# Transactions of the Geological Society, 1st series, vol. 3/Sketch of the Mineralogy of Sky

TRANSACTIONS
OF THE
GEOLOGICAL SOCIETY
I. Sketch of the Mineralogy of Sky,
By John Mac Culloch, M.D. F.L.S. President of the Geological Society, Chemist to the Ordnance, and Lecturer on Chemistry at the Royal Military Academy at Woolwich.
A few weeks residence in the island of Sky having enabled me to give a general idea of its geological structure, I shall make no apology for offering this paper, however incomplete, to the Society. Want of further opportunities may perhaps for ever prevent me from rendering it more perfect, as it may equally prevent others from visiting that island. Nor is a district of such magnitude and interest, of which the distance is so great, and the inconveniences of climate and ground so considerable, likely soon to offer to others greater facilities than those, which enabled me to draw up this sketch. Should it prove otherwise, my object will at least, in its ​principal view, be fulfilled, since I shall have opened a path for others to follow, and traced an outline which some more fortunate observer will with greater ease complete.
The variety of rocks which occurs in this island, as well as the theoretical difficulties in which the connection and order of many of these are still involved, and which are in tome measure at least elucidated by their appearances here, render this island very interesting to the geologist. In attempting to describe them so as to give a clear topographic view of their situations, and at the same time to trace their connections with each other, (that object which is more peculiarly an essential part of geological science,) I have experienced difficulties which other observers must also have felt, and from which I have but imperfectly succeeded in extricating myself. It is to be hoped that our increasing knowledge will at some future day diminish, if not remove, those difficulties, and smooth the path. for the future historians of the earth's surface and structure. In the mean time no greater impediment to the progress of that knowledge of which we are in search can be contrived, than that of assuming a certain regulated order, founded on imperfect and limited, or on prejudiced observations. This proceeding not only renders all investigation nugatory in itself, but accumulates by the establishment and diffusion of false canons, a constantly increasing load of obstruction to the progress of truth. The phenomena which have occurred to me in Scotland, during many years examination of that very instructive country, have so often led to conclusions different from those which have been supposed already established, as to compel my dissent from that system of general rules on which we have hitherto been taught to rely. At the same time, convinced that generalization on this subject was premature, and warned as much by my own discrepancies as by the ​errors of those who have preceded me, I have rarely if ever attempted to draw such general conclusions. My chief endeavour has been to reduce questions of this nature to a narrow compass, where the limits admitted of contraction; and where any thing has appeared uncertain or contradictory, to put those uncertainties and contradictions in the clearest light, that future observers might direct their attention to those points which most stand in need of elucidation.
To place the subjects which have occurred in the examination of Sky, in an order as nearly as possible both conducive to topographical clearness and geological elucidation, I have varied the order of description as circumstances dictated. When the structure was simple, and the geographical details corresponded, the task has been easy to the writer, and will be easy to the reader. When it has been otherwise, I have attempted as far as possible to reconcile these claims; while in the greater number of instances, where the intricacy of the geological structure and the scattered disposition of the materials over a large extent of surface have prevented all possibility of reconciliation, I have been obliged in some measure to divide the subject with a reference to both these objects, preferring to incur the charge of prolixity rather than that of obscurity. I shall therefore commence by giving a general sketch of the several rocks which form the island, noting their geographical positions as accurately as circumstances permit; after which I shall attempt to trace their geological arrangement, entering more largely on those details which lead either to useful doubts or probable elucidation.
The want of accurate geodesic operations has left the form, dimensions, and position of Sky, as yet desiderata in British geography, an inconvenience however, much less in a geological view ​than that which arises from the want of an accurate detailed map of its surface, its mountains, rivers, bays, and lakes; data, without which it is extremely difficult to give such particulars as could be desired of the situations, boundaries, and connections of its several rocks; a difficulty too not a little increased by the utter impossibility of obtaining the vernacular names from the natives, who are neither well informed on the subject nor very well agreed in their application. The want of ascertained distances and of fixed points of reference, arising from the vacant state of many tracts and the loose manner in which the Highlanders compute their miles, add not a little the trouble of giving precise descriptions.
If we may trust to the latitudes and longitudes laid down, and, they are the only documents on which we have to rely, the extreme length of this island is about forty-five miles, and its extreme breadth about twenty. Its form is that of an irregular parallelogram, so much intersected by deep sea lochs that scarcely any point on its surface is five miles distant from the sea. It is divided into geographical districts, which, as they bear some relation to the physical divisions of its surface, and will be necessary points of reference its describing its structure, I shall here enumerate.
Of these, the southernmost is the district and parish of Sleat, a tract of moderate and irregular elevation, terminating in the group of mountains which approaches the main land, and which, in conjunction with Glen Elg, forms the narrow passage of the inner sound. This is bounded to the north by the parish of Strath, an open irregular valley, intersecting the island from N.E. to S.W. and separating the before mentioned district from the remainder of the island, more particularly from the group of mountains which occupies its middle division. A tract of uneven land extends from Broadford along the eastern shore to the eastern Loch Eynort, and ​over a high ridge to Sconser, where it ends at in Loch Sligachan, being bounded on its western side by the terminations of the before mentioned group of mountains. The high hill forming the island of Scalpa is but barely separated by a narrow arm of the sea from this tract. An imaginary boundary drawn from Strath to the head of Loch Slapin on one side, and from Sconser to the western Loch Eynort on the other, includes part of the district of Minginish, and is still more remarkable as a physical division, since it contains the lofty and formidable group of the Cuchullin hills, with other mountains, of which the greater number are either nameless, or only recorded in the traditional geography of shepherds. This is the only part of Sky which can properly be called mountainous, and of which the aspect and character are perfectly distinct from those of the remainder of the island. The small district of Strathaird, more remarkable for its physical structure than its extent, is interposed to the south of this tract, lying between Loch Slapin and Loch Scavig. The northern and larger division of the island which exhibits a great uniformity of physical character considerably distinct from those now enumerated, contains the extensive districts of Trotternish, Vaternish and Bracadale, the former occupying the eastern and the two latter the western side of the island.
The whole of this tract is hilly and uneven, yet cannot be considered as mountainous, except perhaps on its eastern side, where one irregularly elevated and continuous ridge extends from Portree to Ruhunish. Some high ground also occurs about Dunvegan, but the rest is an undulating tract of hill and moor country.
The indentations which form the sea lochs are in some cases determined as on the continental land, by the vallies which are interposed between ridges of hills, and in this case they are also the estuaries of the rivers. But many of them are merely sinuosities of the ​general boundary, receiving no streams of magnitude. To the former belong the narrow lochs Slapin, Scavig, Brittle, Harpart, Eynort, Eishort, Sligachan, and Portree; lochs Bracadale, Follart, and Snizort, belong to the latter.
Although the elevation of the country is considerable, and the climate among the most rainy of this kingdom, Sky affords no rivers of magnitude: their course is too short to admit of the accumulation of large streams. The river which runs into loch Sligachan carries more water to the sea than any other, and after it perhaps follow in order those which run into the lochs Harpart, Slapin, Eishort, Bracadale, Portree, Snizort, and Broadford. The other streams are rivulets scarcely worthy of enumeration. The drainage of the whole country is determined by the positions of the hills and sea lochs, and may readily be collected from the preceding observations. There are three or four freshwater lakes, but of small size, and, except those of Coruisk and Colmkill, hardly worthy of the name. The courses of these streams exhibit but small traces of the wasting of the land. Like other mountain torrents they bring down at times rubbish and stones, but these bear no proportion to those accumulations of loose matte so common on the main land, nor do they offer any example of transported materials of which the origin is not to be traced to some neighbouring rock.
The surface of Sky appears at first sight one continued tract of brown moor, a dreary region of heath, and rock, and bog. Rugged mountains exhibiting naked spires of bare rock of which the sides are covered with ruins, lofty cliffs whose bases are whitened by a boisterous sea, a stormy atmosphere with almost incessant rains, complete the wild picture which first meets the eye of a stranger. Yet a more intimate acquaintance discovers scenes of grandeur and ​sublimity exceeded by no part of Scotland, abounding as it does in the picturesque and romantic, and a more accurate survey of the island shows spots with a fertility and population surpassed by few of the highland districts; together with a climate nearly as mild, and a temperature as equable as that of the western parts of England.
The district of Sleat, consisting of decomposed schistus, possesses along its eastern shore a highly fertile tract with an excellent and deep soil, adapted to the growth of all kinds of grain, and displaying pastures of perennial verdure. Similarly fertile soils are found in the vallies on its western side, while the central division, formed of syenite or quartz rock, produces the usual covering of those rocks, heath. In Strathaird, nearly the whole peninsula of which consists of secondary strata, we find a soil as fertile as we should expect on such a basis and in such a climate; a soil however owing less to art than to nature, whose bountiful efforts are seldom much assisted by highland industry or knowledge. A great part of the district of Strath lies on a bed of limestone, and appears from its natural grasses and its general aspect to possess all the requisites for culture, or at least for an improved system of pasturage; But it has hitherto been much neglected, and remains an almost useless tract of wasted and scanty herbage encumbered with rocks and stones, untrained, unfenced, and untilled.
Nature may be said to have denied a soil to the mountain tract which I have before described as forming the centre of the island, and these hills adapted for no other system are imperfectly occupied by sheep, of which from their rocky and sterile nature they can maintain however but a scanty proportion. A few stags yet remaining in these almost inaccessible regions divide with them this barren range. The lower pastures are more advantageously occupied by the ​well known and celebrated breed of black cattle which forms the staple produce of Sky.
The stony district about the Kyle rich produces like the Cuchullin hills but a scanty covering of heath and grass, and is perhaps among the most unproductive of the island, since the decomposed quartz rock of which it consists, yields a soil even worse than the syenite of the Red hills, or the rocks of the Cuchullin group; yet on the sea shores a few fertile spots are found in cultivation.
The great northern division consists of one entire mass of trap, with the exception of a few narrow lines of limestone and sandstone to be found on some of the shores. In various parts of this tract there are to be seen districts of considerable fertility, admitting, under the highland system of cultivation, a perpetual rotation of corn, with no alteration of green crops or fallow except the occasional one of potatoes; the hay as is usual under the same system of farming being collected from wet meadows and waste patches of land. The most considerable of these corn districts is in Trotternish, which for many miles displays an extent of cultivation exceeded by few highland tracts, and is emphatically called the granary of Sky. The aspect of loch Uig under the new crofting system is in this respect highly interesting. In various parts of the sea shore about loch Snizort and loch Follart, as well as in the vicinity of loch Bracadale, similar fertile tracts occur, and the little retired valley of Talisker might in a drier climate compare in fertility with the most chosen spots of our own southern counties.
Since chemistry has lately, although perhaps hitherto with little success, lent its speculative aids to the improvement of agriculture, it will not be foreign to the views of the geologist to examine how far his science also may bear upon this first and fundamental of all ​arts. From the identity of rocks in different tracts a conclusion may be drawn with regard to the identity of the resulting soils, and the analogies discovered in them may perhaps with less trouble and as great certainty as any chemical analyses, lead agriculturists to hazard those experiments in improvement which seem justifiable from similarity of composition no less than similarity of climate and exposure. I shall therefore I trust be excused for dwelling a little longer on a subject intimately connected with geological science, and in itself among the most practically interesting that can occupy our attention.
It is already well known that many of the rocks of the trap family, like some lavas, afford on decomposition one of the most fertile soils with which we are acquainted. On former occasions I have mentioned this fact, and noticed at the same time the great differences which appeared in this respect in different situations where there was nevertheless a considerable resemblance in the rocky substratum. The different proportions of calcareous earth, of alkali, and of carbon, which latter appears to be an ingredient in some traps, are probably the circumstances which constitute the chief differences in this case; but with this we must also consider that the various rocks of this class differ in composition in perhaps a greater degree than any others with which we are acquainted, and that together with this essential variation in their composition, they also possess infinite varieties in hardness, and in the property of decomposing by the action of air and water. Many considerations must therefore enter into our views before we can decide on the fertility of the resulting soil, unless it be actually exhibited in those places. where decomposition has already taken place.
In the district of Sky which I am new considering, there appears, as far as an accurate mineralogical investigation can determine such ​a point without the aid of analysis, a considerable similarity of composition in the trap throughout the greatest part of the whole tract. In almost every place where the rock approaches the surface, it is found on cutting into it to be rotten to a considerable depth, often reduced superficially to an absolute soil, and although appearing below like a solid rock, capable of being cut without difficulty by the axe or spade. Occasionally it decomposes into a soil perhaps more gravelly in some situations than in others, but in this case there appear only time and a further continuance of the destroying powers requisite for its complete change. Where the decomposition is most perfect it forms a clayey loam of which the aspect at least is favourable, and of which the fertility also is probably not so limited as the appearance of the heath and grasses which it bears would at first sight induce us to believe. The trap which I have mentioned is remarkable for the enormous quantity of zeolites imbedded in it, the mineralogical details of which I shall have occasion to speak more largely of hereafter. In the decomposed soils these are frequently found resisting change long after the rock is rotten and reduced to clay. But in many other cases they also are decomposed together with the soil, and in such quantity as to communicate their white colour to it, and with that colour doubtless a degree of additional fertility derived from the quantity of calcareous earth which they contain. In many places such accumulated beds of decomposed zeolites occur that they have been mistaken for marl, and have when used produced similar effects; although the narrow and slovenly system of cultivation practised by these little highland farmers neither admits of a full trial nor of a fair result.
We have seen that many tracts of this district are characterized by a high degree of actual fertility, while neighbouring ones formed of a soil apparently identical and under similar circumstances of climate ​and exposure, are still covered with unprofitable heath and peat, producing a scanty and almost useless Herbage. It is worth our while to inquire into the causes of this difference, and to see if valid ones cannot be assigned.
In the land already under tillage it is obvious that the subsoil is covered with a soil actually in use, consisting of vegetable mould annually loosened by the spade or plough, and admitting the penetration of light, of air, and of water, to the subjacent and half decomposed matter. In the uncultivated neighbouring tracts we shall on the contrary find it covered with a thick mass of peat, the immediate soil on which the heath, the sphagnum, the carices, and the rushes alternately flourish and die, adding fresh matter to this already impenetrable substance. Neither air, light, nor water can make their way through this dense covering of a substance so notoriously impervious that it has been found of equal use with clay in puddling the artificial banks of canals. It forms in fact an adventitious and useless soil so entire and so impenetrable that it would be of equally little consequence to the land-owner in its present state, whether it where bedded on a rock of solid quartz or on the most fertile garden mould. Fortunately this covering of peat is rarely so deep that the plough or the cashrom (the crooked spade of the Highlanders) cannot reach to the bottom of it. The remedy and obvious method of improvement is pointed out by the relative condition of the fertile and the barren parts of this tract. The few experiments which have been tried have proved eminently successful, and among them I may point out those performed by Mr. Macpherson at Portree. By the admission of air and water the progress of decomposition is accelerated, and the rock is reduced in no long space to a useful soil. The texture of the peat is at the same time loosened by its admixture with the decomposed rock, and being thus ​preserved in an open state and subjected to the changes which pulverization and access of air always produce on it, adds the carbonaceous ingredient to the clay, and forms a true vegetable mould. It is a separate question depending on various other considerations, whether this species of improvement should in all cases be directed to the cultivation of grain; but there is no doubt that it could be easily turned to a system of ameliorated pasturage, a system under which, without exaggeration, one acre of land could be made to produce more than many hundreds in its present state.
It is not my business to inquire into the obstacles which impede this obvious improvement of a country possessing such capability. Want of capital may be one cause, but it is more probable that the chief impediments are of a moral nature, resulting partly from the characteristic indolence and contentedness of the inhabitants, and partly from the difficulties which always attend the introduction of novelty into practices long established.
The want of trees in this island must be attributed to similar causes, since they would grow well in innumerable sheltered situations, and since some parts of the island still exhibit the remains of ancient woods.
In mentioning the artificial divisions of Sky, I have already touched slightly on its general disposition and the disposition of its surface, but as these are intimately connected with the arrangement of its rocks, they require a more detailed consideration.
The principal group of mountains is that of the Culin or Cuchullin, so named from the well known traditional hero and king of this island. This forms an irregular mountain tract covering an area of ten or twelve miles by seven, and skirting the sea shore with a very bold series of declivities from near Loch Eynort to Loch Scavig. The single mountain Blaven forming along ridge between the lochs ​of Scavig and Slapin, may be classed with this group of the Cuchullin, as it resembles it in altitude and aspect as well as in mineral composition, and thus adds about ten more square miles to the space occupied by the particular class of rocks of which this group is composed. The whole of this compound group is strongly distinguished from the associated and neighbouring mountains, as well as from all the other mountains of Scotland, by the spiry and rugged forms of its outline, which presents a series of naked rocks and towering cliffs destitute of vegetation, and rising dark through the mists which seem for ever to hang on its stormy summit. The mountains of Arran, alpine and serrated as they are, bear no comparison with them, and the far famed scenery of Glenco almost sinks into insignificance before the terrific grandeur of the Cuchullin hills.
It is to be regretted that no observations have yet been made to determine the altitude of this group; a want which I was unfortunately in no condition to supply: but from that sort of experience in the elevations of hills which is acquired by long habitude in a mountainous country, and by comparing them with ascertained elevations from different points of view, a rough estimate may be formed of its altitude, which must at least suffice till some more fortunate traveller shall place his barometer on the summit. They do not fall short of three thousand feet, and in all probability exceed it. A general idea of their ground plan may be conveyed by saying that they form an irregular and curved ridge of a very intricate shape, giving rise to numerous streams, the waters of which, for the most part, are discharged into the western sea; this proceeds from the inclination of the hills, the principal escarpements of which, however irregular, look towards the east and north.
This group is intimately connected with another, which for the ​sake of perspicuity I shall distinguish by the name of the Red hills, a name very characteristic of their colour as contrasted with the livid black of the Cuchullin, and excusable in as much as it is applied by the natives to some of the principal hills of the group. I would gladly have given the name of each individual, but I was unable to procure them, no general surveys having been made, and the particular surveys of estates, either neglecting to notice them, or, like the shepherds, differing so much in opinion as to lead to inextricable confusion. Fortunately it is not material, as their uniformity of structure is so great, that the description of one is nearly applicable to the whole of the group. The general outline of these hills forms a character as highly contrasted to that of the Cuchullin as are their respective colours. In place of the lofty spires, the impending precipices and the almost unalterable rocks of those, we see in the Red hills a continued succession of tame rounded outlines, the effect of a decomposition which has covered them with ruins and almost every where concealed from view the natural rock. They also fall far short of the Cuchullin in elevation. Those which are entirely red, and which, as will be hereafter seen, consist of a syenitic rock, do not approach, within many hundred feet, the height of the former; and the loftiest of the group, among which that of Glamich takes precedence, will be found to consist of a mixture of the syenitic rock, and that clinkstone, which, as I shall hereafter show, constitutes a portion of the mass of the Cuchullin.
Comparing from the summits of any of the hills the general aspect of the two groups, the spectator is inevitably struck with the different powers of resistance which the two classes of rock offer to the efforts of time, and looks forward perhaps to a distant day when the red hills shall be levelled with the land below, while the Cuchullin shall still lift its iron summit to the clouds. There is yet another characteristic ​difference between the two groups. We have seen that the Cuchullin and Blaven form one irregular mass, continuous in the ridges and much confounded both with each other and the general mass of the island. But the Red hills form rather a distinct assemblage of mountains, being separated either singly or in small associations by deep vallies, and placed as it were upon the general level. This group is bounded on the west by the vale of Strath, which extends from Broadford to the head of Loch Slapin, and skirting the sound of Scalpa terminates at Loch Sligachan. I may add that except the slight mixture of substances which appears to take place in Glamich hill, and in one or two more of those which come into contact with the Cuchullin and with Blaven, these two groups do not interfere, but are composed respectively of different and well characterized rocks.
The great mountain of Blaven is skirted by lower hills, descending gradually into the table land of Strathaird, which we shall hereafter see to be the largest portion of the secondary stratified rocks existing in Sky.
An uneven land, rising into irregular low hills, occupies the whole tract to the southward of the valley of Strath, with the exception of a nook which may be defined by an imaginary curved line drawn from Loch Oransa to the castle of Inverfuchag. This part of the island forms another distinct mountain group rising into two principal summits, and attaining an elevation apparently of fifteen or eighteen hundred feet.
The general elevations of the remainder of the island may be discussed in a few words.
Having passed Loch Sligachan, an irregular ridge of high land appears branching in two directions. The main ridge occupies the eastern side of the island to Portree, where it is interrupted by the ​harbour of that place. Resuming thence its northern direction it continues to rise gradually for twelve or fifteen miles, and declines as gradually to the northern extremity of the island at Ruhunish. This ridge shows a gentle declivity to the west while to the east it frequently presents a rocky and broken escarpement. As far as its elevation can be estimated by the aspect of the view from its summit, and by comparing it with other known elevations when seen from the sea, it seems to reach at least the height of fifteen hundred feet. It may even be more considerable, since the gradual ascent of the land around it tends to make it appear lower than it really is.
As the abrupt face of this ridge is directed eastward, it thus forms a second and interior range of cliffs which in many places rises to a considerable height. The most remarkable of these is to be seen at the Storr, from whence it extends for some space northwards. The precipices which form the eastern face of the Storr offer scenes to the lover of the picturesque not exceeded either in singularity or grandeur by any thing which is to be seen in Scotland, and almost as little known to the natives as to casual travellers. In the progress of decomposition vast fragments have been detached from the body of the hill, and continue insulated on its slope, resembling at a little distance the remains of ancient castles and the spires of ruined cathedrals. One remarkable conical rock attains a height of about an hundred and fifty feet, its base not exceeding twenty in diameter, forming a sea mark as conspicuous to the vessels which frequent this coast, as it is striking in a picturesque view.
The mineralogist, no less than the admirer of fine nature, will be gratified by the examination of the Storr. It consists of an amygdaloidal rock, containing abundant specimens of the zeolite family, ​which I shall have occasion to describe in their proper place. The other ridge which I mentioned as branching from it, is considerably lower, and consists in fact of uneven undulating ground, stretching away to the north-west till it reaches Vaternish point. The western peninsular division, from Dunvegan head to Macleod's Maidens, is a tract of high land containing the two remarkable flat topped mountains known by the name of Macleod's Tables, the height of which appears to be similar to that of the great eastern ridge. What remains of the island is one irregular table land, offering hills which in any other situation would appear considerable, but which lose their importance in the vicinity of the towering Cuchullin.
I have reason to think that in the general estimate here given of the elevations in Sky I have fallen short instead of exceeding, and shall be glad if future observers shall determine these altitudes with correctness. I scarcely however consider it in any other light than as a question of geography, as the increased examination of geological phenomena has proved that there is no general or necessary connection between formations of particular rocks and given elevations. Equally unfounded appear those rules which have, as recent observations show, been prematurely laid down respecting the outgoings as they have been called of the several classes of rock. The irregularities of these outgoings, not only absolutely but relatively, seem to point out to geologists a department requiring at least a very careful review, if not an absolute elision from the laws of the science.
I have nothing to add to the description of the rivers which I gave in speaking of the geographic divisions of the island, and the remarks I have to make on the waste of the land will be comprised in a short space. ​
I have already said that I did not observe in any part of Sky those accumulations of foreign and transported materials which are of such frequent occurrence on the main land of Scotland, and of which I have already noticed a remarkable example in Staffa. The accumulations of matter which it offers in several places are but trifling, and they are evidently derived from the dauly action of the rivers, or from the ruin of the exposed rocks.
Commencing at Loch Slapin a considerable alluvium may be observed occupying the head of this Loch and extending up Strathmore, encroaching on the top of the bay, and evidently formed by the waste of the Red hills, with additions comparatively insignificant from the naked precipices of Blaven.
As the upper end of Loch Eishort receives no river of note, it presents no alluvial deposit, and no further marks of waste, encroachment, or alteration of the sea line, are to be observed round the point of Sleat till we reach Loch Oransa. Partial depositions of gravel may be traced from hence round the shore to Kyleaken, where a considerable bank of alluvial matter has been thrown up, apparently from the concurring action of the rapid tides which run through the sound of Sky. Opposite the island of Scalpa a sort of delta has been formed, which bids fair at some future day to unite the two islands into one. The head of the eastern Loch Eynort is also the receptacle of considerable masses of rubbish brought down from the Red hills, a waste sufficiently great to alter the courses of the small streams which run through the narrow vallies separating these hills. Similar, but more extensive, accumulations from the same cause have formed a tract of plain ground at the head of Loch Sligachan, subject to frequent inroads and changes from the still varying course of the stream. The waste of the land which ​supplies the river running into Portree harbour appears to have had some effect in filling up its southern branch which the ebb of the loch has no tendency to remove. From below Portree to the northern point of the island, the coast consists of high cliffs of trap, which exhibit the species of decomposition characteristic of these rocks, in the vast slopes which decline from them to the shore wherever the action of the tide has not been sufficient to prevent that accumulation. Continuing round the point of Ruhunish, similar cliffs of trap resting on the secondary strata extend to the bottom of Loch Snizort, at the end of which as well as of Loch Uig the same appearances of encroachment are visible. The parish of Kilmuir offers the only considerable tract of alluvial land in Sky, from which its superior and long established fertility is probably to be accounted for. I cannot speak positively of the shores which form the point of Vaternish, having only seen them from a distance, but as they resemble both in aspect and composition the division of Trotternish just noticed, it is probable they possess no great peculiarity in this respect. Neither does Dunvegan offer any thing worthy of remark.
The shores of Loch Bracadale exhibit when low, considerable portions of clayey alluvial soil, characterised like those of Kilmuir by extraordinary fertility. A similar alluvium may be observed at the head of Loch Harpart, and the little valley of Talisker appears to have been entirely gained from the sea at some distant period, by a combination of the waste of the land with the counteracting efforts of the western swell, which has thus formed a natural embankment for its further protection.
A remarkable difference is to be seen along the whole western shore of the island from Dunvegan head to Loch Brittle, between ​the effects of the eastern and the western sea. I already observed that the eastern cliffs were covered by a slope of alluvial ground descending to the sea; but the western, though formed of the same rocks, offer an almost continued precipice, the foot of which is every where washed by a turbulent swell. These cliffs are in a state of daily ruin, and their bases are beset with enormous masses of rock which from time to time fall from them. The rocks called Macleod's Maidens, the islands in Loch Bracadale, and other detached rocks which skirt this coast, mark equally the gradual waste of the land. But no slope is formed against their faces, nor does any artificial shore accumulate at their feet, except a narrow and almost impassable interrupted stripe composed of fragments. The smaller pieces and the detritus of these larger ones, are probably carried away from the coast by the incessant action of the western swell far into the depths of the sea. As there are no recorded soundings of this shore, I cannot confirm this supposition by any account of the nature of the bottom. But it will not be out of place to enter into a more particular detail of this line.
The coast of Sky from Dunvegan head to the entrance of Loch Brittle is, with but few exceptions, formed of high cliffs; variable however in their altitudes as in their abruptness, but generally very lofty and very abrupt, nay often precipitous and perpendicular from their summits to the water's edge. It is unnecessary to note any but the most remarkable points, as there is a great general resemblance throughout the whole. Between Dunvegan head and Loch Bracadale they are often perpendicular, but variable in height, and seldom attain the great elevation which they reach between Talisker and Loch Eynort. There, as in many other places, they ​possess a singularly striped appearance from the great variety of colours in the several beds which compose them, of which twelve or more may in different places be counted, all horizontal and tolerably equal in their dimensions. The forms of these cliffs are far too monotonous and too square to afford subjects for the pencil, every part being marked by a general similarity of character. Near the entrance of Loch Bracadale some variety is presented by the three detached and pyramidal rocks called Macleod's Maidens, the highest of which appears to reach to about 200 feet. This feature, of detached pyramidal masses, is of frequent occurrence on this coast, a remarkable perforated one being seen in Loch Bracadale, and a similar one not far from Loch Eynort. They are, like all other objects out of the ordinary course of nature, rather singular than picturesque: the strange and the bizarre are seldom legitimate subjects for painting, and rarely please long, after the first wonder has subsided.
In Loch Bracadale some caves are found in the rocks, which have no particular claims on notice either from their beauty, their magnitude, or their singularity. Similar caves are of frequent occurrence between Talisker and Loch Brittle, the low projecting rocks being also often perforated by arches which are sometimes exceedingly complicated and remarkable. With the exception of some, projecting points of high rock the shores of Loch Bracadale are flat, and this tract is among the most fertile of Sky. At its southern extremity the cliffs are perfectly vertical, and without that slope at the foot, which so commonly accompanies the high cliffs of trap, and which are so conspicuous in particular on the eastern side of the island. The retired and green valley of Talisker opens to the sea by a low beach, on which the natural embankment already mentioned has been formed by the western swell. Here ​the cliffs again become high, and shortly rise to the greatest altitude which they attain along the whole line of this coast. In a general sense they may be called perpendicular, but they are seldom without grassy slopes either at their feet or in some intermediate parts; which diminish their vertical appearance when seen in profile, although when viewed in front they still appear perpendicular. The outline is here more varied, the parts more numerous and intricate, and the tone of colour more agreeable, while the foregrounds, which are formed by some high and conspicuous detached rocks, produce with them some of the most magnificent compositions of rock scenery which are to be seen on the coasts of Sky. Not far from this place is a large and very remarkable slide, by which a considerable portion of the cliff has been brought from the summit down to the shore, where it forms a promontory obstructing the further progress of the mineralogist, who in defiance of toil and hazard pursues along this rocky coast the beautiful minerals in which it abounds. I have no measure of the heights of these cliffs, but they are considerable. Comparing them with the masts of passing ships I imagine that the highest can not be much less than 800 feet. The cascades which fall over them are often dissipated in spray before they reach the ground, or descend in a mere shower of drizzling rain.
The cliff continue with more variety of height than of character to Loch Brittle, the habitation of the Osprey, and of what is more interesting to geologists, some rare minerals which I shall describe in their proper places. At Loch Brittle the land runs out into low projecting points, the interior being of small elevation and disposed in terraces. The cliffs rise but little from this place to the entrance of Loch Scavig, where the coast assumes a new character, the declivities of the hills reaching the sea at a considerable ​angle. Here the dark summits of the Cuehullin hills come in view, and new features and new scenes arise to re-excite the interest which so long an extent of an almost uniform character had suffered to languish.
Loch Scavig is an inlet of the sea about a mile in depth, formed by the Cuchullin hills, which rise with all their spiry and naked crags high towering above it. At the bottom of this bay they descend suddenly into the sea, brown and bare, with scarcely a spot of verdure to enliven their dark sides, the only semblance of life they possess consisting in the motion of the few cascades which foam down their rugged declivities. Points of detached rocks projecting into the sea from their base, produce foregrounds for the use of the artist, and relieve that intense depth of shadow which seems ever to reign where the sun beams can scarcely find access. But even the grandeur, the silence and desolation of this place are forgotten, when in a moment on turning the angle of a huge rock, the spectator enters on a scene which suspends the recollection of all which had fascinated him before. He finds himself in a lone valley surrounded by a wall of dark and naked rock, of which the rugged summits are lost in the clouds, intercepting the light of day and casting a twilight gloom over the seat of eternal repose. If ever a sound disturbs this repose it is that of the wind which whistles against the rocks, or of the cascade which rushes down their sides; if ever vestige of life is seen, it is the lone sea-gull dipping its wing in the black still waters of Coruisk. The valley once closed behind the spectator, he sees no more its egress, and calls to mind the tales of eastern fiction, where the victim of magic ​is for ever immured in some profound chasm of the mountains of Caucasus. The lake Coruisk is rather more than two miles in length, being fed by a powerful stream at its upper end, and discharging itself into the sea by a wide and rocky channel, a favourite resort of salmon. Its shores are every where covered with huge fragments of rock detached from the mountains above, and it contains two or three small islands which diversify in some measure the darkness of its surface. The nakedness of the rocks is not poetical. On the declivity of the mountain Garsven in particular, they rise from the base to the very summit, a height of at least 3000 feet, in huge smooth sheets at a very high angle, perfectly bare and of a dark iron brown colour, not chequered even by the growth of a single lichen or by one foreign tint to enliven the uniform gloom of the surface. This rock seems indeed absolutely inimical to vegetation, nor does it appear to undergo the slightest decomposition, or to admit of the formation of soil, the detached fragments showing as little tendency to waste as the mountain itself. Had the globe of the earth been entirely formed of this rock it would still have been lifeless and void. It was among these fragments that I observed a rocking stone of considerable size and easily moved, having to all appearance fallen on such an edge as to allow of the conditions required for producing this effect.
In quitting this scene, for which favourable weather is required, since it is inaccessible by land, and the outer loch is subject to dangerous and sudden squalls, little interest occurs on the, east side of Strathaird, except one fine example of a slide by which a large mass of the hill has descended to the water.
Having passed the point of Strathaird a succession of cliffs commences which extends nearly to the end of Loch Slapin, formed of ​secondary strata, and of an entirely new character. Their upper edge is generally even, and their faces quite perpendicular, but they rarely exceed 60 feet in height. They are most remarkable for the very extraordinary number of the caves which are found in them, and of the fissures by which they are intersected. These are rarely of any great dimensions, but they are so numerous that they sometimes occupy nearly as much space in a given distance as the solid parts of the cliffs themselves. I have counted twenty or thirty of them in the course of a few hundred yards, the interstices having a resemblance to the ends of detached walls placed in a parallel manner. They are the consequences of trap veins which have been washed away, but as they present important geological facts, I shall describe them more particularly hereafter. Few of these caves have been explored, but one of them has acquired historic celebrity from the circumstance of its having been among the numerous places of temporary refuge inhabited by Prince Charles during his concealment. Another has recently become the cause of great resort to Sky on account of its stalactitic concretions; and it is popularly distinguished by the name of the Spar Cave: it lies on the estate of Mr. Macalister, and is too well known to require any more accurate description of its locality. This cave is accessible from the cliffs above, for a short time only at low water, but by means of a boat it may be visited at any time in moderate weather, or with the wind of the shore. The entrance is little less striking than the cave itself, and to the admirer of the picturesque it presents a scene even more attractive. It is formed by a fissure in the cliff, extending for a considerable way and rising into high and parallel walls on each side, its gloom being partially illuminated by reflected light, and its silence scarcely disturbed by the wash of the surf without. A narrow and obstructed opening leads unexpectedly into the cave: hence for a distance of ​about an hundred feet all is dark, wet and dreary, till we arrive at a steep acclivity formed of a white stalagmite. Surmounting this with some difficulty, the whole interior and ornamental part of the cave comes into view, covered with stalactites disposed in all the grotesque forms which these incrustations so commonly assume. Lively imaginations may here indulge in the discovery of fanciful resemblances, and the concretions have accordingly received names more descriptive of the fancies of the spectators than of their real forms. The dimensions of the fissure are in this place but inconsiderable, the breadth not being more than ten feet, and the height scarcely exceeding twelve: it is not long however before the height suddenly increases to forty feet or more, and a declivity of perhaps thirty feet in length, from the surface of the stalagmitical and cascade-like mount which forms the division between the sparry and the dark part of the fissure, conducts to a pool of water occupying a space of twelve or fifteen feet in length, dividing the cave into an outer and inner portion. The dimensions are here somewhat enlarged, and the height in particular is much more considerable. At a distance of about fifteen or twenty feet more from this pool of water the stalactitical ornaments cease, and shortly after the cave terminates, the whole length from the entrance to the end being about two hundred and fifty feet. However beautiful the interior of the cave, from the white colour and ornamental effects of the stalactites which incrust it, the want of sufficient dimensions materially lessens the interest, which in all other respects it is calculated to excite. At the termination of the stalactitical ornaments there is a dark descent for a few yards, filled with rubbish, the ruins of the roof above, which being here naked as well as accessible, is plainly seen to be one of the trap veins. This vein is here about ten feet wide, which is the general breadth of the cave itself, and it is easy ​to see how by the wearing out of that vein the excavation has been formed, having subsequently acquired its present degree of ornament by the infiltration of carbonat of lime in solution. When we consider the great depth of this cave and its distance at present from the sea, we are inclined to inquire by what means so extensive an excavation could have been formed, and how the rock which has fallen from it has been disposed of. It is probable that the depth of water at the face of the cliffs was once such as to allow of the ready access of the sea to them, and that at this period the excavations so numerous on this shore were produced. The subsequent accumulation of rubbish formed by its action, has in later times produced the slope or shore which now excludes it from further access, and protects the cliffs from further demolition.
In the little excavations which are found in the floor of this cave we have the means of seeing the process by which the formation of calcareous spar takes place, the crystallization being carried on in a solution of the carbonat of lime, precisely as it is in the saline solutions in our laboratories. All these small pools are filled with groups of crystals, in a state of constant augmentation, but of very irregular forms. Doubtless these forms must be affected by the agitation which the falling drops occasion in the solution; and it is in all probability owing to some circumstances of this nature, constant in the same place but varying in different ones, that crystals of one form are found to affect certain places, while in others they regularly assume some other modification. As it offers no novelty to the mineralogist, and belongs to a class of geological facts by no means uncommon, it is unnecessary to enter into any further details respecting it. Such are the principal circumstances which I had an opportunity of remarking, and which appeared most worthy of notice in the general face of the island. ​
I shall now proceed to consider the nature of the rocks which compose this island, in as regular a detail as my opportunities and its intricate and unsettled geography allowed me to observe and record them.
It was not without repeated efforts and much careful tracing of the successions of the stratified rocks through coasts difficult in themselves and far distant from each other, that I was able to discover the key to the very great obscurity in which these are involved. It will be seen that there are yet points unexplained, particularly respecting the trap rocks, a circumstance which will not surprise geologists who know that to patience, toil, and good seasons, must often be superadded good fortune; the casual discovery of perhaps the only point over a wide tract which is capable of yielding the explanation of which we are in pursuit.
As the mica slate and its associate the quartz rock are the most ancient of those which I have ascertained in Sky, I shall commence from them as the foundation of the whole structure. I have associated these rocks in this manner, because I have generally found them alternating, and bearing a common relation both to those which appear to hold a deeper, and to those which have a more superficial position with regard to them, the granite and the stratified rocks; but I need not repeat these reasons here as I have discussed them at sufficient length in the account which I formerly gave of quartz rock, I have not however traced any decided alternation between these two rocks in this island, nor, as far as I have observed, is it likely that any alternation will easily be traced, since the beds of both are far less regular than is usual when they are associated in an alternating orders The mica slate is found occupying the district of Sleat, and it extends from the point of that promontory to an irregular line drawn between Loch Eishort and Loch ​Oransa, the point being much less accurately defined on the eastern than on the western shore. The shores which it forms are generally flat and shelving, but in some particular places, as at Tormore, it rises into high cliffs. The whole tract is however but little interesting, since the uniformity of this rock is only varied by the occasional occurrence of veins of trap and of quartz. If I could determine that the beds of mica slate had a regular position, I would say that they were succeeded by quartz rock, and that this rock terminated near Loch Eishort, being followed by a succession of rocks to be described immediately; but to determine such a succession, a regular disposition is indispensable, since, without that, the uppermost in place may not always be the uppermost in order. I must therefore content myself with remarking that the quartz rock follows the mica slate in geographical position, and that its main body will be seen occupying two hills which lie above Ord, from which the skirts reach to the shores of the southern boundary of Loch Eishort. The mica slate has no peculiarity of structure or of aspect, such as to render a detailed description of it necessary, but I think it proper to remark, that like many other districts of the western isles which contain this rock as the basis of their structure, it is by no means so unmixed as it is generally met with in the extensive tracts of it which occur on the main lands. On the contrary, it is frequently found passing into clay slate as well as into talc and chlorite slates, affording an example like Arran, Isla, and Jura, of the intimate geological union which subsists between these several substances. Of the quartz rock I may remark that it is extremely compact, of a highly crystalline aspect in general, and that its weathered surface is so white as to render the hills which are formed of it distinguishable at a great distance by their snowy appearance, while at the same time it is frequently varnished, as it were, with the ​same siliceous enamel which is so remarkable at Balahulish. In most situations it contains the common admixture of felspar in grains. Although not, strictly speaking, stratified, it bears the marks of having once been regularly so, the beds having been rendered obscure, partly by the infinite number of cross fractures which it has undergone, and partly by the bendings and displacements which it has experienced. At Ord indeed some extent of it is to be seen lying directly in contact with the beds of red sandstone and in a conformable direction, and here the stratification is sufficiently regular to preclude all doubts, although the beds are not so continuous and unbroken as those of the sandstone.
But I must not quit this district without noticing a remarkable circumstance, uncertain as I yet am whether the observation was correctly made or not. If I have not an opportunity myself to verify or contradict it, I shall at least point it out to those who may follow me through this country.
There is considerable intricacy as well as obscurity in the rocks which I have now described on the western side of the Point of Sleat, although on the eastern there is neither variety nor obscurity to contend with, this intricacy arising partly from the frequent occurrence of insulated portions of the secondary strata skirting the shores, and of which the further details are cut off by the irregular indentation of the sea line. Thus they become in a topographical sense intermingled with the micaceous schist and with the quartz rock in a very irregular manner, while the general confusion is much augmented by the great number and perseverance of the trap veins which traverse them. It is extremely difficult to find a way through this geological labyrinth. It is near Gillan, where among other places this confusion occurs, that the phenomenon in question is to be seen: a bed of highly compact grey limestone containing ​shells is here included between two beds of quartz rock; unfortunately but a small quantity of the rock is exposed, and as it lies low on the shore it is impossible to trace the series to the extent which is desirable. As far as I could conjecture the quartz rock was a portion of the general series which alternates with the mica slate: if this be correct it offers a solitary example of a rock containing organized remains alternating with the series of mica slate, a fact as yet so anomalous that it must not be received without much more decided evidence than that which I have here produced. I have to regret that the loss of these specimens prevents me from describing the particular shells which I observed in this limestone; yet doubtful as this fact appears even to myself; it is somewhat countenanced by a circumstance not dissimilar which occurs near Borrereg on the side of Loch Eishort, where the white and crystalline quartz rock which alternates with the mica slate is found in one place alternating for a short space with the blue limestone and red sandstone, the two first and lowest members of the secondary strata. With every wish to find that I had committed an error in this examination I could not detect it, and must therefore suffer the whole to remain at present as I have stated it, an example at least of a most deceptive geological appearance if it shall not turn out to be an interesting fact.
The two associated rocks which I have here described are immediately followed by a regular succession of stratified rocks which I shall now proceed to detail; commencing at Loch Eishort, as being the principal point in which I observed their contact with the former, and from which I derived the clue to the order of succession of all the stratified rocks in Sky.
In distinguishing however the rocks which follow by the term stratified, I am far from meaning to say that I consider the mica ​slate and quartz rock as unstratified, but their want of regularity in the present instance, compared with the extreme precision and order of those which follow, render the distinction convenient here for the temporary purpose of describing them. I have in other instances produced sufficient examples of the alternate and regular stratification of quartz rock and mica slate to render it unnecessary to say more on that subject: I have in the same places shown that these rocks alternate with clay slate, and my object at this moment is to distinguish strongly the sudden change from great disturbance to extreme regularity, which takes place here between two rocks which in so many other instances seem to have undergone together the common action of some disturbing force.
The first of these rocks is a compound series of beds consisting of dark blue quartz rock interlaminated with thin layers of clay slate of the same colour: these are to be found in immediate contact with the white quartz rock, and in an order which is conformable to the general position of that substance where it is in contact with them. Often the quartz of this rock prevails to the total exclusion of the schist, and the beds are then much thicker than when the schist interferes. They hold a remarkably even and straight course I for a considerable space on the north side of Loch Eishort, dipping to the N.W. at an angle of about 80°. The thickness of this set of beds appears to be extremely variable, but I know not that it can be ascertained, since the excavation which forms Loch Eishort interferes with the observation at the western point of their exposure, as the sea in that place covers the elevated edges, while in their course eastward they become clothed with the covering of soil. There is consequently a chasm in my attempt to trace their extent eastward, but as the same rock appears on the eastern side with all the same characters, and in the same position with respect ​to the mica slate; I imagine no error will be committed if I consider it as a continuation of this series.
This is the rock which forms the high mountains above the Kyle ri'ch, which have already been mentioned in the general description. The principal summits, on account of their elevation, are visible at a considerable distance in coming from Glen Elg, and they occupy the greater portion of that which may be called the eastern promontory of Sky. Ben na Grien and Ben Ashlaig appear to be the highest of the group: these decline into lower elevations, and with some variation both of character and aspect till they disappear, but I have to regret that I can give no account of the connections of either of the outer extremities with the neighbouring rocks. It is too common for the geologist in the course of his labours to find that he has commenced at the wrong end of his investigation, and to discover, when it is too late to profit by it, that he has probably found the solution of his previous difficulties when the difficulties themselves are no longer accessible to him: I can only therefore offer it as a conjecture, the the rock which forms the mountains in question is a continuation of the one already described at Loch Eishort, since its examination unfortunately preceded that of the latter, and it was out of my power to resume it when I had obtained the information which would probably have enabled me to speak decisively concerning it. I would recommend to those who may follow me to trace it with care from Loch Eishort to the eastern shore, by which proceeding they will probably succeed in determining that which I am compelled so leave in doubt, but I may nevertheless describe its leading features. It is by no means regularly stratified, nor is it possible to trace for any considerable distance even the fragments of a regular prolongation of the edges of beds, such as is so distinctly exhibited in the quartz ​rock of Jura; yet there are evident marks of stratification, which in many places are sufficiently regular and extensive to justify the belief that it has once possessed a greater degree of regularity and extent, which have been disturbed by subsequent changes. The predominant colour is a bluish or greenish grey, and its most general aspect that of a compact splintery quartz, often obscurely granular, and every where traversed by veins of ordinary white quartz. At Kyleaken it has a brown colour and a flat appearance, but becomes white and harsh on exposure to the weather, in consequence of the decomposition of the felspar which it contains. The great thickness of the mass here, compared with the thinness of the similar rock at Loch Eishort, is no proof of non-identity, since in so many other cases we find strata differing, in perhaps as great a degree, in thickness where we have the most satisfactory evidence of their continuity. I have not observed this rock in any other part of Sky, and it will presently appear that it could not be expected any where but beneath the sandstone near Loch Scavig. I shall not be surprised if it exists at the southern side of Soa under the sandstone beds, since that is its place, but this part of that island was unfortunately the only one which I could not reach. That it is a bed of great extent in this country, although so little visible, is confirmed by its being found in Rum, and in the same position, as I have ascertained.
The rock next in order to this is the red sandstone, and the connection between the two is equally distinct at Loch Eishort, where they are found following each other in a regular order and in intimate contact, there being indeed a gradual passage from the blue quartz rock into the red sandstone by a series of intermediate changes both in point of hardness and colour. The same transition will be found near Kyleaken on the eastern side, although I cannot ​point it out so distinctly. The certainty of a transition between these two rocks is equally well marked by another circumstance, although that indication does not appear at Loch Eishort; namely, by the alternation of the beds of schist with the red sandstone, a phenomenon to be observed at Loch Scavig, as I shall presently show. Following this sandstone it is to be seen forming a considerable part of the hills which rise on the northern side of Loch Eishort, and extending along that shore as far as the farm of Borrereg where it ceases. The beds succeed each other with great regularity, and are of considerable thickness; as far as I should judge from a rude estimate formed by comparing the measures of their edges with the space which they occupy, I should imagine that the collective mass does not here fall much short of a thousand feet in thickness, tallying in this respect very nearly with the dimensions which it seems to maintain in Rum as well as in Soa, both of which I have no doubt are different portions of the same rock.
This red sandstone is also found skirting the southern shore of Loch Eishort in different places, and is particularly conspicuous at the castle of Dunscaich, a building of no high antiquity, but situated on a rock, the traditional residence of Cuchullin “ King of the Isle of Mist.” This position is readily explained by observing that the line of Loch Eishort crosses that of the direction of the sandstone in an oblique manner, leaving thus a portion of it on the southern shore; it is here often in contact with the white quartz rock, the intermediate blue rock having disappeared. From this I should be inclined to suspect that there is a real chasm between the conjoined rocks consisting of mica slate and quartz rock, and the stratified ones now under review, how much soever some of their portions may appear consecutive. I have pointed out at some ​length in other places the errors that may arise in these observation, and need not enter largely into the question in this place, but it appears to me a sufficient argument that two sets of rocks are not consecutive if those which correspond in one place do not also correspond throughout. I should expect to find that the same bed of red sandstone exists between Kyleaken and Broadford, where I had no opportunity of looking for it; but beyond that it is not likely to occur, as the limestone which lies at some distance above it in the order of superposition occupies the lowest situation on the shores of Broadford. I mentioned that the same bed is to be found at Loch Scavig, where it is plainly connected with the very large mass of similar sandstone which constitutes the main part if not the whole of the island of Soa; it occupies here the lowest visible situation, being washed by the sea and exposing but an inconsiderable thickness, since it is shortly followed by the trap, when it finally disappears, none of the subsequent stratified rocks which accompany and follow it near Loch Eishort being here found. It is here interstratified with beds of a schist which has sometimes the character of fine clay slate, while at other times, the laminæ being separated by coarser particles of sand and of mica, it puts on the character of that which has been called fine graywacke slate. I have on other occasions shown that these rocks are not necessarily distinguished in position, and that near Loch Ard in Perthshire, and Crinan in Argyleshire, the several sorts of clay slate and graywacke are irregularly intermixed: the same phenomenon is to be seen in many other parts of Scotland, which I cannot here pretend to enumerate; although the question is of importance to geology, this is not the place in which it must be discussed. The fact abovementioned however extends our objections to the commonly received ​arrangement even one step further, since I have shown that perfectly characterized clay slate is here a member (to use the Wernerian language) of the flœtz rocks; the sandstone here described being analogous to the old red sandstone of that school, and a member of a continuous series of stratified rocks, of which the uppermost contain shells and other undoubted characteristics of their rank in the order of rocks.
The extent of this rock on the shore is not great, but it may be seen occupying a continuous line both on the shore of the sound of Soa and within the entrance of Loch Scavig, forming the foot of Garsven, one of the highest of the Cuchullin group.
These are the only places in which I have found this rock, and I have no great reason to expect that it will be found any where else, as its dip does not any where indicate its probable re-appearance; yet I should not be surprised were it to come out at Loch Sligachan, since the limestone which immediately follows it in regular order is to be seen there. These beds, the limestone on the one hand and the quartz with schist on the other, will be guides to these who wish to search for this rock in other parts of the island.
The mineral character of this sandstone is very uniform; it is most commonly of a red colour, but it becomes brown or grayish, when in the neighbourhood of the schist, and sometimes even partakes of the blue colour of this substance. It is of a moderately fine grain and very tractable as a building stone, sometimes possessing the softness of an ordinary sandstone and at others acquiring a flinty kind of hardness, more particularly when it approaches the schist. It is generally mixed with clay, or is of an argillaceous nature, and in some places moreover it is found to contain particles of calcareous matter: in some few places coarser portions are to be ​found, formed of large gravel or even of fragments, but I have not seen either in Sky, in Soa, or in Rum, any regular beds of conglomerate connected with it, although I have in these situations had repeated access to the whole thickness of the rock. Although the general colours of this rock are such as I have described, yet it seems occasionally, like the lowermost red sandstone in other places, to contain portions of a different complexion; one portion of such a bed is seen under Garsven opposite to Soa, where it is of a white colour and intermixed with decomposed felspar, yet still exceedingly compact. Pebbles of quartz and granite are imbedded in some of its laminæ. For the sake of those who contend for the doctrine of universal formations of rock, I have recorded the whole of its characters and connections that they may have opportunities of comparing them with those of other similar rocks, and I can only add to what I have here said, that as I imagine Rum and Sky to contain portions of one common bed, so I am inclined to think that the red sandstone which I have found on the western coast of Scotland in various places, from Loch Ewe to the Ru Storr, appertains to the same series of beds. It is still the same sandstone, as I imagine, which extends to the east coast and finally to Orkney.
The next bed in order is a bed of limestone, the investigation of which was attended with more labour and doubt than that of all the other rocks of Sky, and of which the history I am about to give cannot appear less credible to others than it did at first to myself: the demonstration which I at length procured and shall now lay down is however such as to admit of no dispute. It is important in a general view since it shows us how little reliance can be placed on internal characters in assigning the positions of rocks in the system, and since it may possibly lead the way to the explanation of those anomalous limestones to which the examples occurring near Plymouth, ​and as yet involved in so much obscurity, may perhaps belong, and which have been ranked in the class called transition by the Germans, since they were supposed to be of higher antiquity than the secondary or flœtz strata of the same school. This bed is found at Loch Eishort immediately following the sandstone, with a common dip, and in every respect perfectly conformable to it. It is readily distinguished wherever it occurs, provided its upper surface be exposed to the weather, and this at very considerable distances, the appearance which it here puts on being that of detached and irregular masses with a deeply honey-combed surface, the cavities being from a foot to two in depth, rounded and perpendicular, while the projecting portions are equally rounded and smooth. No lichen attaches itself to that surface, but whatever its hue be, it weathers to a bluish colour, by which it is visible among all the surrounding rocks. I could find no means of ascertaining its thickness at Loch Eishort, but near Kilbride where it is fairly exposed it is some hundreds of feet thick; I have however reason to imagine that this dimension is very variable. It may be traced where it first appears at Loch Eishort from the point where it is in contact with the sandstone, for a considerable way up the hills in the direction of Broadford, but is at length lost amidst trap and syenite, and amongst the mossy and deep soil of this rough ground. Pursuing the line of shore it shortly disappears, other beds coming in the way which will follow next in the order of description; but it is recovered at Kilbride, and hence may be traced through the remainder of its connections to the no small surprise of the geologist. As the sandstone bed does not exist at Kilbride the identity of the limestone cannot here be proved by that of the beds on which it reposes; instead of sandstone indeed its lower surface is found in contact with the syenite already generally ​described. The absolute identity of its aspect, composition, and mode of weathering, might perhaps be sufficient to prove the identity of its nature, but this is put out of all doubt by finding that it is here followed by the same set of beds which succeed to it in that place where the sandstone precedes it, and of which the continuity can be traced between the two points. The drawings (Pl. 2. fig. 1.) which accompany this paper, will explain better than words this very essential circumstance in the geological history of Sky.
There is here an opportunity of tracing by a very perfect natural section the change which it undergoes between the very regular beds which lie near it on the one hand and the irregular surface of syenite with which it is in contact on the other. At this surface it bears no marks of stratification, but is an irregular and almost shapeless mass, while near the former beds, which also consist of limestone, it becomes first vertical and gradually more regular, till at the end its general bearing, although much deformed by counter fissures, partakes most decidedly of the general inclination of the stratified rock, which is here about 25°. In geographical distribution it may be here traced in two divisions, separated from each other by syenite and intersected by trap veins, both of these divisions extending towards Broadford and uniting into one scattered and irregular mass about three or four miles short of that place. The uniformity of its character, as well as its continuity throughout this tract, is such as to leave no doubt that the marble of Broadford already mentioned, and which I shall hereafter more fully describe when I speak of individual minerals, is a continuation of this portion of the limestone beds. When it has once quitted the sandstone that rock is no longer seen, but the whole remaining portions of the limestone, occupying a great space from the hills which skirt the eastern side of Strath to the foot of the syenite mountains on the opposite side, is ​(always with the exception of whin dykes) in contact with syenite; this contact can be distinguished in many places at the surface, and it has moreover been brought to light in the excavations which have been formed at the marble quarries; it is so intricate that the limestone is often divided into insulated portions surrounded by syenite, and were it not for the clue which is given by the shores at Kilbride the whole tract is so obscure that it would have remained as unintelligible to me now as it did in the first examination which I made of it. It is too speculative an inquiry to consider what influence the syenite may have had in producing this irregularity; nay I have not even the means of proving that the syenite is posterior to the stratified rocks; but in the course of examining that rock hereafter, I shall assign reasons for supposing that this is really the case, and that like the trap rocks to which it is associated it is not improbably the cause of all the irregularity apparent in this place.
If even the shadow of a doubt could remain respecting the connection of this, which I shall distinguish by the name of the marble limestone, with the shell limestone, it is removed by the discovery of a regular alternation of the two near the farm of Borrereg, a sketch of which (Pl. 2. fig. 2.) is introduced into one of the sections designed for the illustration of these rocks.
The limestone having completely lost all semblance of stratification where it is involved among the syenite, is found forming large insulated lumps, of which the great structure and general fracture resemble that of the Devonshire insulated limestones, and those of Assynt which I have described in the Geological Transactions, vol. 2. It is fissured in various directions, and can be raised in large irregular blocks only. Hence it has very naturally been considered as a primary limestone, an error into which I was at first inclined to ​fall myself. This is confirmed by its texture as well as its colour, both of them resembling those which are supposed to be characteristic of primary limestones. In these respects indeed it possesses an exact resemblance to the limestones which we find in various parts of Scotland associated with schist, gneiss, and granite, of which I have recorded examples in different places. From this, as I have already hinted, we ought to receive with distrust any attempt to distinguish the primary and secondary limestones by internal characters, nay even by their external forms, since in the respect also the limestone of Strath bears a perfect resemblance to the primitive limestones already alluded to. The texture of this stone is almost every where compact, with a fracture finely granular in the surface, and varying between the splintery and conchoidal. It is generally brittle, a character rare in the stratified limestones, and in many places even breaks with the violence and cleanness of siliceous schist. In its chemical composition it is generally pure; but where in contact with the syenite or the trap veins, becomes overloaded not only with silica but with magnesia and argil also. In such situations it often contains veins and nodules of greenish transparent serpentine, and adopts a variety of colours, a circumstance in which it also resembles most exactly those limestones which in Glen Tilt are found in contact with granite, or at Balahulish, and in Tirey and Iona with mica slate and gneiss. The predominant colour is grey, varying from nearly white through all shades of dove colour to a dark blue grey, sometimes beautifully striped, and mottled or veined; in many situations it is of a pure snow white, forming a perfect variety of statuary marble, which I shall have occasion to describe more particularly hereafter, and affording as yet perhaps a solitary instance of this variety of limestone occurring among secondary strata. When I say a solitary instance, I ​must however add my suspicions, that when more attention shall have been paid to geological investigations, many of the facts which I have enumerated in various parts of these papers will not appear either so solitary or so extraordinary as they now seem.
I have shown that this limestone is in contact with the syenite, and it is also in contact with trap, since a hill of this substance is found surmounting as well as interfering with it near Loch Eishort. I might therefore proceed to describe these rocks according to the order which I have adopted; but nothing certain is as yet known of their real places, and if, as I imagine, and as I think will appear not only here but on almost every occasion where I have described them, they intrude among the regularly stratified rocks, it will be better to defer the consideration of them, as I have done on other occasions, to the last. Before however finally quitting this limestone I must not omit to mention that a similar one is found occupying the south side of Loch Sligachan in a similar manner, and that it has in all probability the same origin and connections, although I had no opportunity of investigating it to my satisfaction.
Returning to the original point at Loch Eishort we find the series of beds which follows this limestone in contact with it. At the very point of contact there is an interference of the two sets, which is highly satisfactory, as tending to establish their perfectly consecutive nature, and consequently of determining the place of the former limestone without the shadow of a doubt. Where they meet, two or three laminæ of the uppermost limestone alternate with an equal number of the lower one, and they are readily distinguished, because the upper ones being mixed with schist have a superior degree of permanence, and project with a sharp edge on the vertically corroded sides of the subjacent one. The upper limestone is equally to be seen following the under one at Kilbride; ​but the same interference is not visible there, although the parallelism is perfect.
From either end of this junction the limestone beds of the upper series may be traced all round the point which separates Loch Eishort from Loch Slapin, intersected every where like the former by trap veins. Neither the thickness, the number, nor the order of these beds can be ascertained, as they are much too complicated and difficult of access to admit of such an examination. The very attempt would be a superfluous endeavour after accuracy, since enough of them is ascertained to prove that only which is important to be known, the order of their connections with the neighbouring rocks, and the characters by which they can be identified with more distant strata. In general they are formed of thin and thick laminæ, composed of a dark blue earthy limestone, at times somewhat more crystalline, and variously interleaved with argillaceous schist of various dimensions. These beds, like the former, are inclined at an angle of about 25°; they seem most regular as well as thickest at the point of separation between the two Lochs Eynort and Slapin, while near the very same place they are also found in very thin schistose laminæ, so that I imagine there is no rule to be laid down respecting them; they are found occupying the island of Heast, as well as a long ridge of rocks which here intersect Loch Eishort; and they may also be traced at Ord; and further down this shore even as far as Gillan, following the red sandstone, for the same geographical reasons, probably, that I assigned when speaking of that substance. Here however the intermediate bed, that which contains the marble, is deficient, and we have already seen that the blue quartz and schist are also deficient, so that the order is here mica slate, red sandstone, schistose limestone; instead of mica slate, blue schist with quartz, red ​sandstone, marble limestone, schistous limestone, two important members having disappeared in so short a space. This is the only instance which has occurred to me in Sky of discontinuous strata, since the Western Islands exhibit but few examples of this class of rocks, and I am glad to have an opportunity of mentioning it, as I am convinced that such discontinuities are common in nature, and that great errors have been the result of the fondness with which geologists have pursued through viewless regions of the earth, continuities of strata, and universal formations. But to return to the upper limestone. It is often remarkable for the cavernous and corroded aspect which it assumes where in contact with the sea, which arises from the falling out of the shells which it contains. The beds are numerous but irregular, and the shells themselves vary much in quantity in different places. I could find only three species, a gryphite, an ammonite, and a cardium, nor do I know whether more are contained in them, since but few of the numerous beds are accessible.
It is now necessary to attempt the tracing of this bed in other parts of Sky, and I imagine that this can be done to a certain extent. This examination might have been rendered more complete by myself had I commenced the survey of the island in the reverse order to that in which I have described it, being guided for want of a better reason by mere geographical convenience. Hence the remarks on the distant and solitary fragments of strata, made when there was no prospect of connecting them, are less perfect than they would have been, had I commenced the survey where I have now commenced the description, and they are insufficient for assigning their general connection. Since however the strata of infest one found at Broadford contain the same animal remains, I have no doubt that they are portions of the same strata, and that ​the beds of this substance, which are visible along various points of this shore as far as Sligachan, are equally continuations of the shell limestone, more particularly as the marble limestone is also found at Sligachan in that position in which it might be expected with respect to the former. But I can give no clue to the history of the limestone which is entangled among the trap to the north of Broadford, having neglected to make such an examination of it as would have been required for this purpose. I must trust to some future geologist for the completion of what I have left unfinished. The most remarkable of these unascertained beds is found to the north of Portree, far inland, and extending towards the Storr on the eastern declivity of that range of hills, being interposed among enormous beds of trap. Others may be seen in various positions accompanying the secondary strata which lie under the trap on the shores of Trotternish; but as I can not pretend to give such a geological description of them as would satisfy myself, I must be contented with this cursory indication of their localities. I have little doubt that with the clue which I have now furnished, the task of assigning their geological relations will be found easy.
I find some difficulty in continuing the pursuit of the stratified rocks upwards from Loch Eishort, because a great geographical chasm now takes place between those already described, and the next most extensive mass which forms the promontory of Strathaird, and because they are connected but by a small indication of their order of succession. In the general description I have noticed that the rocks of Strathaird consisted chiefly of sandstone, alternating with thin and rare beds of limestone: these I conceive to be the next in order of superposition, and the proof tests on this. At the point which separates Loch Slapin from Loch Eishort a series of sandstone beds is found following the limestone lately ​described, and evidently in a higher situation: it is a calcareous sandstone, and bears a precise resemblance to that which constitutes the whole of the eastern side of Strathaird. Now as no sandstone of this description is found among the lower beds, and as we have clearly traced the order of those from the mica slate to this point, I think that we need not hesitate in placing the sandstone of Strathaird in the situation which I claim for it, the last in the series of the secondary strata. However that shall be determined, I must proceed to describe it. It is to be seen reaching from the termination of the trap on the western side of the promontory, round the point of Aird to nearly die upper end of Loch Slapin, preserving a great evenness of direction and consistency of character throughout this space. I have called it in general a sandstone, although it will be seen that it also contains beds of limestone, but in small number and quantity. The position of the leading line of these beds is so generally horizontal that it is never found to vary five degrees, and that variation is so gradual that it will often pass without notice: but it is attended with a disposition so remarkable, that I think it necessary to point it out, and as it is difficult to render it intelligible in words, I have added a diagram for illustration.
Each bed seems compounded of two parts, the one a single horizontal lamina, and the other a series of inclined ones, or, there is a regular alternation of a set of inclined laminæ with one horizontal one. These are perfectly defined, since the intervals are deeply channelled by the weather, the whole having the aspect of some of the carved ornaments of Saxon architecture. This appearance gives on a first view the impression as of a regular series of beds, alternating with each other, of which the one is horizontal, and ​the remainder in an unconformable position to it. But it must rather be considered as the indication of an internal structure, of which however all marks disappear when a fracture is made, a case analogous to what occurs in basalt and many other rocks, of which the internal structure is so often detected by the changes which they undergo on exposure to the weather. A similar case, which I have observed in the argillaceous schist of Isla, serves to confirm this explanation. The appearance in this instance is so precisely like, that the same drawing will serve to represent both, and the nature of the cause is amply confirmed in this, that while the beds of the schist are apparently divided in the direction marked by the horizontal lines, they are fissile only in that which is marked by the oblique ones. It hence also follows, that if the fissile property of clay slate is the result of some internal arrangement analogous to crystallization, we are equally entitled to attribute the structure of this sandstone to the same cause. The inclination of the oblique laminæ upon the horizontal ones varies from ten to thirty degrees; but it is regularly in the same direction, the dip, if it may be so called, being to the south. This appearance is neither rare nor dubious, it is extremely well marked, and predominates throughout the whole range. The measurement of the parts having been mislaid, I speak from recollection when I say that the intervals between the horizontal lines vary from one foot to a foot and a half There is no difference in the quality of the two sets of laminæ, both are of white sand. stone, generally more or less calcareous.
To enter more minutely into the composition of these beds, let me now remark that the sandstone often acquires the aspect of some of the most compact and crystalline varieties of quartz rock, while in other cases it has the lax texture of an ordinary freestone. In some places it is calcareous, and the calcareous matter varies so ​much in quantity that the compound would sometimes be called a calcareous sandstone, while at others it would be described as a siliceous limestone. In the latter cases it is often dark-brown, gray, or even of a dark lead blue. The beds of mere limestone are rare, and those which I observed lie towards its upper boundary; they are of considerable size immediately in the vicinity of the Spar cave, which I described at the commencement of this paper. I think they are much more generally granular than compact, and some of them indeed resemble an aggregate of rounded grains, of the size of mustard seeds, not much differing from some of the oolites, but more compacted, and generally containing, besides these grains, crystallized platy particles. These strata are intersected in a remarkable manner by trap veins; but I shall defer the consideration of those to that which I conceive their proper place, the last in the history of the rocks.
The geographical chasm which intervenes between this and the remaining portions of white sandstone found in Sky is such, the want of accompanying strata so general, and the absence of characteristic indications in the internal composition so great, that I feel quite unable to determine the nature and connections of these detached portions. If any thing can be drawn from such indications as they present, they seem rather to prove that the sandstones which are found at Portree and in the northern parts of the island, appertain to strata different from those last described; but the remarks I have already made on the discontinuity of the limestone and the blue quartz rock, render me diffident in admitting any evidence respecting continuity of stratification, or the reverse, without access to proofs of a more decided nature.
White sandstone is to be found in many places on the eastern coast of the island, and is readily visible at Portree, where it alternates ​with the trap in the same manner as it does in Egg; and in this neighbourhood it also exhibits the same appearance of globular concretions which occur in that little island. It also is found in several places, containing globular masses and fragments of trap, such as are frequent in the similar sandstones, which are seen on the opposite coast of Raasa. The same appearances continue along the shore to Camiskanevig; and although I did not approach the land sufficiently near to be certain of the fact, I am inclined to think that similar arrangements exist all the way to the Altavig islands. Sandstone is found moreover on the eastern side of Loch Snizort; and, I may add, generally on the western shore of Trotternish, particularly at Duntulm, where it contains shells, of which, for the reason mentioned in speaking of the limestone, I neglected to take a sufficient note, not being aware that they might be of future importance in my researches. Here however it alternates with shale; and, unless I am mistaken, with very thin laminæ of coal, such as occur in the sandstone of Egg, while it also contains imbedded fragments of charcoal, and I am therefore inclined to think on these indications, that this sandstone belongs to a deposit still superior to that of Strathaird, and probably the same as that which is found in Egg. The last white sandstone which I met with was in Glamich, where it was penetrated by veins of trap and mingled with the mass of the hill, but the space I saw exposed was so very small that no conclusions could be drawn from it.
I must not terminate the description of these strata without pointing out a circumstance of importance which might otherwise escape the reader's attention, namely, the conformity of all these rocks. I will omit that conformity which in some places appears to exist between the mica slate and quartz rock, and the incumbent ones, because it is certainly incomplete; but between the bed which consists of blue quartz rock and slate, and the red sandstone, it is ​complete, and consequently the same for the whole way upwards. Now in our system of geology, not only is this sandstone considered the first of the secondary strata, but the schistose rock is ranked among the primary or among the transition; and the difficulty will be equal which ever term we adopt. If the nature of the sandstone did not determine its rank, its place after the schistose rock would be sufficient for that purpose: so that here the primary and secondary strata are not only following in conformable order, but that order is demonstrably not accidental as in Mull, where an instance of this nature occurs, since there is a perfect interference and alternation of the two at the point of contact. Those rules therefore, which would define the secondary rocks by their want of conformity to the primary, either have not selected the first of these primary rocks as their basis, or, the law of non-conformity and of a thorough separation between the two classes is exceptionable.
Having thus as far as is in my power described, according to their superposition, the rocks which seem to follow in the most regular order, I shall proceed to describe those which are uppermost, the trap and syenite, which will complete the account of the rocks of this island. In naming these as uppermost, I am far from meaning to say that they are exclusively so, as I shall on the contrary show that they also penetrate the superior strata. No distinct position can in fact be assigned to rocks which are not stratified, and such are generally granite and trap. These unstratified substances interfere with almost all the regularly placed rocks. Although granite is not indeed found passing through or lying near the primary stratified rocks, unless in the form of veins, yet it bears no regular relation to them, and is occasionally found in the same place, in contact with every individual of a series of different stratified substances. The other unstratified rocks, trap and syenite, bear a still more intricate relation ​to those with which they are connected, since they interfere not only with the primary ones, but with almost the latest of the secondary, and are frequently found lying indifferently both above and below these latter. Hence have arisen distinctions which are often merely nominal, and founded on false theoretic views, of traps of one or other age, of first, second, third, and newest formations. The same identical mass will often possess the characters of all these, since it will in one place be found incumbent on the latest, in another on the most ancient rocks. Except this, I know not that any certain marks of distinction can be pointed out among the several traps. In the greater number of instances at least which have fallen under my examination, such a relation between an unstratified rock, occupying the irregular and uncertain position which trap does, and those regularly stratified ones which maintain a constant order of succession, is by no means a criterion by which to judge of its relative order with regard to these. The aid of a diagram is perhaps required to render this statement intelligible to those who have not examined the rocks for themselves. The same mass of trap will be found in one place incumbent on clay slate, in another on red sandstone, in a third on shelly limestone, in a fourth on the uppermost secondary strata. In such a case when the separate portions are either not all accessible, or when they lie far distant and interruptedly, we might be easily led to conclude that they were so many distinct deposits, and thus apply to them terms derived from the particular beds with which they were found immediately in contact. A more intimate acquaintance with them, and with the general nature of trap is required to correct these erroneous conclusions, and the accompanying sketch, will explain that which actually occurs in nature, and of which, if I mistake not, instances are ​to be found in the very island of which I am writing, as well as in many other situations in Scotland. I do not however mean to deny that instances exist where a real distinction of periods in trap rocks can be proved, and I have myself observed some which I shall perhaps have occasion to describe at a future opportunity. I am only desirous to enforce on geologists the necessity of drawing their distinctions from real and not from theoretical views, and of establishing criteria which are better founded, and which rest on more satisfactory evidence than that produced by the mere apparent or even real superposition of an unstratified above a stratified rock. I am even inclined to think that at least two very distinct formations of trap are to be found in the western islands of Scotland, but I cannot discuss the proofs in this place, as the island of Sky has not as yet produced sufficient evidence of them to satisfy me.
Amid the doubts which prevail in the minds of others, the prejudices derived from early habitude with erroneous systems, the natural obscurity of the subject itself, and the extremely inaccessible nature of these rocks, I find it difficult to preserve a consistency in the account which I shall give of them, and to render the details of the history of this family of rocks so distinct, as I am confident it will hereafter turn out to be when the subject becomes better known, and when nature shall have been examined by a greater succession of real observers, who, however desirous of supporting systems, are still more anxious for truth.
In commencing the account of these rocks the trap and the ​syenite, I must premise that I have not found in Sky any indication to denote the relative order of the two. If indeed they are both irregular substances, as I think there is no reason to doubt, any priority or posteriority is out of the question, or at least it cannot be ascertained by examining their juxtapositions. If the one set were proved to be constantly superior to one set of stratified rocks, and the other to a different and later set, the question of rank might be settled between them: but these connections are difficult to ascertain to a sufficient extent, and possibly none such exist. If one stratified rock is in one place superior to another, we are sure that it is every where superior; but if we have ever so clearly proved that a body of trap or of any other unstratified rock is superior and in contact in one place to any given stratum, we have no certainty that it is equally so every where. Thus I shall in this island show that the trap rests in one place on the latest sandstone, in another on the earliest. No means therefore are offered here of determining the relative order of these two unstratified rocks, but I have little doubt from the phenomena which I have witnessed in Rum and Mull, that they are both portions of one irregular mass. I shall therefore commence with the trap as the most extensive.
I was once inclined to make a distinction between the trap which forms so large a part of the Cuchullin hills, and that which is found ​occupying the great mass of the island, from the difference of their external characters and form, and from the existence of some particular varieties in the one, which are not found in the other: but I have not the means of drawing this distinction, nor of saying where or how they are connected or disjoined. I have little doubt that there are distinct deposits of trap rock of different periods, as well as that there are veins of the same; but whether these two leading divisions in the external appearance of Sky are also distinctions in the geological æra of the rocks which form them, is more than I can determine, since they have no distinct set of connections by which such a supposition could be verified. Further, when I attempt to trace their connection with each other, I imagine that I can every where see the mountain trap of the Cuchullin, blending with the stratified trap of the coast; and this opinion is confirmed by every thing which I have observed in Mull, where every possible variety of this rock from Gribon to Ben More, and thence to Loch Don on one hand, and Mornish on the other, appear to succeed each other without discontinuity or interruption, and with changes of character so gradual, that no line of interruption can any where be found. As however I consider this to be an object of the first importance in the history of this rock, I shall still suspend my judgment, pointing it out to other geologists as a subject highly worthy of investigation.
As I consider the term flœtz to be in this case improper, since it implies the hypothesis from which it is borrowed, I have distinguished that trap which is placed in a horizontal form, and of which the terraced edges are so very characteristic, by the name of stratified trap, using the term beds or strata indifferently, and without meaning by the use of either to insinuate any thing respecting ​the mode of its formation. The other I have called mountain trap.
The terraced trap forms by far the greater portion of the surface of Sky; but as it is sufficiently defined in the map and in the general description, I will forbear here to name its boundaries. Its northern and principal tract is every where continuous, but at the southern side of the island there are detached portions, which I shall first notice. The southernmost in position is a hill of no great extent, which is seen above the secondary strata already described at Swenish point, between Loch Slapin and Loch Eishort. This mass is connected with two large bodies like roots, (inasmuch as they have neither the parallelism nor the independence of trap veins) which cut through the whole mass of strata, and disappear below the sea. I consider this place as of great importance in the history of trap, as it shows plainly how a particular mass of that rock may appear fairly incumbent on a given stratum, while it is in fact connected with a much deeper set of rocks.
It is plain that partial views of this mass at any one point of all the various substances which it traverses, would assign to the same rock all the several hypothetical characters according to which trap has sometimes been divided. It would be called in one place primitive, in another transition, in a third flœtz, and so forth. As I do not intend to enter into the well-known questions respecting the origin of trap, I forbear to point out how this appearance bears on any of the hypotheses which regard its formation.
The next of the stratified masses of trap which are seen toward the south are the hills which decline from Blaven to Strathaird, and appear to be similarly incumbent on the stratified rocks of that promontory, but I had no success in my attempts to discover their ​junctions. These are the only detached masses, the remainder of the stratified trap forming one continuous mass, occupying the limits already referred to. It would be an endless toil to follow the varieties of this rock in the narrative as I did in the examination, as fatiguing to the reader as the writer, and equally useless either in a topographical or geological view. After describing such of its connections with the secondary strata as can be discovered, I shall content myself with enumerating a few of the most leading varieties, particularly such as add any illustration to the history of the rock, and with describing the localities of the most remarkable strata.
The contact of the stratified trap with the secondary rocks is seen very frequently on the eastern coast, as well as on the western shore of Loch Snizort, but it never occurs between Dunvegan head and Soa. In speaking of the secondary strata, I have already mentioned the most remarkable points where these contacts are visible, and need not therefore repeat them: they are most accessible in the neighbourhood of Duntulm, and from Portree northwards the alternations can be readily traced. I observed nothing so particular at the junctions in any place as to require a detailed description, except the contact of the basalt at Duntulm Castle, with the siliceous schistus, which I shall describe hereafter.
Among the numerous species of the trap family here existing, basalt is the most conspicuous, and it occurs almost every where, alternating in an irregular manner with all the other species or varieties. It is most frequently amorphous, displaying at the same time so great a variety both in its natural mode of breaking, in its external appearance on weathering, and in its texture and colour, as to form a great number of subordinate varieties much more remarkable in their natural situations than when broken into hand specimens. At Talisker it is found perfectly black, and of an exquisitely ​fine grain, as also in other situations both on the western and eastern shores. Considering the great extent of this rock it is but rarely columnar. The most beautiful and conspicuous collection of this nature is found at Little Brichel near Talisker, and ranges of tolerably defined pillars are also to be seen in many places of this neighbourhood occupying elevated situations in the cliffs. Columns of considerable regularity are also found at Floddigary near the north end of the island, and immense and continuous ranges of a kind imperfectly defined occur in the vicinity of Duntulm.
The next of the most conspicuous varieties of trap in Sky is amygdaloidal. I should perhaps in strictness rather have defined this rock by its base than by its structure or accidents, but mineralogical language has no means of distinguishing, not only the infinite variety, but the perpetual variation of the bases which are seen charged with these nodules of occasional minerals. It is sufficient to say, that in hardness the base varies from basalt to almost the softness of dry clay, and that the colours are black, bluish, brown, dark purple, and gray of different tints, sometimes of a very pale tone. As these varieties often occupy different strata, and are variously intermixed with the solid kinds already described, the strata, when viewed in the cliffs, often seem to possess a variety of composition which when examined into proves fallacious. The nodules imbedded in these amygdaloids are very various, few of all the substances usually met with in the trap rocks being wanting in some part or other of Sky. The zeolites are the most conspicuous, since all the varieties but one, the ichthyopthalmite, is found occupying their cavities, and often in forms so large as to require a separate consideration in the subsequent part of this paper.
Calcareous spar, chlorite, steatite, quartz, chert, chalcedony, and prehnite, occur in other varieties; among which the two latter are ​most rare, and on no occasion have I seen barytes, a substance not uncommon in the trap amygdaloids of other places. At Talisker, mica is to be observed in some varieties, a substance among the least common; and in the vicinity of Scavig, epidote is an ingredient, the rock resembling precisely the specimens found in Caer Caradoc. I have never met with olivin in these traps.
Various complexions of greenstone are found among the strata in different parts of the island; but they are far less common than the basaltic varieties. In one instance I observed specimens in which the crystallization of the hornblende was very perfect. The porphyries of this kind occur also in different places, but like the greenstones they are much inferior in quantity to the uniformly basaltic kinds. The felspar is sometimes glassy, sometimes opake, and the compound at times forms beautiful specimens. I did not observe among the stratified trap any example of clinkstone; and wacke is I believe totally unknown in this island, although some of the more earthy amygdaloids have been improperly designated by this term.
The substance known by the name of trap tuff; and which I have for reasons elsewhere assigned called trap conglomerate, is the last which I shall enumerate of the varieties which come under the general denomination of trap, and indeed no more can be enumerated, since the terms applied to the several members of this family are exhausted. This also occurs every where, and (as is I believe very common) every where irregularly intermixed with the other varieties. I saw no example however of that variety containing rounded nodules and foreign substances which occurs in Canna, the conglomerate in Sky always appearing to form a loose mass of angular fragments of gravel and sand easily mouldering to dust and soil.
It is now necessary to describe some substances which although not appertaining to this family are often found united with it, and ​some of them rarely, if ever, any where else. They were too small in quantity, and too little connected to admit of a place among the stratified rocks. These are iron-clay, coal, siliceous schist, and a particular sort of jasper. They occur separately or together in different places, but the whole are very conspicuous at Talisker. They are all extremely irregular in their positions, and discontinuous in their lateral extent. The iron clay is the most abundant of these, and forms considerable beds in the cliffs about Talisker, and along that coast as far as Loch Brittle. It is of various colours, red, purple, blue, and gray, and these are often very lively, giving to the cliffs the appearance of having undergone the process of calcination. The coal is rare, but occurs in different places, and its character in such situations is so well known that it is unnecessary to describe it. The siliceous schist is not abundant, but it is found in its most ordinary form, and also in that very remarkable concretionary globular shape, which having described at full length in speaking of the Shiant Islands I need not repeat here. The jasper is rare. I have used this term because I know of no other by which it can so well be characterized. It is yellow or brown, with a lustre approaching to resinous, and is well known as a product of the volcanic island St. Helena. The specimens of Sky differ in no respect from those of this island, which have sometimes, but improperly, been called pitch stones. That they are not such, if proof were necessary, would be sufficiently proved here by the regular gradation which they undergo into clay, appearing indeed to be portions of clay which have undergone changes in consequence of their vicinity to the basalt, resembling the well known ones which sandstones experience in similar situations. The succession of these several substances ​stances is often found in the same place, but their order can never be ascertained, for, the cliffs rising to a height of 500 feet or more, they are so far out of reach as to prevent us from forming an accurate judgment respecting the individual parts. It is only by examining the fallen specimens that we can ascertain the number of the varieties in any spot. They seem to me greater from Loch Bracadale to Loch Brittle than elsewhere, but possibly this opinion may only have arisen from the greater facility which I experienced in examining this line of the coast. I have in several places attempted to count the number of strata, and they seem to vary from eight or nine to twelve, fifteen, or even more, but it is not easy to define their boundaries at the distance from which they must be viewed. The way in which the several beds decompose often adds a very remarkable feature to the cliffs: some become scoriform, others moulder into large cavernous shapes, while a third set fall to powder; and these various appearances, combined with the colours of the iron clay, give to the whole that aspect of having undergone the action of fire, which strikes a common observer even more forcibly than a geological one.
I must now proceed to describe the trap which forms the mountains, and which is not entirely limited to the Cuchullin hills, since it is found constituting a great portion of Blaven, as well as parts of the hill of Glamich, and of others whose names cannot be ascertained, but of which the predominant parts are syenite and clinkstone. I have no means of defining their limits with greater accuracy, partly because there is no sufficient map, and partly because the country is nearly impassable in many places, and in others quite inaccessible.
I have attempted to trace at the foot of Garsven, the southernmost of the Cuchullin, the point where the stratified trap ceases and ​the mountain trap commences, because there only are they seen on the shore. Hitherto I have had no success, although there is a decided difference between the disposition of the two if distant points are assumed. I should have been convinced that such a distinction was to be found though I had failed in finding it, had I not been equally unsuccessful in determining the transition of the two in Mull. Yet I am still inclined to suspect the accuracy of my own observations, and I must leave it therefore as a point to be ascertained by those who shall think fit to follow me.
The junction of the mountain trap with the stratified rocks is very visible for a considerable space between Soa and Loch Scavig. It is here found in contact with the red sandstone, which I have fully described in its proper place. No appearance of the upper strata is here to be seen, the whole body of the mountain following upwards immediately after the sandstone. I know not that any thing worthy of notice exists at the junction. All the strata, both of schist and sandstone, are here as equal and straight as elsewhere, or if they are disturbed it is by the trap veins which are independent of the mountain. Nor are these rocks marked by any particular induration or affection of their composition. Yet I may remark that their angular elevation is less regular than at Loch Eishort, and that they are frequently inclined at much higher angles with the horizon than in this latter place.
It will be readily apprehended from the remarks which have been made in the geographical description of Sky, that the peculiarities in the outlines and appearance of the mountains arise from the mode in which the rocks that constitute them are disposed, and from the forms which these assume during the progress of disintegration. In describing the forms of these rocks I can only pretend to detail the ​characters of the southern part of the group, Garsven and Blaven, since I was unable to procure access to the other portions of it.
In other parts of Sky we have seen that the greenstones as well as the other varieties of trap, are disposed in a flat or apparently stratified manner by which the general aspect and outline of all these portions of the island are determined. Here on the contrary the external outline and general features are those of granite, and I may say that to its forms they appear also to add its permanence and durability. They are disposed in huge curved beds of which the external angles are, like those of granite in similar cases, slightly rounded, and they extend over considerable spaces, offering smooth sheets of rock unmarked by a single fissure or indication of past or future fractures. No eye could distinguish them from granite except by examining their composition. Their resemblance to this rock in disposition, and their dissimilarity to the greenstones of the stratified parts of the island, are in every way so decided that no position can be assumed, nor any view taken of them, however general, or on a scale however comprehensive, which can convey the slightest idea of a tendency to stratification. Nor do they in any case that I have seen shew the tendency to vertical fracture so common among green, stones, being in every respect, except that of mineral structure, entirely different from the ordinary varieties of this substance. Their granitic aspect is still further expressed in a most striking manner by the spiry forms of the summits, by their hard serrated outline and their overhanging masses, a disposition by which they are rendered inaccessible even to the stags and the wild goats that roam over this region of solitude and rocks. To this is owing their highly picturesque aspect, which bears a striking resemblance to that of the granite hills of Arran, or the more stupendous masses of the granitic ​Alps. It offers one instance among a thousand others of the little dependence to be placed on the characters of the outline in determining the nature of mountains, and shows how easily geologists, who have assumed the certainty of such a criterion and used it in their investigations, have been led to deceive themselves, and have contributed to the deception of their readers. But I must proceed to particulars.
Many varieties of rock are found in these mountains, of which some appertain to the trap family more strictly speaking, and others to the clinkstones, a set of rocks which, although they are intimately associated with these, possess also some other natural affinities, which may render it more convenient to consider them as members of another division. The phenomena to be observed in Sky are however insufficient to illustrate the views on which I am inclined to allot a place for these rocks in the system, for which reason I shall reserve these remarks till another occasion, when more numerous and more explicit facts will enable me to make the evidence proceed hand in hand with the theoretic arrangements to be established on them. I shall therefore content myself with describing the several rocks in a general way, since an attempt to investigate their connections more accurately, would involve too large and unjustifiable a portion of conjecture.
Greenstone appears to be the most prevalent of the rocks which form this group, and it varies very much in its character in different places. It is often of the most ordinary aspect, consisting of the usual admixtures of hornblende and felspar, and not at all distinguishable from those which appertain to the stratified parts of the island. This modification passes as usual into one in which the constituent parts, from their minuteness and intimacy of mixture, ​cannot be distinguished, and which ought therefore to be considered a basalt, since no other criterion can be established between the two, the variable proportions of hornblende and felspar alone admitting of no better or more defined limit than this. More generally in these mountains the greenstone assumes a large grain and very coarse texture, and in some cases the separate substances exhibit crystals of a quarter of an inch in dimension, while the hollows which are found in the rocks are sometimes occupied by detached crystals of hornblende. The felspar in these examples is often of a greenish hue. This rock appears remarkably permanent, showing few traces of waste or decomposition of the surface, and it is of this particular variety that the rugged summit of Garsven is composed. Among the finer grained varieties a remarkable kind is found on the borders of the romantic lake Coruisk, where it lies in detached blocks rolled down from the surrounding mountains. It is honey-combed into large cavities, which allow the hand to enter deeply within them, while at the same time the surfaces are almost as flesh as if recently broken, showing none of that rusty stain which attends the decomposition of greenstones in general. These blocks are extremely sonorous, and, notwithstanding their thickness, they ring when struck, with a sound as great as, and precisely similar to, that of a thin vessel of cast iron. In other places the same rock is found studded over with large detached protuberances resembling pedunculated fungi, or huge nails driven into it. The last variety which I shall mention is found in the same place, and it is the most remarkable, since it presents modification of trap hitherto undescribed. It forms a great portion of that naked and wild surface which I have already described in the general account of this spot, being disposed in enormous smooth inclined beds extending, without fissure or trace of decomposition ​or fragment and without symptom of vegetable life, from the borders of the lake to the summits of the mountains. Of its interior extent I have no means of judging, since it is inaccessible. This rock is composed fundamentally of felspar and hornblende, the parts being always distinct and crystallized in various sizes, so as to form varieties more or less coarse. The felspar predominates in the compound, and is either glassy or inclining to that variety; at times indeed quite opake. To these is superadded hypersthene, but it is neither so abundant as the hornblende, nor is it found so generally dispersed through the rocks. It is traversed by veins of basalt, and by veins containing hypersthene in a larger and more distinct form.
The other rocks which are found in the Cuchullin hills may all be comprehended under the general term of clinkstone. I find it impossible even to conjecture the relative spaces which are occupied by the rocks of this division and the greenstones, but I have already said that the greenstones appeared to me most prevalent. In the southern parts of the Cuchullin they undoubtedly are so, but I am inclined to think that in Blaven, and in those hills which contain the light coloured syenite mixed with darker coloured rocks and which are as I have already noticed so easily distinguished at a distance, the dark rocks will be chiefly found to consist of clinkstone. The varieties which occur are as numerous as they usually are in the different situations in the western islands where this rock is found, with the single exception of Arran, which presents an infinite and instructive variety of them in all their modifications and transitions. In general the clinkstone is simple in its composition, and of a dark lead blue, which sometimes assumes a brighter hue, and occasionally passes through various tints to a pale whitish gray or ash-icolour. In many places it is porphyritic, the porphyries ​putting on a great variety of aspects varying with the colour of the base, the quality of the felspar which forms the crystals, their magnitude, and the density of their aggregation. Blaven offers a remarkable variety, in which solitary crystals of glassy felspar nearly two inches in length are sparingly disseminated through the ground. The beautiful variety consisting of pure white crystals in a ground of dark blue, which is found in Raasa, also occurs in Glamich. But it would be fruitless to describe these varieties. In the same hill it is found of an amygdaloidal texture, but, as far as I have examined, the cavities contain only crystallizations of epidote similar to those occurring in the greenstone which I have already mentioned.
Such are the rocks which as far as my observations extended form that mass, which, for want of a better general term, I have designated by the name of mountain trap. It is however evident that the rugged aspect and permanency of these hills cannot in any degree be attributed to the clinkstone which enters into their composition. We have examples sufficient in Mull and in Arran to show that hills of clinkstone always form in the progress of their decomposition a feeble and smooth outline. The same indeed occurs in these very hills, since the outline of Glamich, and of the other mixed hills in which clinkstone forms the dark part, is equally tame and rounded. From this knowledge we are led still more strongly to conclude that the hard and serrated summits, and even the main body of the hills which present them, are composed of some of the varieties of greenstone before described, and hence to conjecture that this substance forms their principal ingredient, and that the clinkstone is only found on those outskirts where the passage or change into syenite takes place. We have I fear no means even of conjecturing the causes of the difference either in the disposition or ​in the durability of these mountain greenstones and those of the stratified trap, or of forming any probable conclusion respecting their æras of formation. Much remains to be learnt before we can attain to an accurate knowledge of these rocks, and there is much yet remaining for future observers in the ground which I have now trodden. But it is not an easy task to ascend these pinnacles and to traverse this rocky desart even in summer, and summer seems never to shine on them—at least it has never yet shone for me.
I must now proceed to consider the last rock which remains described, the Syenite.
In describing the Cuchullin, I have for the sake of contrast introduced so many of the most remarkable features of the other group, which I have distinguished by the name of the Red Hills, that the less will remain to be said respecting them. They are invariably characterized by the lumpish roundness of their outline. The cause of this consists evidently in their rapid decomposition, and in the accumulations of fragments which cover not only their sides but their summits so completely, as to preclude in most places any view of the naked rock. Their elevation I have formerly observed is much less than that of the Cuchullin, and I have also remarked that they form a separate assemblage, interfering at their bases only by indentation or approximation, and being always distinguished from the latter both in character and composition. The summits most convenient of access are those of Ben-na-Caillich near Broadford, with its dependencies, and the somewhat insulated mountain of Glamich above Sconser, the highest of the group. As Ben-na-Caillich presents the greatest simplicity of composition, I will describe it first. It forms part of a group which descends into the plain by various acute ridges, reaching into Strathmore and the head of Loch Slapin on the southern side, and extending towards Scalpa ​on the northern. It offers no remarkable variety of composition, but appears to consist of various modifications of that syenite about to be described, to which on a former occasion I proposed that the name should be limited, the syenite connected with the trap formation. Those who are acquainted with the syenite of Arran, of which the main constituent is felspar, will recognize in this a similar rock, of which examples may also be found in the neighbouring island of Raasa, as well as in many other of the Western islands. The hill of Glamich may be selected as specimen of those which are formed of syenite and of clinkstone, both of which are in this case strongly marked, since, as already mentioned, the former displays its reddish yellow hue, and the latter a dark blue tint. In other respects there is no difference between these rocks in the general disposition, which is in both equally irregular and ill defined. I could not discover the contact of these substances, but am convinced from other observations that they are connected by a common bond of transition or of position, and that they present no essential differences in geological relations. As the syenite of this hill presents no peculiar varieties, and as the clinkstone has been already described, one common description of the former will serve for the whole group. There is probably cause for me to regret that I could, procure no more extensive access to the hills of this group, but the uninhabited and desart state of this tract of country renders it nearly impossible, since the limits of the longest day are insufficient for their examination, and since the storms and rains of this wild island form but a dreary canopy to its rocky pillow. I can therefore add nothing more precise to the general ideal I have already given of the connection of the syenite with the trap and with the clinkstone. The light coloured rocks which belong so the group appear in all cases to consist of the several varieties ​of syenite which will be described hereafter, while the dark ones, with the exception of some greenstone, seem to be every where formed of clinkstone. The tendency of these syenitic substances to decomposition, explains the weathered appearance of the mountains which I have characterized by the name of the Red Hills, and the accumulation of the fragments which result from it, accounts for the roundness and tamemess of their outline. These causes very generally impede the view of the natural rock. Wherever it can be seen it appears to possess the disposition which is I believe invariably characteristic of this class of rocks, forming large irregular beds, much rifted and fissured, and never continuous in inclination and extent for any considerable space.
It might be deemed important to determine here the nature of the junction between the dark clinkstone and the light coloured syenite, but if it has not been discovered here, it is fortunately visible in other places, while at the same time the community of these two rocks, in almost every circumstance but colour, seems to bespeak a common position. Ben-y-chat in Mull, and many situations in Arran, present examples of this connection, nay in some cases of a perfect transition. The phenomena visible in Rum, in Sr. Kilda, and in others of the Western islands coincide with these, and offer a proof which may by a fair analogy be extended to Sky, that there is a community of geological position in these two rocks.
I have much more reason to regret the insuperable difficulties which I experienced in attempting to trace the connection between the syenite and the stratified rocks, the sandstones and limestones which were described in the commencement of this paper.
The only real contact of this substance with those strata is to be seen at the marble limestone in Strath, and I have traced it in the ​excavations made for quarrying that marble. I have always found the marble much indurated and very much coloured at the contact, being at the same time contaminated with silica and other earths; while the syenite itself in the same places appears rotten. Although in a general sense the contact of these two rocks may be said to be visible in various parts of this tract, some accident or other always occurs at the point of meeting, to prevent the real contact from being seen. If it is of small extent it is overgrown with soil and herbage; if it is of great dimensions, there is a chasm intervening which is filled with fragments and rubbish. Thus the contact at Kilbride, which would otherwise prove instructive is overwhelmed and invisible. I was equally unable to find any place on the eastern side where its junction with the upper of the secondary strata could be observed, though it unquestionably takes place near Broadford; the soil is every where a deep peat completely covering the rocky surface. It may probably be found on the shores between Broadford and Scalpa. I know not that there is much to regret in not having seen more of its connections with these strata, since, if, as I believe is undoubted, it has the same relation to them as the trap has, no instruction further than that which we already have can be derived from such knowledge. It will be found indifferently interfering with every one of the strata, and consequently no judgment of priority or posteriority to any can be formed respecting it.
The basis of the syenite is a substance which having been generally received as a felspar, I shall describe as such, although not convinced of a sufficient identity in the composition of these two substances. In its softest state it may be considered as a claystone, since it offers no differences in character, while in a state of somewhat greater induration it becomes a clinkstone, and when more ​hardened it is known by the name of compact felspar. As occur in this simple state they must be considered mineralogically as examples of these different substances, although in a geological sense we cannot without troublesome circumlocution and great confusion consider them under any but the general term already adopted. The colour of this base varies from ochrey yellow and dirty flesh colour to gray: it is often cavernous and filled with a ferruginous clay. In other situations it contains crystals of felspar, either of the same or of a different colour, and thus forms various kinds of porphyry. The predominant form however is that whence its name has been imposed, an aggregate of felspar and hornblende, in which the hornblende generally bears a very small proportion to the other ingredient: the porphyritic character is sometimes added to this mixture. In some rare instances quartz enters into its composition, and in such instances it trenches near upon the syenitic granite, a distinction concerning which I have spoken. in the paper on Glen Tilt, to be found in this volume. More rarely still contains mica, and in this case it becomes utterly impossible to distinguish it from those granites which contain crystals of hornblende superadded to the usual threefold mixture of quartz, felspar and mica. Under such circumstances it is quite conceivable that specimens should be met with from which the hornblende was absent, since even in those I have described, it is very thinly scattered through the mass. In such a case, should it occur, mineralogy, unassisted by geological observation, would tend to mislead as in reasoning respecting its position, and we are thus driven to acknowledge, in geological description, the necessity of superadding to mineral characters an accurate knowledge of the connections of a rock respecting which we are reasoning: I must therefore, from a geological knowledge of ​the position of these rocks, refer them to the syenite family, although had I met with the same specimens connected with a mountain of granite, and lying under mica slate, I should have referred them to the granites.
This is far from being the only case in nature where mere mineral distinctions are insufficient to determine the geological situation of a rock. In the stratified classes of rocks, both primary and secondary, these resemblances are frequent, since it is often impossible to distinguish quartz rock from sandstone, the breccias which it contains from the more recent graywacké, ancient clay slate from recent, or, as I have shown in this very account of Sky, primitive from secondary limestones. The same rocks seem in some cases to have been repeated at different epochas, while in others they show variations which may perhaps be the results of posterior changes operating on the first deposits rather than the consequences of original differences.
Two other varieties of this rock occurred to me which may be mentioned, although possessing no peculiar interest. In the one chlorite formed a constituent part, and in the other a greenish compact steatite was intermixed with the felspar and hornblende, the total compound being not much unlike the porcelain granite of Cornwall.
Before I conclude these remarks on the trap and syenitic rocks of Sky, it will not be superfluous to enumerate the striking external features in which rocks so nearly associated differ.
The mountain trap of the Cuchullin is most strongly distinguished from the stratified in the difference of its disposition, in the absence of the columnar forms and decomposing tendency, and in the barrenness of its surface compared with the deep soil and highly clothed vegetable surface of the latter. In its superior permanence, a permanence ​by which it appears to brave the external war of the elements which surround it, it is no less distinguished from the stratified trap than it is from the neighbouring Red mountains, whose rapidly decomposing summits covered with fragments have long lost their characteristic forms, hastening with daily and visible waste to the level of the plain below. It is equally distinguished by its general aspect, being commonly separated by marked features from the terraced trap with which it is associated, and no less so from the Red hills, with which, however associated in almost a common group, it forms no union. In mineral composition it also differs widely from these, hornblende forming its predominant and characteristic ingredient, as felspar does that of the latter. From the terraced trap it differs less in composition, since hornblende is the predominant ingredient of both, but it never like this contains amygdaloids and beds of tufo, neither does it entangle coal, sandstone or ironstone, or exhibit the beautiful zeolites so abundant in the terraced trap.
If there were any theory of the formation of the rocks of this class on which we could rely, it would not be useless to examine how far their distinctness of character and appearance was consonant to it, or how far they differed. Although my chief object has been to describe the geological features of the island, yet I may suggest that such an independence of character and form seems to imply at least that some independence in circumstances, as well as in time, has accompanied the formation of these several portions of a rock associated in so many other essential parts of their leading characters. But the time for discussing questions of this nature does not appear yet to have arrived, and conjectures may be left to the ingenuity of those to whom they afford a source of gratification.
With this rock I terminate the history of the principal rocks of Sky. ​
When speaking of the alluvial rocks, I did not mention granite, as I did not find any rolled stones of this substance. But as I observed it employed in the construction of a bridge not far from Sconser, I conclude that it is somewhere to be found and probably in this state, as I saw no reason to suspect its existence any where, as a fixed rock.
I have reserved to the last the consideration of the trap veins which are found in such abundance throughout the whole of this island, because, on account of their number and the interesting circumstances which attend them, they would have led to a perpetual interruption of that description which required to be unbroken. I have here, as on other occasions, applied to these veins the general term trap, for the same reasons which I assigned in speaking of the rocks of this class, namely, that they vary in composition, although basalt is certainly the prevalent substance in them here and every where else. The order which I have allotted for them in this description is also the order which they hold in nature, since they traverse every rock that lies in their way from the most ancient to the most recent, seldom suffering any change either of direction or composition in this varying course. As the same vein is therefore found to pass indiscriminately through rocks of all ages, it is plain that its association with these can afford no register of the period of its formation. If there were ten different periods in which these veins had been formed, we must be contented in most cases to prove but one, a period posterior to that of the latest stratified substance through which they pass. It is only where they interfere with each other that a register more extensive than this can be found. I have always assiduously sought for such examples wherever these veins abound, and, among other places, in Sky, but have never yet found more than two distinct sets. This number I have also observed in ​Rum. Two distinct sets are perfectly visible both near Loch Scavig and at Strathaird, and the examples are unquestionable, since those of one period hold their course through the other in every direction, with the same pertinacity and distinctness as the first do through the fundamental rocks. We have no means of knowing what distance of time has intervened between these veins. The angle of their courses with the horizon is various, but in a very considerable proportion it is vertical or nearly so.
They are of frequent occurrence in the mica slate of the district of Sleat, and, as far as I have seen them, they are here basaltic. They also abound in the sandstone at Loch Eishort, where they are of considerable size. From this bed they are readily traced through the superincumbent ones as far as the most distant surface of the limestone, and here I have always lost them. In several attempts for that purpose, I never could discover their continuation through the syenite, and am therefore inclined to think that they are prior to it. To ascertain this fact, I caused a portion of the marble bed which was traversed by two of these veins to be cleared away to its contact with the syenite, and found that the whole mass terminated together against it, leaving in my mind little doubt that the syenite was posterior to the veins. This fact has a double bearing: it might be argued that the stratified rocks reposed on the syenite, since the nature of the contact between stratified and unstratified rocks is always such as to admit of a double interpretation, unless where veins of the one are seen decidedly ramifying from its mass into the other. But if, as in this instance, the trap vein, which we know to hold in all cases an unchecked progress through opposing rocks, is cut off by the unstratified rock, the syenite, it is plain that the stratified rock which that vein traverses has also been broken by it, and that the syenite in this particular case is posterior to the limestone, and of course to all that body of rock which precedes it. ​It follows then that the syenite is posterior to the trap veins, and a further confirmation of this is found near Scavig, where a set of trap veins is also to be seen which traverses the red sandstone, but does not pass through the incumbent trap; therefore the syenite, and the mountain trap of the Cuchullin, are each posterior to one set of trap veins. This adds probability to my conjecture, that the mountain trap and the syenite are of one period; it cannot be said to prove it because we have no means of knowing whether or not these separate sets of veins belong to one period or to more. Community of structure proves nothing in this case; they are equally formed either of a coarse basaltic substance, which can scarcely in strictness be called basalt, or of porphyritic trap; and would be with still less propriety denominated greenstone, since the two substances, whose distinct union forms this latter rock, and whose imperceptible mixture is supposed to form the other, cannot be traced in it.
But I must not terminate the history of these veins which appear in such a profusion of intersections in the vicinity of Scavig, without remarking that however numerous they may be, and consequently however large the space which they occupy, they produce little or no disturbance in the regularity of the sandstone beds which they traverse, nor does that rock appear to undergo any alteration in their vicinity. This remark will appear the more necessary hereafter, when I shall describe a similar but infinitely more striking phenomenon at Strathaird.
There is a second set of veins however, which traverse not only the mountain trap but the veins first named, and which are clearly of a posterior date. These are much smaller, often indeed not exceeding half an inch in breadth, and are composed of an extremely fine and hard black basalt. They are less abundant than the first where even they exist, and they are not found in nearly so many ​situations. They are to be observed pretty frequently passing through the rock at Coruisk, and through the greenstone and clinkstone of Garsven, in the neighbourhood of which latter they abound. Their compactness and lustre are frequently so great that they approach in appearance to that pitchstone which forms the basis of the beautiful columnar porphyry of Egg. In addition to this feature they are strongly distinguished from the trap veins that traverse the sandstone by the intimate and almost inseparable union which they form with the body of the rocks which they traverse, whereas the latter are separated with the greatest ease.
There is still another set of veins found in this place, which however even and compact they may appear on a fresh fracture, and thus resembling basalt, appear to consist of clinkstone. This may be concluded from the facility with which they decompose at the surface into a greyish or whitish earthy looking substance, and from the depth of that decomposition. They are further distinguished by the frequency and minuteness of their ramifications, which are often drawn out to the size of a thread. Veins of this description have not as far as I know, been noticed, but they are not uncommon in the western islands, and they abound particularly in some parts of the Long isle; in this place they are chiefly seen on the borders of the small and picturesque lake Coruisk, and are found traversing the hypersthene rock, as well as the veins themselves, which consist of this mineral.
Having now described the trap veins which pass through the mountain trap, and those, perhaps more important ones as far as relates to that rock, which do not pass through it, I must turn the reader's attention to those which are found in the stratified trap. These are not very abundant, but may be seen distinctly among other places on the western shore, traversing the lofty cliffs in various ​but generally perpendicular directions. They are of a very large size, and are frequently stratified, a circumstance not uncommon in basaltic veins. They do not resemble the small ones of the second rank which I have described already.
If, as some have seemed inclined to do, we should adopt a notion that all trap veins were of the same period, the existence of these in the stratified trap, while they were absent from the mountain trap, would lead us to conclude that this latter was a posterior formation to the other. This is possible, and I have certainly no proof to the contrary. But from what I have shown respecting the certainty of two distinct sets of these veins, and the possibility of more, it is plain that they can prove nothing respecting either the difference or identity of these two rocks. If there are two sets of veins there may be three or more.
As no useful purpose is served by describing all the trap veins which abound in these countries, I shall pass over many which offer no interesting features in themselves, and are of no evidence in geological induction, and finish by describing those that are to be seen at Strathaird, which are well worthy of notice on account of their extraordinary number, and of some particular appearances by which they are distinguished. I have already slightly mentioned them in the general description of this coast: I must now be more particular, and, to give an idea of their general appearance, I have subjoined what must rather be considered as a plan than a drawing, since the formality of the subject admits of nothing else.
I pointed out their extraordinary numbers, and may now add that in consequence of their frequency they nearly equal in some places, when collectively measured, the stratified rock through which they pass. I have counted 6 or 8 in the space of 50 yards, of which the collective dimensions ​could not be less than 60 or 70 feet. This remark is not mere matter of curiosity, it leads to geological inferences not unworthy of regard, as will be presently seen. Their direction is almost invariably either vertical or slightly inclined from the plumb, and they present therefore a perpetual parallelism along the coast. They are equal throughout, and never ramify, and although they vary from 5 feet to 20 in breadth, they more commonly are of a dimension not exceeding 10 feet. It is well known that the permanence of trap veins is sometimes less, sometimes greater than that of the surrounding rocks. Hence they sometimes project like walls, while at others their ruin produces fissures or caves. This latter effect has taken place here, marking a great tendency to decomposition in these veins, since the including rocks do not seem to be of a very durable nature. The depth to which they have been excavated is often very considerable: in the case, of the spar cave including the external fissure, it cannot be less than 250 feet. In consequence of this wasting it happens that the intermediate cliffs which remain, have, as I have noticed in the general description, the appearance of the ends of walls; and as they also sometimes yield and fall away behind, in such cases they present the appearance of insulated square pillars of masonry, the resemblance being rendered perfect by the channelled marks of the strata formerly described. These veins are often stratified, or more properly speaking, laminated in the direction of their length. They are generally formed of a bluish black basalt; at times they are porphyritic, or vary in other ways, which it is unnecessary to describe. I observed in one nodules of prehnite, the only occasion on which I have found that mineral in veins of trap. In another I found a second vein, holding a serpentine course through the first in somewhat parallel direction, and readily distinguished by being formed of a much more black ​durable and compact basalt. I have given a sketch of it. (Pl. 2. fig. 3.) I may finish the description by saying that the continuations of these veins are to be seen between Swenish and Kilbride, at least, veins similar in appearance and equally numerous, but that there is no trace of them on the opposite shore of Sleat in the places which their prolongations would cut: they are not therefore extensive in their courses.
It has been so general an observation that the courses of trap veins are attended with disturbances of the accompanying strata, that it seems almost to have passed into a rule among geologists. The present instance is a most remarkable one to the contrary. Although they are here so numerous, not the slightest disturbance takes place in the evenness and the horizontality of the strata of sandstone which they intersect. There is neither contortion, bending, fracture, or displacement, nor do they appear to have affected the texture of the rock, since it is the same both at the place of contact and at a distance from it. Of exceptions to general rules it is rare to meet with any so pointed and so strong; and though there is no room for any long commentary on it, I cannot entirely quit this very remarkable place without pointing out one extraordinary effect which must have resulted from the intrusion of these veins. If the lateral dimension of the collective veins is assumed at one tenth of that of the stratified rock, (and I have reason to think this estimate not excessive) it is plain that the stratified rock of Strathaird must have undergone a lateral extension equal to that quantity; a motion so great that it is extremely difficult to reconcile it with the present apparent repose and regularity of the whole. ​
I shall now proceed to describe the mineral substances which I observed in Sky, having reserved these details for separate consideration, lest they should interrupt the connection of the geological remarks which form the preceding part of this paper. The most numerous, and not the least interesting of these minerals, are those of the zeolite family. They are to be seen in various parts of the island, but are to be found in the greatest beauty and variety in the cliffs of the western shore between Loch Bracadale and Loch Brittle. Talisker, as it is the most accessible of these places, so it presents the richest assortment to the collector of specimens. But in general the mineralogist can have no access to any specimens but those which fall from the cliffs, and have long been exposed to the violence of the sea and the injuries of the air. However splendid therefore, they my once have been, they are not always to be found in a state of good preservation. It is moreover often difficult to gain access to them on any terms, particularly along the other points of this wild shore, since it is so beset with rocks on which a dangerous surf is almost always breaking, that it requires neither common good weather, nor common dexterity in the management of a bear, to effect a landing and retreat without hazard.
Analcime is the most commons of all these minerals on the shore to which I have new alluded, and it is found in the greatest profusion at Talisker. It sometimes occupies cavities of considerable size, in different varieties of the trap, but seems to be by far the most abundant in those, earthy and little compact sorts, for which there is no name in our catalogue of terms. In other cases it forms flat druses of considerable extent, occupying the walls of fissures, while in a third, a single crystal is sometimes seen in a cavity just sufficient to contain it. In the greater number of instances the remaining part of such cavities is filled with the filamentous mesotype ​hereafter to be described, and the crystals thus seem to be imbedded in a mass of cotton.
The size of the crystals varies from that of a pin's head to the diameter of half an inch; but in general they present only one modification, the twenty-four sided crystal with trapezoidal faces, of greater or less regularity. The only other form which I found was the primitive, and of that I procured but two specimens, while a ship might be loaded with the trapezoidal variety. The crystals described are sometimes opake and white, at others they are mottled with a mixture of opake and transparent parts, while in a third, but less common case, they are transparent. In this latter case, when minute, they sometimes transmit the black colour of the subjacent basalt to which they adhere, so perfectly as to resemble a velvety surface of black crystals. In similar circumstances, transmitting the greyish or ochry colour of the substance to which they are attached, they appear to possess a colour which a more narrow inspection shows to be fallacious. A few specimens however occur of a flesh red, a colour frequently found in almost all the minerals of this family, and very predominant in the different zeolites which occur at Glen Farg in Perthshire. I also found a solitary specimen of a pale sea green colour, but did not observe that variety of a pale bluish grey, which, in company with the flesh coloured and yellow green, I have seen in the rocks at Larne in Ireland.
Chabasite is found in similar circumstances on the same shore, but it is comparatively of very rare occurrence: it abounds however in the rocks at the Storr, which for a considerable space consist of an amygdaloid containing it accompanied by stilbite. It is here so common, occupying cavities of greater or less magnitude, that a fourth or fifth part of the total bulk of the rock is sometime constituted by the chabasite. ​As far as I have observed, the chabasite which I have described is never, like the analcime, imbedded in the filamentous mesotype, but is not unfrequently associated in the same cavity both with stilbite and with analcime, nor is it unusual to find minute and well formed crystals of this latter substance, imbedded in the crystal of chabasite. In some cases perfect crystals of chabasite are lightly sprinkled over the surface of crystals of stilbite, adhering so slightly as to fall off on the slightest concussion: in other cases crystals of this mineral, as well as of the analcime, are confusedly mixed with rhomboids of carbonate of lime, hereafter to be described.
The primitive crystal of this mineral is by much the most common, its modifications being rare and offering but few varieties; it is very frequently twinned, the angles of the one crystal appearing on the faces of the other, nor is it uncommon to meet with it in triplets, or even in more complicated groups, displaying an irregular mixture of prominent angles. The most common modification consists in the truncation of one angle; sometimes two neighbouring angles are truncated, and occasionally this defect extends to three, the truncations being often so deep as to remove a third part of the rhomb. In other modifications a single angle and a single edge are removed, or the truncations extend to two angles and two edges; but I have not observed any specimens in which these defects were extended to a greater number of edges. The edge is in some cases replaced by two or by three planes, or even by a greater number, so as to appear nearly rounded; and each face of the rhomb is also frequently replaced by two planes meeting in an edge diagonally extended and sometimes rising by successive stages of planes parallel to the. original face.
These crystals are sometimes opaque, more frequently transparent, but in by far the greater number of instances they acquire a ​fallacious aspect of opacity in consequence of innumerable minute fractures by which they are pervaded throughout; they are most commonly white, but the flesh coloured variety is also found: their magnitude varies from the twentieth to that of three tenths of an inch in breadth.
Stilbite is perhaps the most abundant of these substances which Sky produces, and it appears to be the most generally diffused throughout the island; it occurs along the shore which I have described, but in less quantity than in the northern district. It is so common in some places in the northern of Kilmuir and Snizort that it is scarcely an exaggeration to say, that the roads are sometimes almost made of it. In some situations the decomposed trap falling into a powdery soil, leaves large accumulations of it resisting the action of weather long after the rock has mouldered away, while in other places it has been converted itself into a friable mass, which, as I already remarked, has been mistaken for marle and used as manure.
It presents scarcely any varieties of crystallization: the predominant, I might almost add the universal form, is that most common one consisting of very flat tetrahedral prisms, terminating in tetrahedral pyramids, of which the faces are placed on the edges of the prism. These are aggregated in distinct fasciculi, parallel or divergent, of which the groups sometimes affect the form of the constituent crystals. In the neighbourhood of Loch Eynort I observed some specimens of great beauty, consisting of large and distinct square prisms terminated at each extremity by truncated, tetrahedral pyramids arising from the edges, the crystals being transparent and nearly an inch in length, adhering slightly by its side to the quartz crystals of the chalcedonic nodule in which it was formed. ​
In the same place I found nodule of great size, and of a variety which is far from common, but which has I believe been found in the Faroe islands. The nodules in question are either hollow or solid, and sometimes reach the enormous dimension of four and even five feet. The hollow ones are crystallized within in the fasciculated forms already described. The peculiarity of this variety consists in its extreme frangibility: the least effort is sufficient to detach the plates of which its structure is formed, and it therefore falls to pieces in the very act of procuring it, unless great care is taken; its fracture is fresh, and distinguished by an uncommon degree of the pearly lustre which is so characteristic of this mineral. So great is its frangibility, that the jarring of the hammer at one end of a large nodule is often sufficient to destroy the whole; and it not unfrequently happens, that when a large piece is obtained entire and has been laid down, although it appears uninjured and resists a strong effort of the hands to break it, yet in a few minutes it falls to pieces with a sort of violence not unlike that which is known to happen in unannealed glass that has received an injury. This variety is sometimes white, and much resembling spermaceti in its translucency, but in the greater number of cases it is of a delicate flesh colour.
The next of these minerals is mesotype, and it is found in three states, a compact, a mealy, and a crystallized form. Of these the compact varieties sometimes recede so far in character from the mineral in its most acknowledged forms, that it is only by tracing the gradation of the several varieties, that we are enabled to determine the names of those which occupy the distant points of this range. The opake whiteness, the toughness, and the radiated disposition of those specimens which may be considered as forming the first remove, serves to connect them with the best characterized ​ones. By a series of gradations the radiated structure disappears, while the mineral acquires additional toughness, verging in its aspect first to chalcedony, and lastly towards chert; while in some cases it would be difficult to distinguish it, without trial of its hardness, from the white limestone of the north of Ireland: in this state it is not scratched by hard steel, while its toughness is such that a heavy hammer makes no more impression on it than it would on a similar mass of iron. The last transition is into a perfect chert, scarcely to be distinguished from those which in other situations occur in trap, and which are so frequently to be seen in those traps where nodules of calcareous spar and of chalcedony are found together.
If we were to reflect on the causes of this gradual change, we should attribute it to the successive diminution of the proportions which the other constituent earths of this mineral bear to the silica which it contains. I need not point out the difficulty of reconciling such a supposition to the general theory of mineral species and of definite proportions, since mineralogists are already aware of it, and since many other cases, attended by similar doubts, are well known. It is a question too important to be discussed without much more numerous and better established facts than those which we yet possess, and it will hereafter become an object of serious investigation to mineralogists, when their science shall have made further progress.
That variety which is called mealy is also here presented under different aspects, by which its nature is in some measure illustrated. This condition has, I believe, been generally attributed to the loss of its water of crystallization, the result of decomposition. It is obvious however that this is not the cause, since the specimens of this variety are found in the centre of solid nodules, of the glassy ​kind, where neither air nor water can have access, and where they are accompanied by crystals of absolute transparency: they are also found intermixed with and investing solid nodules of the toughest varieties, deeply imbedded in large masses where the elements are effectually excluded from them. This mealy variety appears in three different forms: in the first it is disposed in a radiated or rather in a ramose manner, in fine fibres possessing the peculiar lustre and softness of the finest white pulverulent talc. In a second case it forms distinct globular concretions of extreme minuteness, not to be discovered without the aid of the lens; and in a third instance, which I observed near Loch Eynort, a mass of globules of solid radiated mesotype, very much resembling some of the oolites, is intermixed throughout with farinaceous scales of the same substance, having the greasy aspect and lustre already described.
In speaking of this substance I have, according to common usage, ranked it with the mesotypes, it appearing to have been thus placed, partly because it is found associated with them, and partly because of the theoretical views which have been held respecting its origin. It will be for mineralogists of more authority to consider whether it does not deserve a separate place as a species: the question is evidently of a nature not to be determined by geometrical analysis, as far at least as the varieties already found extend; and the delicacy and uncertainty of unassisted chemical analysis in questions of this nature, are far too great to tempt us to seek a new place for it by this kind of investigation.
The last variety of mesotype exhibits a distinct crystallization; but crystals of tangible magnitude are so rare that I only procured one specimen in which the forms could be determined. They sometimes consist of a square prism, considerably elongated: in ​one case all the edges of the prism are replaced by planes, while in another, by the unequal truncation of three angles of a parallelopipedon, a pentagonal prism is produced. These prisms sometimes terminate in a pyramid, the faces of which correspond to the planes of the prism, and which is either complete or truncated: in other cases greater irregularities take place, but the extreme minuteness and transparency of the crystals frequently renders it impossible to ascertain their exact nature.
To compensate for the deficiency of large crystals of this substance, a profusion of that variety is to be found, which bears a general resemblance to amianthus, and it is popularly known in the country by the name of cotton stone. These filaments occupy the cavities of the trap, and are sometimes accompanied by analcime, as has already been remarked. They vary much in minuteness and delicacy, as well as in their state of aggregation, and hence many variations in their external aspect may be observed. At times they are placed in distinct straight needles, in other cases they are crowded into a dense mass, while in a third they are so entangled as to resemble a lock of cotton wool. Frequently they have the lustre of common silk, with its apparent dimensions, while they are in some instances so far attenuated as to resemble the silk of certain spiders. When the trap has fallen into powder, they are occasionally detached in light compacted balls, which are blown away by the winds and float on the surface of the water: in all these cases the microscope discovers their glassy transparency, but its powers are insufficient to determine their form, from the dazzling play of reflected and refracted light which they exhibit. In some rare instances this variety seems as if it passed into the mealy; in reality it becomes opaque and puts on to the naked eye a mealy aspect, which is however readily distinguished by the lens, from ​the specimens which I have described above. The last specimens I in point of structure which I shall notice, consist of radiated mesotype intermixed with crystals of hornblende, and producing a compound of an unusual appearance.
The specimens which I have now described are almost invariably colourless and transparent, or white, but occasionally they assume a brown tinge. One specimen occurred of a sea greed hue and of perfect transparency; but the flesh colour not uncommon in this mineral did not fall in my way, although found in the other members of this family which are seen here.
With respect to the exact locality of this substance, I have only found it at Talisker and at Dunvegan, although it is probable that it exists in many other parts of this very extensive island, which the labour of years would scarcely suffice to examine with the scrupulosity necessary for this purpose.
It was at Dunvegan that I observed a solitary crystal of ichthyophthalmite, nor did I succeed in discovering a second. It appears that laumonite has been also found in Sky: in the course of my researches I observed some minute specimens of it associated with stilbite, but scarcely worthy of notice, unless for this slender record of their habitat.
Prehnite, a mineral so nearly allied to the zeolites, is also found in Sky, but it is far from common, while the specimens are at the same time of trifling magnitude. It occurs in the trap at Portree, and at other points along the eastern shore, as well as at Strathaird, in the trap veins which traverse the sandstone, as I have already mentioned. On the shores opposite to the point of Clachan in Raasa it is found in a rock, which, although not very common, occurs in different parts of the western islands. This rock is a compound of augite, glassy felspar, and common felspar, the ​two latter having frequently a greenish hue. Besides the decided. nodules of prehnite contained in it, the same mineral is intermixed throughout the rock, forming an integral part of it, and often passing into mesotype, as it appears to do in other more decided instances. It has been said by Haüy that prehnite has not been found forming an integrand part of rocks; but as a compound of a similar nature occurs in the Kilpatrick hills near Glasgow, an exception must be made in favour of these instances. I may here add, that a corresponding rock may be seen on the opposite coast of Raasa.
It is perhaps superfluous to say that nodules of chalcedony, often hollow and containing crystallized quartz, are occasionally found in the trap rocks of Sky, since they are of such common occurrence in this substance. The cavities are sometimes, in addition, sprinkled with crystals of stilbite, of analcime, and of chabasite.
Of those mineral substances which are the least frequent in trap rocks, steatite occurs in considerable quantity; it is tender, and always of a greenish dirty hue. It is sometimes found in very small nodules; but in other places, as near Dunvegan and in the parish of Kilmuir, it is so abundant that it has been dug up with the intention of exporting it for economical purposes.
I have already mentioned that epidote is found both in the clinkstone and in the trap, but in too small quantities to render any further account of it necessary. It is thus far worthy of notice, as it is one of the few minerals which seems to appertain to rocks of very different characters and periods of formation.
The carbonate of lime to which I alluded when speaking of the chabasite, is found in company with this mineral and with the analcime, occupying along with them cavities in the trap: it presents but one form, that rhomb which is called the inverse, and ​it varies considerably in its dimensions: it is sometimes white, but most generally of a honey yellow colour, but I must add, that in either case it is of rare occurrence.
The last and the rarest mineral which I discovered in Sky is hypersthene. This occurs at Scavig, in that singular variety of trap which I have already described in the account I gave of the Cuchullin hills: it forms veins of different dimensions and much blended with the rock in which they lie, but they are neither numerous nor large, nor are the veins simple in their composition, since they resemble the containing rocks in the different substances of which they are composed. The most prevailing mixture is however that of hypersthene, and of a dark felspar precisely resembling that of Labrador in its general aspect, but not possessing its iridescence. This felspar is frequently crystallized, but as the crystals are always completely imbedded, nothing further of their form can be discovered than the outline which is displayed by the fracture: together with the dark felspar, white and glassy felspar also occurs in the mixture, and the common opake white variety is sometimes, but more rarely, intermixed with all the other substances. It has been already noticed that these veins are traversed by veins of clinkstone and of basalt, and they are also intersected by veins consisting of common white felspar and quartz, sometimes confusedly intermixed, and at others disposed in the graphic form. This opake felspar is sometimes distinctly crystallized in cavities. Rarely mica occurs in the compound, and in one specimen I observed transparent green crystals so minute and so imbedded that their nature cannot be ascertained: they have the aspect of olivin. Pyrites is occasionally seen, interspersed among these substances, but it is also rare.
The hypersthene presents specimens of great magnitude and ​beauty, which, although they seem to resist the injuries of time far longer than the accompanying substances, at length also become rotten, and fall into an ochry powder. Distinct concretions are to be found exhibiting the primitive form, and which appear to be true crystals, since they are detached from the surrounding substances. More generally however, it is without form, while in many cases it is intermixed with the dark felspar so as to present the graphic character, the crystals of felspar being defined, and the hypersthene occupying the interstices. The lustre of this mineral is always highly metallic, but the specific gravity of the specimens which I examined did not exceed 3.342. The colour is various; in general it is of a purplish black, sometimes steel grey, and more rarely of a pale whitish grey, while it often assumes the hue, together with the lustre of polished brass, when it has long been exposed to the air.
Hypersthene has been found in Aberdeenshire, but the circumstances which accompany it have not been described, nor the nature of its connections ascertained. As far as can be determined by this instance, it must be considered as an inmate of the trap family. Having also found it in the island of Rum associated with the same class of rocks, additional confirmation is afforded of this connexion. That of Labrador is known, like the present, to be accompanied by dark felspar; but the rock which is the common repository of both has not been described by the missionaries, to whom we are indebted for the only knowledge we possess of that country. Mr. Giesecké considers the Labrador felspar of Greenland as belonging to what he calls the “ Syenite formation,” and it is not improbable that his syenite formation resembles the rock which I have already described, and that there is a correspondence in the repositories of this substance in both countries. ​
Among the rocks, for which I could not find a place in the geological description without disturbing its order, pitchstone requires to be noticed. Although not found in situ it offers as a mineral specimen some appearances which are interesting, and which I shall therefore describe. It was on the hill of Glamich that I found the specimens in question, and it is probable that they had been detached from some veins which I was unable to trace. There are two varieties, a black one very little differing from that of Rum, except that it contains a few dispersed crystals of glassy felspar; and an olive green one, which as it offers some apparently important peculiarities hitherto unobserved, I shall describe more fully. It is often of a granular combined with a small conchoidal fracture, and is generally disposed in distinct concretions which are either of the flat or curved lamellar form. It is remarkable for containing irregular rounded cavities similar to those of the amygdaloids, filled with compact grains of a grayish hue. The structure of these is so singular as to be deserving of notice. On breaking the smaller ones, they are discovered to consist of a grayish white enamel similar to that which is formed by the fusion of felspar. But if we break the larger grains we can distinctly see that the center is composed of glassy felspar, the crystalline transparency and platy fracture of which are perfect, while the surface to a certain depth is converted into the white enamel I have described. I have not observed this very peculiar and striking appearance in any other pitchstone which has come under my notice, although there are appearances not much unlike it in some of the varieties found in Arran.
Those who conceive pitchstone, like basalt, to be of igneous origin, will have little difficulty in explaining this phenomenon, and will even find in it strong evidence to support that theory. It is unnecessary to enter on a reasoning so obvious. ​
I have concluded, perhaps without sufficient evidence, that the pitchstone both of Ben-na-Caillich and of Glamich, has been detached from veins. This deduction is made from the small quantity of fragments which are to be found, and from the circumstance that all the pitchstones of Scotland hitherto observed actually occur in veins.
A large portion of the summit of Glamich has the power of affecting the magnetic needle at even a considerable distance, a property extremely common in the rocks of this family. I was desirous of ascertaining if any regularity existed in the position of the magnetic poles, similar to that which Humboldt has observed in a rock of serpentine which he has described: for which purpose I observed the affections of the needle over many parts of the space as far as that was accessible, and I have represented them in the accompanying diagram, Pl. 3, fig. 1.
A consideration of those positions in the diagram will explain immediately that which would require much circumlocution to describe in words. In five places, extending from the eastern to the south—western side of the ground included in the circle, it will be seen that the needle remains uninfluenced. At the northern limit of the hill its position is equally true, but from the strength of its polar tendency, I conceive it was there affected by the coincidence of its position with the meridian of some neighbouring magnet, although by some oversight I neglected to take any other bearings near the same spot which might have verified this supposition. On the north east rhumb in five several places taken at the distance of five yards from each other, the positions are such as could not result from the influence of any one magnet, however ​that may be conceived to be placed. This discordance of position is still more remarkable in the several parts of a neighbouring line running on the E. by N. rhumb, and proves clearly that several magnetic bodies exert their influence in producing the disturbance which is there visible. Were it necessary to confirm this by any other observation, it would be fully proved by its deviation at a distant point situated near the N.W. part of the circle, where its position is such as to be evidently produced by some magnet unconnected with those which cause the great irregularities crowded together in the two places immediately before described. If I understand rightly the account which Humboldt has given of the affection of the needle by the hill of serpentine which he describes, it would appear that the whole rock consisted of a single magnet acting with great regularity on the needle. But in this case it is plain that its deviations from the true meridian are the consequences of the actions of several magnetic bodies dispersed over the summit of the hill, from the intricacy of whose combined influences it will evidently be impossible to determine the precise position or extent of any of them. It is probable that the meridians of all these and similar magnets occurring in the basaltic rocks, or in other rocks, are coincident with the magnetic meridian, and that they acquire this virtue as masses of iron are often known to do by long continued rest in favourable positions, combined probably with circumstances of which we are as yet doomed to remain in ignorance, little acquainted as we are with the history and causes of this obscure power. But I need not dwell longer on this particular example, since in a paper on Glen Tilt, which will be found in the present volume, I have entered at some length into the general question. ​
Among other matters I have reserved for this division of the present paper, the very little information which I was able to procure respecting the coal of Sky. Appearances of it are to be seen in several places in the trap, as I have already cursorily noticed, and among the rest a solitary mass of some thickness is found near Talisker, but in a part of the cliff nearly inaccessible. It is here as in most similar cases mixed with bituminous wood. Coal has also been found at Portree, and some fruitless and expensive attempts have been made to work it. Thin edges of seams of coal may also be observed in different places in the parish of Kilmuir, together with the carbonaceous rubbish which so generally indicates their presence. Under the direction of Lord Macdonald some borings and examinations have been made by coal surveyors, but, for reasons of too frequent occurrence among the itinerant professors of this branch of surveying, I found that no dependence could be placed on their reports, nor did my time permit me to institute such an examination as would have been required to ascertain the true state of things. The anxiety of the inhabitants and of the proprietors of the western islands in general for the discovery of this mineral is such that they are readily misled into the search, and coal mines have even been pointed out to me in micaceous schist. Nevertheless it is not unlikely that the district of Kilmuir may contain coal, since the sandstone and accompanying strata which I have already described are such as we should expect to find it conjoined with, and since in the island of Egg similar strata are actually accompanied by very minute laminæ of that mineral. But there is no great probability that this coal, even if it were proved to belong to these strata and not to the accidental fragments dispersed in the trap, like the one at Talisker, could be worked as a matter of ​œconomy, since the perpetual interference of the trap rocks and the disturbance occasioned by them, would render it too expensive an undertaking.
Among other substances I have reserved to this place the description of the siliceous schistus which is to be seen at Duntulm, on account of its partial nature and because its history would have interrupted the connection of the geological details. I have already mentioned that the trap found in this vicinity is obscurely columnar, forming high and picturesque ranges of cliffs surmounting the hills and extending to the sea shore. I have also noticed in general that beds of shelly limestone and of sandstone, containing shells and carbonized wood, alternating with shale, are seen under it. But the most interesting appearance is that of a disrupted portion of a thick stratum of the schist, known by the name of siliceous schist, and of that particularly hard and black variety which has been called Lydian stone. This rock forms a portion of a bed the base of which is covered by the fragments of the shore, but its visible thickness is about twelve or fifteen feet. It is surrounded on all sides by, and lies under, a mass of obscurely columnar trap, the junction being in many places attended with great confusion, (Pl. 2, fig. 3.) It is divided into thin laminæ, of which the upper ones alternate with similarly thin lamina of sandstone, precisely resembling those alternations of shale and sandstone which are so common and so well known. It is not indeed till fragments of the rock are examined in the hand that the spectator can discover that he sees any thing but a bed of shale alternating with sandstone: but on thus examining the schist, it is found to be an extremely brittle and hard substance, of a black colour, giving fire freely with steel, sharp in the fragments, and with an obscurely rhomboidal fracture; this last character being the only one by which it can be distinguished from ​the fine grained basalts, particularly such as I have mentioned as occurring in the form of veins in the Cuchullin. The sandstone laminæ possess at the same time the hardness and jaspideous aspect of that which I formerly described as lying in contact with the greenstone of Stirling castle. Considering therefore the analogy of these two sand-stones both in aspect and position, we may fairly conclude that they have in these instances been altered from their original texture, in consequence of the proximity of the trap rock. The alternation of the two substances here described, which have doubtless been originally shale and sandstone similar to that of the unchanged specimens which we meet with in various parts of this shore, gives us an equal right to conclude that the same influence of the neighbouring trap which converted the sandstone into hornstone, also converted the shale into Lydian stone. This in fact is the position of every specimen of siliceous schist or Lydian stone which I have seen in Scotland. In Cruachan, in Raasa, in Shiant, at Talisker, it forms beds, in contact with and involved in trap, which, from their connections and positions, appear to have been common clay slate, in those cases where it belongs to the primitive strata, and shale in those where it has formed a constituent of the secondary ones. It is possible also that the gray varieties of this substance may have originated from slate, and the black or Lydian stone from shale: additional facts however would be required to prove this part of the theory.
I have said that the resemblance between this Lydian stone and fine grained basalt is so perfect that there is no assignable difference of character save that of the large fracture, a circumstance of difference perhaps necessarily resulting from the unaltered stratification ​of the Lydian stone. Nor is there any reason to doubt this resemblance, since the same materials under a different form probably compose both rocks. Chemical analysis unfortunately offers us no temptation to try this analogy further, since the variable composition of basalts as well as of schist, a variation necessarily arising from the circumstances of their formation, would prevent the possibility of comparing any two specimens even of the same substance. It may be an interesting matter of speculation to inquire by what power the vicinity of trap operates in influencing the change from shale to Lydian stone, as well as in producing the much better known changes which occur in sandstone bordering on trap. If basalts have been in any case produced by the fusion of beds of slate, the necessary analogy between the Lydian stone and basalt will appear conspicuous, and we have only to consider it as a shale brought into the state of basalt by fusion, without such further disturbance as to destroy its original stratification.
Although both the neighbouring sandstone and alternating shale which have undergone no change from the vicinity of the trap contain shells in considerable abundance, I did not perceive any marks of them in this bed of Lydian stone. They may nevertheless exist, although they escaped my observation. I have little doubt that the instances of basalt containing shells which have so often been described, have sometimes been cases analogous to this: the observers, attending to the composition rather than to the disposition of the rock, having easily been led into error from the perfect similarity of the indurated shale to fine grained basalt. I may venture to point out these instances as calling for re-examination.
I shall terminate this account of the minerals found in Sky, by a more detailed description of the marble of Strath, adverting to those economical uses to which it seems applicable. ​
The following varieties are the most remarkable of those which are to be seen in this tract.
1. Pure white marble, the fracture intermediate between the granular and small platy.
2. The same with a scarcely discernible shade of gray.
3. The same with variously disposed veins of grey and black, resembling the common veined marble used in architectural ornaments.
4. The same with narrower veins well defined, and often reticulated with a great semblance of regularity: very ornamental.
5. The same, distinguished, independently of the veins, by a parallel and regular alternation of layers of pure white and grayish white.
6. White marble variously mottled and veined with gray, yellow, purple, and light green. This is also s very ornamental variety.
7. Marble, exhibiting various mixtures of white, pink, purple, light green, dark green, and black, of a rich sombre effect, and highly ornamental.
8. White marble, beautifully mottled and veined by yellow transparent serpentine.
The ornamental coloured marbles here described, scarcely yield in beauty to many of the similarly constituted specimens of ancient marbles, and like many of the marbles of Scotland they will be found to owe their colours to serpentine. This is also the case in Glen Tilt, at Balahulish, and in Iona. But the most obviously valuable variety is the white, which seems to possess most of the qualities requisite for the purposes of statuary.
Few substances in the catalogue of those with which economical mineralogy is concerned have excited more interest than statuary ​marble, from its rarity, its beauty, and its indispensable necessity in the art of sculpture. It has at different times formed an object of anxious research in this country, and premiums have been held out for it by the Society of Arts. It has consequently been found in various parts of Scotland, as well as in Ireland, but no native specimens have yet been introduced into the arts. As the causes which have impeded their introduction have hitherto been such as may be considered adventitious, being of a commercial nature and not founded on any experience of their physical defects, it has been hoped that they might by perseverance and time be removed, and that the statuary marbles of this country might at some future day supersede the necessity of importing this article. It will not therefore be a misplaced inquiry to examine the several properties of those marbles which have at different times held a place in the estimation of artists, and to compare them with our own specimens, more particularly with that of Sky now under review, the most abundant and certainly the most specious of all those which have yet been found in Britain. The inquiry is the more necessary, as the Several circumstances in which white marbles differ, do not appear to have been generally attended to, and as an undue value seems in some instances to have been fixed on our own in popular estimation, although not in that of sculptors themselves.
The value of this substance in those distant periods when the arts of Greece flourished, occasioned an industrious research after a material in which the sublime ideas of its artists could be embodied, Accordingly many quarries have been wrought in ancient times, of which little has descended to us but the names, and a few of the works which were executed from their produce. These marbles were of various qualities, and examples of them are still to be seen in ancient statues, although with regard to many of them, a species of ​evidence, often little better than conjectural, has guided sculptors and mineralogists in their attempts to determine the quarries from whence they were derived. Among these, the quarries of Paros afforded a marble (the often quoted lychnites of Pliny) in which it is asserted that the celebrated Venus was wrought, as well as some others to which we have not access. But there are many specimens of sculpture in the British Museum which seem to have been executed in this stone, or in one at least of analogous character.
Of the nature of the Parian marble we are enabled to speak positively, since some blocks of it have been quarried during the last few years, and are now to be found in the shops of the sculptors of this city. The grain of this marble is large and glistening, while at the same time its texture is loose and soft, and its colour of a yellowish and watery white. It possesses considerable translucency on the edges, a quality which, however desirable in statuary marble when of a fine grain, from the softness which it gives to the outline, only increases the disagreeable aspect of the Parian, by the angular reflections of light which take place on the pellucid edge and surface, from the innumerable faces of the small plates. The specimens of sculpture which I am about to quote, will exemplify this fault. It is certain indeed that the Greek sculptors abandoned the marble of Paros after the quarries of Luna and Carrara were discovered, the superior fineness and whiteness of these marbles which at present cause them to excel any with the places of which we are now acquainted, rendering them also at least equal to the best of those ancient ones of which the native places are now unknown.
Independently of the injurious effects which the large grain of the Parian marble produces on the transparent surface of sculptured works, and the false lights which it thus introduces into the contour, it interferes materially with the requisite correctness of drawing ​in the lesser works, and is thus inapplicable to the details of small sculptures in relief. It is nevertheless susceptible of a good polish, a quality however, of little value in the eyes of the statuary, and one which in this variety only serves to render the defects of its texture more apparent. It is also said to have been deficient in size, since it was so intersected by fissures as to be incapable of yielding blocks of more than five feet in length. I may add that in the present state of the public habits with regard to white marbles, there is no demand for modern works executed in Parian marble. Its celebrity is consigned to the metaphors of poets.
It will afford satisfaction to those who are interested in the arts to point out such works in the British Museum as appear to have been executed in Parian marble, or in one of similar character.
A Cupid bending his bow. This specimen is rather of a finer grain than the generality. It may perhaps belong to that marble called by the Italian sculptors marmo statuario, but this question cannot be determined without a fresh fracture.
A bust of Minerva.
Aratus, a bust. This also is of a fine grain like the Cupid.
A Venus, of a similar grain, and agreeing with the character of the marmo statuario.
Zeno, a bust, of a very coarse grain.
A terminal head of Bacchus.
A terminal head of Mercury.
A jupiter Serapis.
Bacchus and Ampelus.
Marcus Aurelius, a bust.
There are others, but it is not requisite to enumerate them.
A marble of a much finer grain, and capable of a high polish, is described by the ancients, as found near the river Coralus in ​Phrygia, as well as in some of the Greek islands: it is supposed to be the variety known to statuaries by the name of marmo Greco, and some ancient statues are described as being formed of this marble. It is possible that specimens of it may exist in the British Museum, but our sculptors are, as far as I know, incapable of distinguishing it at present, and it is much too hazardous to assign the place of a particular specimen from the contemplation of a polished and often of a stained surface. Mr. Tennant has found that the marmo Greco is a magnesian limestone.
I am equally unable to point out specimens of that variety known to the Italians by the name of marmo statuario, of which the quarries are also lost, but which, with greater translucency of surface, resembles the Parian marble in the largeness of its grain, unless those which I have conjectured to belong to this variety, when describing the specimens of sculpture in Parian, do in fact appertain to the latter.
The quarries of Luna produce a compact white marble susceptible of a high polish, and capable of being wrought with the most minute accuracy. Hence it is preferable for the finer operations of bas relief, either to the Parian, of which the aspect interferes with the delicacy of finish and of surface required in these works, or to the Pentelic, which was subject to accidents from veins of mica and of serpentine, or to that of Carrara, in which dark veins are of frequent occurrence. It was accordingly preferred by the antients, and among many other works, the Apollo (Belvedere) is said to have been executed in Luna marble. We have no other knowledge of the marbles of Hymettus and of Arabia than their names.
Of all the marbles employed in the works of the antients, and of which many specimens have descended to our days, that of ​Carrara is almost the only one which is at present held in estimation, or is now accessible to modern sculptors. This marble is of a very fine grain and compact texture; it is also susceptible of a high polish when required, and is consequently applicable to every species of sculpture, except when, as is too often the case, dark veins intrude and spoil the beauty of the work. Notwithstanding the general apparent uniformity of its texture, it offers different varieties of aspect. It is always of a fine granular fracture, yet this fracture is sometimes combined with a slight tendency to the flat splintery, in which case the stone is harder and more translucent than when it is purely granular. When merely granular it is sometimes dry and crumbly, precisely as if it had been exposed to a high heat; it then loses much of its transparency, and is called woolly by sculptors. Its transparency is various, and in some cases nearly equal to that of alabaster, (granular gypsum.)
The bust of Marcellus in the Museum offers an example of a very fine grained and extremely translucent marble, apparently of this kind. The specimen employed in the bust of Messalina is equally remarkable for the fineness and beauty of its texture. In at bust of a youthful Hercules in the same collection the identity of the marble is marked by the dark veins which are to be seen in it; but it is unnecessary to quote individual specimens, as the greatest number of the sculptures in this collection appear to have been executed in Carrara marble.
The last of the antient marbles which I shall describe is that of Pentelicus, of which the quarries are probably still to be found in the vicinity of Athens, although they have not been investigated by modern travellers. But we are in possession of numerous specimens of sculpture in this stone, from which we are able to ​determine its qualities; two are to be seen in the British museum. Of these there is the bust of a Philosopher, of, apparently, antient and very dry workmanship: the other is the celebrated Discobulus. It is known that Myron the Athenian, who flourished about 440 A.C. executed a work of this character in bronze: but we have no evidence respecting the marble statue, and artists have therefore remained in doubt whether it was executed by himself or was a copy by another hand. This question cannot be positively decided by the sculpture itself, however high its merits. In the mean time a step is gained by the mineralogical investigation of the material, and thus mineralogy is capable of throwing light on the history of the arts. The substance in which it is wrought must therefore be considered a sufficient proof of the antiquity of the copy, if it be such, as well as of its having been executed at Athens, since the quarries of Pentelicus were abandoned in consequence of their defects, as soon as those of Carrara and Luna were known. Although it is difficult or impossible to determine this period, yet as so few works in Pentelic marble posterior to the time of Phidias and of Myron have descended to us, it is probable that little use was made of those quarries after the period of these artists. We are therefore, perhaps, entitled to conclude that the Discobulus of the Townley collection is an Athenian work of the best age of sculpture, and not a copy by any more modern artist; and that if it was executed neither by Myron himself nor under his direction, it is yet not likely to have been much inferior to the original, while it may serve, at the same time, as some proof of the esteem in which that work was held at Athens.
But the most numerous examples of Pentelic marble are to be found in those works of Phidias which form the collection of Lord Elgin, and which afford easy access to examination. In the ​present corroded and tarnished state of the surfaces of these statues we cannot trace the nature or the defects of this variety, but an examination of its texture and composition in the broken fragments, serves to excite the deepest regret, that the genius of the greatest sculptor whom the world has seen, should for want of better materials have been condemned to bestow its energies on so perishable and so defective a stone. This marble is of a loose texture and moderate sized grain, coarser than that of Carrara, but finer than that of Paros; in colour it is exceedingly imperfect, being tinged with gray, brown, and yellow, and mottled with transparent parts, which give it the appearance of having been stained with oil. But its most formidable defect is its laminated structure, and the quantity of mica with which it is contaminated: to this we are to attribute the corrosion and almost entire ruin of so many of the specimens, the action of the weather dissolving those parts of the stone where the mica is most abundant, and eating deep fissures through many parts of the work. It is peculiarly unfortunate that the two most admirable specimens, specimens which are calculated to excite in the minds of artists a mixed feeling of wonder and despair, the horse's head and the Theseus, should be those which have suffered most. Had they been fortunately executed in the more uniform and durable stone of Carrara, these works might still have been preserved to us in all, their original perfection of drawing and surface. Even the hammer of the Turk would have rebounded with little injury from the marbles of this texture, while the micaceous stone of Pentelicus splitting in the direction of its laminæ, has permitted the complete mutilation of many valuable sculptures.
We have no geological information with regard to the relations of these stones. The great resemblance of the Pentelic marble to ​that of Glen Tilt in aspect and composition, renders it probable that like this it lies in mica slate, forming beds parallel to and interstratified with that rock: that the others have similar relations to the primary rocks, we should have concluded on general geological principles, had we not already seen that the white marble of Sky which has given rise to this discussion belongs to the secondary strata.
We have now to examine the white marbles which have been discovered in our own islands, for the purpose of comparing their relative properties and the value which they are likely to possess in sculpture. I am unfortunately unable to give any account of those found in Ireland, neither having seen their places, nor being possessed of any specimens.
That which has been found at Cape Wrath in Scotland, is of a grain much larger than even the Parian, and is consequently useless for the purpose of sculpture; and this indeed is by much the most common character of the Scottish specimens. Those of Blairgowrie, of Glenavon, and of Balahulish, are all equally characterized by this large sparry texture, and are all equally unlit for sculpture, however applicable to the purposes of architecture. The marble of Iona has been long since exhausted, and consequently requires no particular notice: however valuable from the purity of its colour and compactness of its texture, yet the uncertainty of its splintery fracture before the chisel, (that tool without which no spirited work was ever finished,) combined with its great hardness would probably have rendered it useless in the arts, even if it were still to be procured.
In a paper on Assynt I have already described the white marble of that district: it is of a very close texture, and although it contains ​no earth but lime, is of unusual specific gravity and hardness. It is incapable of being polished, a circumstance, it is true, of no consequence in statuary, since the polish only gives a false light to the surface and is not admitted of in modern sculpture; but it labours under the concomitant disadvantage of want of transparency, producing nearly the same dead effect and dry outline as is seen in a plaster cast, a fault in itself sufficient to prevent it from ever being adopted as a good material in the arts: its extreme hardness also renders it very expensive to work.
The marble of Sky, the more immediate object of this discussion, is of a pure white colour, and appears sufficiently extensive and continuous to be capable of yielding large blocks. The purity of its colour is seldom contaminated; its fracture is granular and splintery, and its texture fine, less fine than that of Iona, but more so than that of Assynt: its compactness, hardness, and gravity, are greater than those of the marble of Carrara, which it in fact resembles in little else than colour. It is apparently well fitted for all purposes of sculpture, as it can be wrought in any direction, and has sufficient transparency, while at the same time it assumes even a better polish than is required for statuary. With these good qualities, however, is combined an uncertainty arising from its unequal hardness. While some parts of the stone are nearly as easy to work as that of Carrara, many other specimens turn out so hard as to add a charge of near 50 per cent. to the cost of working: this appears to arise from the influence of the syenitic and trap veins which traverse it, as I have before mentioned, but which however produce no change in its chemical composition, nor any other effect than that of induration. This addition of price to the current charge of working is sufficient in the harder specimens to counterbalance in a great degree the superior cheapness of the material, ​and the advantages derived from lower freight duty and insurance. Such are the difficulties which oppose the introduction of the most perfect marble which has yet been found in Britain, difficulties which, slight as they are, ought, together with the prevalence of established habits and of a commercial routine, to check the extravagant hopes which have been entertained in this country of superseding by its own produce the importation of foreign statuary marble. But it will not be rendering justice to the marble of Sky if I do not add, that it possesses a property not found in that of Carrara, and one of considerable importance, at least in small sculptures. This is, that compactness of texture by which it resists the bruise which so often takes place in marble at the point where the chisel stops, an effect known to sculptors by the technical term stunning, and of which the result is a disagreeable opake white mark, generally in the very place where the deepest shadow is wanted.
I have little to add respecting the marble of Glen Tilt, as I have spoken of it in another place. Except the somewhat larger size of its grain, it is scarcely to be distinguished from the Pentelic: in colour it is precisely similar; but as the character and defects of the Pentelic, which I have already given, are equally applicable to this variety, we may fairly abandon all hope of rendering it useful in the art of sculpture.
* ↑ Coruisk, the water of the mountain hollow, not Coriskin.
* ↑ Geological Transactions, vol. 2d.
* ↑ Pl. 4. fig. 1.
* ↑ See Pl. 4. figs. 2 & 3.
* ↑ I think it necessary to say that I have throughout user the term Trap as the name of a a family, including basalt, greenstone, tuf, amygdaloid, trap porphyry, and many other varieties of rock which have as yet obtained no names, and which constitute a class equally distinguished by their geological as by their mineral characters. I have preferred it because as it is derived from the external outline so common and characteristic of this class of rocks, it is in no danger of misleading by producing any confusion of individuals, and because it was already in use as the name of many in this family without having been rigidly limited to any one species. I have also chosen the term syenite as the generic term of a set of rocks generally allied to these, and which had already been applied to that rock by Werner, excluding from this denomination the original and classical syenite, which, as well in geological connection as in mineral character is a mere modification of granite. The compound term syenitic granite may be applied to this, as I have remarked in a former paper. Varieties intermediate between common trap and syenite may be called syenitic trap.
* ↑ I have since received similar specimens from Guadeloupe, where they occur among the lavas of that island, adding one more to the numerous analogies already existing between rhe volcanic rocks and the trap family.
* ↑ See Pl. 4. fig. 3.
* ↑ Geol. Trans. vol. 2.
* ↑ Vol. 2.