[from Descriptions of the Western Isles, 1819]
A VERY slight topographical sketch of the rocks which form the Isle of Man, will be sufficient for the purpose of preceding the geological details ; and, as they are disposed in a very simple manner, these details will hereafter be attended with little geographical confusion.
The inspection of the coloured map will assist the reader in comprehending their situations and general boundaries.
Granite occurs in one situation in the immediate vicinity of the Doon river, at a very short distance from the road side, between Laxey and Ramsey. The quantity exposed does not extend to many yards, and having been subjected to the action of water, it is every where decomposed. It was also observed in Foxdale mine, when that was formerly wrought, but there is now no opportunity of ascertaining its existence.
It will be readily perceived on inspection of the map, that the principal part of the island is composed of schist, which occupies at least five-sixths of the surface. The boundary of this rock, on the northern side, is the alluvial tract already described. On every other, with the two exceptions which will presently be stated, it terminates in the sea. This mass of schist presents several varieties, and is in many instances applied to economical purposes. These I shall here state ; referring to a distinct place, the account of its geological relations and mineralogical characters.
It is every where obvious that the more tender varieties are in a state of rapid decomposition, and that they are converted, in the last stage of this process, into the clay which forms the leading division of the agricultural soil of the island. These clays differ in aspect and composition ; as do the rocks whence they are derived; their Colour varying in proportion to the quantity of iron contained in the original rock, and according to the degree of change that metal has undergone during the progress of decomposition : their ingredients also differ more or less, as quartz, mica, or calcareous earth, have taken a part in their formation. Hence the several soils resulting from them vary materially ; more however to the experience of the agriculturist than to the eye of the mineralogist. Fissile slate is not very abundant among these tocks, but it appears in different places. It has been wrought at Balla gawn, in the neighbourhood of Peel, and possibly in other situations which did not fall under my inspection. In many places, and among others at Laxey, the fissile varieties are thick, and excellently adapted for the purpose of flags ; containing much quartz, and bearing a considerable resemblance to those which are found in Isla at the places where the quartz rock and the clay slate approach each other. The size of such flag slate, when in its native place, is sometimes enormous ; large plates of twenty or thirty feet in dimensions, or even more, being visible in many parts of the island. No attempt is however made to raise these entire ; there being no demand for specimens so large. They are generally broken in the act of quarrying, and applied to the purposes of building stone ; the flatness of their surfaces, and the angles of their joints, adapting them particularly to dry stone work, as well as to the ordinary objects of masonry.
A more rare variety of schist, applicable, like the former, to economical purposes, occurs at Spanish head and in its vicinity, where quarries have been opened. It is of a fibrous texture, and is capable of being raised in long beams, extending from twelve, even to eighteen feet in length. These are used for lintels and similar objects, and are admirably adapted for many other architectural purposes, too obvious to require mention. As yet they have not become the subjects of export ; the duties on the internal commerce in stone, producing here, as in so many other instances, the. effect of a prohibition. They possess a property not often observed in rocks, namely, that of flexibility ; returning to their places, like other elastic substances, when the, bending force has been removed. In some trials, made for this purpose, it was found that a beam of fifteen feet in length was forced five inches out of the straight line before it broke ; its thickness being two inches.
Among the other economical varieties of schist, a kind of hone slate occurs at a place called Montpellier. It is of a whitish colour and soft texture, better adapted for the polishing of metallic plates than the uses of the cutler. It has not been exported.
The last variety of this rock which it seems necessary to notice at present, is drawing slate. This is to be seen near Peel, at the point where the junction of the red sandstone with the schist is visible. The specimens which I procured were not of a very good quality, but as the bed is of considerable thickness and varies much in its texture in different parts, it is possible that, with a patient research, better might be found. This substance forms also a good substitute for other colours as a paint in oil ; but,: being unknown to the inhabitants, it has not been applied to any purposes.
The next most extensive and important rock after the schist, is the limestone. The boundary of the principal mass cannot every where be defined with absolute precision, since in the interior parts it is covered with a deep soil. This however is easily done on the shore, and it is indicated in the accompanying map ; while, by tracing its termination in the schist near Ballasalla, its limit at that place is also determined. The. intermediate line on each side of that point, towards the Santon river on one hand and Pool vash on the other, is laid down as well as the map would allow, from a consideration of the form of the land ; since there is a sensible difference between the undulation of the surface above the limestone, and that above the schist. A small detached piece also occurs on the point near Port la Marie, which is there, like the former, capableof being accurately defined, since its junction with the schist is visible on both sides.
There are two very different kinds of limestone in these places, and there are also several varieties of each of them, to which more or less importance in an economical view is attached. The two leading distinctions are, a stratified limestone containing shells, and an unstratifled and irregular one rarely containing any, and commonly of a crystalline texture. These are mixed in an irregular manner over the whole tract ; a mass of the unstratified rock occurring unexpectedly in the midst of the stratified, without any certain order or arrangement. In general, all the varieties of the unstratified rock are rejected by agriculturists. In some cases it seems to have been ascertained that they are ill adapted for land ; in others it has been so imagined without sufficient trial. There is another objection to the use of the unstratified limestone, arising from the greater difficulty of quarrying it ; while it is also less easy to burn. Differences in mineral composition will explain the causes of these variations. I may here remark, that, in many instances, these varieties present an ornamental disposition of colour, some of the rocks also, consisting of breccias of considerable beauty ; but their irregularity, their hardness, and the numerous fissures by which they are intersected, prevent any temptation to work them as marbles.
The chief variations in the stratified limestone are those in the colour, in the quantity of organic remains which it contains, in the greater or less admixture of argillaceous and siliceous earths, and in the quantity of shale which occurs interlaminated with the beds. In consequence of these variations, the different beds are more or less adapted to the several purposes, whether of mortar, or of agriculture, and different quarries are accordingly opined in various places, to which, according to the purity of the produce, more or less value is attached. Those of Ballasalla are by far the most extensive ; but it is unnecessary to enumerate all the quarries which have actually been wrought.
At Pool vash, and within extreme high-water mark, is found a series of beds, of a very dark colour approaching to black, which have long been wrought as marble.. They possess a sort of historical celebrity from having furnished the steps which ascend to the entrance of St. Pauls Cathedral ; a present, as is well known, from the bishop of this diocese. They are now wrought for grave-stones only, since they are, from their argillaceous nature, incapable of receiving a polish.
The next and last rock is a small tract of red sandstone, which, on account of its insignificant geographic extent, is here described last, although, in a geological view, it must rank before the limestone. It commences about half a mile to the north of Peel, at the end of a small sandy bay, and is continued along the shore for the space of about two miles, where it terminates, being immediately succeeded by the schist already described. Its interior extent seems never to exceed half a. mile, but the boundary must remain conjectural, since it scarcely appears at the surface and has not been there quarried. This judgment respecting its boundary is only founded on the external form of the land. It contains one or two small patches, or portions of beds, of white sandstone, but of no value : the red, being compact, easily quarried, and raised in large blocks, has been used for building.
Such is a brief and popular view of the rocks which are found in the Isle of Man.
In proceeding to those scientific details more peculiarly interesting to geologists, I shall, as on other occasions, describe the rocks according to their superposition; that order which, whenever it can be followed, is by far the most instructive and intelligible.1
The granite already mentioned is situated two or three hundred feet above the sea, on a steep declivity, and has been laid bare in one place by a small farm road, in another by the mountain stream which has worn a deep channel in it. Unfortunately, the place where it appears is surrounded by various small streams, and lies under a rapid slope of the mountains ; from which causes perhaps, among others, it is every where so overwhelmed by alluvial matter as to be completely insulated among a mass of rubbish. This accumulation is so extensive and at the same time so deep, since it in many places exceeds twenty or thirty feet, that the schist is no where visible in the vicinity of the granite. The streams, which have. caused such deep channels, have not yet succeeded in. reaching the solid rock, except at the point just described; the masses of granite, which occur at the small bridge over the Doon river, being transported blocks ; not the fixed rock, as they appear to be on a superficial view. No information can therefore be obtained respecting the junction of the granite and the schist, nor can it be determined whether the former substance is part of a fundamental mass or an exposed portion of a vein. On this subject it can only be remarked, that the structure of the granite is even, and small grained ; and that it resembles in general aspect those which are known to form fundamental rocks ; being totally unlike that which, in Scotland at least, is found in independent veins, and which is there always characterized by a peculiarity of structure. In describing the alluvial rocks, it was already hinted that the granite might possibly extend to a much greater height in the mountain ; from the great number of rolled pieces and angular fragments which are found in a position far superior, and which. are identical in structure with it, while they at the same time differ materially from those demonstrably transported blocks found in different parts of the island. Hereafter perhaps, some accidental increase of a mountain torrent may, by sweeping off the alluvial covering, lay bare a larger extent at a higher elevation, and confirm this conjecture.
Although the actual contact of the schist and granite is no where visible, there are indications present to show that this contact is probably not remote from the spot where the latter appears. In the rolled fragments are found pieces of schist entangled in the granite ; which, when large, preserve their usual character, and when small, put on the aspect of micaceous schist : minute fragInents of these are even found in the fixed rock. I have already mentioned this phenomenon as occurring in Mull, in Rannoch, in Braemar, and in Glenco. In all these cases, this appearance is limited to the vicinity of the schist ; and it thus becomes probable, that here also, the junction of the schist and the granite takes place in the vicinity of the Doon river.
Felspar and mica predominate in the composition of this granite ; the quartz is in small quantity. The felspar is of a brownish hue, . and the mica is white or grey. In the decomposed rock, the former is converted into a yellow clay, and the latter into a greyish one. In addition to this, small strings and fragments of a fine dark clay are found in it. These appear to result from the decomposition of the entangled fragments of schist already described, and have been sought after with some anxiety for the purpose of a polishing powder : they form the Dun earth of the islanders. The mode in which the granite decomposes, is not very common : at the same time it is well known to geologists. In trap, this mode of decomposition is frequent, and is not the only point of resemblance which occurs between the older and the most recent unstratified rocks. Although the general mass of the rock is at first uniform, and apparently homogeneous, the commencement of decomposition is marked by circular lines which respect a central nucleus, or rather different centres. A. section exposes the whole process, which takes place without the contact of air. While the parts most distant from the centre of each nucleus, are completely detached in successive crusts and converted into clay, lines, gradually becoming more feeble, are found marked in succession towards the centre, where the solid rock, ultimately destined to undergo the same change, is still in a state of integrity. 2 The intervals between these irregularly globular bodies are occupied by curved and by straight lines, the granite in certain parts decomposing in schistose laminæ. In a general view there is nothing to be remarked on this subject which has not frequently been discussed already, and the appearance in this case, as in that of trap, has been by all geologists referred to the concretionary structure. But a circumstance, which . has recently occurred in my observations, proves that in certain cases at least, the exfoliation of granite is independent of any internal structure. It was already alluded to on a former occasion.
Among the granite columns lately brought from Leptis, and now in the British museum, it thay be remarked that many are decomposing in crusts concentric to the axis of the shaft ; each crust varying from one-sixth to one-fourth of an inch in thickness, and still possessing considerable tenacity, with an appearance scarcely changed from that of the fresher specimens. However difficult it may be to explain this mode of decomposition, it appears quite similar to that which occurs, in many cases, in natural blocks of granite, as well as in the trap rocks ; and may perhaps, in some measure invalidate, not only the preceding opinions, but, to a certain degree also, the observations formerly made respecting the schistose granite of Arran, where a reference was made to this fact.
The next rock in order to the granite is the argillaceous schist, the general extent and disposition of which have already been stated. In describing the leading features and relations of this mass, the general term of schist has been adopted ; since the whole, though differing in many places in mineral composition, bears a common relation to the rocks which follow it, and doubtless also to that which precedes. With such a term, the history of this body of strata will be more intelligible, and the general bearings of the whole more easily apprehended, than if in describing it, the several varieties were considered separately ; a mode calculated not only to produce a tedious repetition, but in danger of conveying fallacious ideas respecting its character. As on former occasions, it will be more consistent with clearness of description to consider these several varieties as so many mineralogical distinctions ; taking care to notice their relations to the general mass wherever they are such as to present any useful information, or where they tend to illustrate the history of the argillaceous schists.
This proceeding appears the more necessary, since, as was already hinted in the account of Bute, some obscurity appears occasionally to have been produced by confounding two circumstances which ought, as far as possible, always to be distinguished in description, namely, the geological connexions and the mineralogical characters, or by deducing the former as a necessary consequence from the latter~ With respect to the particular class of rocks under review, namely, the argillaceous schists of the primary class, a practice .has prevailed, with what propriety appears doubtful, of making geological divisions where Nature appears only to have made mineralogical distinctions, and of founding essential characters on cir.. cumstances which ought perhaps only to be considered as accidental. In the case of micaceous schist, a class of rocks of as great frequency and extent as the argillaceous schists, and exhibiting varieties in composition much greater than those which occur. among the latter, no such division has been thought necessary ; every variety occurring in this series being admitted to stand in a common relation to the surrounding rocks, although differing from each other in a much greater degree than fine clay slate does from those coarse varieties which have been designated. by the term graywacke. If it is judged expedient by geologists that the same practice should be extended to the rocks immediately under review, the confusion which has hitherto so often attended them will probably be removed, and the history of the argillaceous schists reduced to that simplicity so much to be desired in these investigations. . .
It is unnecessary here to repeat the different observations scattered throughout this work, which form the arguments for the view now taken of the common relation in which all the argillaceous schists stand to each other and to the surrounding rocks. These are supported by the examples of Cornwall, Wales, and Cumberland ; the structure of which countries resembles that of the Isle of Man, and in all of which the several argillaceous schists form part of a common deposit. It is certain that graywacké is often predominant, that it is even sometimes exclusive ; yet, in those cases also, it maintains the same connexion with the rocks that precede and follow it. As already remarked in the account of Bute, if any such series can be ascertained, which is distinct in geological characters, both from the primary and secondary rocks, it would be advantageous to confer on it a specific designation ; but even then it would be necessary so to distinguish it by an appropriate term, as to prevent any confusion from applying that of graywacké in a double sense.
It is evident that the presence of organic remains does not distinguish between clay slate and graywacké, as they are found in those deposits which contain both these rocks. Neither does this form the constant geological distinction, even in the graywacké, as they are frequently absent in those districts which have been assigned to . the class of transition rocks. It appears difficult, indeed, to discover what are the characteristics of a class which is to be defined by circumstances that are oniy occasional, which occupies the same positions and maintains the same relations to the associated rocks as the primary, and which does not form an intermediate member between the primary and secondary strata, since it is often altogether absent.
This is not however the place to discuss so important geological question. It is sufficient to have thrown out these hints to geologists of more experience, to whose consideration they must now be left, while I proceed to describe the structure of the island under review.
Assuming now, that which will shortly be proved, namely, that the schist of the Isle of Man constitutes a single mass, its character as a primary rock is determined by its position with respect to the granite, since it immediately follows that rock. This is therefore another instance, in addition to those already known in our own country, of the immediate succession of argillaceous schist to granite, a fact too common to require any remarks on the inconstant order in which the primary strata succeed that fundamental substance.
The next question respecting the schist is, whether its position in different places bears any relation to the granite mass, or rather to the forms of the mountains ; since the quantity of granite here visible is too trifling to admit of any comparison of this nature. To determine this point, it would be ñecessary to assign the positions of the strata on different sides of the mountain range ; an object which renders it first indispensable to ascertain to what extent proofs of the stratification of the schist can here be obtained.
The indications of this order are seen in many instances so distinctly, that no doubt can be entertained respecting the stratified nature of argillaceous schist. But the disposition of the strata, as formerly observed, is sometimes easily confounded with that of the laminar structure, which is not always coincident, but is, on the contrary, sometimes placed at considerable angles with the plane of stratification. This error is easily committed in examining . the schist of this island, but it may be corrected by careful examination. But that examination, although sufficient to determine that the laminar tendency of the schist is here often different from the plane of stratification, has not enabled me to assign the real positions of the strata, if indeed they can be discovered to such an extent as to determine the object in question, namely, the relation of the strata to the forms of the mountains. To understand this part of the subject, however, it is necessary first to describe the schist in some detail, and thus to state the causes of this uncertainty.
The substances which appear to be predominant in the general mass, are those which mineralogists have agreed to rank with clay slate ; although many of them differ materially in their composition and appearance, from the best defined specimens of that rock. Together with these, there occurs, but in comparatively small quantity, that schist distinguished by the term graywacké ; generally of a fine texture, and presenting a much more limited variety of appearance than is usual in similar situations. It is found either massive or fissile, or appears under the two modifications of graywacké and graywacké slate.
This rock, in its different forms, is found in every part of the island, but the massive is every where more rare than the fissile. Wherever it occurs, it is irregularly interposed among the clay slate, often in portions extremely small, sometimes forming larger tracts, and therefore alternating with it in an irregular manner. This alternation appears, ii some cases, to take place by beds; in others, the portions of the graywacké mixed with the clay slate, are so irregular and so dispersed, that it is impossible to determine whether or not they possess any fixed relation to it. Examples of these occurrences are so numerous, that it is superfluous to enumerate them : I shall describe only two, because of their distinctness and of the facility with which they may be examined by those who may be so inclined.
Where the junction of the red sandstone with the primary rock at Peel takes place, a fine fissile clay slate is found in immediate contact with the sandstone. A few thin beds of a non-fissile graywacké, extremely reticulated by veins of quartz, alternate with this slate ; and it may be remarked that the quartz veins are limited to the former, never passing into the latter. The graywacké breaks into irregular angular fragments, having no apparent relation to the general fracture of the schist. In the same place, a repeated alternation is also visible, of an extremely fine and soft clay slate, of a blue and of a black colour, with a similar graywacké. The clay slate is in thin beds, which are prolonged, and, as it were, corn-pressed between the graywacké ; being at the same time bent or twisted, so as to put on the deceptive appearance of veins.3 The unequal action of the sea and weather on these two substances, renders this appearance very conspicuous. There can be no doubt of the nature of the rocks engaged in these alternations, as one of the beds of slate contains the black chalk, or drawing slate, which was mentioned in the general description.
I need take no notice of the flexion and apparent compression of the clay slate in this instance, as I can add nothing to what I have on former occasions remarked on this subject.
The next instance is to be seen in the vicinity of Douglas. Here, the clay slate is also the predominant substance, and is disposed in laminæ at a high angle, often nearly vertical, and of great extent. These, from their magnitude, convey the idea of beds. In examining them more narrowly, a belt of rock, resembling at first sight a trap vein, is seen crossing them in a nearly horizontal direction. On investigation, this is found to be a mass of graywacké, not divided by any fissure or discontinuity from the slate, which lies above and below it ; although when broken, the one rock readily separates from the other. The fissility of the slaty rock is in this place very remarkable, since it is easily split into large tables, and also exfoliates naturally in flakes of extreme thinness. But this fissile quality is interrupted at the place where the graywacké appears, being again renewed as before, above it ; the breadth of the graywacké zone not exceeding two or three feet. The fracture of this latter is angular and irregular, without the slightest appearance of a slaty structure, and its texture is rather fine-grained. The subjoined sketch will afford a clearer elucidation of this circumstance.4
It will be rendered more intelligible by calling to mind a similar circumstance occurring in Isla, and already described, as the one will serve to illustrate the other. In the present example, the fissility is indeed suspended where the band of graywacké appears, although, in Isla; it extends through both ; but there is no difficulty in explaining this circumstance, when it is recollected that many of the gray wackés occurring with clay slate, are of a solid and non-fissile structure. The band in question may therefore be considered as the edge of a bed of this rock, deposited in alternating order with the clay slate. This bed, on the foregoing principles, will therefore serve to determine the position of the great strata of schist, of which the seams are here invisible ; and they will thus be found to. lie nearly at right angles to the direction of the fissile tendency. Instead therefore of considering the schist in the neighbourhood of Douglas as placed in an erect position, it must be considered as disposed in horizontal beds ; and, if the same rule could be proved to hold good every where in this island, we must in every instance reverse the most obvious appearances ; considering the beds to be erect where the fissile tendency is horizontal, and horizontal where this is erect. Such an universal conclusion however would not be justified ; but these considerations inculcate. the necessity of caution in assigning the positions of the beds of schist, unless, either the indications just mentioned are visible, or the positive existence of the seams is ascertained.
The seams themselves are visible near Spanish head, añd they are horizontal or nearly so ; while they are at the same time slightly undulated : but there is here no ready access to them, so as to determine at what angle the fissile tendency lies. In most other parts of the island, extreme confusion in the fissility is visible, but the seams of the beds are scarcely ever to be discovered. If we cannot safely conclude from this circumstance, that the beds themselves are disposed in an undulating and irregular manner, that point must remain unsettled, since there are no other indications from which to form a judgment.
In now reverting to the original question; namely, whether the beds of schist bore any relation to the exterior forms of the mountains, and whether their irregularities have determined the disposition of the group, it is evident that no light can be thrown on thé subject, unless future observers shall determine, throughout the mountains, that disposition of the beds which eluded my examination. The laminar tendency is obviously incapable of furnishing any unexceptionable evidence on this subject. Yet the preceding discussion will not, imperfect as it is, be without its use, since it will point out Some of the causes which prevent the solution of similar questions in other cases, and excite useful doubts whether, in many instances, general Conclusions to this effect have not been founded on imperfect views of the nature and bearings of the schistose rocks:
As it is in vain to endeavour to convey an idea of the varieties of rocks by description, ; I shall content myself with a very superficial notice of the principal modifications of schist occurring in the Isle of Man; . enumerating rather their leading featùres than attempting to describe individual specimens. Such as are applicable to useful purposes, have been already sufficiently pointed out in the topographical part of this article.
The clay slate appears frequently in its simplest form, as a soft fissile uniform substance, of different . colours, passing from a greyish white through various darker tints to lead blue, and Ultimately to an imperfect black. In certain cases it possesses a silky appearance, ap.. proaching at times to a plumbaginous lustre ; and, these appearances by degrees increasing, it assumes the aspect of a fine micaceous, and of a talcy slate. In structure, it is occasionally fibrous, and this variety predominates at the southern end of the island, while in other cases it shows neither fibrous nor foliated tendency. It is proper to remark that these varieties are found indifferently in all parts of the island, on the shores as on the summits of the highest mountains.
In certain cases the clay slate contains a greater proportion of siliceous matter, which, according to the mode of its disposition, produces numerous other modifications. The whole acquires at times a flinty hardness with an uniform aspect, and, in proportion to the increase of the siliceous ingredient, becomes at length con-verted into a quartzose clay slate, a substance well known to accompany ordinary slate in other situations, and not unfrequently confounded with indurated schist, or flinty slatç, as it is called. Where the grains of quartz are distinct, ~it acquires a coarser texture, and thus passes gradually into a graywacké, or at least into a substance which cannot, either by aspect or definition, be distinguished from it. Interspersed grains of mica õccasionally diversify these varieties still further, but a sufficient idea of them has already been given. To describe the more extensive masses of graywacké in all their modifications, would be superfluous.
It is almost equally unnecessary to say that the whole of these schists are V every where traversed by yeins of quartz. One detached and solitary fact occurred during the examination of the sohist, which must not be passed over ; although most geologists will now consider ~ the argument which j~ offers respecting the mechanical deposition of argillaceous schist as superfluous. It was that of a rounded pebble imbedded in the clay slate, at the mouth of the Santon river. This was of an oval shape, evidently rounded by mechanical action, and decomposed on the surface ; consisting of a harder variety of clay slate with a few interspersed grains of hornblende. It has on other occasions been shown that the other two leading members of the primary strata, namely, quartz rock and micaceous schist, sometimes contained bpth fragments and rounded pebbles, of the same or of different substances, imbedded in the finer mass of the rock ; pointing out a certain analogy to the secòndary strata, and indicating the share which mechanical arrangements have taken in their formation. In the case of graywacké, the same structure is abundantly obvious, and in this instance it is equally extended to clay slate. I must now proceed to examine the red sand-stone, as next in order of arrangement.
The general position and extent of this rock have already been sufficiently detailed, and it has also been seen that the principal tract occurs in the vicinity of Peel. At the southern extremity of this mass, the beds are elevated at an angle of forty-five degrees, dipping to the north. In proceeding from this point along the shore, they are V found, after a short interv~i, to decline from this elevation, until at length they become nearly horizontal. As far as this point, no marks of disturbance are visible ; but after again being elevated into a high position, and once more returning to the horizontal plane within a very short space, they become exceedingly irregular, and then suddenly terminate in the schist ; the junction being perfectly visible. It is apparent, on considering the extent and form of the whole mass, that where the beds are most regular in their position, they are furthest removed from the schist, and that their irregularity becomes conspicuous only in proportion as they approximate to that rock. The actual junction is most easily examined within the high water mark, at the recess of the tide ; where it is washed clean so as to be exposed to view with. the same freshness it would exhibit on fracture; a very desirable circumstance, but not often to be found,. Under the uncertainty aireatty stated respecting the disposition of the schist, it is impossible to determine in what way the two rocks are disposed at the junction, as far as their relative stratifications are concerned ; yet it may be remarked generally, that the issue tendency of the schist is more or less opposed to the seams which separate the beds of the sandstone. But a great change in the regularity of both, takes place at the points of contact between the two ; the schist being much twisted and bent, and the regularity of the sandstone beds being nearly obliterated. Similar confusion occurs in the cc!mposition of the substances themselves ; the schist becoming in some cases coarse, and charged with sand, and the sandstone being. for a short space so inter-mingled with the argillaceous matter of the schist, as to put on the aspect and character of graywacké. These appearances however, extend on either side, only to a few feet ; but it may also be remarked that the schist acquires a mixture of red in this place, while some parts of the sandstone, on the contrary, become blue.
The probable causes ~f this confusion of position and character, which not unfrequently occurs at the junctions of the primary and secondary rocks, have been subjects of discussion among geologists, but no satisfactory explanation of the appearances has yet been given. It may however be remarked that those causes which have been supposed to operate in producing similar effects, where the junction is that of granite or of trap with the stratified rocks, do not seem applicable to the present case ; the junction here, being that of the primary with the secondary strata. These disturbances are also more interesting, since they are either of less frequent occ~urrence, or have been more rarely noticed. In most of the instances which have occurred in my experience in Scotland, such junctions are undisturbed and regular, the secondary rock lying on the primary, either in conformable order or otherwise. One coñclusion however may perhaps be drawn from a consideration of the appearances in question. Whether the schist has acquired its present position, either in consequence of elevation or subsidence, it has not only been so disturbed after the deposition of the sandstone, as the elevated position of this latter appears to demon-strate ; but the causes which led to that change of position have been such as to extend their action, whether chemical, mechanical, or both united, through the primary to the secondary strata.
The mineral character of the sandstone now described, presents little variety. It is more commonly of a compact than a loose texture, and not unfrequently possesses the hardness and crystalline appearance of the primary sandstone, or of quartz rock. It is formed of sand intermixed with ferruginous clay, and is generally of a deep purple-brown. Occasioríally it displays those white spherules so frequently o~curring in the sandstones of this description, and already noticed in the account of Arran. The beds are often divided by seams of red or blue shale, or of clay, generally very thin ; and they occasionally present that undulated surface which so accurately resembles the effect produced on hard sand during the retiring of the tide. Rare and thin beds of the coarser grit and breccia usually associated with this rock, are also to be seen, intermixed with the finer variety, and mica is not an unfrequent ingredient. The clay found in the seams has often the character of ironstone, and lumps of genuine red iron-stone, apparently detached from some of the beds, are scattered on the shore in its vicinity. Two or three insignificant beds of white sandstone are also found alternating with it. The last and most remarkable substance occurring
in this sandstone, is magnesian limestone. This is highly
V crystalline, white, and of a pearly lustre, the scales, of which it is composed, being of considerable size and presenting curved surfaces. It forms flat masses of inconsiderable dimensions, which, being parallel to the stratification of the sandstone, might be considered as beds,
but, as they frequently ramify into small filaments, they ought rather perhaps to be viewed as parallel veins.
Although the calcareous strata which are found at the sonthern side of the island following the red sandstone, are not here visible in the place where they might naturally be expected, it is probáble that they exist in the neighbourhood, concealed by the sea which flows over them This probability rests on the nnmerous fragments of a sVjmilar limestone which are thrown up in unexhausted succession on the western alluvial shore ; often so little rounded as to render it probable that they have not been long detached from their native beds.
Such is the history of the principal tract of red sand-stone occurring in this island. But there is another mass, iiientioned in the topographical description, and noted in the map, which, however insignificant in dimension, is not so in a geological view ; while, at the same time, it serves iñ some measure to connect the limestone and the schist, and to establish that order of succession from the primary to the secondary strata, which is most common, and to be esteemed most regular.
On examining the map it will be seen to occupy a small portion of the western side of Langness point. This rock is here classed with the red sandstone, rather from it~ geological position than its mineral character, since, strictiy speaking, scarcely any real sandstone is to be observed. That substance occurs in one or two places only, seldom exceeding a few inches in thickness, and occupyiiìg a very small portion of the conglomerate beds of which it forms a part. But as the whole mass possesses a distinct analogy to that rock whic~h is so often found occupying a more considerable extent, and separating the primary from the secondary rocks, there need be no hesitation in considering it as a portion of the lowest red sandstone, the remainder of which has possibly been removed by the natural causes of waste, of which this shore presents abundant indications.
The total dimension of the conglomerate is but trifling.
Consideral in its superficial dimensions, it forms a belt on the shore of perhaps 300 or 400 feet in bréadth, and scarcely a greater number of yards in length. Its breadth is greatest towards the south, where its junction with the schist is to be seen ; the line occupied by the limestone, and that of the sandy bank which forms thé shore, ex~ cluding it at length by their approximation. * The thickness of the collective beds is also inconsiderable, and might be computed, were it necessary, by a measurement of the exposed edges of the strata. The dip is to the north, at an angle of about fifteen degrees, being nearly in a reverse direction to that of the stratified rocks which lie above it at Castletown ; whence it would appear that the strata are here disposed in a sort of concave or basin-like form.
The junction of this conglomerate with the schist may be traced in the high banks which form the shore, where it is indicated in the map. If the schist is here disposed in beds, and not merely in laminæ, at the high angle of eighty degrees, at which it appears to lie, the position of the conglomerate is unconformable to it ; the dip of the schist being also here to the south, as that of the con-glomerate is to the north, although neither of them are constant in this respect. The schist in this place is an ordinary fine clay slate, of a foliated texture.
The conglomerate is generally very coarse and loosely aggregated, containing larger and smaller fragments of quartz and schist cemented by clay and sand : thinner strata of a sandy clay also occur in it. In colour, it is either red or grey, and in some places is further mixed with calcareous matter and wjth fragments of lime-stone.
This conglømerate is immediately followed by the beds of limestone which are next to be described , some interference of the two, consisting of an occasional repetition of
~ rt ~ represented in the map as if crossing the~i~tbmus at Langness point, but itiS oVray clearly visible on the southern shore, where the beds are t~iirly exposed.
the conglomerate, taking place at the points where the change occurs.
It will hereafter be seen that the limestone beds in Derby haven which are continuous with these, come into contact with the primary strata without the intervention of the conglomerate. It is therefore apparent that the bed here described is of a very partial nature, and it may be conjectured to diminish in a wedge-like form till it disappears. It presents an example, analogous to those cases occurring in Sky and elsewhere, of the extenuation and disappearance of the lowest secondary strata, and of wth consequent contact of a superior set with the primary rocks.
[sorry I do not have copies of the maps or plates]
1 The Sections, Plate XXXII. fig. 6, 7, will serve to explain the general relations of all the rocks of the island.
1 Plate XXVIII. fig. 5.
3 Plate XXVIII. fig. 2.
4 Plate XXVIII. fig. 1.