278 THE ORIGIN OF SPECIES 



We may, I think, conclude that sediment must be accumu- 

 lated in extremely thick, solid, or extensive masses, in order to 

 withstand the incessant action of the waves, when first upraised 

 and during successive oscillations of level, as well as the subse- 

 quent subaerial degradation. Such thick and extensive accumu- 

 lations of sediment may be formed in two ways; either in pro- 

 found depths of the sea, in which case the bottom will not be 

 inhabited by so many and such varied forms of life as the more 

 shallow seas; and the mass when upraised will give an imperfect 

 record of the organisms which existed in the neighborhood dur- 

 ing 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 sub- 

 sidence 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 1845, I have watched the progress of 

 geology, and have been surprised to note how author after au- 

 thor, 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 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 oscilla- 

 tions 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 pe- 

 riods of subsidence, but only where the supply of sediment was 

 sufficient to keep the sea shallow and to embed 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 shallow parts, which are 

 the most favorable to life. Still less can this have happened dur- 

 ing the alternate periods of elevation; or, to speak more accu- 

 rately, the beds which were then accumulated will generally have 



