Sept. 14, 1876] 



MATURE 



435 



SECTION C. 



GEOLOGY. 



The Duke of Argyll read a paper On the Physical Strtulure of 

 the Highlands, in connection with thiir Geological History. He 

 said : — 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 which have bepome extinct to those 

 which are of more modern origin and which still survive. No 

 bone caverns have been discovered of importance, and, with one 

 exception, even our river gravels and estuarine deposits have not 

 been especially productive. That exception is, indeed, a feature. 

 It was in this valley of the Clyde that the late Mr. Smith, of Jordan- 

 hill, first discovered those indications of Antarctic climate recently 

 prevailing, which have ever since constituted a large and important 

 branch ot geological inquiry, and the full interpretation of which 

 still presents some of the most curious problems with which we 

 have to deal. But our Palseozoic areas, except the Coal Measures, 

 are to a large extent singularly unfossiliferous ; neither the 

 Scottish Oolite nor Lias have 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 connection 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 civilised 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 ot 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 the 

 Silurian age, but so highly crystalline as to be almost wholly 

 destitute of fossils, and so upheaved, twisted, contorted, and 

 folded into a thousand difterent 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 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 whole — a direction from north-east to south- 

 west, 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 counteracted along anti- 

 lineal axes ; but, nevertheless, there are also innumerable cases 

 where no such relation can be traced, where the mountains seem 

 to have been cut off 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 the 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 uy 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 debris 

 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 whoUy 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 different material. More- 

 over, 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 

 whicii give its peculiar outline to the adjacent coasts. 



It may serve to bring more vividly before you the facts of the 

 physical geography of the country (for which it is one of the 

 duties of geologists to account if they can) if I give you some 



statistic^ fac's affecting the single county of Ar,'yll, which begins 

 on the northern shore of the Firth of Clyde. Following the coas"- 

 line 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 2,289 miles in length, of which 

 about 840 represent the sinuosities of the mainland, and 1,449 

 represent the coast-line of its larger islands. There are, besides, 

 valleys, which are now inland, and are occupied by fresh-water 

 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 eleven principal arms of the sea, each of them measur- 

 ing from one to thirty-six miles in length. Two of these arms of 

 the sea exceed the 100 fathoms 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 the other 

 valleys vary in elevation from hills of very moderate height 

 to the ranges of Cruachan, which immediately beyond the 

 boundary of the country 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 inter- 

 folded on the western coasts comparable with that which is 

 gained by some of the many beautiful views that 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 Lome. 



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, in the times which, geologically 

 speaking, belong to the existing age ; but the quesli m I put has ^ 

 reference to the epoch of past time when the main ou' lines of 

 hill and valley were determined, when the great mass of the 

 country (which has been, I believe, correctly identified as com- 

 posed of metamorphosed Silurian beds) was elevated into the 

 various mountain chains which now constitute its characteristic 

 features. 



If the question had been asked some five-and-twenty years 

 ago I should have said that the evidence pointed to an age ot 

 great geological antiquity, for the central group of highland 

 mountains was in some shape 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 and 

 great lakes in which that great system of deposit was lai'l down ; 

 and that they had lifted their heads above those waters in forms 

 not wholly unlike those in which we now see ihem. The total 

 absence over almost the whole country of any other or later 

 rocks, the absence among the debris 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, havin.. the same general elevation, partaking of the 

 same general trend 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 185 1 of the leaf-beds in Mull, it has become clearly 

 ascertained that these islands are the remains of volcanoes of that 

 geological age to which an ever- increasing interest seems to 

 attach— that middle age of the great Tertiary division of geological 

 time to which Lyell gave the name oi Miocene. The mountains 

 of Mull and of Eigg and of Rona and of Skye, in all their 

 valleys and intricate Imes of coast, have unquestionably an origin 

 later than the Miocene — how much later is the question of 

 physical geography which geologists are called to solve. It is 

 possible, indeed, to suppose that the hills of the mamland 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 adjust- 



