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 claystone, 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 specimens 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 proof, if such were wanting, of the ne cessity 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." I. 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 diversity 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

without a name, than to designate them by terms that have been already too often used in a lax and ambiguous sense; since the negative confusion that may result from the want of appropriate appellations, is much less inconvenient than the positive one which attends their misapplication; inasmuch as it admits of future amendment, and as it is much easier to add to a nomenclature than to change its signification.' II. 122, 123.

Throughout the whole range of the Western Islands, there is scarcely a rock, whether belonging to the primary or secondary strata, which is not more or less intersected by veins or dykes of trap. In every part of the Long Island, the gneiss is penetrated by these veins, varying in thickness from many yards to the diameter of a thread: One in the little island of Hamersa, off North Uist, is about fifteen or twenty feet in thickness; passing through the gneiss in a position nearly vertical, splitting into minute ramifications, and very much confounded and mixed with the including rock. In Barra, they are of very small size, but are subdivided into branches of extraordinary tenuity, and traverse the gneiss or granite veins in the most intricate manner. Veins of the same degree of tenuity may be observed in the adjoining islands of Hellesa and Gia, branching off from dykes of great dimensions. One off the shore of North Uist, of about twenty feet in thickness, is composed of two or three beds, each of which has a peculiar structure, and might be mistaken for a succession of strata, were it not seen to cross the beds of clay slate through which it passes, in a curved and somewhat waving course. A vein somewhat similar in the variety of rock, is found at Loch Oransa in Skye, traversing gneiss, being a fine basalt at the edge, and passing by degrees into greenstone, porphyry, and amygdaloid. A very remarkable vein occurs on the shore of Loch Scresort in Rum, where the mass is columnar; but the directions of the columns is not at right angles to the direction of the vein, but parallel to it, and their position horizontal, divided into rude joints of irregular length. In Rum, there is a vein of ordinary dark basalt, enclosing fragments of the adjoining red sandstone, scattered at considerable distances through it, and varying from an inch or two to a foot in diameter. Analogous instances occur in Muck, Seil, Lunga, and Bute. In Seil, the vein runs between strata of clay slate; and, parallel to them, and at each side, it is intermixed to the depth of an inch or two with distinct fragments of the slate. In Arran, there is a vein of pitchstone, including fragments of the adjoining red sandstone; a circumstance that has not been before observed in a pitchstone vein, and pointing out an interesting analogy between that substance and trap, in addition to those already known. Where these included frag

ments occur in a vein of small dimensions, they are rarely changed in appearance from the adjoining rock; but where they are found in the larger masses of rock, they are considerably altered, having the appearance of those gradual changes which mark the commencement of fusion. Some very remarkable instances of the conversion of shale, containing organic remains, into lydian stone, by the contact of the trap rocks, similar to what occurs at Portrush on the coast of Antrim, have been noticed by Dr Macculloch in Skye and the Shiant Isles.

One of the most plausible arguments of those who maintain that the trap rocks have been deposited in the same manner as the strata with which they are associated, is founded on the assumed fact, that they are frequently found alternating in regular and parallel succession with the strata. Numerous instances of this alternation have been observed by Dr Macculloch in the Western Islands; but he has had frequent occasion to discover, that this seeming alternation is quite fallacious, so much so, as to lead him to doubt whether, in the cases that have been quot ed as a proof of slow deposition like the other strata, a similar fallacy may not exist.

The interference of the trap with the strata presents, as I already remarked, every modification that has yet been described. Some of them require a few words; but the greater number will be sufficiently and even better illustrated by the drawings, which have been so selected as to contain the principal details of the whole line of disturbance the general aspect of larger portions of the coast being given in other sketches taken from a distance, where the minor disturbances were invisible. One of the objects is to show that there is no persistent parallelism between the trap and the stratified rocks, and that the occasional regularity of alternation is deceptive; since, by extending the examination, we always arrive at some point where that regularity ceases. This fact has often been noticed on a smaller scale; but there is here a display of the whole arrangement on a scale so magnificent and extensive, since it occupies many miles in length, and so free from all chance of error, since the sections are as perfect as if made by art, that it would be unpardonable to pass it over.

6

The instances of fracture, separation, displacement, flexure, and entanglement, are sufficiently visible in the drawings: those of irregularity in the stratified disposition of the trap, require a few words. In one case, which occurs not far from Holme, there is a bed extending for a great way, surmounted by a parallel series of the secondary strata in contact with it; but, on a narrow inspection, innumerable veins are seen branching into the strata in every possible direction, illustrating in a very perfect manner the origin of at least one order of veins. In a second case, three beds of trap can be traced in a parallel direction for a considerable space, separated by the regular strata, when suddenly the whole unite into one mass.

Had not this occurrence at length betrayed the true nature of these beds, there would have been no hesitation, from a limited observation, in describing them as unquestionable instances of alternation. In the last case which I shall enumerate, one regular bed of trap may be traced for more than a mile, lying in a parallel and undisturbed continuity between the secondary rocks. On a sudden, however, it bends downwards, so as to pass through the strata immediately in contact, and then continues to hold its regular course for a space equally great, with a thickness and parallelism as unaltered as before. I need make no commentary on these several facts, since the conclusions that may be deduced from them have long been familiar to geologists. I. 382, 383.

In describing the limestone of Broadford, I formerly remarked that it contained beds of trap, often so equably interstratified as to be generally undistinguishable from regular alternations. An excellent example of their real nature, and of their identity with the analogous appearances in the north-eastern coast of Trotternish, is afforded by a circumstance occurring among similar beds at Borrereg. In one of these, the bed, after a very extensive parallel course among the strata of limestone, undergoes a sudden flexure into an oblique position; which, shortly becoming vertical, it is then continued beyond reach of investigation, under the usual form of a common trap vein; intersecting at right angles in one place the strata to which it was parallel in another. I. 400.

There is perhaps no point in Geology that has been more satisfactorily made out, than the history of the trap rocks; and the great mass of evidence which has now been collected regarding them, appear to warrant the following conclusionsThat they are all of posterior formation to the stratified rocks with which they are associated, whether primary or secondary, and have been consequently intruded among the strata-that this intrusion has been accompanied, in many cases, with such force as to fracture and displace the strata-that they have been ejected from below in a fluid state-and that this fluidity was produced by the action of heat. Throughout the whole of this work, wherein an extensive series of the trap rocks are described, Dr Macculloch almost invariably has come to these conclusions. We refer our readers to the work itself, for the numerous instances where these opinions are delivered; and must content ourselves with one or two short extracts.

Numerous trap veins are seen traversing the strata of this island. They have no certain direction, being in some places erect, in others inclined; at one time intersecting the beds in angular directions, at another insinuating themselves in a parallel course between their laminæ. In some cases, the same vein will be found to occupy both positions, changing its course from a transverse to a parallel one. -The deep and perfect sections of the rocks, both here and in Scarba,