sive crystalline protrusions have been converted into perfect schists. The dykes of dolerite have been transformed into hornblende-schists and the granitic pegmatites have been reduced to a kind of powder which has been rolled out so as to simulate the flow-structure of a lava. There is evidence that most, if not all, of this dynamical change was effected long before the deposition of the Torridonian series, for the latter rests in nearly horizontal sheets, with a strong unconformability upon the crushed and sheared gneiss.

Torridon Sandstone. This group of rocks covers only a limited area in the north-west of Scotland, but it must once have spread over a far more extensive region. It reaches a thickness, as I have said, of 8,000 or 10,000 feet, and consists almost wholly of dull, purplish-red sandstones, often pebbly, and bands of conglomerate. Dark grey shales, already alluded to as occurring towards the base of the series, are repeated also in the highest visible portion, and have yielded tracks of what seem to have been annelids and casts of nail-like bodies which may have been organic. I have said that the Torridonian deposits which were classed by Murchison as Cambrian, have been proved by the discovery of the Olenellus zone in an unconformable position above them, to be of pre-Cambrian age. Except along the line of disturbance to which I shall immediately refer, these strata are quite unaltered. Indeed, in general aspect they look as young as the old red sandstones with which Hugh Miller identified them. It is at first hard to believe that such flat undisturbed sandstones are of higher antiquity than the very oldest Palæozoic strata which are so generally plicated and cleaved.

The interval of time between the deposition of the Torridon Sandstone and of the overlying Cambrian formations must have been of enormous duration, for the unconformability is so violent that the lowest Cambrian strata, not only transgressively overspread all the Torridonian horizons, but even lie here and there directly on the old gneiss, the whole of the intervening thick mass of sandstone having been there removed by previous denudation. At Durness, in the north of Sutherland, about 2000 feet of

Cambrian (possibly in part Lower Silurian) strata can be traced, the lower portion consisting of quartzites, the central and upper parts of various limestones, sometimes abundantly fossiliferous. Nowhere else in the north of Scotland can so thick a mass of early Palæozoic rocks be seen. Elsewhere the limestones have been in large measure replaced by a complex group of schistose rocks which rest upon the Cambrian strata, and like them dip, generally at gentle angles, towards the east. It was the opinion of Murchison, and was commonly admitted by geologists, that these overlying schists represented a thick group of sediments, which, originally deposited continuously after the limestones, had been subsequently altered into their present condition by regional metamorphism. They were variously named the "Eastern schists," the " younger gneiss," the "gneissose and quartzose flagstones." Nicol, who at first shared the general opinion regarding them, afterwards maintained that they did not belong to a later formation than the limestones, but were really only the old gneiss, brought up again from beneath by enormous dislocations and over-thrusts. We now know from the labors of Professor Lapworth and the officers of the Geological Survey, that Murchison and Nicol had each seized on an essential part of the problem, but that both of them had missed the true solution. Murchison was in error in regarding his younger gneiss as a continuous sequence of altered sedimentary rocks conformably resting on the Cambrian (or to use his terminology, Lower - Silurian) formations. But he sagaciously observed the coincidence of dip and strike between the schists and sedimentary rocks below them and inferred that this coincidence, traceable for many leagues, proved that the metamorphism which had given these schists their structure must have taken place after the deposition of the Durness limestones. Nicol, on the other hand, with great insight recognized that there was no continuous sequence above those limestones, but that masses of the old gneiss had been thrust over them by gigantic faults. But he failed to see that no mere faults would account for the coincidence between the structural

lines just referred to in the Cambrian strata, and in the overlying schists, and that the general tectonic structures and lithological characters of the eastern schists differed in many respects from those of the Lewisian gneiss.

The problems in tectonic geology presented by the complicated structures of the northwest of Scotland have been ably worked out by the officers of the Geological Survey, to whose report in the Quarterly Journal of the Geological Society for 1888, I would refer for full details. It has been shown that, besides stupendous dislocations and horizontal displacements, the rocks have been cut into innumerable slices which have been driven over each other from the eastward, while at the same time there has been such a general shearing of the whole region that for many hundreds of square miles the original rock-structures have been entirely effaced, and have been replaced by new divisional planes, which, when they approach the underlying Cambrian strata, are roughly parallel with the bedding planes of these strata.

In this region, therefore, we have striking proofs of a stupendous post - Cambrian regional metamorphism. But there is still much uncertainty regarding the geological age of the rocks which have been affected by it. There can be no doubt that large masses of the old gneiss, torn up from below, have been thrust bodily westward for many miles, and are now seen with their dykes and pegmatites resting on the Durness limestones and quartzites. It is equally certain that in other districts huge slices of the Torridon sandstones have been similarly treated. But where all trace of original structure has disappeared, we have, as yet, no means of definitely determining from what formation the present eastern schists have been produced. The ordinary gneissose and quartzose flagstones do not appear to me to be such rocks as could ever be manufactured by any chemical or mechanical process out of the average type of Lewisian gneiss. I have long held the belief that they were originally sediments, but whether they represent altered Torridon Sandstone, or some clastic formations which may have followed the

Durness limestones, but which have been everywhere and entirely metamorphosed, remains for future discovery. For my present purpose, it is sufficient to observe that, in the meantime, as we can not be sure of the origin of most of the rocks, which, between the West Coast and the line of the Great Glen, have been subjected to a gigantic post-Cambrian regional metamorphism, it seems safest to exclude them from an enumeration of the pre-Cambrian rocks of Britain.

Dalradian. East of the line of Great Glen, which cuts the Scottish Highlands in two, another group of crystalline schistose rocks is largely developed. It consists mainly of what were undoubtedly originally sedimentary deposits, though they are now found in the form of quartzites, phyllites, graphitic schists, mica-schists, marbles, and various other foliated masses. With them are associated numerous eruptive rocks, both acid and basic, sometimes still massive and easily recognizable as intrusive, sometimes more or less distinctly foliated and passing into different gneisses, hornblende-schists, chloritic-schists, etc. Though it is not always possible in such a series of metamorphic rocks to be certain of any real chronological order of succession, those of the Highland tracts have now been mapped in detail over so wide an area, that we are probably justified in believing that a definite sequence can be established among them. These masses must be many thousand feet thick. Their succession and association of materials are so unlike those of any of the known older Palæozoic rocks of Britain, that they can hardly be the metamorphosed equivalents of any strata which can be recognized in an unaltered condition in these islands. Some traces of annelid casts have been found in the quartzites, but otherwise the whole series has remained entirely barren of organic remains.

What then is the age of this important series? I must confess that in the meantime I can give no satisfactory answer to this question. I have proposed, for the sake of distinction and convenient reference, to call these rocks "Dalradian." Murchison supposed them to be a continuation of his Durness quartzites,

limestones, and "younger gneiss." His belief may still prove to be in some measure well founded. But at present we have no means of deciding whether the quartzites and limestones of the Central Highlands are the more altered equivalents of the undoubtedly Cambrian strata of the north-west. It is possible that in the vast mass of metamorphosed rocks constituting the wide stretch of country from the northern headlands of Aberdeen to the south-western promontories of Argyllshire, there may be portions of the old Lewisian gneiss, tracts of highly altered Torridon sandstone, belts of true counterparts of the Cambrian quartzites and limestones of Durness, and, what should not be forgotten, considerable portions of some later sedimentary series which may have followed these limestones, but which, by the great dislocations already referred to, have disappeared from the north-west of Scotland. We are gradually learning more of these rocks, as the detailed mapping of them by the Geological Survey advances, and when the ground on either side of the Great Glen is surveyed, it may be possible to speak with more certainty regarding their true geological relations.

A glance at a geological map of the British Isles will show that the metamorphic rocks of the south-western Highlands of Scotland are prolonged into the north of Ireland, where they spread over a region many hundred square miles in extent. They retain there the same general character and present the same difficult problems as to their true stratigraphical relations. Quite recently, however, a new light seems to have arisen upon these Irish rocks. My colleagues on the Irish Branch of the Geological Survey have detected several detached areas of coarse gneisses, which in many respects resemble parts of the Lewisian gneiss of north-west Scotland. In some cases these areas lie amidst or close to "Dalradian" rocks, but with that obstinacy, which so tries the patience of the field-geologist, they have persistently refused to disclose their true original position with regard to these. Some fault, thrust-plane, tract of boulderclay or stretch of bog is sure to intervene along the very junction-line where the desired sections might have been looked for.