PAL^EONTOLOGICAL COLLECTIONS. 303 



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 degra- 

 dation as it has as yet suffered, but which will hardly last 

 to a distant geological age, was 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 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 subsidence, but 

 only where the supply of sediment was sufficent 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 

 during the alternate periods of elevation ; or, to speak more 

 accurately, the beds which were then accumulated will gen- 

 erally have been destroyed by being upraised and brought 

 within the limits of the coast-action. 



These remarks apply chiefly to littoral and sub-littoral 

 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 thirty or forty to sixty 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 

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

 would the deposit be much consolidated, nor be capped by 

 overlying formations, so that it would run a good chance of 

 being worn away by atmospheric degradation and by the 

 action of the sea during subsequent oscillations of level. It 

 has, however, been suggested by Mr. Hopkins, that if one 

 part of the area, after rising and before being denuded, sub- 

 sided, the deposit formed during the rising movement, though 

 nut thick, might afterward become protected by fresh accu- 

 mulations, and thus be preserved for a long period. 



Mr. Hopkins also expresses his belief that sedimentary 



