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 quoted 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.

The instances of fracture, separation, displacement, flexure, and éntanglement, 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 di rection, 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,

enable us to trace distinctly those arrangements in the veins, which, in most instances where the surfaces only admit of examination, are nearly as much matter of induction as of observation. It is from those veins which are entirely or in part conformable to the planes of the strata, that we are enabled to understand the manner in which the character of the including rock has affected the form and disposition of the vein. Where its course lies at angles with the direction of the strata, it is generally straight and persistent, while its breadth is equal throughout; as if the violence and suddenness with which the original fissure was formed, had been such as to disregard all impediments arising from inequalities of hardness, these bearing no proportion to the force exerted in the separation. Where, on the contrary, the fissure has taken place in the direction of the beds, the vein will often be found unequal in thickness, and sometimes curved, or even tortuous; from the greater facility with which these have yielded in that direction, and from the inequalities originally existing in the disposition of the laminæ, which, by their separation, have given a passage to the fluid-intruding material. Such appearances are here both frequent and remarkable; and analogous, if not equally well marked facts, occur in other parts of the Western Islands.' II. 168, 169.

'If we examine any other portion of the strata, we shall find similar, although not equal deficiencies, attended by consequent irregularities; all of them, doubtless, equally caused by the trap rocks, the effects of which seem to consist in the overwhelming of some of the strata and the displacement of others; the whole being on a scale so large and distinct as to leave no doubt respecting their nature, and producing a connected train of appearances that cannot be traced in any other place with which I am acquainted. I shall not attempt to point out the revolutions that must have taken place in these parts of the earth's surface before such effects could have been produced; still less to inquire into causes, in a work intended as a simple record of the physical structure of the places examined, as far as it is possible to give such a record its due value without a certain proportion of theoretic connexion. While the changes are as obvious as their magnitude is striking, it is evident that they have been produced long after the greater part of the materials here forming the surface of the earth had assumed their regular distribution.' I. 393.

If the connexions of these rocks with those on which they lie are examined, it will be found that they are sometimes in contact with the clay slate, at others with the graywacké; a proof of their posteriority to both, and of their irregularity of position. They are also found branching into large veins, which again ramify into smaller; penetrating the schist in various directions, and sometimes for a long space conformable to its disposition. Where these parts only are visible, and where their connexion with the superincumbent masses cannot be traced, they have the appearance of beds alternating with the schist; but this appearance, traced in one instance to its true origin, justifies us in assigning the same to all similar masses.

The relation of the trap to the sandstone and conglomerate, is by no means so clear: yet as some decided instances occur of its overlying position with respect to these also, we are perhaps justified in concluding, that the whole of the trap is posterior to the stratified rocks, whether primary or secondary; and that, wherever it appears inferior to the latter, the apparent priority implied by this, is only an example of intrusion similar to that which occurs between the trap and the schists.' II. 123, 124.

The similarity between the trap rocks and those of volcanic origin has been too often noticed to require a repetition of that remark. It is equally known to geologists that volcanoes are situated in this class of rocks; a position which appears to countenance the opinion that there is a connexion in the origin of the two, and that the recurrence of trap in any given place, such as it is observed in the island under review, is a phenomenon closely linked with the actual existence of volcanoes in districts of trap rock.

It is still necessary to point out a circumstance of no uncommon occurrence in the trap of this island, as well indeed as in many others which I have examined; partly because it confirms this conjecture, and is an additional proof of the resemblance between the traps and the lavas, and partly because it has been doubted or denied by many geologists who have treated of these rocks. This is the existence of cavities resembling in every respect those which are contained in the scoria of volcanoes, or in cellular lavas. They occur also abundantly in Mull and in the trap near Oban; and are particularly conspicuous in some parts of the Little Cumbray, where I shall have occasion to describe them more fully.' I. 458.

And in describing these cavernous traps of the Little Cumbray, he states that, it is often impossible to distinguish them from scoriform lavas, so identical are their characters with those of the volcanic rocks. Similar specimens occur among the trap in many other places, but they are rarely so perfect. That any other cause but the extrication of air should have produced these cavities, is highly improbable; and on the igneous view of the origin of those rocks, the existence of such a cause is sufficiently proved.' II. 487.

The impossibility of making any arrangement that should perfectly combine geographical with geological convenience, has rendered it necessary to form a separate division for the Sandstone Islands. They are of very limited extent; but the rock itself forms a very important feature in the geological history of the country, and prevails to a great extent on the Mainland, occupying an uninterrupted line from Glen Elg to Cape Wrath, and extending inland nearly thirty miles. It occurs also in the centre of Sutherland, and forms a large portion, if not the whole, of Caithness. Notwithstanding its name of Red Sandstone, it must be classed with the primary rocks, for it is found alternating with gneiss; but, until this important discovery of