62 THE POORXESS OF OUB [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 formations, 

 wliich are tlu-oughout the greater part of their thickness 

 rich in fossils, have thus been formed during subsidence. 

 Since publishing my views on this subject 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 con- 

 clusion that it was accumulated during subsidence. I 

 may add, that the only ancient tertiary formation on 

 the west coast of South America, wliich has been bulky 

 enough to resist such degradation as it has as yet 

 suffered, but which will hardly last to a distant 

 geological age, was deposited during a downward 

 oscillation of level, and thus gained considerable 

 thickness. 



All geological facts tell us plainly that each area 

 has undergone numerous slow oscillations of level, 

 and apparently these oscillations have affected ^\idQ 

 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 



