106 



But we have now reached the foot of the escarpment of the 

 sand-hills overlooking the Weald, and if we cut a section 

 downwards we should see that lying immediately on the fresh- 

 water clays is another bed of clay containing marine fossils. 

 Among the earlier geologists this was regarded as a most 

 astonishing occurrence, for they could not imagine how a 

 river deposit could become covered by one purely marine ; 

 but they were not acquainted at that time with one of the 

 fundamental principles of Geology which the two formations 

 in question, the Wealden and Lower Greensand, illustrate 

 exceedingly well. This principle is Oscillation of the Earth's 

 Crust. 



We are now quite familiar with the fact that the Earth's 

 crust is perpetually undergoing an up and down motion, one 

 region sinking, while another, perhaps not far distant from 

 it, is rising ; and this up and down motion very rarely takes 

 place spasmodically, but is a process so slow and gentle that 

 it is only by the most careful observations that we can detect 

 that it is going on at the present day. But we see in the 

 accumulation of immense sedimentary deposits proofs that it 

 has been perpetually ia action, and it is to this slow sinking 

 of our Wealdeu continent that we owe the presence of the 

 marine beds now overlying the deposits of our Wealden river, 

 which had accumulated to the thickness of 2000 feet. For 

 this thickness of material to have been deposited it was of 

 course necessary that the delta should have sunk 2000 feet 

 below its original level, and it may be asked — How is it that 

 this great sinking went on without the water getting any 

 deeper? for throughout the whole of the beds we have no 

 evidence of any conditions existing other than those accom- 

 panying shallow water, wet marshes, and mud-banks. The 

 reason is obvious : the deposits of sands and muds of the 

 river grew upwards at about the same rate as the land was 

 sinking, until at last, probably fi-om the rate of subsidence 

 becoming more and more rapid, the sea gained on the river 

 and gradually overflowed the land. 



We have evidence that the submergence was gradual from 

 the occurrence of oyster-beds here and there in the upper 



