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 lamina, 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 irregula rities; 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

Dr Macculloch, it has always been considered as belonging to the secondary rocks, or at least to that class of secondary rocks to which some geologists apply the term transition. It is prin cipally distinguished from the red sandstones of the centre of Scotland, by its extreme hardness, and by its crystalline texture. In this respect it often equals common quartz, no distinction between the constituent grains being visible, but the whole appearing as if cemented by a general solution of silica. It is often, however, in these cases, gravelly, while it occasionally consists of large fragments, angular as well as rounded, of different colours, compacted into one crystalline mass. It is always stratified. In the predominant examples, the strata are very thin and equal; and in all these cases the angle of inclination is low, deviating in some instances but slightly from the horizontal position; and the surfaces of the flags sometimes bear these marks of undulation which occur in the secondary sandstones, and resemble so much the marks left by the sea on sandy shores. Where the angle of inclination becomes considerable, the distinctness of the stratification diminishes; and where the beds assume the vertical position, it requires great care to discover any marks of that order; the rock acquiring the aspect of some granites, or that of the irregular gneiss by which it is accompanied, and being split into prismatic or angular fragments. Both in the islands and on the Mainland, it graduates into gneiss; and the transition is effected sometimes by schist, and grey indurated sandstone, and sometimes by quartz rock. In a large proportion, however, of the Mainland, no such transitions are found; but there is a sudden and complete alternation of the two rocks. The alternations with gneiss are visible at Loch Carron and Great Loch Broom; the sandstone in the latter place lying beneath the gneiss, and both having a common dip to the south-east. It forms hills of all dimensions, and of every variety of aspect-round, conical, ridged, or serrated, alternately rising to the greatest average altitude of the Scottish mountains. Kea Cloch, in Ross-shire, presents in itself exam-ples of all these forms; the summits being in some places no less strongly serrated than those of the Arran mountains, while the height is between 3500 and 3700 feet; and, what is remarkable, the strata of this mountain are horizonal, and consist of the red sandstone from the base to the summit; and as it is separated by a wide valley from the adjoining mountains, which are composed of the same rock, it stands as an index of the enormous waste by which the land has been worn down into its present outline.

It is scarcely necessary now to say, that this sandstone must be ranked in the class of primary rocks. On no other view can the preceding facts be explained, notwithstanding the occasional points of resemblance which it presents to those of the secondary division. It must therefore be considered as a red primary sandstone; and, that it is not even the latest of the primary strata, is evident from the preceding history of its connexions.

No objection need arise with respect to the use of the term primary, as applied to a rock composed of the reunited fragments of former rocks. That term has here, as on other occasions throughout this work, been substituted for the word primitive, and is purely relative; implying nothing theoretical respecting the origin of any order of rocks, whether stratified or amorphous. Nor is this sandstone a solitary example of the mechanical recomposition found among rocks of the primary division. The instances of this structure occurring in the quartz rock of Jura and the associated islands, are equally remarkable; and there are indeed striking analogies between that series and the present, in many important particulars. With respect to the existence of an unquestionably mechanical structure in a primary rock, it may also be remarked, that the micaceous schist of Isla, which contains fragments of granite and quartz rock, presents a decided example of it; as do also those rocks of the graywacké and conglomerate structure, which are found among the argillaceous schists in various parts of Scotland. If even the red sandstone of this district be considered as a modification of quartz rock, no alteration will follow in the nature of any of the views that have here been brought forward. The same history will be transferred to a new term, not to another substance; and the same consequences will follow, that so often result from the substitution of one word for another; in the best cases, that of leaving every thing precisely where it stood before; and in the worst, confusion and obscurity, instead of light and order.' II. 98, 99.

In further illustration of the important inferences to be deduced from this history of the primary sandstone, we may notice the occurrence of a conglomerated rock formed of fragments of quartz, imbedded in a micaceous schist, in the island of Coll

of a conglomerate composed of rounded and angular pebbles. of quartz and gneiss, imbedded in a gravelly clay, found contained between two portions of gneiss in Lewis-of a rounded pebble imbedded in the clay state of the Isle of Man-but, above all, that most interesting discovery, of a rock containing organic remains alternating with gneiss, at Loch Eribol. It is scarcely necessary to point out how entirely these facts demonstrate the truth of that great and fundamental position of the Huttonian theory, the most important in its consequences in the whole range of geological inquiry, That in all the strata we discover proofs of the materials having existed as

elements of bodies which must have been destroyed before the 'formation of those of which these materials now actually make a part.' That Dr Macculloch coincides in those views, is sufficiently evinced by the following passage.

With the exception of granite, it is not probable that geologists have yet discovered a rock beneath which organic remains may not be found. As they diminish in number, in a general sense, the further we recede from the most recent strata, it is plain that, among the lowermost rocks, they may occur so rarely as still to have escaped observation; a circumstance of which the chances would be increased by their more limited variety, more complete loss of texture and shape, and more simple forms. Their gradual disappearance in those cases where the secondary limestones assume the massive structure and crystalline texture, described in the account of Skye and of the Isle of Man will illustrate this opinion, and suggest the possibility, that even the common primary limestones may originally have contained organized bodies.

Perhaps when observations have been further multiplied, it may yet be ascertained, that there has been no portion of time during the deposition of the stratified rocks, however ancient, in which animals have not existed. In concluding this subject, it is unnecessary to point out the importance of the preceding facts to geological science ; and it is almost superfluous to say, that to account for them by calling this gneiss a transition rock, is merely to substitute a term which leaves the fact and its consequences precisely where they stood before II. 514, 515.

The Schistose Islands comprehend Isla, Jura, the smaller isles immediately contiguous, and those which lie between Mull and the Mainland. Their strong resemblance to each other in structure, the obvious repetitions of similar strata which occur throughout them,-the mutual correspondence of their outlines and of the bearing of their strata, together with the intimacy of their geographical positions,-render this association as natural as it is useful for the illustration of the whole. They consist of all the primary stratified rocks except gneiss; and, among them, quartz rock and clay slate appear predominant, micaceous schist being less abundant. These are accompanied by a series of rocks composed of chlorite schist, of felspar, and of hornblende, in various states of mixture and alternation; and, lastly, by graywacké and limestone, the last being the smallest in quanti ty. The same alternations of rock which occur in any one or more islands, are found also nearly throughout the whole group, although in different proportions. In contemplating these alternations, there is nothing more remarkable than the frequent change of substance, and the tenuity of the strata which are thus intermixed with each other; and, in consequence of this perpe