and moreover the state of the science has so much changed, both in regard to our knowledge of the structure of individual rocks, and the mode of conducting geological investigations, that we can hardly compare the observations of the two geologists, far less infer anything to the disadvantage of the learned Professor, where they do not agree. It is indeed satisfactory to find, that Dr M. bears testimony to the accuracy of his account, as far as the facts have been described. But the work before us forms a manual by which the structure of each island may be examined with minute detail, and will prove a most valuable guide to the mineralogists who may hereafter visit any part of that most interesting and instructive coast. It is obviously quite impossible for us to enter at all into the detailed descriptions contained in the work;-it will be sufficient if we give a rapid sketch of the general structure of these islands, and notice some of the more remarkable points of geological speculation which appear to us to have been elucidated by the labours of this very accurate observer. We do not apply the term accurate, without knowing that it is deserved; and it will be very satisfactory to our geological readers to be informed, that Dr Macculloch's description of the Island of Arran has been compared with the notes which a very eminent scientific person made upon the spot a few years ago, with the view of drawing up a geological account of that remarkable island, in conjunction with the late Professor Playfair and Lord Webb Seymour; and he has found that, with a single exception, Dr Macculloch has pointed out every object of importance-and that exception occurs at a spot which is only laid bare at low water.

Although there is a great deal of low land in the islands, their general aspect is hilly and mountainous, and their shores are generally bounded by bold and precipitous cliffs. In some there are mountains, the elevation of which almost equals the more conspicuous of those of the mainland. Ben More in Mull, Dr Macculloch ascertained, by barometrical measurement, to be 3097 feet above the level of the sea. Hecla in South Uist, is 3000; Clisseval in Harris, 2700; Goatfell in Arran, 2865; Ben an Oir in Jura, 2500; and the Cuchullin Hills, in Skye, range from 2000 to 3000 feet. The general direction of the coasts, the mountainous ridges and the intervening valleys, is between N. E. and S. W., a disposition analogous to what prevails in the mainland; and, in general, these prevailing directions will be found to correspond in a great degree with the directions of the strata of rock. There appears, moreover, a remarkable identity between the strata of the is

ands and those of the mainland; and, in a great variety of cases enumerated by Dr Macculloch, it may be distinctly ascertained that they have at one time been continuous, although now entirely detached by the deep sea which rolls between them. There is, indeed, no part of the geological history of those regions more interesting or more instructive, than the numerous and irresistible proofs they exhibit of that gradual but sure decay which is in unceasing operation over the whole surface of the globe. Nor is it only from the identity of the strata of the islands with those of the mainland that we are led to infer that an intervening portion has been worn away. A proof no less conclusive exists, in the rounded blocks that are sometimes scattered upon the surface of the islands, or are embedded in the alluvial soil, of rocks totally distinct from any existing in the island itself, but identical with rocks found in situ on the adjoining islands or mainland: such blocks are very numerous on the surface of Staffa. Many of the facts described clearly indicate, that the separation has not been sudden, but the slow action of destroying forces; and if it is legitimate to measure the process of past decay by the standard which our own experience supplies of its advancement, there are none of the phenomena of geology which speak a language less equivocal as to the high antiquity of this earth, than those to which we have now alluded.

In determining the plan he should follow in describing the several islands, Dr Macculloch has not confined himself to any geographical order, but has classed those islands together which are of analogous structure, and are connected by their natural affinities. By this arrangement, unnecessary repetition has been avoided; the islands mutually assist in explaining the structure of each other; that which is obscure in one, being elucidated by some other of the group; and the interest of the whole is increased by the comparison. The general relations of the different individuals of each group with the adjoining mainland, are pointed out; and, at the conclusion of each great division, the principal facts are brought together in one condensed view, that the reader may be enabled to draw such general conclusions as he may consider to be made out by the evidence that has been laid before him. The descriptions are aided by numerous coloured maps, plans, and views-a species of illustration which Dr M. has more at command than any geologist we are acquainted with; a very important branch of the education of a geological traveller, and which, if more frequently attended to, would save many an unintelligible description.

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 strata. 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 contortions 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 which 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, particularly 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 itself 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 phenomenon so very palpable and so frequent. As a fact proved, it may