[from Descriptions of the Western Isles, 1819]
IT is now necessary to proceed to the examination of the limestone, being the uppermost of the stratified rocks found in the island.1 Its general extent is already understood, from the topographical description which has preceded, and from the accompanying map. However different in mineral character the stratified and the unstratified limestone may seem to be, there will be sufficient proof in the course of this examination that they are only different parts of one common deposit. At the same time this examination will throw light on many obscure points in the history of the unstratified limestones.
By far the greater portion of this limestone is disposed in thin beds, in a parallel manner. The thickness of these varies every where ; rarely exceeding two feet, and seldom descending to three or four inches. In consequence of the general flatness of their position, and that of the shore, large portions are in many places completely exposed by the recess of the tide ; and, being thus also perpetually washed, they admit of being examined in the fullest manner. In many places indeed, access may be had to the edges of the beds, from the surfaces even down to the schist. This is more particularly the case between Derbyhaven and Langness peninsula ; the lowest points of the limestone being visible in contact with the schist near the Santon river on one side, and with the conglomerate near Langness point on the other; while the uppermost beds are exposed in many intermediate points, if these can be determined, as they appear to be, by a particular breccia hereafter to be described, which covers them. The thickness of the collective mass is necessarily greatest at some of these intermediate points, and diminishes in approaching the schist on the one hand and the breccia on the other. That thickness, were it an object of any moment, might be estimated by carefully tracing. the exposed edges of the beds. In some attempts at boring which were once instituted in pursuit of coal, forty-five or forty six fathoms were passed through without reaching the fundamental rock.
Although the general position of the limestone beds is flat, it is rarely quite horizontal. Such a disposition however occurs in two or, three places, where, in consequence of a slow and gentle undulation of the strata, the tendency of the dip becomes reversed. In these cases a variation of the angle of inclination takes place, amounting to ten degrees or more,and the tendency of the dip necessarily varies : at Santon river it is south, near Port la Marie east-south-east, while in other places it respects other points of the compass.
In a few places, the inclination of the limestone is much greater than that above mentioned, and these cases either in the immediate neighbourhood of the fundamental rock, or in that of the trap veins. They are attended with marked curvatures of the beds, and will be described in examining the general connexions of the different rocks hereafter.
The beds of limestone are found to alternate with breccias of different characters, above noticed ; but these are, generally speaking, either at the bottom or at the top of the limestone mass.
Besides these breccias, beds of shale, or of clay, more or less approaching to that substance by its induration, are found interposed among the limestone beds. This is far too common an appearance to require any further notice.
The colour of the stratified limestone varies slightly in different places, but is generally limited to different shades of dark and blueish grey ; the darkest, Which approaches near to pure black, being the marble of Pool vash formerly described.
The differences in composition lie also within narrow limits, and consist in the greater or less predominance of argillaceous earth, as well as of silica, in the calcareous rock.
Magnesia is also occasionally found as an ingredient, but it is unnecessary to dwell on circumstances of so ordinary a nature, as similar limestones are familiar to geologists.
It is equally superfluous to detail the texture, fractures and general appearances of this rock, which vary from smooth and conchoidal, to granular or earthy, as in other instances of almost daily occurrence.
The organic remains are abundantly dispersed through almost every part of the calcareous strata, and of the interposed shales ; being however much more numerous and entire in some places than in others. Occasionally they are absent altogether, but they are rarely wanting for any considerable space. their cavities are in almost every instance filled with calcareous matter; to which rule one very remarkable exception is found. This occurs in the black beds from which the marble formerly described is procured. Here, the animal remains are all converted into pyrites, while at the same time their forms are more destroyed than in those situations where they are calcareous. It is uncertain whether the vicinity of a considerable trap vein, which traverses the limestone at this place, may have had any influence in producing this change.*
The unstratified limestone, which accompanies the ro. regular beds, is found irregularly interspersed in detached masses throughout the whole calcareous tract, of which it forms a part. It is neither placed above, nor, as might more naturally be expected, below the strata; but is irregularly intermixed with them, forming a portion of the common deposit. Although it is here called unstratified, on account of its predominant character, this feature is not always definite ; since it is found in certain places to pass gradually from a shapeless mass into the form of regular beds, and, in certain cases, may even be traced into the ordinary stratified rock.
It is commonly of a crystalline texture, often indeed highly so, like some varieties of the primary limestones; the plates being of large size, and at times translucent oil the edges of the fracture. It is extremely refractory under the hammer, and all these distinguishing characteristics are most perfect where the semblance of stratification is most completely wanting. Where it passes into the stratified limestone, ~- its mineral character also undergoes a gradual alteration, until it ceases to be distinguishable from that variety.
In colour, it displays nearly the same modifications as the stratified limestone, with the exception of black, of which no instance occurred during the investigation. The paler greys seem to predominate, and in some partial spots the rock even becomes of an impure white. This circumstance is interesting, and is analogous to that pointed out in Sky. If the unstratified is a portion of the stratified rock, changed in character by some operations posterior to its deposition, it would hence appear that the causes, whatever they may have been, have also dissipated the bituminous matter from which the colour is derived ; as they also seem to have destroyed the organization of the shells once imbedded in it. A breccia consisting solely of different coloured fragments of the same variety, and entirely different from those formerly mentioned, is found connected with it near the contact with the schist in the neighbourhood of the Santon river.
The unstratified limestone is often so changed, and that to so great a depth from the surface, that its true character can only be discovered after a deep fracture. In these cases it becomes yellow and dark brown ; thus assuming, in some instances, the aspect of the yellow magnesian limestone of England.
Brown spar is occasionally found in the rifts, and also in nests occupying irregular cavities. It must however be remarked that the same mineral exists generally in the rifts also of the stratified limestone, being in both cases crystallized in its most ordinary form ; the only difference being, that in the latter case it is white with the usual pearly lustre, while, in the unstratified rock, it is brown, and may often with equal propriety be termed calcareous iron ore.
In general, no shells or organic remains are to be found in the unstratified rock ; but as some of the exceptions are important, it is necessary to be more particular in tracing them.
Near the Santon river, at the junction of the schist, and where one of the most conspicuous and interesting of these masses is to be seen, no shells occur for a considerable space. But as the unstratified rock approaches to the stratified, it undergoes a gradual alteration, the crystalline texture gradually passing into the earthy, and the change being complete when the beds have put on that which may be called their natural form. The organic remains increase in the same proportion, being sparingly found at first and in the intermediate parts of the change, and becoming abundant when that is complete, as they were absent before its commencement. In one place only they were found imbedded in the crystalline limestone, but in this place their forms were less accurately preserved than usual ; a remark which was confirmed in other instances where they were found in a similar situation. It is important to note, that in these instances the organic substances are similar to those found in the stratified limestone ; and they prove the original identity of the calcareous deposit under both its present forms, even in those cases where the actual gradation between the beds and the shapeless rock cannot be traced.
The next, and by much the most decided examples of the absence of organic remains in the unstratified rock, are to be seen in the neighbourhood of the black strata at Pool vash. Both kinds of limestone are here found together, the irregular being predominant ; but they are so situated that it is impossible to trace any connexion between them beyond that of juxtaposition ; a circumstance which is probably to be attributed to the form of the Shore, which is flat, and to the imperfect manner in which the beds are exposed. Had no other portions than these been visible,: the nature of the important ~ connexion between the two, might still have remained unknown It must he remarked that although tim irregular rock possesses here no marks whatever of stratification, it cannot he called crystalline, except in a few points. A perfect idea of it may be given to those who have visited Devonshire, by saying that it resembles the two. Stone near Stonehouse ; and perhaps an idea of the crystalline variety more accurate than can be formed from the preceding description, may also be conveyed by comparing it to that which is seen near the same place at the Devils point. Other analogies will be apparent hereafter. In the rock thus described, the organic remains abound, as they do in~ that which is stratified, but their forms are neither so entire nor so decided. It is not however certain at in this instance, as in the former, they have themselves undergone any: change, since their obscurity may arise from the difficulty of procuring specimens of the rock, as fair and as perfect as those which can. be obtained from the regular limestone.
. It is unnecessary to quote any more of the places where similar appearances occur, as they offer no material differences, and the present object is rather to select the ~most important circumstances, than to enter into a detail of every minute particular which was examined.
But before quitting this part of the subject, it is. proper ho point out the very remarkable forms which the unstratified limestone assumes on weathering ; by which, together with its superior durability, it is in all cases readily distinguished, even at a distance, from the stratified. In almost every instance, its superior hardness, and the greater resistance it offers to the sea and weather, cause it to project in rough masses, often many feet in height, above the surrounding stratified rock. It presents in these, as in most other cases, an angular large fracture ; consisting of an aggregation of small irregular pyramidal bodies with intervening cavities ; not the least semblance even of displaced stratification being visible. In some instances it decomposes by weathering, into round honeycombed cavities separated by irregular ridges ; resembling that lime stone which occurs at Broadford and at Kilbride in Sky. Its resemblance to that rock in other circumstances, will be apparent from the whole of its history, and each serves mutually to illustrate the other.
In attempting to determine whether or not the limestone of this island is conformable to the schist on which it lies, it becomes. immediately obvious that it must occur in both ways, namely, parallel to some of the strata of that rock, and at angles to others ; since, in a general view, it is placed at a common low angle, while the schist occupies every intervening one from the horizontal to the vertical line. Under the circumstances of obscurity. formerly stated, respecting the. real position of the schistose beds, and the difficulty of distinguishing them from the Laminæ, it would be impossible to determine this point more accurately ; since they may often appear conformable where they. are reversed, and the contrary.
It would be superfluous to trace all the points where the actual junction of the schist and limestone can be observed, or their probable meeting inferred. The geological conclusions which may be drawn from these facts; will be equally deducible from one or two selected examples.
This. junction is displayed in great perfection near the mouth of the Santon river, where the boundary of the limestone, towards the east, is marked on the map. The position of the schist is here very irregular, but the laminæ are placed at high angles, varying from forty-five degrees to the perpendicular.
Although here of the usual blue colour and ordinary degree of hardness which it displays elsewhere in the vicinity, it is red and decomposed at the immediate line of junction ; and the same appearance may also be observed in the limestone for a similar space ; while neither does it extend beyond a few feet. The appearance exactly resembles that already described as occurring at the junction of the red sandstone near Peel.
The limestone is here seen under both its forms, the stratified and the unstratified, but the former is mod.ified in a manner which requires further explanation. . In one part of the junction, the schist and the irregular limestone are in immediate contact, the character of the latter oor.. responding so exactly with the general account already given, as to require no further notice. But where the stratified rock abuts against the schist, it is suddenly diverted from the low position in which it is found. at a small distance ; being turned upwards in a curved form till it acquires a position considerably erect.* At the same time, there takes place a gradual change in ~ that accurate parallelism and evenness of the beds which were before predominant. They become rough, undub~ted, unequal in thickness, and deficient in that apparenfly nice adaptation to each other which they possess ~ where in~ their natural or stratified position. In the same circumstances, their mode of weathering is altered, from the former flaky and even, to the irregular cavernous and pyramidal manner by which the unstratified rock is distinguished. Their composition and general characters undergo corresponding changes. As already remarked, the organic substances gradually disappear, and the earthy aspect gradually also becomes converted into the crystalline, so that the stratified and the unstratified rocks cease at length to possess any mineralogical distinction.
It is further necessary to observe, that these changes occur in two distinct modes. The first may be traced by following the progress of each individual stratum from a point near the schist to one at a distance from it, and Is seen in those cases where the edges of the strata come into contact with the fundamental rock.* The next occurs where the unstratified rock lies between the schist and the stratified one, and takes place, according to the order of succession in the strata, in the following manner.~ By degrees the irregular appearance of the unstratified mass diminishes, and it is then succeeded by an irregular bed having a rude conformity to those of more regular form which are to follow. The succeeding strata become gradually more even, and more resembling the ordinary stratified and prevailing limestone, till a perfect uniformity at length takes place. A complete transition may therefore be traced, in this direction also, from the unstratified to the stratified rock ; and that transition is accompanied by the same circumstances with regard to the mode of weathering, the appearance and number of the organic remains, and the texture, as have just been pointed out in the other cases .
This account of the junctions, and of the transitions of the two limestones, must not however be concluded without mentioning, that other marks of change, and indications of violence, are impressed on the unstratified rock. In the stratified one, the usual appearance of straight joints, filled with ordinary or magnesian carbonate of lime, occurs, but in no great abundance. The unstratified is, on the contrary, penetrated by innumerable fissures crossing each other in irregular directions, and filled with the same materials. Here also, at the entrance of the Santon river, is to be seen the breccia which was formerly mentioned. This is a part of one of the irregular beds, and it consists of fragments of the crystalline rock, cemented by the same substances which fill the rifts of the limestone in other places ; as if in this particular spot the rock had been broken into fragments, instead of yielding in a body to the impression of the disturbing force.
After thus minutely detailing the changes apparent at this junction, it is proper to say that, in many other instances, it is impossible to trace any passage from the crystalline rock to the ordinary one, nor from the unstratified to the stratified disposition ; but that the change is sudden and perfectly defined ; while, at the same time, the proximity of the schist cannot be detected.
There is yet another distinct set of appearances, connected with the change from the stratified to the unstratified limestone. . There are two examples of this, each of which is sufficiently distinguished from the other to require a separate detail.
Near Scarlet point, the successive beds of stratified limestone are to be traced in a flat position ; following each other in a very even and regular manner for a considerable space. On a sudden they undergo an undulation in repeated curves, and are then intersected by a number of trap veins.* These veins are repeated at intervals for some distance along this shore in proceeding westward. They all lie in a position more or less erect, and their general tendency is towards the south-west but as they are not parallel to each other, their direction cannot be more accurately assigned. They ramify at times, in a manner which appears somewhat complicated, many of the branches being very minute. Two or three are more remarkable than the rest, but it is unnecessary to describe either their dimensions or numbers very particularly.
Where they first appear to the eastward, the stratified limestone terminates abruptly, and is succeeded by a very conspicuous mass of the unstratified rock, The same rock, under circumstances of much confusion, is continued throughout the greater part of the space intersected by these veins ; that confusion being much increased, and considerable difficulty thrown in the way of the examination, by the occurrence of a breccia, which, wherever the limestone is undisturbed, is found in a position superior to it, but is here irregularly intermixed and confounded with the calcareous mass. Hence it is difficult to determine whether the unstratified limestone and the trap veins are every where co-existent ; but whether they are or not in this particular instance, is of no moment, since, in the next example, the latter are found traversing a stratified limestone. The immediate junctions of the veins and the limestone, do not any where appear to be attended with circumstances in the texture of either, sufficiently remarkable to require particular notice. Such changes as are apparent, resemble those now so often described as to have ceased to be objects of curiosity.
Although the stratified limestone recurs beyond Scarlet point, it never again becomes as regular as before, but continues undulated, distorted, and broken by deep gullies, as far as Pool vash. Along this shore it is covered by the same breccia, consisting of fragments of schist and clay intermixed ; by which its real position is much obscured.
A considerable trap vein is found near the black marble quarry at Pool vash, forming the third and last example of this junction which it appears necessary to describe.
In this case the passage of the vein is not connected with any change from the stratified to the unstratified limestone since the former is found lying on both sides of it. At some little distance from this vein it however ceases, and is replaced by the unstratified, which prevails along the remainder of the shore towards Kintracht, where it terminates.
The works of the quarry have laid open the junction of the black strata and the trap. They are here found firmly united ; a disturbance also taking place in the beds, but extending to a distance of two or three feet only from the planes of contact.
A more remarkable change is visible where the super-incumbent breccia and the limestone meet, and this change, as far as I could trace it, seems limited to the neighbourhood of the trap. In other places the breccia seems evenly disposed on the plain surface of the limestone ; but here, the latter is elevated into ridges, undulations, and acute : points, the former adapting itself to them. In different places also, fragments of the lime stone are entangled in the breccia, and processes resembling short veins rise from it and traverse that rock. All these circumstances seem to bespeak some disturbance since the breccia and limestone were deposited. The pseudomorphous veins are similar in their characters to those formerly noticed in treating of Isla, and are doubtless to be attributed to analogous causes. it is unnecessary to make any further remarks on the phenomena attending the passage of the veins, excepting as far as the unstratified limestone is concerned ; since nothing new can added on a subject concerning which so much has already been said throughout this work.
In reviewing the whole of the preceding facts which relate to the stratified and unstratified limestone, the following conclusions seem to be among the most interesting.
The whole limestone mass, whether stratified or unstratified, consists of one deposit, or of one series, as is proved by its continuity, and by the identity of the organic remains. The absence of the stratified disposition is therefore . no proof that any given limestone does not belong to the class of the secondary rocks, a conclusion equally deducible from the history of the calcareous district of Sky. There is consequently no reason a priori why a limestone of even much more recent formation than this should not be found unstratified .
The absence of organic remains proves nothing respecting the comparative antiquity of limestone, and the crystalline texture is also an imperfect criterion of the geological relations of any given mass of that substance ; deductions which are equally to be made from the same example.
There appears no definable limit of the extent to which a limestone, really secondary, may exist in an unstratified state ; and the various unstratified limestones which have been described in different parts of the world, as of a period prior to the secondary rocks, require therefore to be re-examined ; excepting those which alternate with gneiss, micaceous schist, quartz rock, or argillaceous schist.
Some of these conclusions will, in a practical view, be equally valuable, whether the limestone be supposed to have been originally formed in the mixed manner in which it now appears, or to have undergone the change from the stratified disposition to the unstratified, in consequence of posterior alterations.
It is now necessary to remark that the confusion of structure here described, appears hitherto to be limited to limestone, a circumstance which may suggest the explanation that has already been indicated by the corresponding facts formerly stated, namely, that it is the result of the fusible nature of that rock. Nor is it possible to avoid remarking the analogy which exists between this case and those formerly noticed, where masses of trap are to be found intimately associated with schistose rocks, as in the hill of Kinnoul, and entangling fragments which bear the marks of partial fusion. Considering the different qualities of the respective substances, there is a perfect correspondence between the changes in both cases. There is however one important circumstance present in Sky, which is here wanting, at least throughout the greater part of the space where the unstratified limestone occurs. The existence of overlying syenite in that island, was there supposed to offer a solution of the phenomena in question; as the passage of trap veins has been shown, in other instances, to produce analogous effects. Although the veins of Scarlet point may be supposed to have caused the irregularity and change of structure there apparent in the limestone, no such veins exist throughout the greater part of the space where the unstratified limestone occurs. Nor is even granite here visible ; the calcareous rock being in contact with the argillaceous schist wherever it admits of examination. No apparent and analogous cause therefore exists in the Isle of Man, for that change in the nature of the limestone, which, in Sky, is supposed to have been the consequence of the presence of syenite. Yet there are circumstances, in the characters both of the sandstone and the schist, which seem to indicate that they have also undergone some analogous changes ; differing, it is true, but these differences depending on the relative nature of the different substances.
It will be remembered that, in the vicinity of the schist, the sandstone of Peel possesses an unusually indurated texture ; and that the two rocks are fractured and intermixed at the junction, while the characters of both are changed. In other places, the schist presents similar marks of some posterior influence, by which it has been fractured where in contact with the secondary strata, and its iron oxydated to redness. It is easy to imagine that such effects might, in these two rocks, be produced by the same causes which were capable of destroying the original forms and characters of the stratified limestone.
Thus the arguments must remain ; as in attempting to assign causes for the changes" in this place, of a nature similar to those which have been supposed to have acted in Sky, it can only be conjectured that the granite of the Isle of Man exists beneath these strata, at a distance sufficiently small to have produced the effects in question.
It is enough to have suggested this hypothetical explanation of an obscure and very interesting circumstance; but before entirely dismissing the subject, it will not be useless to inquire how far the history of this limestone can be applied to the illustration of other instances where the same or similar obscurities exist. Of these I am practically acquainted with two only ; the case occurring in Sky, already sufficiently considered, and that of Plymouth. The difficulty which occurs in respect to the characters and connexions of the unstratified limestone of Plymouth, arises, probably, in a great degree, from an insufficient acquaintance with it. In its disposition and general characters, it strongly resembles the examples here described, and, like the whole of the limestone of the Isle of Man, it follows an argillaceous schist which is incumbent on granite. It is probable that a closer investigation of its connexions may hereafter enable us to draw the comparison still nearer, and to ascertain its resemblance in the more important particulars still wanting to complete its history.
This suggestion will not be fruitless, if it shall stimulate those who may have an opportunity, to trace it with the care and perseverance which it merits and requires. To those who are inclined to undertake this pursuit, I may point out the propriety of endeavouring in the first place to trace its contact with the fundamental rocks; secondly, of searching for the stratified limestones which nìay exist in the same district, and of comparing the organic remains contained in each ; and lastly, of attempting to determine, at some intermediate points between the two, whether the real transition of the unstratified into the stratified rock cannot here also be found.
The obscurity which attends the different breccias that accompany the limestone, renders it expedient to reserve a separate paragraph for their description.
No continuous beds of either of them can be traced; since even that one, of which the greatest extent is visible, is much interrupted and confused, either in consequence of the trap veins or from other causes. With respect to the others, they are only to be seen occasionally, where their inclined position causes their edges to appear on different parts of the shore, or where they are detected by some casual fracture of the ground. There is consequently much difficulty in discovering their relations to the limestone, which can only be deduced from careful comparisons of their internal characters and positions together, at the several interrupted points where they appear. They seem to admit of being divided into two classes, those which are found below the lìmestone, and those which lie above it.
In describing the sandstone at Peel and at Langness, it was shown, that although the schist and the limestone are in some places separated by the conglomerate, in others, the contact of these two substances is immediate.
In these cases no breccia intervenes, but a sort of approximation to one may be occasionally seen in the numerous fractures of the limestone, afterwards filled up by calcareous spar. In one instance, already pointed out, this fracture becomes so complete, and the fragments are separated by intervals so wide and so irregular, that a perfect breccia is the result.
Another similar substance, apparently intermediate, both in position and structure, between the limestone and the schist, is found at Derby haven, and in other situations which it is unnecessary to detail. The basis of this is calcareous, and it contains . irregular fragments of schist and quartz. Its analogy, both in position and in origin, to the breccia of the red sandstone is obvious ; and it may be conceived to be the evanescent edge of that rock. It is perhaps easily explained by examining existing alluvia, which are found commencing by a thin edge, and gradually increasing in depth its circumstances favour their accumulation. If it be supposed that calcareous matter was deposited on such a compound surface, capable of forming a solid rock, it is evident that at a point above the alluvia it would be immediately united to the fixed and original rock, while these would be converted by its infiltration in some places, into breccia of the present structure ; the remainder forming, by the unknown processes of induration, a bed of ordinary conglomerate. Thus the wedge-like form and partial extent of similar rocks are also explained.
The last breccia which I observe as inferior in position to the limestone, is remarkable for its structure. It is visible near the Abbey at Ballasalla, where its position however can only be conjectured. At Port la Marie, it is actually visible beneath the limestone. It consists of rounded grains and pebbles of white quartz cemented by limestone, and undergoes a slight alternation with, and gradation into that rock, before it finally disappears. It is of inconsiderable thickness, and its analogy to those already described, in origin and nature, need not be more particularly pointed out.
Such are the varieties of conglomerated rock inferior in position to the limestone. It is possible that there may be others still unnoticed, but there will be no difficulty in classing them, if attention is paid to their structure. When they consist of fragments of the fundamental rock, whether rounded or angular, cemented by a calcareous base, or indurated without that cement, it is probable that they are all inferior to the limestone ; and whenever they contain fragments of the limestone itself, it is equally probable that they have been formed by subsequent changes, after a certain portion or the whole of the calcareous beds had been deposited. In the latter of these cases, they will be found alternating with some of the upper beds, or superior in position to the whole of them.
Of such beds, posterior to the calcareous deposit, I only remarked two. One of these is visible at Derbyhaven, and also in the neighbourhood of Pool vash. It consists of fragments of schist, quartz, and limestone, united by a calcareous base, and is of a coarse texture. It does not seem to extend far beyond the boundary of the schist, and the explanation of its origin cannot be attended with much difficulty. The fragments of limestone serve to distinguish it from those already enumerated, even where its true position cannot be discovered.
The other is far more obscure, while it is perfectly dissimilar from all the rest, and indeed from any rock of this description with which I am acquainted. It consists of minute fragments of schist cemented by a basis of clay, but varies slightly in different places. The fragments are sometimes conspicuous, and, at others, the whole is reduced to a scaly powder. In certain cases it also contains fragments of limestone, as well as of quartz, and,the, argillaceous base is occasionally more or less calcareous.
It is found extending interruptedly from Scarlet point to Pool vash, being almost every where visible at the surface of the limestone, but alternating also with some of its beds. The confusion, which prevents it from being easily understood, arises partly from the action of the sea in an unequal manner on a substance so tender, but still more from the general irregularity of the calcareous sub-stratum where the trap veins are found, and from the apparent disturbance produced at the places of their intersections.
Although it is not easy to explain in a satisfactory manner the formation of this rock, I may still point out the resemblance which it bears in its materials to the rubbish and clay which result from the action of the atmosphere on the present exposed surfacesof the schist, and which are found accumulated in beds in almost every part of the island. It is not difficult to imagine that such substances were deposited during the progress, or after the termination of the calcareous formation, and that they were distributed over the surface of the beds and indurated into their present condition, before the sea had quitted the rocks, or these had emerged from the waters.
I must now proceed to the last of the rocks, namely, the trap veins ; the particular description of which was reserved to this place, to avoid interrupting the more interesting parts of their general history.
The places of those which were observed, have already been pointed out in speaking of the limestone. The general connexion of these with the stratified rock, and the influence, apparent or real, which they may have possessed over its character and disposition, have already been sufficiently described. It only remains to notice the individual specimens in greater detail than could then have been conveniently adopted.
Some of the smaller, which occur near Scarlet point and at Pool vash, are composed of an ordinary fine greenstone, which in certain places becoming perfectly compact, smooth, and uniform, may perhaps be considered as a basalt. These basaltic specimens are, in general, of a dark lead blue, but I observed one vein not exceeding an inch in breadth, of a glossy black, nearly resembling in lustre and compactness the basis of the well-known porphyry of Egg. It traverses a smooth conchoidal limestone, with which it is firmly united at the edges.
The principal veins near Scarlet point present a porphyritic texture. The basis is here of a pale or dark grey, and the crystals of felspar are of the same colour, being very conspicuous, and possessing the common as well as the glassy lustre. In some places these veins are decomposed ; or rather, they have undergone the change which precedes decomposition, to such a depth, as to put on the deceptive appearance of a common yellow felspar porphyry ; the fallacy being only detected by penetrating to a considerable depth in the stone. This case is parallel to those formerly noticed in treating of the Slate isles.
Some of the veins also possess the amygdaloidal cha racter, the compact greenstone basis being filled with rounded grains, which, as far as I observed them, seemed always to consist of calcareous carbonate ; and in certain cases the porphyritic and amygdaloidal characters are combined in the same rock.
At Pool vash, the trap veins are of an earthy character and very pale grey colour, not much unlike some of the tufaceous rocks which so often accompany the great deposits of trap. Even in these cases, certain parts of the vein assume the more ordinary character of greenstone. All these are very firmly united to the limestone which they traverse, and contain grains of pyrites interspersed throughout. In many places, they possess a foliated or schistose structure, which is detected by the progress of decomposition ; the whole exfoliating in thin earthy laminæ, the direction of which is always parallel to the side of the vein.
The last variety which remains to be described, is remarkable for containing olivin dispersed in considerable abundance throughout it, in small grains, and of a yellow colour. This variety is also schistose, and is found at Scarlet point.
Before closing the account of the rocks of this island, it is necessary to point out one substance, which, although it may be viewed as incidental, is nevertheless interesting; since it serves to throw light on the composition of certain mineral veins which have been the subjects of discussion, and may also perhaps explain an appearance occurring in Lewis, which was noticed in the account of that island.
Near the point where the schist and sandstone unite, in the neighbourhood of Peel, a conglomerate of rolled pebbles is seen adhering to the face of the schist, which is here vertical and even. At first sight the appearance is somewhat extraordinary, since it looks like the exposed surface of a bed in a vertical position, placed in the midst of the schist, and unconnected with the sandstones in its vicinity. On a more careful examination, the deception is explained by discovering that a fissure has formerly exisled in this place, and that in consequence of the fall of the rock which formed one side of it, the opposite and remaining wall has been exposed to view. This wall is covered by the conglomerate in question, which has apparently been formed while the fissure was still entire and empty, by the falling down into it from above, of such loose materials, consisting of rolled and other fragments, as were in the vicinity, and by the subsequent infiltration of calcareous and ferruginous carbonates, of which the marks are in other places apparent on the surface of the schist. It is in fact a conglomerate vein. It is impossible to form any conjectures respecting its antiquity ; nor to discover, from any thing visible, whether, like the empty fissures mentioned near Spanish head, it is of recent origin, or whether it may be ranked in point of age with the mineral veins of this island or of other countries. It is certainly of no moment as far as respects the general theory of metallic veins ; but, whether recent or ancient, it serves to illustrate in some degree the origin of those veins, which, like the well known example at the Relistian mine in Cornwall, contain rounded conglomerates.
As in most other places, great anxiety is manifested in the Isle of Man for the discovery of coal. It is not however easy to understand the value of the discovery,even were it made, when the trifling expense of freight from Whitehaven, and the facility of delivering a cargo on so many points of this extended coast, would cause the produce of those mines to rival it in a competition, almost in every instance where land carriage was concerned. It is asserted that coal has been found in the red sandstone near Peel ; but there is little reason to doubt that the fragments in question had either been washed ashore by the sea, vessels laden with this article having been lost on the coast, or that they have been the consequences of fraudulent views on the part of interested coal surveyors or miners. Since my visit to the island I have however received fragments of coal, which appear, on good authority, to have been found under the limestone or in the conglomerate of Derbyhaven, where some expensive borings for that purpose were formerly made. The specimens are of a dry and splendent quality, resembling the coal of Arran, and they are, very probably, analogous to it in situation. They do not appear to offer any temptation in an economical view, even abstracting the commercial objection just stated.
1 It is here proper to remark that many beds of breccia are found connected with this calcareous deposit, and occasionally, superior in position. But these are of a partial nature, and are no where of such extent or importance as to interfere with the claims of the limestone to the rank here assigned to it. in other respects the perpetual inter-ference of the different breccias with the limestone, renders it impossible to describe the latter without occasionally adverting to these. It will hereafter be necessary, even at the risk of some repetition, to collect the whole of them into oue general view.
2 The following list contains a general catalogue of the organic remains which I observed in this rock
Pectenthree species, but neither of them so perfect as to admit of being determined.
Madreporitæturbinated ; conical ; ramose with small, and ramose with large stars.
Trochitæsmall and large.
Ammonitathe species undescribed ? The shell is smooth, the back flat; the sides longitudinally sulcated, one groove running near the outer edge, and the other, a third larger, running nearly on the middle, but rather towards the outer than the inner edge.
Corallinitæramose, flustriform, and straight filiform, resembling those found in the limestone of Dudley.
Productus.A species of this genus as formed by Sowerby, but not definable. It resembles somewhat the Conchyliolithus (anomites) scabriculus of Martin.
Another species, remarkable for the fineness of the striæ.
A turbinated shell, the mouth of which being broken, the genus cannot be ascertained ; but the form of the wreath and the clavellæ disposed on the upper part of the whirls, and passing off into fine plicas, give it a resemblance to the genus Cerithiura.
Another turbinated shell, equally obscure.
Several unassignable bivalves ; besides other fragments so obscure as not to admit even of a conjecture respecting them.
4 Plate XXVII. fig. 1.
5 Plate XXVII. fig. 2
6. Plate XXVII. fig. 3.