196 THE RIVER VALLEYS OF 



Topley's paper on it, as these observers have care- 

 fully examined the geology of the country. 1 



The Weald is bounded on the north, west, and 

 south by chalk-hills or Downs, while near its central 

 line is an axis of hills, which in places is higher 

 than the average height of the Downs. These central 

 hills are of rocks the oldest in the area ; consequently 

 they are hills of elevation, which, however, have been 

 greatly modified by denudation. If on the rocks 

 forming these hills the absent strata were replaced, 

 the hills would be between 2500 and 3000 feet in 

 altitude^ or in round numbers, about 2000 feet higher 

 than at present. The North Downs are, on an aver- 

 age, 700 feet high, and the South Downs 800 feet ; 

 consequently the central hills would be 1300 and 

 1200 feet higher than the present North and South 

 Downs respectively. These are about 18 and 12 

 miles from the central axis, and there are dips north- 

 ward and southward (ignoring all minor flexures and 

 rolls) of about 78 feet and 100 feet to a mile, equal 

 respectively to about 1 in 68, and 1 in 53, or about 

 an angle of one degree both ways. 



If subgerial agencies are as powerful as some 

 suppose, it is quite unnecessary to call in any 

 aid from marine denudation, and it may be supposed 



1 " On the Superficial Deposits of the Valley of the Medway, with 

 Kemarka on the Denudation of the Weald," Quarterly Journal of the 

 Geological Society of London, November 1865, p. 443. 



