196 Geological Society of London. 



agencies of waste have produced great modificatioDS of the surface, 

 the form of the hills and valleys has in the main been determined 

 by the action of subterranean forces. 



In illustration of his opposition to Mr. Geikie's theory, he de- 

 scribed a supposed case of the formation of a valley by atmospheric 

 agencies, observing that, if the crumplings of the strata have not 

 affected the present surface, a subsequent submergence and a fresh 

 unconformable deposition filling in all the inequalities must have 

 ensued, and that these new deposits must have been again raised 

 along different lines of elevation. Taking this new deposit to be 

 the Old Eed Sandstone, the author asks how it was removed, and 

 points out difficulties in the way of supposing the removal to have 

 been either by submergence or by subaeriel agencies. 



His Grace then stated that Mr. Geikie admits that the agencies 

 of erosion have been guided in their work by the prevailing strike 

 of the strata, which strike is followed along the same line by the 

 larger faults, and by the anticlinal and synclinal axes, — at least as 

 regards the general trend. He then pointed out that in reality all 

 the great physical features of Scotland take the same N.E. and 

 S.W. direction. He therefore considered that Mr. Geikie had 

 understated the case of the coincidence of certain physical features, 

 and had entirely omitted all mention of others, such as the appear- 

 ances of subsidence and dislocation to be observed in the Western 

 Islands, and the relations existing between dislocated sedimentary 

 strata and apparently intrusive rocks. 



In supporting his argument by special facts, the Duke of Argyll 

 endeavoured to show that the whole valley-system of Argyllshire 

 may be accounted for either by faults, foldings, subsidences, or anti- 

 clinals, mentioning in particular that Loch Tyne occupies the bed of 

 an enormous fault ; that Loch Awe lies along the line of a great 

 subsidence of the metamorphic slates, and that the gorge of the 

 Brander Pass lies along the line of a great fracture connected with 

 the subterranean movements which brought up the granites of Ben 

 Cruachan ; with many other instances of a like nature, in discussing 

 which he especially demurred to Mr Geikie's theory that the trans- 

 verse valleys and gorges have been formed by two streams, each 

 working backwards towards its own source, until the ridge which 

 divided them was finally destroyed. 



His Grace also remarked that the mineral condition of the granites 

 at the time of the subterranean movements was such as would facili- 

 tate the transmission of earthquake waves ; and the condition of the 

 slates was such as necessitated fracture when those waves were pro- 

 pagated beneath them. 



In conclusion, the author contested Mr. Geikie's statement of the 

 symmetry of river- valleys and uniformity of mountain heights ; and 

 contrasted the philosophy of the older geologists with that of the 

 advocates of subaerial denudation. 



