineral proves to be needle stone; a distinction not understood when the original paper was drawn up.