On the Physical Structure of the Highlands in connexion with their Geological History. By His Grace The DUKE OF ARGYLL, K.T., F.R.S., F.G.S.*

The questions dealt with by Geological Science have now become so vast and various, that no one district of country can be expected to furnish illustrations of more than a very few of them.

The West of Scotland, in the capital of which we are now assembled, is not rich in deposits which illustrate the passage of animal life from the types that have become extinct to those which are of more modern origin and which still survive. No bone-caverns of importance have been discovered, and, with one exception, even our river-gravels and estuarine deposits have not been especially productive. That exception is, indeed, a great one. It was in this valley of the Clyde that the late Mr. Smith, of Jordan Hill, first discovered those indications of an Arctic climate recently prevailing which have ever since constituted a large and important branch of geological inquiry, and the full interpretation of which still presents some of the most curious and difficult problems with which we have to deal. But our Paleozoic areas, except the Coal Measures, are to a large extent singularly unfossiliferous. Neither the Scottish Oolite nor Lias has yielded any remarkable additions to the curious fauna of which in England and elsewhere they have yielded abundant specimens.

But, on the other hand, perhaps no area of country of equal extent in any quarter of the world presents more remarkable phenomena than the West of Scotland, in connexion with those causes of geological change which have determined the form of the earth's surface, and have given to its physical geography those features of variety and beauty which are the increasing delight of civilized and instructed men. We cannot descend the course of this river Clyde to the noble estuary in which it ends without having presented to us mountain outlines and an intricate distribution of sea and land which raise questions of the highest interest and of the greatest difficulty. From the northern shores of that estuary to Cape Wrath, in Sutherland, the country is occupied mainly by rocks of Silurian age, but so highly crystalline as to be almost wholly destitute of fossils, and so upheaved, twisted, contorted, and folded into a thousand different positions, that, except in one great section, it is most difficult to trace any persistent succession of beds. It is one great series of billowy undulations traversed by glens and valleys, some of which are high above the level of the sea, but many of which are now so deeply submerged that through them the ocean is admitted far into the bosom of the hills. These glens and valleys lie in many different directions; but there are so many with one prevalent direction as to give a general character to the map, a direction from N.E. to S.W., or parallel to the prevalent strike of the Silurian rocks. The shapes of the hills and mountains are not by any means wholly without relation to geological structure-because in a thousand cases the sloping outlines will be found to be determined by the inclination of the beds, and the precipitous or steeper outlines to be determined by the upturned or broken edges. In like manner there are cases where a crumpled or knotted outline is the index of beds deeply folded and contorted along anticlinal axes. But nevertheless there are also innumerable cases where no such relation can be traced; where the mountains seem to have been cut out of some solid mass, all the rest of which has been removed by some agency which left these great fragments standing by themselves, and of which the contours cut across the lines of structure at every variety of angle. Along the whole western face of this country it is guarded from the open ocean by an archipelago of islands, some of which are separated from the mainland by submerged valleys no broader than those which separate one hill from another in the inland glens. Many of these islands are wholly occupied by the débris and the outbursts of extinct volcanoes. The mountains which are thus composed bear, in many cases, the characteristic forms of lava-streams; but many others are not readily distinguishable in outline from the mountains of wholly different material which are near them. They reach the same general average level of height, here and there rising into peaks very similar to others of a widely different age and of a widely

1876.

Printed in full by order of the Council.

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different material. Moreover all the islands partake largely of the general character of the mainland in having their deeper valleys submerged, and in being thus deeply indented by arms of the sea similar to those which give their peculiar outline to the adjacent coasts.

It may serve to bring more vividly before you the facts of the physical geography of this country (for which it is one of the duties of geologists to account if they can) if I give you some statistical facts affecting the single county of Argyll, which begins on the northern shore of the Firth of Clyde. Following the coastline of that county from the head of Loch Long, which is its southern and eastern boundary, to Loch Aylort, which is its northern and western boundary, and including its islands, we find it measures no less than 2289 miles in length, of which about 840 represent the sinuosities of the mainland, and 1449 represent the coast-line of its larger islands. There are, besides, valleys which are now inland, and are occupied by freshwater lakes which evidently, at a recent period, were arms of the sea; and these represent a further line of coast, measuring 276 miles. There are 11 principal arms of the sea, each of them measuring from one to six and thirty miles in length. Two of these arms of the sea exceed the 100fathom line in depth-Loch Fyne and the Linnhe Loch; and it is very remarkable that these deep soundings do not occur near the points where these lochs join the more open sea, but, on the contrary, far up their course or bed among the mountains. The ridges dividing these and other valleys vary in elevation from hills of very moderate height to the range of Cruachan, which immediately beyond the boundary of the county culminates in Ben Nevis, which rears its head almost on a level with Ben MacDhui, now ascertained to be the highest summit in the British Isles. But no statistics can give an idea of the intricacy with which sea and land are interfolded on our western coasts comparable with that which is gained by some of the many beautiful views which abound on the heights in the vicinity of Oban, whence the visitor can command the entrance of Loch Etive, with the course for many miles of the Linnhe Loch, of the Sound of Mull, the Sound of Kerrera, and the Firth of Lorne.

Now the question naturally arises to what geological ages and to what geological causes do we owe, in its main features, this curious distribution of land and sea? I say in its main features, because, of course, the more superficial sculpturing of every mountainous country is undergoing incessant modification; and this modification may have been, and probably has been, very considerable indeed within times which, geologically speaking, belong to the existing age. But the question I put has reference to the epoch of past time, when the main outlines of hill and valley were determined; when the great mass of the country (which has been, I believe correctly, identified as composed of metamorphosed Silurian_beds) was elevated into the various mountain-chains which now constitute its charac teristic features.

If the question had been asked some five and twenty years ago, I should have said that the evidence pointed to an age of great geological antiquity for the central group of Highland mountains, in some shape very like that in which we see them. All round the edges of the country there are the remains of the Old Red Sandstone, which often fit into the contour of the valleys and have left fragments in nooks and recesses of the hills. It would almost seem as if they had been the shores of the seas or great lakes in which that great system of deposits was laid down, and that they lifted their heads above those waters in forms not wholly unlike those in which we now see them. The total absence over almost the whole country of any other or later rocks, the absence among the débris of any material other than that of which the hills are themselves composed, would seem to confirm the same general conclusion.

Some doubt, however, may seem to have been thrown on this conclusion, since it has become certain that it cannot be true of at least one district of our western mountains, which is nevertheless closely related to all the rest, having the same general elevation, partaking of the same general bend of coast-lines, cut up by similar valleys, and fitting into the same contours of denudation. The district to which I refer is that of the volcanic islands which stretch from the south end of Mull to the north end of Skye. Since the discovery, which I was fortunate enough

to make in 1851, of the leaf-beds of Ardtun, it has become clearly ascertained that these islands are the remains of volcanoes of that geological age to which an everincreasing interest seems to attach-that middle age of the great Tertiary division of geological time to which Lyell gave the name of Miocene. The mountains of Mull, and of Eigg, and of Rona, and of Skye, with all their valleys and intricate lines of coast, have unquestionably an origin later than the Miocene-how much later, is the question of physical geography which geologists are called upon to solve. It is possible, indeed, to suppose that the hills of the mainland might be of a very different age from those of the adjacent islands; and against this, until some two years ago, there would have been nothing to advance except the suspicious similarity and adjustment between the two groups, the coincidence of their outlines, and of the way in which they have been cut and carried. But the admirable researches of Mr. Judd, in 1874, have brought one little fact to light which speaks volumes for the enormous changes which must have taken place since the volcanoes of the Miocene over a portion at least of the Highland area, and which may, therefore, have taken place over the whole of it. The land upon which the Miocene vegetation flourished, and upon which the lava-streams of its volcanoes were poured out, seems to have been for the most part a land consisting of Cretaceous and Secondary rocks. The fragments of that country which remain are generally consistent with the supposition that they were deposited in a sea which washed round the bases of the Highland mountains, but which never covered them. Like the fragments of the Old Red Sandstone, the remains of the Secondary rocks lie along the margins and fringes of the Silurian hills. But Mr. Judd has made the startling discovery of an outlier of the whole series of the Secondary rocks, including representative beds of the Trias, Lias, Greensand, and Chalk, together with deposits, probably Lacustrine, all lying on the top of one of the mountains of metamorphic gneiss which constitute the district of Morven. This fragment has been preserved by having been covered by a sheet of lava from some great neighbouring volcanic centre, the position of which is probably indicated by Ben More in Mull. But the mass of volcanic trap which has covered up and preserved this relic of the Cretaceous land is itself a fragment occupying the top of a mountain of gneiss, separated from the remainder of the sheet of lava to which it belongs by deep valleys, precisely similar to those which divide the hills from each other throughout the whole area of the Highlands. This position of an outlier of the Cretaceous rocks on the summit of a mountain of gneiss is rendered still more curious by the circumstance that in that position the beds are not tilted or in any way apparently disturbed. They are arranged horizontally, as if the ocean floor in which they were deposited had occupied that level, or as if its deposits had been lifted up over so large an area that any small section of that area could retain its original horizontality. The Lower Silurian gneiss beds on which these Secondary deposits have been laid are violently twisted and contorted; and this structure must have belonged to them when they constituted the floor of the Cretaceous sea. The position of the Miocene basalts capping the Secondary deposits proves that the whole mountain, as a mountain, is of later date than the Miocene agehow much later we cannot tell; and thus that the causes of geological change which have cut up the country into its present form, though they doubtless began in very remote epochs, have at least been prolonged into a comparatively late age in the history of the globe.

It would, I think, be affectation to pretend that our science enables us to follow, with any thing like distinctness of conception, the exact nature and sequence of operations which through such a vast lapse of time have brought about the final result. But I believe in something like the following general outline of events.

First. That subsequent not only to the consolidation, but probably also to the metamorphism of the Lower Silurian deposits, the whole area of the WesternCentral Highlands became an area of that kind of disturbance which arose from lateral pressure due to secular cooling and consequent contraction and subsidence of the crust of the earth.

Second. That the crumpling, contortion, and tilting of the Silurian beds which we now see arose from that disturbance.

Third. That then were determined those great general lines of strike running

from N.E. to S.W. which are to this day a prominent feature in the physical geography of the country.

Fourth. That during that period of disturbance, and as part of the movements which then took place, the disturbed rocks fell inwards upon materials at a great heat, which rose in a pasty state along the lines of least resistance, and thus came to occupy various positions, sometimes intercalated among the sedimentary beds.

Fifth. That to this period, and to this method of protrusion we owe some at least of the masses of granitic material which are abundant in the Highlands. In particular, that to this period belong the porphyritic granites on the northern shores of Loch Fyne.

Sixth. That during the later ages of the Palæozoic period, volcanic action broke out at various points, accompanied by great displacement and dislocation of strata, and that to this, with the denudation which followed, we owe much of the very peculiar scenery of the south-western coasts, especially in the district of Lorne in Argyllshire.

Seventh. That we have no proof that the Central Highlands were ever under the seas which laid down the deposits of the later Palæozoic age.

Eighth. That such evidence as we have points rather to the conclusion that they were not under those seas, since such fragments as remain of the Old Red and of the Carboniferous rocks appear to have been deposited round the bases and in the marginal hollows of the Silurian hills.

Ninth. That in like manner we have no evidence that the great mass of the Western or Central Highlands was ever under the seas of the Secondary ages, which on the contrary, appear to have deposited their sediment upon an area outside of, but probably surrounding, the area of those Central Highlands, and certainly upon their north-eastern and western flanks.

Tenth. That the whole area of the Inner Hebrides and of the waters dividing them, together with some portion of the mainland, as in Morven, was an area occupied by Secondary rocks.

Eleventh. That in the Tertiary ages, probably in the Eocene, and certainly in the Miocene, these rocks formed the basis of a great land of unknown extent, very probably extending for a great distance both to the east and west of the present coasts of Scotland, and embracing the north of Ireland.

Twelfth. That this country became in the Miocene age, and possibly earlier, the scene of great volcanic outbursts, which covered it with vast sheets of lava and broke up its sedimentary rocks with every form of intrusive plutonic matter.

Thirteenth. That later in the Tertiary periods, and perhaps as late as the Pliocene, this volcanic country was itself broken up by immense subsidences and upheavals, giving both occasion and direction to the agencies of denudation and to enormous removals of material.

Fourteenth. That this Tertiary country had been thus broken up and nothing but its fragments left when the Glacial epoch began, and that the main outlines of the country, as we now see it, had been already determined when glacial conditions were established.

Fifteenth. That thus the work of the Glacial period has been simply to degrade and denude preexisting hills and to deepen preexisting valleys.

Sixteenth. That during the Glacial epoch there was a subsidence of land to the depth of at least 2000 feet below the level of the present sea, and again a reelevation of the land to its present level.

Seventeenth. That this reelevation has not restored the land to the level it stood at before the subsidence began, but has stopped greatly short of it; and that the deep arms of the sea or lochs which intersect the country, and some of the deeper freshwater lakes, such as Loch Lomond, are the valleys still submerged which at the beginning of the Glacial epoch where high above the sea and furrowed the flanks of loftier mountains.

Eighteenth. That during the Glacial period the working of denudation and degradation was done, and done only by ice, in the three well-known forms :—1st, of true glaciers descending mountain-slopes; 2nd, of icebergs detached from the termination of these glaciers where they reached the sea; and 3rd, by floe or

surface ice, driven by currents which were determined in direction by the changing contours of the land during the processes of submersion and reelevation.

It would be impossible on this occasion to illustrate or support these various propositions by going into the evidences on which they rest. But as those of them which relate to the operations of the Glacial epoch express a decided opinion upon questions now involving much dispute, I must say a few words in explanation and defence of that opinion.

It will be seen that I disbelieve altogether in the theory of what is called an Ice-cap; or, in other words, I hold that there is no evidence that there ever existed any universal mantle of ice higher or deeper than all the existing mountains, covering them and moving over them from distant northern regions.

In the first place, this theory presupposes conditions of climate which must have prevailed universally over the whole northern hemisphere; whereas over a great portion of that hemisphere west of a certain meridian on the American continent, all traces of general glaciation and of any general distribution of erratics disappear. In the second place, the theory assumes that masses of ice lying upon the surface of the earth, more than mountain-deep, would have a proper motion of their own, capable of overcoming the friction not only of rough level surfaces, but even of the steepest gradients, for which motion no adequate cause has been assigned, and which has never been proved to be the natural consequence of any known force, or to be consistent with the physical properties of the material on which it is supposed to have acted.

In the third place, as a matter of fact there do not now exist anywhere on the globe masses of ice which can be proved to have any motion of this kind, or to be subject to forces capable of driving and propelling it in this manner and with the effects which the theory assumes. The case of Greenland, which is often referred to as an example, does not present phenomena at all similar to those attributed to the ice-sheet.

In the fourth place, all the phenomena of glaciation which are exhibited on the mountain-ranges, including the distribution of erratics, can be adequately accounted for by the three conditions or forms of moving ice which have been above enumerated, and all of which are now in actual operation on the globe, namely:ice moving, not up, but down mountain-slopes by the force of gravitation, and ice floated by water and driven by currents as icebergs or as floes.

In the fifth place, these phenomena of glaciation are essentially different from those which would result from the motion of a universal ice-sheet, even supposing it to have existed and supposing it to have had the (improbable) motion which has been ascribed to it.

In the sixth place, and in particular, the mode in which erratics are distributed and the peculiar position of perched blocks are demonstrative of the action not of solid but of floating ice; whilst the surfaces of rock, which have escaped glaciation on one side and retain the deepest marks of it upon another, are equally demonstrative of exposure to moving ice under conditions which did not enable it to fit into the irregularities of surfaces over which it passed.

In the seventh place, the phenomena seem to me to prove that some of the very heaviest work done by ice has been done towards the close of the Glacial epoch-when the land was emerging again from out of a glacial sea, and when all the currents of that sea, loaded with bergs and floes, were determined entirely by the outlines of the rising land.

In regard to the much disputed question of the glacial origin of Lake-basins, the conclusion to which I have come is one which, to some extent, reconciles antagonistic views. I do not, indeed, believe that glaciers can ever dig holes deep under the average slope of the surface down which they move; but, on the other hand, they are the most powerful of all abrading agents in deepening their own bed and cutting away the rocky surfaces which lie beneath them.

If valleys thus deepened by the long work of glaciers and glacier-streams are afterwards submerged along with the whole country in which they lay, and if that submergence is accompanied by partial and unequal rates of subsidence, they would inevitably become hollows into which the sea would enter, or in which fresh waters would accumulate. In this sense, and in this way, it can hardly