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The five distinct groups into which Dr Macculloch has divided the Western Islands, he distinguishes by the names of the Gneiss, the Trap, the Sandstone, the Schistose, and the Clyde Islands.
The Gneiss Islands comprehend the whole of the Long Island, Coll, Tiree, Iona, Rona, and a portion of Rasay. The Trap Islands are, Mull and Skye, with Egg, Muck, Rum and Canna, and the numerous islets which lie around them. The Sandstone Islands are of very limited extent; but are taken in conjunction with the adjoining coast, where this class of rock prevails from the point of Sleat in Skye to Cape Wrath. The Schistose Islands include Isla, Jura, and the smaller islands which skirt the shore of Argyleshire from Lorn to Cantyre. The Clyde Islands are, Arran, Bute, the Cumbrays, and Ailsa.
Dr Macculloch uses the term Gneiss in a much more extensive sense than that to which it is usually confined by the school of Werner. But this extension of the term appears to be fully warranted; for the varying composition of the rock, and the gradual nature of the changes it undergoes, render it impossible to apply separate terms to each variety, without accumulating titles which would incumber rather than elucidate the subject. The chief varieties described have a composition identical with granite, sometimes characterized by a large grain and imperfectly foliated structure, with frequent partial transitions into granite; at other times having a schistose structure, and graduating into micaceous schistus and quartz rock. Hornblende, as well as mica, enters into the composition of it; and is more prevalent than mica in the gneiss of the Western Islands.
With the exception of Arran and a part of Mull, granite is not found in mass in any of the Western Islands; but, throughout the whole of the Gneiss islands, it is of constant occurrence in the form of veins traversing the accompanying strala. These are exhibited under so many varieties of form, that they throw considerable light upon the history of this rock; and it is very important to have the testimony of so accurate an observer as Dr Macculloch, upon a point upon which the opinions of geologists have been so much divided. To those who are less familiar with the controversy, it may be proper to state, that the point at issue is this--whether there is sufficient evidence to prove that granite has been brought into the situations it now occupies, posterior to the deposition of the rocks which lie over it. Dr Hutton, who was the first to maintain this bold and ingenious theory, considered it to be demonstrated by the numerous veins and ramifications which are seen to proceed from the mass of
granite into the superincumbent strata. His opponents maintain that these veins are of cotemporaneous formation with the strata in which they occur. We cannot afford space for more than the general conclusions to which Dr Macculloch has arrived ; and must refer to the work itself for the abundant and very distinct evidence upon which his reasoning is founded.
• All the varieties of gneiss are occasionally intersected by granite veins, and they are indeed almost characteristic of this rock ; being rarely absent for any considerable space, and seldom traversing micaceous schist unless under circumstances where they can be traced to some neighbouring mass of granite. They are, however, most abundant in the granitic division. They are infinitely various in size, and in the number and intricacy of their ramifications; and it is further worthy of remark, that the contortions of any mass of gneiss are always proportioned to the number and importance of those which it contains. Hence it is that the schistose is more free from contora tions than the granitic variety. It is nevertheless proper to make an exception respecting those beds of gneiss which alternate with other rocks, such as clay slate; these, as far as I have observed, never containing veins. In some varieties of gneiss they are so abundant as nearly to exclude the original rock, so that the mass presents little else than a congeries of veins. An instance of this nature occurs in the Flannan isles ; but the most striking are to be seen on the northwest coast, between Loch Laxford and Cape Wrath. The latter spot is no less remarkable for its picturesque grandeur, than for the perfect manner in which it displays this circumstance, the cliffs being free from lichens, and unaltered by the weather ; so that all the parts are as visible as in an artificial section. The hornblende schist and the gneiss are broken into pieces and entangled among the veins in the same manner as the stratified rocks are in the trap of Skye; but with infinitely greater intricacy, so as rather to resemble a red and white veined marble with imbedded fragments of black. These fragments do not seem to form a twentieth part of the whole mass ; while the progress of the different veins, and their effects in producing the disturbance, are as distinct as in an ordinary hand specimen. If the intricacy of the ramifications, and the intersection of one set of veins by a second and a third of different textures, present an argument in favour of a succession of these at several periods, there is here no want of such evidence,
• Whether these granite veins are connected with masses of granite in all cases, cannot be determined. In some instances, as in Perthshire, where the gneiss reposes on granite, it is probable that they proceed from it; but it has already been seen that there are no traces of that rock in the Long Island. That is, however, no proof of its non-existence; and the circumstance of the veins being always present when the gneiss reposes on granite, and absent when another rock is interposed, renders it probable that in these cases granite, though invisible, is still present.
The following considerations render it probable that the granite veins which traverse gneiss are posterior to the including rock, and formed under circumstances analogous to those under which other granite veins have intruded into the schistose rocks with which they interfere.
• They are accompanied by fractures or contortions of the gneiss, of such a nature as to prove that it once possessed a condition capable of yielding in different ways to external force, while these appearances are also proportioned to the number and intricacy of the veins. In the schistose varieties which yield easily in the direction of the laminæ, the veins frequently hold a parallel course to these, while an occasional flexure occurs in those cases where the vein crosses them; the edges being incurvated from the thicker part, or the root of the vein, towards its termination. Lastly, in cases, of which an example was described in Tirey, where a vein traverses a mass of limestone included in the gneiss, it disturbs that substance as well as the surrounding rock; and in another parallel instance noticed in Scalpa, where the included substance is serpentine, the vein itself undergoes a change, by participating in the nature of that rock during its passage.
The schist is in these islands occasionally traversed by granite veins, similar in aspect and composition to those whieh traverse the gneiss of the neighbouring parts. This may be adduced as a proof of the posteriority of these veins to the rocks which they intersect; since they are here, as in Coll, found to pass indiscriminately through two different rocks, of which the one appears, from its position, to be of more recent formation than the other.
• Although unwilling to repeat the trite arguments derived from the nature of granite veins, I cannot avoid remarking the strong support they receive from the circumstances now mentioned, particular. ly from the fact of the imbedded fragments; an appearance which cannot be reconciled to any supposition yet offered, except that of the posteriority of granite.' I. 218–220. 144. 556–557.
The Island of Arran was the great source from which Dr Hutton drew the proofs in support of his theory of Granite, and has been since appealed to by his illustrious commentator, Mr Playfair; and as this island has been so minutely examined by Dr M., it is important that we should notice, that his views entirely coincide with those of Hutton and Playfair upon the phenomena in question.
It would in itselt be sufficient evidence against the stratification of the granite of Arran, to point out its connexion with the superincumbent schists. This has been brought to light at Loch Ransa in a most distinct manner, by the removal of the soil where the junction of these substances takes place. In numerous other situations in Scotland, it can be seen already exposed to view ; insomuch that no rational doubt can be entertained respecting the true nature of a phepomenon so very palpable and so frequent. As a fact proved, it may
also be considered a rule, not an exception. The ramifications which proceed from the mass of granite into the schist at the place above mentioned, are numerous and intricate, and they diminish as they recede from the main body; while their mineral character undergoes a change; the granular structure becoming more minute, almost in proportion to the minuteness of the vein, until the true character of granite entirely disappears. It is superfluous to repeat the conclusions which have been drawn from these appearances, relative to the origin of granite, since they must be familiar to every one.' II. 345, 346.
The term TRAP, by which the next group of islands is distinguished, is also used in a generic sense, and comprehends that extensive variety of rocks to which the names basalt, greenstone, syenite, claystone, clinkstone, and porphyry are applied ; all of which are found to pass into each other by insensible gradations, and are associated by a common set of geological relations. They can nowhere be studied to greater advantage than in the Western Islands, from the various forms in which they occur, and the facility with which their connexion with the accompanying strata may be observed. The facts which Dr Macculloch has recorded in regard to this class of rocks, are highly important. A detailed account of them we cannot attempt to give; but we shall endeavour to state the general results which he has deduced from his extensive examination of this interesting class of geological phenomena. One of the most important, is the geological identity which he has traced between syenite and the other members of the Trap family; for the resemblance which this rock very often bears to granite, may lead, and probably has often led, to very erroneous conclusions, where its geological position has not been fully investigated. In the island of Skye, there is a very extensive district where common greenstone, amygdaloidal claystone, common pale syenite, micaceous syenite, and simple blue claystore, are found irregularly recurring throughout the whole group. The predominant variety of the syenite, is 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 assumed by this mixture; while, in some rare instances, quartz enters into the composition. More rarely still, it contains mica; and, in this case, it cannot be distinguished from those granites which contain crystals of hornblende, superadded to the usual threefold mixture of quartz, felspar, and mica.
• The character of this syenite gives rise to some conclusions that are not unimportant. At present, it is easily mistaken in hand speçimens for a variety of those granites which are entirely subjacent to
the older rocks, and divested of any pretensions to the overlying character. With a very slight change of composition, it could not be distinguished. That such a change occurs in other situations, seems proved by the observations of Mr Von Buch in Norway, who has described granite lying on black conchiferous limestone. This granite is, according to that author, connected with porphyry; and there is no reason therefore to doubt that the instance quoted by him is analogous to this, although he has not entered into a full examination of its connexions. His overlying granite will therefore prove, like this, a mere variety, in a geological view, of the syenite and porphyry formation ; another proot, if such were wanting, of the necessity of great caution in drawing geological inferences from the examination of mere specimens of rocks, and of the absolute necessity of tracing the actual connexions of all those rocks which are subject to similar variations of character.
• In the next place, this syenite may serve to prove, that, in many other cases, the granites, which we have been accustomed to consider as prior in formation to the secondary strata, if not to the primary schists, may be often posterior to both : the opportunities for ascertaining their relations being wanting ; sometimes from the total absence of the secondary rocks in the places where they occur ; at others, from the impossibility of obtaining sufficient access to them, to enable us to ascertain a point of great delicacy and difficulty; and in a third case, perhaps from the demolition and disappearance of those portions which may have once been overlying, and have, as being the most limited and the most feebly supported, been removed through a long course of time by the ordinary causes of waste. 1. 371, 372.
Similar instances of a gradual and imperceptible transition, from a perfectly characterized greenstone to an equally distinct syenite, are of frequent occurrence in Harris, Rum, Mull, and Arran; and in every part of the Western Islands, where the trap rocks prevail, abundant proof may be found how utterly unimportant, in regard to their geological history, is that distinction which is founded upon the variety in their mineralogical structure. In describing the trap rocks of Kerrera, Dr Macculloch makes some very judicious remarks upon this subject.
• As in other cases, the different modifications or members are here found gradually changing their characters, and passing into each other. Thus, greenstone passes into basalt, or into clinkstone, or compact felspar, or into porphyries and amygdaloids of various aspects. Many of the simpler varieties occur, of a brown, grey, reddish, or white colour, with different degrees of hardness and much diver: sity of fracture ; offering specimens, to none of which, in the present state of our nomenclature, it is possible to apply names that could be understood. It is, perhaps, better to leave such substances