o!6 SUMMARY OP GEOGRAPHICAL EVOLUTION. [CHAP. XIV. 



From the fragments of terrestrial surfaces preserved be- 

 neath the Eocene lavas of Ireland and the Western Isles, 

 we know that rivers were actively engaged in carving 

 channels out of the Secondary rocks which surrounded the 

 volcanic district, and that during the course of the Eocene 

 period a large amount of material must have been carried 

 ;seaward by these streams. 



Further, if we may assume that the plateau of Antrim is 

 still approximately in the same position which the whole 

 volcanic district originally occupied with regard to the sur- 

 rounding country, we may infer that the rest of the plain 

 or plateau over which the lavas were poured has since sunk 

 considerably below this level ; we may suppose, in fact, that 

 when the volcanic activity finally ceased, large parts of the 

 .area began to sink inwards, the subsidence being greatest 

 to the west of Scotland. This conclusion is confirmed by 

 the actual proofs of inward subsidence round the volcano 

 of Mull, as observed by Professor Judd, by the inclined 

 position of the lavas of Skye and Eaasay, and by the exis- 

 tence of powerful post-Eocene faults which have dropped 

 portions of the volcanic masses below their original level. 1 

 To this local subsidence we may attribute the great depres- 

 sion which is now filled by the waters of the Minch and the 

 Sea of the Hebrides, large parts of which are deeper than 

 the outer plateau on which the Hebrides stand, and would 

 form extensive lakes if the whole region were raised to the 

 extent of 500 or 600 feet. 



We have no further record of the progress of the great 

 denudation which was continued uninterruptedly through 

 Eocene, Oligocene, Miocene, and older Pliocene times, but 

 we know that it resulted in the removal of all the Creta- 

 ceous and Jurassic rocks (except the few remnants which 

 .survive), together with large portions of the Trias from 



1 "Quart. Journ. Geol. Soc.," vol. xxx. p. 257, and vol. xxxiv- 

 jpp. 673, 671. 



