62 THE POOENESS OF OLE [Chap. X. 



which existed in the neighbourhood during the period 

 of its accumulation. Or, sediment may be deposited to 

 any thickness and extent over a shallow bottom, if it 

 continue slowly to subside. In this latter case, as long 

 as the rate of subsidence and the supply of sediment 

 nearly balance each other, the sea will remain shallow 

 and favourable for many and varied forms, and thus 

 a rich fossiliferous formation, thick enough, when up- 

 raised, to resist a large amount of denudation, may be 

 formed. 



I am convinced that nearly all our ancient forma- 

 tions, which are throughout the greater part of their 

 thickness rich in fossils, have thus been formed during 

 subsidence. Since publishing my views on this subjeci 

 in 1845, I have watched the progress of Geology, and 

 have been surprised to note how author after author, 

 in treating of this or that great formation, has come to 

 the conclusion that it was accumulated during subsi- 

 dence. I may add, that the only ancient tertiary forma- 

 tion on the west coast of South America, which has 

 been bulky enough to resist such degradation as it has 

 as yet suffered, but which will hardly last to a dis- 

 tant geological age, was deposited during a downward 

 oscillation of level, and thus gained considerable thick- 

 ness. 



All geological facts tell us plainly that each area 

 has undergone numerous slow oscillations of level, and 

 apparently these oscillations have affected wide spaces. 

 Consequently, formations rich in fossils and sufficiently 

 thick and extensive to resist subsequent degradation, 

 will have been formed over wide spaces during periods 

 of subsidence, but only where the supply of sediment 

 was sufficient to keep the sea shallow and to embed 



