OF THE WESTERN ISLES OF SCOTLAND. 371 



of very different geological ages. We can readily compare the 

 altered or unaltered rocks of the Tertiary periods with Palaeozoic 

 lavas like those of Lome, and of the great Central Valley of Scotland. 

 At the time when I wrote, all these Palaeozoic lavas were generally 

 considered by geologists to be of Devonian age ; but since the 

 publication of my paper in 1874 I have had the opportunity of 

 studying the remarkable volcanic rocks and conglomerates of Eallan- 

 trae in Ayrshire, and I cannot help thinking that a part at least 

 of the Lome rocks may prove to be of the same age. I am at all 

 events fully prepared to subscribe to the opinion so clearl}^ expressed 

 by Mr. Dugald Bell, namely, that the question of the age of the 

 Lome lavas is still sub judice *. 



Among these Palaeozoic lavas we find just the same contrast 

 between almost wholly unaltered and greatly altered rocks, as in 

 Skye, Mull, or Rum in the case of Tertiary volcanic rocks. Mr. 

 Teall has shown that the glassy rocks of the Cheviot Hills are 

 really enstatite-andesites, which differ in no essential respect from 

 the recent lavas of Santorin and Krakatoa. Mr. Durham and 1 

 have described in Fifeshire enstatite-andesites and glassy dacites, 

 which, though as old as the Carboniferous, are as fresh and unaltered 

 as the modern enstatite-andesites of Japan. In the Garlton Hills 

 near Haddington there occur sanidine-oligoclase-trachytes, of Pre- 

 Carboniferous age, which are strikingly analogous with those of the 

 Siebengebirge. 



On the other hand, we find in the Pentland and Braid Hills 

 rocks, which, while of the same general ultimate chemical composi- 

 tion as the modern andesites, are remarkable for their obscure 

 structure and peculiar mineralogieal constitution. A comparison of 

 these w^ith the Tertiary propylites of the Western Isles of Scotland 

 is most instructive, for it shows that some of the " porphyrites " 

 are really andesites that have been subjected to the propylitic 

 modification, and then further modified by surface-agencies. It may, 

 indeed, be asserted of many of the propylites of Mull that, if their 

 abundant magnetite-granules, some of which are original and others 

 secondary, were changed to a red colour by peroxidation, they would 

 be quite un distinguishable from the obscure porphyrites to which 

 I have referred. These latter have such a peculiar constitution 

 that they have been classified by Dr. A. Geikie as " felspar- magnetite 

 rocks." 



IX. The Younger Augite-andesites (<' Tholeites," 



" PiTCHSTONES," &C.) OF THE WESTERN ISLES OP SCOTLAND. 



While the older Tertiary andesitic rocks which we have been 

 describing are remarkable for the extraordinary and often extreme 

 changes which they have undergone, there exist other lavas of 

 similar composition in the district, which present the most marked 

 contrast with them, by the wonderful freshness of their appearance. 

 That these lavas are younger than all the plateau-basalts is shown 

 * Trans. Geol. Soc. of Glasgow, vol. viii. (188G) p. 111). 



