Chap. IX. GEOLOGICAL KECOKD. 291 



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, sedi- 

 ment may be accumulated 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 supply of sediment nearly balance each other, 

 the sea will remain shallow and favourable for life, and 

 thus a fossiliferous formation thick enough, when up- 

 raised, to resist any amount of degradation, may be 

 formed. 



I 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 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 certainly deposited during a 

 downward oscillation of level, and thus gained consi- 

 derable 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 



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