xix. SINKING AND RISING OF LAND. 483 



1500 feet in certain parts of the north temperate zones ; much less 

 would they account for the vertical up and down movements of 

 older periods in the world's history of ten times this amount a . 



Assuming then, according to the general consent of geologists, 

 a constant or but slightly variable sea-level, we find the following 

 results. 



1. During the whole eocene period the area which is now the basin 

 of the lower Thames was undergoing depression, so as to have re- 

 ceived 500 or 600 feet of sediments upon the chalk floor, which had 

 been previously worn by waves and currents at small depths, and 

 drilled by resident boring shells. This movement must have extended 

 far beyond the Thames drainage ; in the basin of Hampshire it 

 must have measured vertically some 1500 feet ; yet between the two 

 basins it is thought the anticlinal of the weald may have been 

 rising and delivering detritus both northward and southward. If 

 so, the movement, though perhaps due to a general subterranean 

 cause, must have had local determinants which theory may possibly 

 discover. 



2. During meiocene and pleiocene periods it may be believed 

 that the Tamisian area was undergoing re-elevation, because the 

 crag deposits of the eastern coast, the latest pleiocene known in 

 Britain, are found to have been deposited for the most part in 

 shallow sea-water, and unconformably with reference to eocene 

 strata 400 or more feet above their level. At the close of the 

 pleiocene period, as represented by the crag, the general level of 

 the island must have been nearly what it is now, for the crag is 

 only elevated a few feet above the sea. 



3. But again depression took place; for first, in the crag dis- 

 trict, beds of fresh- water shells, and a ' forest bed' a little more 

 recent in date than the crag, and formed at a level somewhat 

 below that bank of shells after its elevation, are followed and 

 covered up on the Norfolk coast, near Cromer, by an hundred or 

 two hundred feet of boulder clay and gravels associated therewith, 

 which contain some bands of marine shells, and many marks of 

 marine action, and probably ice-floating on a wide and deep sea. 

 In the district round Oxford, we may satisfy ourselves that this 



a It is chiefly with reference to climate that Mr. Croll has so well employed the 

 astronomical considerations referred to. See Phil. Mag. and Annala. 



I i 2 



