Ch. XIV.] 



SUBTERRANEAN MOVEMENTS. 



317- 





< 



attended by the commencement of a new class of sedimentary 

 deposits, characterised by a new set of fossil animals and 

 plants, while the reconversion of the bed of the sea into land 

 may arrest at once and for an indefinite time the formation of 

 geological monuments. Should the land again sink, strata 

 will again be formed; but one or many entire revolutions 

 in animal or vegetable life may have been completed in the 



interval. 



As to the want of completeness in the fossiliferous series, 



which may be said to be almost universal, we have only to 

 reflect on what has been already said of the laws governing 

 sedimentary deposition, and those which give rise to fluctua- 

 tions in the animate world, to be convinced that a very rare 



circumstances can alone g 



testimony 



the gradual passage from one state of organic life to another. 

 To produce such strata nothing less will be requisite than the 

 fortunate coincidence of the following conditions : first, a 

 never-failing supply of sediment in the same region through- 

 out a period of vast duration ; secondly, the fitness of the 

 deposit in every part for the permanent preservation of im- 

 bedded fossils; and, thirdly, a gradual subsidence to prevent 

 the sea or lake from being filled up and converted into land. 

 It will appear in the chapter on coral reefs,* that, in cer- 

 tain parts of the Pacific and Indian Oceans, most of these 

 conditions, if not all, are complied with, and the constant 

 growth of coral, keeping pace with the sinking of the bottom 

 of the sea, seems to have gone on so slowly, for such indefi- 

 nite periods, that the signs of a gradual change in organic 

 life might probably be detected in that quarter of the globe, 

 if we could explore its submarine geology. Instead of the 

 growth of coralline limestone, let us suppose, in some other 

 place, the continuous deposition of fluviatile mud and sand, 

 such as the Ganges and Brahmapootra have poured for 

 thousands of years into the Bay of Bengal. Part of this bay, 

 although of considerable depth, might at length be filled up 

 before an appreciable amount of change was effected in the 



mollusca 



* See last chapter of Vol. II. of this Work. 



