IMPERFECTION OF GEOLOGICAL RECORD 261 



must be accumulated in extremely thick, solid, or 

 extensive masses, in order to withstand the incessant 

 action of the waves, when first upraised and during 

 subsequent oscillations of level. Such thick and ex- 

 tensive accumulations of sediment may be formed in 

 two ways ; either, in profound depths of the sea, in 

 which case, judging from the researches of E. Forbes, 

 we may conclude that the bottom will be inhabited by 

 extremely few animals, and the mass when upraised 

 will give a most imperfect record of the forms of life 

 which then existed ; or, sediment may be accumulated 

 to any thickness and extent over a shallow bottom, it 

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

 long as the rate of subsidence and supply of sediment 

 nearly balance each other, the sea will remain shallow 

 and favourable for life, and thus a fossiliferous forma- 

 tion thick enough, when upraised, to resist any amount 

 of degradation, may be formed. 



1 am convinced that all our ancient formations 

 which are 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 conclusion that it was accumulated during subsid- 

 ence. 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 distant 

 geological age, was certainly deposited during a down- 

 ward 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 wide spaces. 

 Consequently formations rich in fossils and sufficiently 

 thick and extensive to resist subsequent degradation, 

 may 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 and 



