PAL^ONTOLOaiCAL COLLECTIONS. 323 



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 appar- 

 ently these oscillations have afEected 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 sub- 

 sidence, 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 sta- 

 tionary, 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 generally have been de- 

 stroyed 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 thirty or forty to sixty fath- 

 oms, 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 foi'mation could not be great, for owing to the ele- 

 vatory movement it would be less than the depth in which 

 it was formed; nor would the deposit be much consoli- 

 dated, nor be capped by overlying formations, so that it 

 would run a good chance of being worn away by atmos- 

 pheric 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, subsided, the 

 deposit formed during the rising movement, though not 

 thick, might afterward become protected by fresh accumu- 

 lations, and thus be preserved for a long period. 



Mr. Hopkins also expresses his belief that sedimentary 

 beds of considerable horizontal extent have rarely been 

 completely destroyed. But all geologists, excepting the 



