Chap X.] PALEOXTOLOGICAL COLLECTIONS. 273 



when upraised will give an imperfect record of the organisms which 

 existed in the neighbourhood during the period of its accumulation. 

 Or, sediment may he deposited to any thickness and extent over a 

 •shallow hottom, 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 upraised, to resist a large amount of denudation, 

 may he 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 author, in treat- 

 ing 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 oscillations 

 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 periods 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 favourable to life. Still 

 less can this have happened during the alternate periods of elevation ; 

 or, to speak more accurately, the beds which were then accumulated 

 will generally have been destroyed by being upraised and brought 

 within the limits of the coast-action. 



These remarks apply chiefly to littoral and sublittoral deposits. 

 In the case of an extensive and shallow sea, such as that within a 

 large part of the Malay Archipelago, where the depth varies from 

 SO or 40 to 60 fathoms, a widely extended formation might be 

 formed during a period of elevation, and yet not suffer excessively 

 from denudation during its slow upheaval ; but the thickness of the 

 formation could not be great, for owing to the elevatory movement 

 It would be less than the depth in which it was formed ; nor would 



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