Transactions of the Geological Society, 1st series, vol. 2/On the Geology of various parts of Scotland
By J. Mac Culloch, M.D. F.L.S. Chemist to the Ordnance, and Lecturer on Chemistry at the Royal Military Academy at Woolwich.
Having had an opportunity during the Society's late recess of collecting some specimens of the mineral productions and rocks of Scotland, I have transmitted them with a hope that other members who may have the means of so doing, will make still greater exertions in aid of this very necessary department of our museum. Such collections, however useless in an insulated state, acquire a real importance from their union, and form by their accumulation one of the leading objects of such an institution, a point of reference for authority whether geographical or mineralogical.
To these specimens I have been induced to add the miscellaneous remarks which form the present paper, with a view of attaching to them more interest than would accrue from the mere mention of a naked habitat, and for the further purpose of rendering the catalogue which must accompany them, more intelligible. These remarks include such geological observations on the nature and connections of the rocks from which the specimens were detached, as occurred to me at the time, or were the result of comparison with the observations of other writers who have treated the same subjects. Where the specimens themselves are deficient in the requisite purposes of elucidation, the references to the places where they were found will enable future visitors to verify or correct them. In this view the remarks which I have hazarded are even more necessary than in that of perfecting a catalogue of hand specimens, since of many important geological facts, it is impossible to preserve a sufficient record in a mere collection of rocks. A great portion of the country which is the subject of the miscellaneous notice, has been already surveyed by various geologists, and particularly by Professor Jameson in his tour to the Western Isles. Those who are accustomed to geological investigations will be as little surprised to find me occasionally differing from those observers as I shall be to find future observers differing from me. The science is as yet far removed from the class of accurate ones, and must still owe a great deal to that free enquiry which alone can lay the foundation of a precise and stable induction. It is peculiarly necessary in its present state that the labours of many should co-operate, as the wide diffusion and difficult accessibility of its leading facts put in out of the power of any single observer to add much from his own stock.
To the more extended geological remarks which seem to be called for by the peculiar circumstance of some of the rocks included in the catalogue, I have added a few words on specimens which scarcely involved any novelty either of a mineralogical or geological nature. Yet it will not be useless to describe their situation, since our Society among other objects offers itself as a deposit of miscellaneous information on those subjects, and of such detached facts as must otherwise perish in the portfolios or memories of those whose fortune it has been to notice them. It is not one of its least advantages that it forms a school of practical knowledge for those whose opportunities are circumscribed by the scarcity of scientific institutions or schools of this nature, and with this view, even the humble offering of anew habitat is deserving of record.
In the mass of geological reading which is in the hands of every one, most of the places referred to for characteristic examples are in foreign countries, always of difficult access to the many, and rendered more so at present from the circumstances of the times.
Our own country, perhaps, exhibits a greater variety of mineralogical products and geological facts than any equal space on the globe, condensed in their position and easy of access. It is desirable therefore that a record should be kept of such circumstances, that the student may not be sent to Siberia or Bohemia for that information which he may acquire in Arran or the Grampians; and that the humble task of recording the domestic habitats of interesting particulars may be countenanced by a Society which we trust is destined to add somewhat to the interest already taken in this pursuit, and to the general progress of geological knowledge.
In matters of terminology I have felt an inconvenience not peculiar to myself, which I have been unable to remedy without appearing to attack hypotheses that I would willingly have let alone, as the language of controversy is unpleasing, and appears rather calculated to retard than promote the progress of an infant science; not only by warping the impressions which the mind receives, but by diverting it from legitimate observation to the more amusing occupation of attack and defence.
I allude here chiefly to Werner's great divisions of rocks. If we describe the several rocks by the terms which he has applied to them, we begin by admitting the very matter to be proved. A worse consequence follows; the adoption of the terminology insensibly leads to a belief in the hypothesis, and becomes inimical to that independent and free spirit of observation which the infancy of any physical investigation more especially requires. To describe appearances without adopting this systematical language requires a circumlocution which it would be desirable to avoid, yet till either a system is firmly established on a wide and fair induction, or a set of terms can be produced independent of all system, such circumlocution is perhaps inevitable.
I have only to add, that since the following remarks did not admit of any useful or methodical arrangement, they are placed with little regard to order, and as the subjects of them occurred. That they are so detached, and often so superficial, must be imputed partly to want of knowledge, partly to want of time, and still more to the uncontrollable elements, to which the best laid projects of the mineralogist as of the husbandman must bend.
I should scarcely have introduced any remarks on Rona, were it not for the purpose of mentioning that wolfram, hitherto unnoticed in this spot, is found in the granite veins that traverse the gneiss of which this island is principally formed. I may however remark at the same time, that these veins exhibit that variety of granite called graphic, a rock of much more frequent occurrence than it was once supposed. The graphic granite of Rona is distinguished by the great size of the crystals of felspar which enter into its composition, and consequently by the equal magnitude and distinctness of the quartz which fills their intervals. An accurate survey of Rona is still a desideratum for future mineralogists, as the number, magnitude, and peculiar character of the granite veins might give us hopes of detecting in them some of the rarer minerals known to be inmates of such veins, and observed in similar situations in other parts of the earth. I cannot however dismiss this subject of Rona without pointing out to future observers the singular contortions exhibited as well by the gneiss as by the hornblende rock where they come into contact. I may also venture to suggest that the rock of Blue Bay, described as a mixture of hornstone, chalcedony and quartz, appears to be a modification of gneiss, exceedingly intersected by veins of quartz and felspar, and probably of granite. It exhibits no genuine hornstone; that part of it which has at first sight this appearance, being an intimate mixture of quartz and felspar. The chalcedony too is only that modification of quartz which occurs in graphic granite, and which is well characterized by its peculiar waxy look. Exposure to the weather produces on its surface a glossy enamel, of an opacity and lustre still more nearly resembling that of chalcedony, but which does not penetrate into the stone. A similar enamel is to be observed investing the sandstone of Jura and of Schihallien, and I have also specimens of the granite of Rockall, which exhibit the same appearance. It seems to arise from a partial solution of the silex on the surface, and has been observed in certain foreign sandstones, being quoted as an instance of the solubility of silex in water. This quartz occasionally assumes a high red hue, which is evidently to be traced to the penetration and intimate mixture of red felspar. In some places it is of a green colour, being penetrated by threads and laminæ of a substance bearing a resemblance to steatite rather than to hornblende. It thus forms a greenish stone, probably different from quartz coloured by actinolite which is called prase, from the same stone penetrated by epidote which has obtained no name, or from quartz tinged by green earth, which occurs in Rum and which, as well as chalcedony when similarly coloured, is known by the name of heliotrope. Yet, as it possesses the external characters and aspect of prase, it may be safely referred to a species of which colour seems the only very discriminating character. I think it necessary also to mention that the hornblende rock exhibits the two subordinate varieties of hornblende slate and greenstone slate.
Since I have already indulged in some cursory remarks on that sort of nomenclature which tends to mislead, by confounding substances, I may be excused for suggesting a reformation in the terms primitive greenstone and greenstone slate. These consist of mixtures of felspar and hornblende, in the latter case disposed in a laminated form, and in the former without that regular structure. Their title to greenstone, as far as colour is concerned, is still less than that of the varieties of the trap family, as the felspar is (in Scotland at least) always of a red colour. By the use also of the term greenstone, and by the series which is formed of primitive, transition, and flœtz greenstones, the mind is naturally led to look for a geological and natural connection between these rocks. But the primitive greenstones belong to that class in which gneiss and mica slate form the principal features, and which constitute that great system of rocks of which hornblende in its several modifications of hornblende rock, primitive greenstone, hornblende slate, and greenstone slate, is only a subordinate and occasional member. The geological character of the flœtz greenstones, is their independence with respect to the rocks with which they are associated, and their intrusion, if I may so call it, among a regular system of stratified bodies. The transition greenstones are, I conceive, of a character merely arbitrary, and only so called because occupying the same situation with regard to the transition, as the former do with regard to the flœtz rocks.
Thus also, greenstones may be classed among the primitive, where they occupy similar positions among primitive rocks.But the primitive greenstones to which I now allude, and which
constitute a class of rocks entirely different, have been evidently formed together with the rocks which they accompany, and are, like them, referable to the same epoch, never intruding to their disturbance, but occupying situations and maintaining characters, in every respect conformable to them. It would tend to the accuracy of geological language, if they were to receive designations derived from their simple parent, hornblende rock, and it is to be desired that the occasional mixture of felspar with hornblende, whether that rock be massive or slaty, should be designated by a term, if it requires a distinct one, derived from its true basis, so as to prevent the confusion which their present names introduce into our ideas and descriptions.
The Shiant isles having escaped the notice of mineralogists, I shall make no apology for giving such a cursory description of them as my opportunities of observation allowed me to draw up, for the purpose of explaining the specimens which I have transmitted.
These islands are situated to the north of Ruhunish point, in Sky, from which they are distant about 15 miles, and they lie about 8 miles nearly south-east of the entrance of Loch Brolum, in Lewis. There are three principal islands, forming a nearly equilateral triangle, besides a few subordinate rocks of considerable height, but of small superficial dimensions. The names of the three largest are Gariveilan, Eilanakily, and Eilan Wirrey. Of these the two former are about a mile and half in circumference, and the latter appears to be less than a mile. They constitute one farm, the house appertaining to which is situated on Eilanakily. In approaching them on any side they exhibit high columnar precipices, surmounted by grassy irregular plains, and surrounded by rocky shores of difficult access, which are whitened by a rapid and generally turbulent sea. It is easy to perceive that they are all of a trap formation, and with that general knowledge of two of them, Eilanakily and Eilan Wirrey, I was obliged to remain content, as a gale of wind with thick weather coming on soon after I landed on Gariveilan prevented me from extending my examination further, without endangering the boat and her crew. I trust that some future mineralogist, with better fortune, may complete the investigation which I was obliged to leave undone, and fill up the blanks which will be found in my brief account of this very picturesque and interesting spot.
The boat landed in a bay on the north side of Gariveilan, where a noble façade of columnar trap descends perpendicularly and without a break into the sea, rising to an elevation, as I should guess, of 200 feet. In picturesque effect, and in continuous profundity of shade, it excels even the celebrated cliffs of Staffa. The columns are however neither so regular nor so perpendicular as those of that island, but rather resemble the obscurely formed ones which occur on the shores of Ulva. A few here and there present nearly the same degree of regularity as those well-known specimens on the south side of Arthur's seat. This colonnade extends along a great part of the northern shore, plunging under the water to the eastward, while towards the west it is so elevated as to allow an examination of several strata, which lie beneath it. At the south side of the island the ground slopes so as nearly to meet the sea. It is on this south side, and at the eastern end, that these different strata come into view. The lowermost bed is a dark purple hornstone, of which the thickness cannot be determined, as nothing is seen below it. The accompanying specimens will preclude the necessity of describing a rock of no uncommon occurrence, whether the name by which I have designated it be one about which mineralogists are agreed, or not. It has unfortunately been applied to so many different stones, that it is utterly impossible to steer clear of difficulties. Among the reforms of nomenclature, in the departments of rocks, the term hornstone, and the various substances which have been classed under it, call loudly for examination. The adoption of petrosilex might have gone some way to remove this confusion, as we should then have had two names instead of one, by which to designate four or five different substances; but as this latter appellation has been equally misapplied with the former, and different authors have called the same substance by both these names, the confusion has, if any thing, been increased. Mineralogists may inquire whether by a due appropriation and limitation of the three terms, hornstone, chert, and petrosilex, already in use, some progress may not be made in removing this obscurity without any material additions to our nomenclature. I forbear even to hint at the apparently, and only apparently, corresponding French terms, pierre de corne, and cornèene, lest I should drive the reformer to despair.
But to return. This bed dips to the south-west, and appears to have about 15 degrees of elevation. Immediately above it lies a bed of rock, about 20 feet in thickness, and of a structure so peculiar as to require a more detailed description.
It is formed of globules, every where adhering together, not merely by their touching surfaces, like most of the pisolites, but in general by large segments, similar to the globular limestone of Sunderland. These are occasionally compressed as if by a superincumbent weight, but in many places are absolutely spherical, and by their accumulation become botryoidal, smaller spheres growing out as it were from the larger ones. Detached specimens that have been washed by the sea, exhibit the appearance of a bunch of grapes, but in their native bed the intervals are filled with an ochry clay. Their fracture shows the uniform aspect of siliceous schist, being neither coated nor radiated; and the rock agrees in every respect, except its structure, with the bed of siliceous schist, which is found not far off.
I have only observed one other instance of a rock similar to this, and it occurs also under amygdaloidal trap at Talisker, in Sky. I have reason to suppose it of rare occurrence. In various parts of the rocks which I have now described there are found natural joints, which on being separated exhibit two smooth surfaces, absolutely flat, and appearing as if they had been cut through by a sharp tool. The globular bodies themselves are divided in various parts, just as they happened to interfere with the section. In the greater number of the sections which I examined, no substance was interposed between the touching faces. But in some I found wavellite, a mineral as yet not so common, but that this new habitat will be acceptable to the mineralogist. It is remarkable that the circles of wavellite occupy precisely the surface of each segment of the several spheres, varying in size according to the varying dimensions of these segments. This bed is covered by one of trap, or to use the term in its present great latitude, of greenstone, forming a parallel bed of a more considerable thickness, but with the same inclination as the subjacent strata. This bed of greenstone weathers so as to exhibit distinct globular concretions, although in its entire state the eye can trace no difference between the amorphous portion and the columnar mass which forms the upper part of the island. This very common fact should serve to convince us how little information we can obtain of the true and fundamental structure of rocks, from their natural fracture, while in a state of integrity.
The bed of trap which I have now described, is followed by a bed of schistus, which may be called a siliceous schistus, a term likewise under which many substances have been very improperly confounded, although the characters of this rock are sufficiently defined to render such carelessness of description inexcusable.
But it is not only in mineralogy that the desire of accounting for that of which the cause is unknown, and of describing that which is not understood, predominates over those severer habits of investigation, which can alone render scientific description accurate.
It is too hopeless a task to attempt to develope the confusion of rocks to which the terms cornèene and trap have been applied, names which will probably be given to this rock by those who follow the French nomenclature. I believe I go along with Mr. Jameson, in calling it flinty slate, or siliceous schist, and it will perhaps be considered a variety of that which is called Lydian stone. It breaks naturally into prismatic and rhomboidal fragments, but its forcible fracture is flat, approaching to the conchoidal, with a small degree of lustre, and without grain. It is exceedingly brittle, and cuts the hands like glass. Its gives fire readily with steel, and the file scarcely touches it. Its colour is a dark lead grey.
Natural joints, similar to those I noticed in the botryoidal schistus, (if I may venture thus to distinguish the rock described above) occur also in this schistus, and circular stains may be observed upon the planes of contact. I did not find any specimen of wavellite in this variety; but I cannot pass on without remarking that the habits of this rock, and its connections, in all the cases where I have had an opportunity of seeing it, seem to point it out, as an argillaceous schist, altered and indurated in consequence of its proximity to greenstone.
The last bed is the immense mass of columnar trap already mentioned, which surmounts the whole. The character of this rock is not that of a greenstone, to use the word in its more restrained sense, as it does not contain distinct grains of hornblende and felspar, but is an obscure mixture of hornblende, felspar, and quartz.
I have therefore both here and in other places, (where I have had occasion to describe the varieties of this rock, of which the characters are not so decided as to admit of their being placed either under the heads of basalt, wacke, or greenstone,) preferred the general name of trap to the use of a specific one with which the characters of the individual cannot be made to agree. In so doing I have left out of view the confusion which must arise if the siliceous schist, and Lydian stone, which appear to be the trap of some authors, be considered as forming a part of this great natural division, and have taken the term, as I imagine, in the sense intended by the followers of Werner. It is thus used by them as the title of a family including basalt, wacke, greenstone, and clinkstone, a family well expressed by the vernacular term whin, now, I believe, and perhaps unfortunately, obsolete. The characters of basalt, wacke, greenstone, and clinkstone, are tolerably decided, and the geological relations of the trap family are so remarkable that we ought to find little difficulty in assigning proper terms of distinction to its respective members. But, independently of these four well characterized rocks, innumerable varieties occur not properly referable to either, and which cannot even be truly expressed without much circumlocution, and reference to those well-known members which they may chance to resemble. If in these cases we apply the name of any of their species in place of a generic term, we render our description of individuals unintelligible, and often contradictory and absurd. Thus the name basalt has been given to greenstone, as well as to many other modifications of trap, to which no name has, or perhaps ever can be applied. But the chief confusion has resulted from the application of the term greenstone, as a generic term for these rocks. In this way we have had greenstones of all colours, from red to white, and in which neither hornblende nor felspar were to be distinguished.
It would assuredly be better in describing the geological appearances of these rocks, when the description of individuals is not necessary, to make use of a generic term which should distinguish the character of the family, without confounding the individuals. The peculiar aspect of the rocks of this family renders the name of trap as expressive as any which can be applied, and in this sense, if it be limited to that well-known class of unstratified rocks which includes the above-named individuals and their numerous modifications, we shall escape the confusion which must otherwise ensue, and which unfortunately already exists too much.
It is evident that the other modifications of this rock consisting of amygdaloid and of the various trap porphyries, are still further subordinate species, if I may style them so, ranking according to their particular basis, under the several species above enumerated. It is said that in the Shiant islands ammonites and other animal remains are discovered in beds under the trap, but I did not observe them. They may very probably however exist here, as they are of frequent occurrence in Sky.
Among the rolled pebbles on the shore I met with specimens of pink coloured quartz. I did not observe any foreign matters imbedded in the trap, except minute drusy veins of calcareous spar.
Such are the very limited observations, which an inspection unavoidably much hurried, enabled me to make on this part of the group. Although the general aspect of the other two islands would lead me to expect in them an identity of structure, yet they may afford differences worthy at further investigation.
A stormy sea, a dangerous shore, want of harbours, a trackless country, and a scanty and wretched population, render it as difficult a task to reach this island as to examine it.
It forms one mountainous tract of rugged and barren aspect, and of considerable extent, although its highest mountains, which lie towards the west, do not appear to exceed two thousand feet in elevation. My observations were limited to the only two parts of the island which are tolerably practicable, Loch Skresort, and Scuirmore.
Loch Skresort forms a semicircular indented bay, from which the land shelves gradually upwards by a moderate acclivity, till it approaches the middle region of the mountains, from whence the further ascent is steep, rugged, and often precipitous to the very summits. These terminate in craggy and broken points, exhibiting abrupt faces, and piles of ruins. The rock which incloses the bay of Loch Skresort, is a reddish argillaceous sandstone, disposed in beds and elevated to a low angle, varying from ten to fifteen degrees, occasionally fractured, bent, and displaced. It appears to be the result of the decomposition of granite, since the argillaceous grains are often found to consist of felspar in a state of integrity, although at times putting on the appearance of clay. The quartz is of a brownish red colour. My examination of it was by no means sufficiently extensive to enable me to say what varieties may exist in different places, but it has an aspect and disposition so perfectly analogous to that of the sandstone which lies under the Cuchullin mountain on the opposite shore of Sky, and which forms the greater part of the island of Soa Vretil, that I consider them as belonging to one bed. As I shall have occasion to describe this rock again in the remarks which I propose to offer on Sky, I forbear to dwell on it here. The basalt veins, so common throughout all the islands of this coast, traverse this sandstone, but are not so abundant immediately about Loch Skresort, as is common on these shores. Where the sandstone terminates on ascending towards the mountain, it is succeeded by a mass of rock, which bears the general aspect and features of the mountain trap rocks, of the rocks for example which constitute the Cuchullin mountains, and which are perfectly distinguished in their general characters from those which, on account of their bedded and parallel appearance, are well designated by the term floetz trap.
But the composition of this rock is peculiar, and, as far as I know, resembles no other in Scotland. It consists of two ingredients only, and has at first sight the aspect of those syenitic traps so common in the Cuchullin, which are formed of hornblende and felspar, in large crystals: but instead of hornblende it contains augite. These substances, the felspar and augite, are confusedly crystallized together; and although the concretions vary much in size, they are most generally large, and appear to increase in dimensions on approaching nearer to the summit of the mountain. The crystals of augite are not infrequently an inch or more in length. Although I traversed a considerable space over these mountains, I did not discover any variation from this general character. I should add, that the felspar is the glassy variety. The cliffs from Loch Skresort to Scuir-more, exhibit the same general appearances of sandstone skirting the shores, and intersected by trap veins, or with masses of trap rocks superimposed on them. A more particular knowledge of this coast is scarcely attainable either from sea or land. At Scuir-more a small beach affords a point of access, and here some variations occur which are worthy of notice.
The sandstone is in this place elevated at very high angles, varying from forty to sixty degrees and upward, and it appears to dip uniformly toward the west. In composition it differs more materially, being ferruginous from oxide of iron, and in place of clay, containing calcareous earth. It is also traversed by numerous minute veins of calcareous spar, and is intersected by innumerable basaltic veins, of different sizes, and often of great magnitude. By these and the subsequent disintegration of the rocks, it is hollowed into caves, and wrought into arches, producing strange and picturesque scenes of ruinous grandeur. I may remark, that the junction of these different rocks does not appear to be attended with any change in the nature of either. In one of the veins there is a portion containing a compound of an uncommon nature, but somewhat resembling the rock above described, being a mixed mass of crystals of augite, hornblende, and felspar. Loose specimens of black pitch-stone are also found on the beach, arising in all probability from some vein of that substance. But the production for which it is most remarkable is the green chalcedony, or heliotrope as it is generally called.
This substance, which exists in large masses, is also found occupying cavities in the basaltic amygdaloid, in nodules varying from the size of a mustard seed to that of a pea. These are accompanied by similar nodules of calcareous spar, often also penetrated by green earth (chlorite baldogée.) This amygdaloid accompanies the larger masses of green chalcedony, and seldom extends many inches beyond them. The great masses are either in the form of irregular nodules, reaching from the size of an egg to that of a man's heed, or they resemble short veins, of which the opposing sides are flat, and somewhat parallel, and which might perhaps more properly be called angular nodules. Their colour is as various as their other external characters, but the most frequent varieties are comprehended in the following list.
- White chalcedony? approaching to quartz, and sometimes to hornstone, in its aspect, fracture, and other qualities.
- Pure chalcedony of a more or less milky hue, constituting the chalcedony and the white carnelian of lapidaries.
- The first variety of chalcedony, of a brown colour with narrow parallel stripes of green.
- The same, irregularly mottled with green, white, and brown.
- The opaque, or first variety of chalcedony, of an uniform dark sap green, with the dull fracture and aspect of wax, and like that substance, translucent on the thin edges.
- The translucent variety, similarly coloured, but with a more conchoidal fracture, and of a glassy aspect.
- Varieties which in one specimen exhibit a perfect gradation from pure white or colourless chalcedony, to the deepest green.
All these varieties contain imbedded spherical bodies, varying from the size of the minutest poppy seed, to that of a mustard seed. These are frequently formed of pyrites, and although they are often entirely decomposed, yet they now and then exhibit the remains of a crystal of pyrite within them, and their external surfaces retain a ridgy and crystallized aspect, the remains of the edges of the plates which have formed them. But the most common of the spherical bodies are, when fresh broken, of a white colour, becoming at first yellow by exposure to air, and subsequently of a dark brown. Their internal texture, as far as it can he determined by high magnifying powers, appears platy, and although it is difficult to speak with certainty of bodies so minute, and so much out of the reach of examination, they seem greatly to resemble brown spar; a conclusion rendered more probable, by the changes which they undergo on exposure to the air. The whole of these chalcedonies are occasionally penetrated by laminæ of calcareous spar, which are often so numerous as to form nearly one half of the stone, and which from its yielding; in their direction, rather than in that of the chalcedony, do often seem on a superficial view to form the whole of it. In the rifts of these chalcedonies, we may occasionally find detached messes of green earth, which by its combination with the stone itself appears beyond all doubt to be the colouring ingredient. Its unequal diffusion through the stone, is equally demonstrative of the same supposition.
We have now to compare the green chalcedony of Rum, with the stone known by the name of heliotrope, and commonly called oriental, since that name has been applied to it, and as it will appear, with perfect propriety. The fracture and translucency of the true oriental heliotrope, show, as Brongniart has well remarked, that it has been improperly ranked among the jaspers; and the description and natural analysis, if I may so call it, which I have now given of the stone of Rum, confirm the propriety of his remark, and refer it to its true place among the varieties of chalcedony. It is true, that the stone of Rum very rarely exhibits the red spots which so often occur in the oriental kinds, but these are purely accidental. An examination of many specimens of the latter in my possession, has proved the existence in it of variations in regard to lustre, and other characters, precisely similar to those I have noticed in the stone of Rum. In these too, the fracture is now and then waxy, flat, and dull; occasionally it is conchoidal, shining, and highly translucent, but it never approaches to the earthy aspect of true jasper. The irregular diffusion of the green earth is also equally remarkable in some of the oriental specimens, since among those which have fallen under my examination the green colour is often diffused in partial stains through a pure chalcedony, so that the distinct grains of green earth are discernible by the lens. These also being accompanied by the blood marks esteemed characteristic of this stone, leave the determination of its name free of doubt.
We may therefore conclude, that the green stone of Rum is the true heliotrope of lapidaries, and that this stone is merely a mixture of green earth with chalcedony, its external characters varying either from the variations in the quality of its base, or from the quantity of the green matter with which it is combined.
I did not perceive among the green chalcedonies of Rum any specimen resembling the plasma of the Italians, unless perhaps that in some of them small parts of a more remarkable transparency might occasionally be traced. Yet, as among specimens of the real plasma which I received from Egypt, there are green spots in the stone, resembling minute crystals of chlorite, independent of the general green stain which pervades the whole, I think it not unlikely that this substance is also the colouring matter of plasma, and that the only difference between that mineral and heliotrope, will be found to consist in the different transparency of the siliceous stone which receives the colour. It is not therefore improbable but that plasma as well as heliotrope may be an inmate of the trap rocks of Scuir-more.
A substance is occasionally brought from India, known to lapidaries by the name of brown carnelian, and it is esteemed peculiarly rare. This substance also is found among the chalcedonies of Rum, occupying the same situation as the green varieties, and differing from them only in colour. It probably owes its stain to iron. Motley mixtures of brown, and green, of considerable beauty, add to the variety of ornamental stones which these rocks contain.
The apparently inaccessible nature of the southern shore of Rum, prevented me from extending further my observations on this interesting island. But in coasting it slowly along, it offered the same general appearance as the cliffs I have now been describing which look toward the island of Canna, exhibiting one formidable wall of basaltic aspect, reposing on a base of sandstone. It will be for more successful geologists to examine whether circumstances equally interesting, and of a different nature, may not be found among the caverns and ruins of this repulsive, if not absolutely inaccessible coast.
I have little to add to the description of Egg given in the “ Mineralogy of the Scottish isles,” but the following fact relating to the situation of the promontory called the Scuir of Egg, which is not noticed in that work. The columns which form this most magnificent precipice, exceeding in grandeur and picturesque effect even the far famed Staffa, are of a black pitchstone porphyry. They are disposed in various perpendicular, inclined, and horizontal directions, and are either straight, or curved, but never jointed. The felspar which they contain is the glassy variety. The lines which separate the different sets of columns, together with the varying inclinations of these beds, seem to mark different sets of deposits. The whole promontory lies on a bed of compact grey limestone, generally indeed approaching to the character of a stone marle. This bed is three or four feet thick, and rests on a still lower bed of hard reddish sandstone, beneath which nothing is visible. Large masses of wood., bituminized and penetrated with carbonat of lime, are found in the made stratum, not at all flattened. Portions also of trunks of trees retaining their original shape, an seen in the same bed, silicified, and their rifts filled with chalcedony, approaching in aspect to semi-opal.
I have separated the account of the limestone which accompanies the quartz rock of Assynt hereafter to be described, and which forms so conspicuous a range of hills, because its connection with that rock is tolerably obvious, and throws no light on its history. But its peculiarity of character, and the great space which it occupies, render it highly deserving of notice, as I believe Scotland no where affords a tract of limestone so extensive.
A low chain of hills commences about Achamore, and accompanies the Tain road towards the east for four or five miles, lying as it were in a large valley bounded on both sides by an interrupted range of quartz mountains. It appears to rise gradually as it extends eastward, and about the Kirk of Assynt attains a height of 1200 or 1400 feet, forming a large and magnificent continuous mountain ridge, exhibiting a great mural face of precipitous rock to the south, and shelving away to the northward. The dip and direction of the quartz mountains appear in this place to be similar to those of the limestone. The uppermost part of this ridge seems to consist of one great uninterrupted bed of limestone, some hundreds of feet in thickness, or rather, of an accumulation of beds; well marked horizontal lines being observed to extend in a parallel direction along the precipice. It is also completely traversed by perpendicular fissures.
Below this great mass a bed of quartz or sandstone, of considerable thickness, is to be seen running parallel with, and separating the incumbent from a similar subjacent body of limestone. I did not observe whether this alteration is carried further, but it is probable that it might be discovered in some other parts of the ridge. The limited extent of my investigation also prevented me from determining the position of the limestone relative to the great sandstone or quartz formation; whether it lies upon the sandstone which rises to the south of it, and under that which rises to the north, or whether it is a partial deposit, occupying the valley alone, and incumbent on the whole mass. The examination of an extensive tract of very difficult access, would be requisite to decide this question, as well as to ascertain the magnitude of its extent, and the nature of the limits by which it is bounded. Yet, the appearance of a quartz bed alternating with it, renders it probable that it is also interposed among some of the larger masses. The beds which I have described are elevated to a considerable angle, and although in some places their section forms a continuous and even line, in others they are curved and broken, and tossed about to such a degree, that their stratified position can scarcely be perceived. The stone itself is a dark grey or nearly black bituminous limestone, of an earthy aspect, and minute granular fracture, smelling offensively when rubbed. I did not any where observe animal remains in it. In many places it is fissured into small fragments, the intervals of which having been subsequently filled up by red or white sparry matter, a beautiful variety of marble is produced, which is now wrought for the purposes of ornament. Its cavities also occasionally contain large and perfect crystals of that form of calcareous spar called equiaxe. Its surface is in most parts covered with a loose calcareous tufo, which in some places being solidified by an infiltration of calcareous matter, forms beds of solid breccia. It contains grains of sand dispersed throughout, and for this reason, commonly gives fire with steel, a circumstance of very frequent occurrence in all stratified limestones, whether referred to the rocks called flœtz, or transition, and generally most conspicuous in those which contain animal remains in abundance. Such are the circumstances of chief importance in the character and position of this limestone.
I shall now, perhaps, be expected to assign a place to this rock in the usual division of primary, transition, and flœtz, distinctions which I am inclined to think are more easily made in the closet than in the field. In the present state of geological science, it would appear a safer practice in this case, as in many others, to describe that which actually exists, without the use of hypothetical terms, which only serve to perplex the observer, and to mislead the student, who either boldly pronounces on the character which suits his particular creed, or modestly supposes himself incapable of sound observation, because he is unable to see that which is not visible. If the quartz rock of this tract be a primitive rock, then the limestone is necessarily a primitive rock also, notwithstanding its bituminous nature, and accurately stratified structure, since it alternates with it. But, if this rock is not a primitive, but a transition rock, then the limestone also must be removed from the first class of rocks to the second.
In examining its right to a place among the transition rocks, we must compare its characters with those which have been assigned as the essential distinction of those rocks. I read in the exposition of the Wernerian system best known to us, that "transition limestone contains petrifaction's of marine animals," and "that it rests immediately on newer clay slate," and in the numerous distinctions assembled for the purpose of defining it, these are the only two which appear capable of being rendered truly definitive. Certainly neither of those characters is found in this limestone. Equal difficulties oppose its admission among the flœtz rocks, when we consider its connection with the quartz rock, which, whatever division it may be referred to of the older formations, can scarcely be supposed to belong to this class. Having no anxiety on this head, and conceiving that we are still deficient in information, both on the nature of this district, and on the subject at large, I shall willingly leave the determination of its artificial place to those who are either better acquainted with the unwritten laws by which these matters are regulated, or to a period of further information.
To a mere observer, uninfluenced by systems, it exhibits the remains of a disposition originally stratified and horizontal, but disturbed, inclined, and broken, by subsequent changes;—a disposition, which however uniform in more distant times, has been altered to its present one with a partial, not a total loss of its original character, by revolutions of which the antiquity and magnitude are unknown to us, and by agencies which we are ill able to explain. Any system of arrangement may be useful, which, although artificial, assists us in classing and describing phenomena more satisfactorily; but if we are to adopt the system of arrangement to which I have alluded above, it is much to be desired that geologists would furnish us with such characters as shall enable us to decide on the different bodies appertaining to the several divisions of their system, without which, an artificial arrangement is not merely nugatory, but injurious to correct observation, chaining down our conclusions in an obedience to particular dogmas, and producing impediments destructive to the progress of legitimate science.
Another tract of limestone, of a distinct character, occurs in the same valley, occupying a position nearly parallel, at some distance from it. Of this I have unfortunately nothing to record beyond the detail of specimens. The whole inclination of the hill on this side of the valley is covered with a deep bed of soil, such as to render it impossible to trace the position of the rock, of which, all that is visible appears in the form of detached masses, apparently without order or semblance of stratification, rising through the grass.
The rock itself is an object worthy of attention from its œconomical qualities, affording various specimens of marble, of a colour from pure white to grey, which have alredy been introduced into commerce, and for which a premium has been assigned to Mr. Joplin, of Gateshead, from the Society of Arts. It is the same marble described by Williams. The principal varieties are the following:
- Pure white of a milky opaque aspect, with a crystalline texture, and large grained granular fracture, approaching to the splintery. Acquires a smooth surface on the polisher, but remains of a dead hue like the marble of Iona, reflecting no light. Hence its uses as an ornamental marble are much circumscribed.
- White mottled with gray, of a large platy fracture, and capable of receiving a high polish, forming a marble of some beauty.
- Highly crystalline and translucent, with a large platy fracture, and of a gray colour, capable of being applied to the purposes of ornament in sepulchral sculpture.
- Of an uniform dove colour, very compact and translucent, assuming an excellent polish.
- Pure white, of a large platy fracture, and translucent appearance, capable of being used in plain ornaments, but too transparent for sculpture.
- White, with irregular yellow stains, from serpentine intermixed; extremely fine, compact, and fine grained splintery.
- Intersected by thin laminæ of schiefer spar, of an exceedingly splendent appearance.
My object in laying before the Society the series of specimens collected in Isla, is the correction of some mistakes which have been entertained about the nature of its limestone district; a district of some importance in more than a speculative view, as it forms a mining field in which lead ores have been wrought to a considerable extent. My researches in Isla having been limited to this particular part of it, I shall confine myself to that which I have seen, referring to Mr. Jameson's book for such further information as may be found in it. I may, however, venture to point out this island as a spot in which the very important circumstance alluded to in another place, of the connection of the quartz rock with the mica slate, and clay slate, might be studied to great advantage. This deduction is not drawn from my own observations, which were not sufficiently numerous to enable me to decide on a question so important, but from the narrative of Mr. Jameson, who appears to describe frequent alternations of those different classes of rocks. Extensive and correct observations on Isla, would probably determine the true relative æra of the quartz rock, and elucidate still further the disposition of Schihallien, of Jura, and of the north of Scotland. I shall merely indulge in a few very cursory remarks on this part of the formation of Isla.
I have stated in another place that in Jura the quartz rock assumes towards the shores a disposition far less regular than that which it exhibits in the elevated districts. On the sides of the narrow strait which separates this island from Isla, this irregularity is particularly remarkable, and it is also very easy of access. These rocks possess all the varieties of structure which I have formerly described, and with them mica slate and clay slate are found in beds, which, even if this question had not already been decided by evidence drawn from other places, I should have no hesitation in calling alternate.
The shores of Isla which form the other side of this narrow strait, resemble those of Jura so perfectly, and correspond with them so nicely in their various characters, that a spectator would not, without a geographical mark for his guide, be able to determine which of the two he was examining. We may, without hazarding too strong a conjecture, look back to a time when these islands formed one continuous tract. Veins of basalt and granite are to be observed in both, the former by their superior permanence bearing a resemblance to high walls, which at a distance emulate ancient ruins; appearances similar to that which forms the supposed volcanic amphitheatre of Mull, so highly pictured by Faujas de St. Fond. Some varieties of this basalt exhibit by an incipient decomposition, a structure consisting of thin lamellæ, and occurring, although in a less remarkable degree, at the Giant's Causeway. There is an example of this structure among the specimens which are the subject of the present brief commentary.
But to proceed to the great limestone district of Isla. Mr. Necker's map in the possession of the Society, founded on the general opinion of the mineralogists of Scotland, calls it a flœtz limestone.
A very cursory survey of it is sufficient to show, that it consists of irregular and broken fragments of beds disposed in every possible position. Occasionally it is flat, but is now and then elevated at a small angle, and often appears as if it consisted of strata perfectly vertical. Almost every where it is disturbed and contorted to a great degree. The beds are easily traced in several places, where quarries have been dug, or mines wrought; and in many others the inclination of the soil and the action of water, have laid the edges bare for some hundreds of yards together. The perfect similarity of its disposition to that of the limestone which accompanies clay slate all over the Highlands of Scotland, and more particularly to that of the island of Lismore, would suffice to determine its true character, even without the detail of specimens. There are two predominant varieties of it. Of these, one is dark blue, and the other greyish white. They are both compact, and hard, but the aspect of the whiter varieties is more crystalline than that of the dark, which indeed often possess a merely earthy and granular appearance. In many places the blue varieties, where most crystalline, are traversed by numerous veins of a white colour, producing an ornamental marble worthy of attention.
The blue varieties of this limestone appear to be deposited in contact with clay slate, the white on the contrary, which are much more rare, are found in contact with a micaceous slate.
When the blue variety approaches the clay slate, it first assumes a schistose fracture. Shortly after it appears penetrated with delicate laminæ of straight or undulating silky schistus, which by degrees increase in number, and at length so predominate, that it is only by examining the cross fracture, and even with some care, that the presence of limestone is discovered in the mass. Ultimately the limestone disappears from the schistus.
The whiter varieties, which I have described as more rare, are found in contact with a micaceous schistus consisting of thick flexuous laminæ of quartz separated by thin layers of mica. Here the gradation from the limestone to the micaceous slate takes place, by an alternation of the layers of mica with the limestone, forming a rock easily mistaken for mica slate, and resembling the Cipolino marble of the Italians. These limestones are no where bituminous, and it is scarcely necessary to say, that I did not observe in them any animal remains. A breccia formed of both the varieties is found here and there, and I observed detached specimens of a variety filled with minute pores, which although it exhibits no decided animal appearances, has yet a suspicious character. A specimen is before the Society for the examination of those who possess superior experience in these modifications.
It now remains to determine the place of this limestone in the system of Werner: a determination which probably involves the character of all the limestones so frequent in the Highlands of Scotland, which, like this, are deposited with schistus. The instances which occur at Balahulish, at Loch Earn, at Lismore, at Dun MacSniochain, and at numerous other places, are too familiar to be pointed out to those who are acquainted with Scottish mineralogy.
It is evident that the character of the limestone must, in this case, depend on that of the rocks which it accompanies, and not on the mixture of mica or other foreign minerals which it may contain. If the clay slate and mica slate of Isla are both considered primitive, as they probably will be, then this limestone must also be ranked among the primitive rocks. In all the other instances in which I have observed this kind of limestone, it has appeared to me to appertain to the same system of rocks. Yet I have reason to suppose that the beds of it which exist in the district of Aberfoyle, alternate with a sort of intermediate rock, which I am inclined to consider as the latest of the primitive, or the oldest of the transition series, a rock approaching at least as near to the character of graywacke, as to that of micaceous schist, if it be possible, as I suspect it is not, to draw any line between these two classes of rocks. In this case, the same limestone will, like clay slate, bear a share in both these artificial divisions, for artificial I must needs consider divisions which nature has separated by a boundary so feeble and so undefinable.
The craig of Ailsa, from which the specimens now exhibited are taken, resembles the summit of a huge mountain rising abruptly out of the deep. Such in fact it is, and not a mere rock, as its name might induce one to suppose. It shelves rapidly into the sea, and is surrounded by deep water on all sides except the south eastern, where a small beach has been formed by the accumulation of its ruins. I regret that the derangement of my barometer, a derangement unfortunately too frequent, prevented me from ascertaining its altitude; but by comparing its appearance with that of Arthur's seat, and computing from the time it required to ascend it, it cannot be much less than 1000 feet in height. It is called 940 in some of the sea charts, but on what authority I do not know.
Its circumference cannot be less than two miles, and it therefore forms a large island, which is covered with verdure, and is the habitation of gulls, awks, gannets, goats and rabbits. Its shape is round and cumbrous when viewed from the north-west, but whenseen from the north it assumes an elegant conical figure.
It is bounded on the north-west by perpendicular cliffs, 200 or 800 feet in height, but on the other sides it declines by a rapid grassy slope to the sea, intermixed however with rocky faces, and covered with heaps of fragments, which are perpetually falling from the bare rock. The square ruined tower which remains at about a third part of its elevation, offers nothing interesting to the antiquary, but the botanist will be delighted with the profusion of lychnis dioica and silene amæna which covers it with a dense coat of flowers; to the exclusion of even the grass. It contains springs at about 200 feet below its summit, which unite in a small marshy plain, covered with enormous plants of hydrocotyle vulgaris.
The rock which forms this insular mountain is in general amorphous, and breaks into large irregular masses, sometimes approaching to a rectangular shape, sometimes without that tendency, and resembling the fragments of quartz rock. In many places it approaches to an obscure columnar structure, and this occasionally acquires great regularity. It is on the north-west side that the columns are most perfect. They are here well defined in their angles, yet adhere together, so as to appear to form one continuous mass, their true structure being only detected by the occasional falling of the huge fragments which strew the narrow beach on this quarter of the island. They vary in the number of their sides, but like basaltic columns, the most general forms are the pentagonal and hexagonal. I could not any where perceive that they were jointed, but they break at right angles to their axes, forming those flat summits which are tenanted by clouds of gannets. Their dimensions are universally large, as they are from six to eight feet in diameter, and extend in height as far as the eye can judge, to a continuous altitude of 100 feet and upwards. Nothing can exceed the magnificence of the columnar wall on this side of the rock; even the high faces of Staffa sink into insignificance on a comparison with the enormous elevation and dimensions of Ailsa. With that elevation is combined an air of grandeur, arising from the simplicity of their aspect, which the pencil and pen are equally incapable of describing. To the lover of picturesque beauty, they possess a requisite, of which the want is perpetually felt in contemplating the basaltic columns of Staffa, or Egg. This is their gray colour, catching the most varied lights and reflections, when the iron cliffs of basalt are confounded in one indiscriminate gloom. He is an incurious geologist, or a feeble admirer of line nature, who is content to pass Ailsa unseen.
This rock is traversed in various parts by large veins of greenstone or basalt. Among these I observed one which was horizontal, but the greater number are vertical, and of large dimensions: these lie on the west side. There is no apparent alteration in the rock at the points of contact. The whole of this island consists of one substance, in which slight differences of appearance here and there occur, but are unworthy of particular notice, and scarcely sufficient to constitute a variety. Its basis is an even and small grained mixture of white felspar, and transparent quartz, in which the former appears to predominate. This mixture constitutes more than three-fourths of the stone, and is mottled by minute and indistinct stains of a black colour, which through the magnifying glass are seen to consist of small grains of hornblende. These appear as if diluted through the stone, and arising from a common centre; and as they vary in proportion to the other constituents, so the rock assumes various shades of colour, from a whitish to a blackish gray, but the lighter varieties predominate.
The stone which I have described may safely be called a syenite, as it accords with Werner's definition of that compound rock, and is a term pretty generally admitted. We may, however, venture to question the propriety of applying to rocks of this nature, a term borrowed from the ancient naturalists, when it seems certain that the rock of Syene was, in fact, a true granite, containing hornblende merely as an occasional and accidental ingredient. Yet, as we are in want of a generic term for the various rocks which consist of hornblende and felspar, and which are not considered as greenstones, it may be advisable to admit of its use for this purpose, and to sacrifice a little of our classical accuracy for the sake of convenience, rather than by the introduction of another new term, to introduce new confusion into that which already reigns in the denominations of rocks. I hold it necessary however that the word syenite, if it is to be used as a generic term, should be limited, as rigidly as the ever varying and mingling composition of rocks will admit, to those compound rocks which are akin to greenstone in their chemical and essential characters, or consist of certain notable and distinct proportions of felspar and hornblende, but in which the felspar is either the predominating ingredient, or is crystallized in masses so large and distinct as to remove the compound from those similar compositions now generally known by the name of greenstone. When the condition of the stone is of that intermediate nature which renders its precise place uncertain, the modified term of syenitic greenstone is easily applied. In difficult cases where these rocks are still more varied in their composition, and in all others where a multiplication of terms would lead to an unbounded and unnecessary nomenclature, it will even be preferable to distinguish the variation from the more rigid form by a detailed description rather than to encumber the science with a useless neology. It is easily understood that no doubts respecting the place of these rocks in a system, need arise from the intermixture of quartz, or even of mica in the stone, provided their proportions are not conspicuous, nor their influence on its general aspect remarkable.
But the rocks themselves, to which I am thus desirous of limiting the name of syenite, are so associated in their habits and formation with the rocks of the trap family, and with those porphyries which appear to have originated in similar circumstances, that they have a strong claim on us for one generic term, as well from their geological, as their chemical, or mineralogical relations. If then we consider the term syenite as, founded on this double view of the nature and position of the rocks to which I allude, it will be obviously convenient, if not absolutely necessary, to separate from them the rocks of a granitic character; of these the most simple in its construction is that which consists of quartz, felspar, and hornblende, or of those three ingredients with mica. Such rocks are not of uncommon occurrence, nor is it uncommon to meet a rock in which quartz, felspar and mica are so compounded, as to form a true granite with crystals of hornblende superadded to the compound. Great confusion must follow the separation of these from the granites, and their union to the syenites above described. Their structure and characters are in all cases granitic, nor does the eye readily detect the difference till the darker parts are minutely examined, when the crystals of hornblende may, and that often only with much care, be distinguished from the mica. Their geological affinities associate them also with the granites, and never with the greenstones, since they occur in the formations of the granite æra, as hornblende rock and hornblende slate are known to do. The obvious distinction would be to call these compounds syenitic granites, thus preserving their analogy with the syenitic greenstones, and preventing us from confounding into one mass two formations of widely different characters and epochas, merely from the accidental presence of a single ingredient.
But to return to our subject. We have here an example of the columnar form existing in a very perfect manner in a rock, which however it may bear a considerable analogy to greenstone, and through that to those rocks which more commonly exhibit the columnar shape, is still sufficiently distinguished from them all by the great predominance of the felspar. There is no difficulty in conceiving that the same causes presided at the formation of both, when we consider the similarity of geological structure and position which pervades all the rocks distributed under the heads of basalt, greenstone, and syenite.
The insulated position of this rock precludes all possibility of tracing its connection with the contiguous ones, and the chasm which exists between Ailsa and the neighbouring shores, is such as to insulate it as much in a geological view, as it is in its geographical position.
As yet nothing resembling it in structure has been observed in the neighbouring islands, nor on the main land of Scotland, yet the syenites and greenstones of the motley island of Arran, with the porphyry of Devar, and the extensive trap formation of the neighbouring shores, point out to us its natural affinities, and enable us to guess at a connection possibly once more intimate, but long since submerged in the depths of the sea.
I cannot entirely quit the Craig of Ailsa, without remarking that it possesses in a high degree one of the conditions requisite to the solution of the interesting problem of the earth's density. This is, the absolute uniformity of its structure, which, with the exception of the few basaltic veins above described, is of one unvarying rock. Its mass is also sufficiently considerable to fulfil another of the conditions required for this problem, while its form and situation would appear to afford sufficient facilities for measuring its solid contents. As this idea had not suggested itself to me when on the rock, I did not examine, and cannot now recollect, whether it affords situations adapted for the erection of the requisite observatories, a condition without which its other conveniences would be of no avail.
The beautiful specimens of porphyry taken from this rock may render a short description of the rock itself interesting, and I shall therefore make no apology for occupying the Society a few minutes with an account of it. It is the more worthy of notice, as it is the most accessible, and the best characterized mass of porphyry which I have seen in the various parts of Scotland that I have visited. It is an island and a peninsula alternately, according to the state of the tides, and lies off the harbour of Campbelltown, which it covers from the south and east winds. It is about half a mile in length, and a quarter in breadth. From the N.W. it rises in a pretty rapid slope, and terminates to the S.E. in a precipitous face, reaching as far as the eye can guess, from one to four hundred feet. It consists of one entire mass of porphyry, which however varies much in its colour and texture in different places. It contains no other rock or vein, nor is its contact or connection with any other to be traced, but it appears to extend on all sides below the water. On a first and cursory view, it seems to be formed of upright beds, but a more accurate examination shows that this appearance is a deception, and arises from the tendency which the rock has to split in a vertical direction. Such is its outline and general aspect that it might at a distance be taken for a mass of trap, similar to those in the vicinity of Edinburgh, its leading features being the abrupt face and perpendicular fracture. On examining this great fracture more closely we may perceive that it is of a large lamellar form, or such that one of the lateral dimensions is always much greater than the other, the leading lines being nearly vertical. The cross fracture by which it is separated into small masses and becomes detached, is comparatively so rare as not to give any general feature. If it is possible to transfer a term from a hand specimen to a rock, I might say that it was formed of distinct lamellar concretions, and a perfect idea may be acquired of it by conceiving the lamellar pitchstone of Arran magnified to the requisite size. It contains some caverns, formed in a vertical direction, but whether by a falling out of some parts, or a subsidence and consequent separation of contiguous ones I could not determine. I have already said that its texture and aspect are very various, its varieties occurring as far as I could perceive without any certain order or regularity. Its basis appears to be a felspar, occasionally of the compact sort, but more generally crystallized in a confused manner. It differs much in colour, and offers the following remarkable varieties, brown red, umber brown, iron grey, purple brown, ash colour, yellow green, olive green, pale grey green, and grass green. The crystals of felspar which are imbedded in the mass are of a small size, rarely exceeding the tenth of an inch, and are either grass green, brown, or white. Nodules of green earth are also found imbedded in it, and the green varieties appear to derive their colour from this substance.
I have contented myself according to the established language with calling this rock by the simple name of porphyry, a name so vague, and so carelessly applied to rocks of widely different characters, as to be a cause of unspeakable confusion. Since the term porphyry is now applied to all rocks, whatever they may be, which in a given base contain imbedded crystals of felspar, or of any other substance, it is evident that it can determine nothing with regard to the nature or true place of these rocks. An accident common to many species is not the characteristic of any. As well might granites and micaceous limestones form one species from their participation in a common ingredient. Thus we are told that Cruachan consists of porphyry, that the Ochil hills consist of porphyry, that the Calton hill consists of porphyry, and that Rona consists of porphyry. Yet these four rocks are essentially different, and separated from each other as widely in composition as they are in geological relations, bearing, in short, no resemblance except in the common and unessential circumstance of imbedded felspar. For, the hornstone, the claystone, the trap and the felspar, which constitute the bases of these several rocks, although often containing crystals of felspar, are yet perhaps as frequently destitute of that accidental substance. I am not an advocate for increasing words, and encumbering our language with unnecessary terms, but that confusion which arises from applying one term to many substances, is at least as faulty and a greater cause of error than the accumulation of an unweildy stock of synonymous ones. If we must retain the ill applied term of porphyry, let it at least be returned to its place among the adjectives, and used as the trivial name to those species, which, for the purpose of description, can only be truly characterized by their permanent composition, not by their accidental features. Thus we shall have porphyritic traps and claystones, or, if our grammatical ears are not offended by it, we may have felspar porphyry, and hornstone porphyry. These are associations of substantives at least as tolerable as that of a line of battle ship.
I trust it will not be considered beneath the dignity of the Society if I here take notice of the assistance which the arts may derive from some of these porphyries. The green specimens are capable of assuming a good polish, and form ornamental stones of equal novelty and beauty. We have long been indebted to foreign countries for substances which our own island produces in abundance, and in no instance have the powers of established habits and prejudices been carried further than in the preference given to many of their insignificant marbles and porphyries, while our own, either similar or superior, have been neglected. To encourage commerce and the arts is not unworthy of a Society, which, like ours, has in view among other objects, the investigation of an important branch of the natural history of our own country. Let us not suffer ourselves to be misled by the imposing aspect of independence and disinterestedness which philosophy assumes when employed in investigating general principles, and, contented with the splendour by which science is surrounded, refer to others the humble task of converting it to useful purposes. It is the chief boast of science, that while it occupies the mind in pursuits by which it is exalted and interested, it tends at the same time to better the condition of life, by adding to its enjoyments, or taking from its inconveniences.
The general disposition, as well as the particular details of the geology of Arran being universally known, I need not enter into any description of them. I may merely remark, that the group of mountains which constitutes the most elevated part of this island, consists of a mass of granite, of which Goatfield is the highest point. Various ridges, separating deep vallies of sudden declivity, branch from the higher parts of the group in different directions. These mountains are succeeded by a very systematical arrangement of consecutive rocks, terminating at length, on one side, in the sandstone which forms the shore from the Cock to Brodick Bay, and on the others, in various alternations of rocks, which it is foreign to my purpose to describe.
The granite of this group differs essentially in external features from that of the central highlands, and it equally differs from it in character, resembling almost precisely, in hand specimens, the well known granite of Devon and Cornwall.
Like that also it has the bedded appearance which in detached parts so much resembles stratification, and has not unfrequently been mistaken for it. If we were to be guided by the look of a single rock or jutting mass, taken here or there, it would be very difficult to avoid being misled by the impression of stratification which it gives. But if we examine it in a wider view, we shall see that this appearance is fallacious, and that the laminæ, rather than beds, in which it is disposed, are placed in every possible direction; exhibiting even in the immediate neighbourhood of each other, an irregularly tabular form, or lamellar texture, and not a stratified deposition. It is not even possible that these masses should be portions of disrupted strata, as their positions are complicated in a way in which no subsidence, or other subsequent disturbance could have placed them. At any rate it has no better claim to stratification than the granite of Dartmoor and Cornwall—with these it must be classed in geological as it is in mineralogical character.
As the stratification of granite has been a subject of much controversy, it is worth our while to examine those doubtful cases which may be explained by other considerations, and to see what analogies in the disposition of other rocks can be brought to bear on this question. Thus we may possibly find that this disputable appearance forms part of a series of very common phenomena, and that although it may possess a certain general resemblance to the effects of mechanical action, it is, in fact, produced by chemical agencies.
There are two circumstances to be regarded in describing the fracture and texture of a rock, its small or artificial fracture and microscopic texture, and its large or natural fracture and texture, as they are determined and exhibited by the effects of time or decomposition.
The instances which might be adduced to illustrate these differences are sufficiently familiar. In the case particularly of the large disposition and fracture of rocks, a striking difference occurs in the often approximated beds of trap and sandstone. Both form parallel beds, and both on the application of force break into nearly similar fragments, yet the natural division of the sandstone bed shows an horizontal tendency, or one parallel to the plane of the bed, while that of the trap is vertical to it. From the vertical fracture a series of gradations occurs which at length assumes the perfectly geometrical form of polygonal columns. In this case then, we have a form still more perfectly mechanical and regular than that of the most even stratification, and produced by a species of crystallization, a tendency to decided forms in those particular rocks, with the laws and causes of which we are at least as well acquainted as we are with the laws that determine the figure of a quartz crystal. At present they are both equally inexplicable. There is no further difficulty in conceiving that a rock may constitute a huge bed separable into horizontal laminæ as regular as the strata of a mechanical deposit, than in conceiving that the island of Staffa is separable into columnar fragments, or the rock of Devar into vertical laminæ. It is true that we have not yet produced any instance of continuous, horizontal laminar concretions, which are incontrovertibly not mechanical, yet its existence implies no chemical impossibility. That which occurs on a small scale may occur on a large. The terms are but comparative, and the works of Nature are not to be limited by a measure taken from our own confined dimensions.
Thus we find on the small scale, that antimony is divisible into laminæ, while its sulphuret splits into columns; that mica has a lamellar texture, and asbestos a fibrous one. Should a mountain of the size of Goatfield occur, formed of a solitary crystal of mica, we should not be entitled to call it stratified, while we considered a neighbouring mountain of asbestos to be columnar.
Such I conceive to be the analogies by which we may safely guide our reasonings on this subject. Nor is the great lamellar texture which has so often been considered as the effect of stratification, peculiar to granite. In examining the Cuchullin hills in Sky, I have observed that the syenite and greenstone are bedded as it were in layers, either curved or straight, either horizontal or slightly inclined, resembling so much the disposition of granite beds, that even an experienced eye would at a distance be deceived by them. It is not necessary to illustrate this view by any further analogies, as every person's memory will afford examples in the disposition of some kinds of porphyry. My design is merely to suggest the necessity of considering the greatness of the scale on which Nature operates, and the probable occurrence of a chemical texture, if I may use such an expression, different from, and additional to that which may be observed in the smaller masses, and probably as constant in the larger masses which constitute the different rocks, as the minuter crystallization is in the smaller parts which unite to form them.
Thus far I have only argued on the probability that Goatfield does not consist of stratified granite, by analogical reasoning and arguments drawn from theoretical principles. We have, however a perfect certainty (as far as we have any certainty in geological induction) that it is not stratified, because veins are found arising from it, and entering the mass of incumbent schistus in the well known junction at Loch Ranza.
I have not thought it necessary to take notice of the probable cause of the prismatic form which occurs so frequently on the summits of the high ridges in this granite, having before spoken of them in my remarks on that of Cornwall and Devonshire.
I cannot quit this subject without mentioning that many of the masses of granite on the summit of Goatlield, are magnetical, affecting the poles of the needle in situ, and influencing it also even in detached pieces. This occurrence may probably be more common in granite than we are aware of, but as yet, I believe it has been observed only in the Hartz mountains.
The sappare (disthène) said to have been originally discovered in this place, is now known to be inherent in many different rocks. Thus, it has been found in granite, in mica slate, and in talc. The specimens of it seen here, occur in a vein of quartz, which traverses a talcy clay slate, a slate accompanied by a singular rock, which I will also describe. It only offers the common aspect of this mineral, which hitherto has exhibited no remarkable varieties, and is of a very decided character. The crystals of disthène occur in the quartz vein, as well as in the clay slate which bounds it, and pass indifferently through both, without any change of their direction or appearance, marking clearly a common condition in the schist and the quartz, at the period of their formation. It is also worthy of remark, as I have observed, in another place, in speaking of the rutile of Killin, that although the crystals of disthène in general penetrate and impress the quartz, they are sometimes bent and waved as if they had accommodated themselves to its irregularities. Ignorant as we still are of the mode in which various minerals are crystallized together, it is not unworthy of our attention to collect all the facts which may tend ultimately to throw light on this obscure process.
The clay slate which I have mentioned above, constitutes a large portion of the mountains which here form the termination of one of the Grampian branches in this direction; it is succeeded by a hard quartzy sandstone, similar to the rock of Assynt, before described, and by an exceedingly hard quartz breccia, in which the paste is red, and the fragments white, and which is noted for containing agates and jaspers. Immediately after this follows the red sandstone, which constitutes a great part of the flat tract of Moray, and bounds the course of the Spey in this place. It is accompanied by the singular rock above alluded to, which I am about to describe. This rock consists of a talcy clay slate, so penetrated with hornblende as to render its character for an instant doubtful. On an accurate examination, it will be seen that the body of the rock is a clay slate, and that it is interspersed throughout with lamellar and thin crystals of hornblende. These lamellæ are generally disposed at right angles to the lamellæ of the schist, and are sometimes short and straight, and variously placed, interfering with each other in every direction. More commonly they diverge from a sort of central axis, in curved planes, so that their section according to the lamellæ of the schist, exhibits an appearance of curved penicilliform groups of acicular crystals frequently an inch in length, assuming an aspect of great singularity. In this direction the schist is visible, and appears to form the largest part of the stone, while in the cross fracture the lamellæ of hornblende alone being seen, the whole rock seems to consist of this mineral. Occasionally the hornblende displays crystals disposed in so many different ways, that the schist is discernible even in the cross fracture, but this variety is the least common.
The veins of granite found at Portsoy are well known, not only for the peculiar character of their crystallization, called graphic, but also because they contain crystals of schorl (tourmaline) of great magnitude. The singular disposition, and mutual relations of the crystallized substances which form this compound rock, are known to have afforded Dr. Hutton an argument for its igneous origin, and its peculiar character has been supposed to arise from a simultaneous, or nearly simultaneous crystallization of the several substances contained in it. The specimens which I have to enumerate, are such as not only throw considerable doubt on this explanation, but are in fact, sufficient to prove a sequence of epochas even in this limited space, and to show that the compound rock in question has been formed by successive operations, the nature of which however I fear we shall not easily determine. I would entreat the pardon of the Society for occupying so much of its time on the fracture of a schorl crystal, were I not persuaded that much light must at some period inevitably be thrown on the greater geological phenomena, by considering the chemical and mechanical relations existing among the smaller portions which constitute them, and that the language of Nature is often as intelligibly spoken in the minute space of an inch, as in the immensity of a mountain. It is scarcely necessary to add, that the other prevailing theory, that of aqueous formation, supposes the substances which constitute this rock to have been crystallized from a watery solution, and that the fundamental objection to the igneous explanation, is the apparently chemical impossibility, that of a rock compounded of two crystallized substances, the one which is fusible at the lowest temperature, and therefore would be the last to crystallize, should by crystallizing first, have made its impression on the other. An analogy has, I know, been offered in explanation of this difficulty, but it will immediately be seen that it is at least incompetent to explain the particular case under review.
The first specimen which I have to describe is a detached crystal of a flattened and irregular figure. It has been broken into four parts, by transverse fractures, which have again united without the intervention of any intermediate substance. Previous to this reunion however, they have all been slightly shifted, in such a way that the several parts of the fractures project, and the whole crystal has undergone a slight deviation from its original straight line. If it be alledged that this appearance could arise from a disturbed crystallization, the next specimens will remove any doubt on this head.
In these, the crystals have not only been fractured in the same way, across their axes, but the fractures are filled by the quartz and felspar which constitute the body of the rock. The granite veins of Arran do not show more clearly the ramification of a central substance through the fractures of the neighbouring rock, than these specimens show the veins of quartz proceeding from the mass, and penetrating every fissure which had been formed in the crystal. It is perfectly evident, that whatever is true of the above cited granite veins, must also be true of this rock, that the schorl has been crystallized, then broken, and penetrated by quartz in a state of fluidity. Nor is there any intermixture of the matter of quartz, with the matter of schorl, but the line of separation is most accurately drawn between them. It follows then from these circumstances, that the rock in question is not a simultaneous formation from a state of fusion, nor can we readily understand how it can be the effect of fusion at all, consistently with the chemical principles we acknowledge. Had such a mass of fused quartz invaded the minute fragments of schorl which the specimen exhibits, the latter must either have been fused into a shapeless mass, or at least the asperities of fracture could not have remained in a substance whose fusibility is so much lower than that of quartz. Those who attribute the formation of this rock to aqueous solution, will perhaps in the above mentioned specimens find arguments in favour of their hypothesis. I will not repeat the often quoted difficulties which attend this theory also, but request from it an explanation of the remaining specimens.
The first of these is from the same vein at Portsoy, and contains an acicular and detached crystal of schorl, which is bent without fracture, so as to form a considerable curvature.
As chemistry produces no examples of incurvated crystallization, I may safely conclude that this crystal has been bent by external force after its formation. The noted fragility of schorl will not allow us to suppose that it could be bent without breaking, unless it had been previously rendered flexible by some chemical agent possessed of powers which we have not hitherto discovered in water.
The other specimens are not from Portsoy, but from an interior part of the country, and exhibit appearances equally irreconcilable with the theory of the aqueous formation: but the objection on this ground has been urged before. In one a crystal of schorl passes through the centre of a garnet, and the whole is suspended (if I may use such a phrase) in the quartz. In others the crystals of schorl are simply suspended in the quartz or felspar, and have perfect terminations. It is evident in the first, and strongest case, that the schorl crystals must have been supported in a fluid of equal gravity, possessing no action chemical or mechanical on it, while a garnet was allowed to crystallize round it, and that this extraordinary state of things must have continued during the time which it would require to deposit a mass of quartz from a watery solution around the whole, a period, which if we may judge from the slow formation of chalcedonies from water, involves a supposition little short of miraculous.
Such are the difficulties which beset this very simple and probably very common occurrence. I do not mean by adducing it, to say, that no theory is worthy of attention which cannot explain all the phenomena. My wish is rather to excite the industry of those who cultivate geology, to the investigation of the still recondite chemical actions, from which alone we can hope for the solution of these and numerous other difficulties which attend us.
I am merely desirous of pointing out under this title a bed of limestone, lying in a country so little visited, that it has not yet been observed by mineralogists, and of sufficiently rare occurrence to render it an object of notice.
The whole country about Loch Laggan is of that composition which forms the far larger part of the highland districts, namelymicaceous schistus.
The bed of limestone has been quarried near the eastern end of the lake, and if the reports of the quarry-men are to be trusted, extends in a direct line for many miles. This part of its history merits investigation, as the limestones which lie in the highland schistus, are generally of very limited extent. The stone itself is remarkable for containing hornblende. The sides of the bed are in contact with hornblende slate, or rather with that unnamed and common rock, consisting of a slaty mixture of quartz, mica, and hornblende.
Where the limestone comes in contact with the schist, the hornblende crystals penetrate it in such quantity as to blacken the compound. Towards the centre of the mass they diminish in number, and at length disappear.
Mica also accompanies the hornblende to a certain extent within the bed, and the limestone being itself of a pure white, a marble is thus produced of an ornamental nature, and bearing a considerable resemblance, as well as analogy, to the celebrated marble of Tirey. A substance very much resembling massive garnet may be perceived here and there united with it, but in so small a quantity as to render the determination of its true nature difficult. It is worthy of notice, that the limestone is frequently of a large platy fracture where in contact with the hornblende schistus, and that, these plates are not straight but flexuous.
Craig Cailleach near Killin.
This summit forms a part of the ridge separating the vales of the Tay, and Lyon, and is the next highest in elevation to Ben Lawers, the most conspicuous part of that ridge. The predominant rock of this ridge, is a very well characterized chlorite slate, which is, however, intermixed with rocks, consisting of hornblende, and felspar, and of micaceous schist mixed with hornblende. It exhibits therefore, specimens of the syenite of Werner, but such as are evidently associated with the mica slate family, and perfectly distinguished in their connexions, from the greater bodies of syenite occurring in Scotland, which appear in an independent position, resembling that of the trap family. The mica slate is often remarkable for containing numerous and very large cubical crystals of pyrites, an occurrence much less frequent in this rock than in clay slate.
Veins of quartz traverse it in innumerable places, and it is also interspersed with compressed nodules of quartz, which bear no marks of attrition, but are intimately united to the mica slate, or chlorite slate, in which they occur. They are frequently characterized by a beautiful transparent brown colour, which renders them objects of much research and high price among lapidaries.
Massive chlorite also appears in abundance, and it is generally interposed between the chlorite slate and quartz nodule. But the object for which I have chiefly noticed this mountain, is the occurrence in it of Rutile, a mineral as yet of sufficient rarity to deserve a record of all its habitats. It is found in the larger, as well as in the smaller quartz nodules, and exhibits many of its well known varieties.
The crystals generally penetrate the quartz, and often appear to have their bases fixed in the investing chlorite. It is worthy of remark, that although they most commonly penetrate the quartz, as if crystallized at perfect liberty, yet they are frequently bent, so as to accommodate themselves to its occasional elevations and depressions.
Our knowledge of the chemical laws by which mixed minerals are crystallized together is as yet so imperfect, that we can offer no conjecture on an appearance so complicated as this, which however, is not the less deserving of our attention.
The formation of a new road on the banks of this lake, has exhibited some instances of the contortions of mica slate, which are deserving of notice. I have attempted to give a notion of them in the accompanying sketches (Pl. 31. fig. 1, 2.—Pl. 31*, fig. 3 only.—Pl. 32, fig. 1) as it is not possible to procure specimens of the magnitude requisite for that purpose.
It would be superfluous to add any thing to the observations on the inflexion of strata, in the "Illustrations of the Huttonian theory," where this subject is ably treated. But as they consist chiefly of remarks on the continuous inflexions of extensive strata, it will not be useless to notice the more complicated curvatures which take place in the smaller masses. In a brief notice on certain waving lines of colour occurring in killas at Plymouth Dock, transmitted last year to the Society, I suggested the difficulties which attended a solution of this question, the disposition of the colour having no relation to the laminæ of the schist. But in the contortions of the mica slate, the laminæ themselves are waved, and we have only therefore to enquire into the conditions requisite to the production of this appearance.
It is evident on inspection of the sketches, and will be equally so to those who shall inspect the rocks themselves, that, if the several laminæ which compose any given mass, were to be now rendered flexible, they could not be reduced to continuous straight lines, without materially changing the relations of their several lengths, and thus altering the figures of the rocks which now contain them. It is therefore evident, that no general force acting on a large mass has been the cause of these curvatures, but that they have been produced by the application of numerous partial forces acting on different parts, and capable of stretching those sets of laminæ on which the forces have acted, independently of the neighbouring ones. It is equally evident, that a mere state of softness in the stratum of schist is insufficient to account for this partial and complicated effect, but that it must have taken place when the rock was in a state of tenacious fluidity. The effect produced by mechanical disturbance on fluid slags which consist of differently coloured laminæ, will illustrate my meaning, or, to leave out of the question any illustration which may appear to involve a cause, similar appearances may be produced by the disturbance of tenacious compounds of clay and water, or of other semifluid mixtures. It is not perhaps essential to the igneous theory, that this fact should be explained by an igneous softening; but it is necessary that the aqueous theory should admit of a disturbing force capable of producing a mechanical effect of this nature, since nothing short of mechanical force acting on yielding matter is capable of explaining it.
I may here add a fact somewhat illustrative of these partial contortions in schist, which is to be observed occurring in the basalt that forms the hill of Dun Can, in Raasa. Innumerable instances prove to us that the decomposition of basaltic as well as of granitic and other rocks, detects in them that structure which we should in vain seek in the fresh and entire mass. The weathered basalt to which I allude discovers appearances of contortion similar to those now under review, its surface exhibiting parallel prominences, separated by deep lines, which are waved in very complicated curves, and give reason to suspect a mechanical disturbing force, acting in this case also, upon a semifluid mass. As analogous appearances of curvature are also of frequent occurrence both in veins of quartz, and in veins of granite which traverse mica slate and gneiss, I have given figures of a few among the most remarkable of the innumerable specimens to be seen all over Scotland (Pl. 31*, excepting fig. 3.).
I have nothing to add in explanation of them; they merely suffice to prove, that these substances like the schistose rocks, have been in a state which admitted of bending; the capricious and intricate contortions visible in some of the figures (which were drawn with great care) not admitting of any fissure of this form to be subsequently filled either by injection or infiltration. One of the figures, Pl. 31*, left hand figure at top, is perhaps particularly worthy of notice, on account of the distinctness with which it demonstrates the action of partial forces, by the remarkable difference of curvature occurring in two veins so nearly approximated.
A specimen of a compound vein is before the Society, which traverses the well known amygdaloid of this shore. This vein consists of a highly crystalline brown limestone, mixed with a ramified compact earthy black basalt. These substances are so intermingled, that it is scarcely possible to conceive that the basalt is posterior to the limestone, or that a basalt vein should have found its way into a vein of limestone without also traversing the amygdaloid. It is probable that the whole vein is of simultaneous formation, and that the substances have separated from each other in consequence of those obscure chemical affinities which regulate the crystallizations of compound rocks. This however is a subject deserving of further investigation, as, if it is admitted, it will prove that the same cause, whatever that be, may preside over the formation of basaltic veins, and those veined limestone deposits in trap rocks, which are called calc sinter, and are held to arise from the posterior effects of a watery infiltration.
In the same rocks a singular circumstance accompanies the imbedded calcareous nodules. Their surfaces are occasionally marked with sets of concentric circles, consisting of line black lines most mathematically drawn, with their intervals often filled up by portions of parallel circular segments, an appearance similar to that produced on the surface of some of the agates imbedded in trap, and which on that account are called ocular. These black lines are perfectly superficial.
The land about this harbour is remarkably disposed in small elevated hillocks, producing an irregularity of aspect similar to that of the Oban shore. The mode adopted in making the canal, by cutting away the edges of these hills, so as to form a continuous embankment against them for a considerable space, has brought their structure to light, and given great facility to the investigation of their composition. They are formed of a continued alternation of beds, elevated to an angle of 80° or upwards, and in a general view appearing to be nearly vertical. On examining these beds, they are perceived to consist of the following substances.
- Coarse graywacke, very well characterized, and precisely corresponding to the definition, inasmuch as it consists of grains of quartz cemented by clay slate.
- The same rock, in which grains of felspar as well as of quartz are cemented by clay slate.
- Coarse grained graywacke of a slaty fracture, or graywacke slate.
- A similar rock of a much finer texture.
- A perfectly homogeneous clay slate, not to be distinguished from the finest varieties of the primitive sort.
- A similar slate of a pale greenish gray colour, and silky lustre, approaching in character to chlorite slate.
All these beds are traversed by numerous veins of quartz, and they alternate with each other without any regular order, the beds not forming a series passing from the coarse to the fine grained, but a fine clay slate often following close upon a coarse graywacke, and being succeeded by a similar rock.
Here then we have an instance of a fact, of which the observations of every geologist will furnish many other examples, namely, the occurrence of clay slate among those rocks called rocks of transition, their alternation rendering this part of the fact indisputable. The nomenclature of rocks therefore which is derived from geological situation, is here at variance with that which results from mineralogical character. If the unnecessary multiplication of distinctions and names in mineralogical nomenclature is productive of toil by introducing a cumbrous apparatus into the science, it has at least the merit of conducing to accuracy of description. That is a much worse extreme, which by rejecting all such distinctions confounds together under one sweeping term, all sorts of substances, which, however differing in individual character and however constant and uniform in the character each severally assumes, are associated by only one common circumstance, the accidental one of position. A mineralogical nomenclature, like that of the other branches of natural history, must either be derived from the appearances and properties of the individual species, or from the character of the species combined with some generic or family character, either natural or artificial, which may render it of more easy classification and description. But we are not at liberty in the nomenclature of mineralogy, to derive our terms sometimes from the appearance of the species, and sometimes from the accidental circumstances which are found to belong to it. This is to acknowledge two distinct principles of nomenclature, and to claim a privilege of using that which happens to suit any particular hypothesis which we may wish to support. The accurate description of mineralogical species, must be the base of all geological reasonings, but if we intermix characters derived from geological circumstances with true mineralogical characters, we set out upon a petitio principii, and end by reasoning in a circle. In the case of limestone, the chemical properties and well known popular characters of which have given it a very decided and constant name, we act rightly. The name of the species is retained, be its geological situation what it may, and its geological accidents are distinguished by the addition of that name which suits its position, whether primitive, transition, or flœtz. But on the other hand, a mineral equally common, whose characters too are sufficiently familiar, is distinguished by two names, although even more identical in structure than the different tribes of limestone, and it is called clay slate, or graywacke slate, for no other reason than that in the one case it is associated with the rocks called primitive, and in the other with those which go by the name of transition. It is obviously necessary that the same practice should be adopted in this case as in the former, and that, adhering to the name of the species, we should distinguish its geological position, if required, by the superaddition of the corresponding term. We shall thus have primitive clay slate, and transition clay slate, if we find it necessary to retain these geological distinctions.
It is almost superfluous to quote instances of the existence of genuine clay slate occurring together with well characterized graywacke slate, as they may be seen in all the slate counties of England, and among others remarkably in Cornwall. I trust that the geologists who have attended to these rocks, will see, with me, the necessity ofadopting this distinction; a distinction, without which all accuracy
of discrimination will otherwise be sacrificed to the maintenance of an hypothesis, and the student whom it is a duty incumbent on our Society to assist, will learn to adopt that hasty and slovenly nomenclature, which is destructive of correct description, and scarcely less inimical to accurate observation.
Having said thus much on the very ill apprehended and often ill applied term graywacke, I shall be pardoned for suggesting the propriety of limiting it by a certain fixed definition. Different observers have classed under it, substances the most discordant, looking either to their general geological hypothesis, or finding it a convenient repository of rocks for which no other name was at hand. Thus it has become a chaos of ill associated substances. Because Cumberland and Wales are supposed countries of transition, almost every rock found in those districts has been occasionally called graywacke, and thus we have had breccias of all possible modifications, sandstones, and clay slates, confounded with genuine graywacke under one common designation.
The definition of Werner appears precise, and I believe I do not misapprehend it, when I state that its essential part is to possess clay slate as the cement of certain mechanically altered grains or fragments of different rocks. These may vary materially in size, and thus form the two leading varieties of fine and coarse graywacke, and if they also possess a fissile structure, they will then constitute fine and coarse graywacke slate. It is true that in the definition of Werner, as given us by Jameson, the grains are stated to be quartz, indurated clay slate, and flinty slate, but since felspar and fragments of other rocks do occasionally occur in the best characterized graywacke, it would probably be desirable to extend this part of the character so far as to include all grains and fragments, of whatever nature they may be, and to consider the cementing substance, and obviously mechanical structure, as the essential part of the definition. I shall take an opportunity in some remarks on another district, to enquire whether it would not be also convenient to extend the definition so far as to permit mica slate to participate with clay slate in the office of cement, the other parts of the character remaining the same.
Whatever definition be ultimately adopted, we cannot too strongly inculcate the necessity of accuracy in the application of terms. Accurate mineralogical knowledge is an indispensable condition to accurate geological description, and the errors of very modern and celebrated authors, arising from the want of this fundamental quality, are too well known to call for the invidious task of pointing them out.
The cacophony of the term graywacke has excited a desire in some late observers to discard it altogether, and substitute one more vernacular, and less liable to that objection.
The multiplication of synonyms is itself an evil of so crying a nature, and has unfortunately become a disease of such magnitude in our science, that we ought to consider well before we venture to add another to the unwieldy and vexatious stock. It is so great an advantage to possess one term well understood, and understood as the language of philosophy should be, in all countries, that we can scarcely find a motive sufficiently powerful to induce us to change this received name. Under these circumstances we have every reason to retain the term graywacke, however jarring to English ears, and it is the excess of fastidiousness to reject on account of its sound, one word from such a polyglot of unmelodious and ill compounded Greek, French, and German terms, as assail the mineralogist on every side. If there is in any case a choice among equally established terms, it is in our power to choose the most musical, but we are not justified in changing them for so slight a convenience. The late introduction of a new chemical nomenclature, has possibly, in conjunction with other causes, excited a taste for neology, which it behoves us to restrain by every method in our power, and it is the duty of our Society to watch over and protect the science from those changes which will, if not restrained, shortly inundate us with as many names as we have writers.
The word killas, a vernacular and Cornish term, has been proposed as a substitute for graywacke. If killas were actually graywacke, we might have a fair plea for using the name given to it by a Cornish miner, in lieu of the corresponding one of a German miner. But this is not the fact, as those who are conversant with Cornish terms, well know that killas is applied to all the soil and fissile rocks occurring in Cornwall, whether clay slate, or graywacke slate; and that it is never used for either when they acquire the more compact and laminated form of roofing slate, a term as commonly applied to this variety in that county, as in other parts of England. The harder and more granular graywacke, is also called elvan, in common with trap, and other hard and dark blue stones.
It is well known that the ridge of which Ben Lomond forms a part, consists of micaceous schistus, which terminates near Drymen in the highly elevated range of breccia that separates the primitivefrom the secondary country, and which may be traced from this
point in various places round by the east coast to Troup Head. It is visible at the pass of Aberfoyle, and possesses a character and situation similar to that which it exhibits at Drymen.
A ridge, parallel to that of Ben Lomond, is separated from it by the valley which includes the Forth. A second parallel valley contains Loch Ard and Loch Chon, which pour into the Forth at Aberfoyle a tributary stream of equal magnitude. To the east this is again bounded by a second ridge, nearly parallel to the first, of which Ben Venu is the principal elevation. The valley which lies at the foot of this ridge, contains Loch Ketterin, having for its eastern boundary a double ridge, of which Ben Ledi is the chief eminence. These four ridges appear to ramify from a central point, situated between the top of Loch Ketterin and that of Loch Lomond.
The whole of this country has been supposed to be of the same formation as Ben Lomond, namely micaceous schistus, a supposition which I have found to be groundless. I am ignorant of the nature of the ridge which separates the vale of Loch Ard from that of the Forth, the nearest in order to the ridge of Ben Lomond. But on examining the vale of Aberfoyle, as far as from the breccia to the bottom of Loch Chon, I found it to consist of alternations of graywacke, and graywacke slate, with clay slate, similar to those I have described as occurring at Crinan, and placed with equal irregularity of alternation. The beds are also elevated to high angles, but I believe that the positions are various. In traversing the ridge which separates Loch Ard from Loch Ketterin, the same rocks appear, and quarries of fine roofing clay slate are wrought in various parts of it. These, I believe, are of the same class, and belong to the transition clay slates, but the whole ridge is so extensive and broken, that it is extremely difficult, if not impossible, to obtain access to it, except in a few places. On reaching the shores of Loch Ketterin, the nature of the rock alters, and it assumes an aspect approaching here and there to mica slate, but it is still characterized in most places by the true graywacke structure, that is to say, by grains united by a cement, the cement in this case consisting of micaceous schist instead of clay slate. At Ben Ledi the structure still more resembles that of mica slate, insomuch that but for the previous examination of the adjoining district, we should not suspect it to consist of any thing but genuine mica slate. Beyond this, and to the east of Loch Lubnaig, occurs true mica slate which constitutes the whole of this district as far as Perth. I may remark too, that the graywacke of which the character is so perfect in the lower parts of the vale of Loch Ard, becomes micaceous as we ascend the course of the stream, and that in the upper parts of the valley, it is often very difficult to ascertain under which head it is to be ranked.
Those who have been engaged in similar observations on an extensive tract of country, well know how difficult it is to fill up all the chasms which the nature of the ground forms in the pursuit of a series of rocks. Yet I have very little doubt that a true gradation exists here, between mica slate and graywacke slate, and that the usual intermediate member, primitive clay slate, does not occur as a necessary part of the series, but that it is irregularly alternated with the graywacke. It was with a view to these rocks, that I suggested the propriety of extending the definition of graywacke, so as to include those which contain mica slate, talc slate, and chlorite slate, as the cement of the grains, instead of limiting it to clay slate alone. In many of the rocks which occur here, the cementing ingredient appears to consist of these several substances, the rock in other respects preserving a perfect graywacke character, and being composed of distinctly rounded as well as crystallized grains of quartz, joined by a common cement.
It is plain that there will be a point of gradation where it will be impossible to say to which of the two the rock in question ought to be referred, although the extremes are perfectly characterized, in the one case by the laminar form of the quartz and mica, in the other by its granular disposition. I shall not be surprised if future observers discover that the rocks of the graywacke structure alternate here with those of the micaceous schist. Should this be the case, it will confirm the supposition which I have suggested in other parts of these papers, that no real and well defined line of distinction exists between the transition and primitive rocks, but that they form a graduating series of one single formation; a series so gradual as to render it expedient once more to return to the most simple division of rocks, into primary and secondary.
- I have since observed a similar modification in the schist of Devonshire in which wavellite is found.
- I have since observed it in the mountain Cruachan.
- In the Mineralogy of the Scottish isles, it is said that the summit of Ben Lomond consists of gneiss. It is necessary to correct this oversight, the whole of the mountain being formed of micaceous schistus, and no gneiss occurring in the neighbourhood.
- Ketterin the same as Kerne used by Shakspeare—banditti; hence Loch Ketterin and not Kathrine or Catharine as it is erroneously spelled.