68 THE OmOIN OF SPECIES 



this nitter case, as long as the rate of subsidence and 

 the supply of sediment nearly balance each other, the sea 

 will remain shallow and favorable for many and varied 

 forms, and thus a rich fossiliferous formation, thick 

 enough, when upraised, to resist a large amount of 

 denudation, may be formed. 



I am convinced that nearly all our ancient formations, 

 •which are throughout the greater part of their thickness 

 rich in fossils, have thus been formed during subsidence. 

 Since publishing my views on this subject in 1846, 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 subsidence. I may add, 

 that the only ancient tertiary formation on the west coast 

 of South America, which has been bulky enough to re- 

 sist 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 appar- 

 ently these oscillations have affected wide spaces. Conse- 

 quently, formations rich in fossils and sufficiently thick 

 and extensive to resist subsequent degradation, will have 

 been formed over wide spaces during periods of sub- 

 sidence, but only where the supply of sediment was suffi- 

 cient to keep the sea shallow and to imbed and preserve 

 the remains before they had time to decay. On the other 

 hand, as long as the bed of the sea remains stationary, 

 thick deposits cannot have been accumulated in the shal- 

 low parts, which are the most favorable to life. Still less 



