1 5 o ORIGIN OF SCENERY OF BRITAIN 



for it, and would be mainly employed in clearing out 

 the corroded debris, though likewise finally deepen- 

 ing, widening, and smoothing the basins in the solid 

 rock. 



THE HILLS AND ESCARPMENTS. 



The Hills and Hill-groups of Britain have all 

 emerged during the gradual denudation of the country, 

 and owe their prominence to the greater durability of 

 their materials as compared with those of the surround- 

 ing lower grounds. They thus represent various stages 

 in the general lowering of the surface. In many cases 

 they consist of local masses of hard rock. Such is the 

 structure of the prominent knobs of Pembrokeshire 

 and of Central Scotland, where masses of eruptive 

 rock, formerly deeply buried under superincumbent 

 formations, have been laid bare by denudation. In 

 connection with such eruptive bosses, attention should 

 be given to the ' dykes ' so plentiful in the north of 

 England and Ireland, and over most of Scotland. In 

 numerous instances, the dykes run along the crests of 

 hills, and also cross wide and deep valleys. Had the 

 present topography existed at the time of their pro- 

 trusion, the molten basalt would have flowed down 

 the hill-slopes and filled up the valleys. As this never 

 occurs, and as there is good evidence that a vast num- 

 ber of the dykes are not of higher antiquity than 

 the older Tertiary periods, we may conclude that the 

 present configuration of the country has, on the whole, 

 been developed since older Tertiary time a deduction 

 in harmony with that already announced from other 

 independent evidence. 



Escarpments are the prominent outcrops of flat or 



