CHAP. XIV.] SUMMARY OF GEOGRAPHICAL EVOLUTION. 317 



the west of Scotland and from the area of the Irish Sea, as- 

 well as from the north-western counties of England. Por- 

 tions of the older Neozoic rocks were doubtless removed 

 during the earlier part of the Cretaceous period, but the 

 greater part of the work was doubtless accomplished in 

 Tertiary times. 



With regard to the formation of St. George's Channel 

 between Wales and Ireland we have very little evidence to 

 guide us, but it may have originated in a minor valley 

 like that of the Clwyd in North Wales opening northward 

 into the basin or plain of the Irish Sea. All the proba- 

 bilities and analogies of the case lead us to conclude that 

 its origin was post-Carboniferous and pre-Cretaceous, and 

 that Wales was completely isolated during the great Creta- 

 ceous subsidence. We may therefore assume with much 

 reason that a chalk-filled channel (narrower of course 

 than the present one) existed at the beginning of the 

 Eocene period, and that during the course of the ensuing 

 epochs it was re-excavated by subaerial agencies and 

 widened to nearly its present dimensions. 



In the south of England also great changes were in pro- 

 gress during the interval between Eocene and Pliocene 

 times ; the curvature of the London and Hampshire basins 

 was completed, and the intervening Wealden axis was 

 largely denuded of its original coverings, receiving its final 

 planation from the waves of the older Pliocene sea. Thence 

 it emerged to form a tract of elevated land 600 or 700 feet 

 above the sea-level of newer Pliocene time, and stretching 

 continuously across the Straits of Dover through the north 

 of France to the region of the Ardennes. 



We now arrive at another important epoch in the 

 development of British geography, the time which is 

 indicated by the break between the older and newer Plio- 

 cene deposits, and during which the movements that led to 

 the formation of the North Sea basin appear to have taken 



