CHAP. VI GEOGRAPHICAL AND GEOLOGICAL CHANGES 89 



way every part of a continent may again and again have 

 sunk beneath the sea, and yet as a whole may never have 

 ceased to exist as a continent or a vast continental archi- 

 pelago. And, as subsidence will always be accompanied 

 by deposition, of sediments from the adjacent land, piles of 

 marine strata many thousand feet thick may have been 

 formed in a sea which was never very deep, by means of a slow 

 depression either continuous or intermittent,or through alter- 

 nate subsidences and elevations, each of moderate amount. 

 Supposed Oceanic Formations ; — tlic Origin of Chalk. — 

 There seems very good reason to believe that few, if any, of 

 the rocks known to geologists correspond exactly to the de- 

 posits now forming at the bottom of our great oceans. The 

 white oceanic mud, or Globigerina-ooze, found in all the great 

 oceans at depths varying from 250 to nearly 3,000 fathoms, 

 and almost constantly in depths under 2,000 fathoms, has, 

 however, been supposed to be an exception, and to corre- 

 spond exactly to our white and grey chalk. Hence some 

 naturalists have maintained that there has probably been 

 one continuous formation of chalk in the Atlantic from the 

 Cretaceous epoch to the present day. This view has been 

 adopted chiefly on account of the similarity of the minute 

 organisms found to compose a considerable proportion of 

 both deposits, more especially the pelagic Foraminifera, of 

 which several species of Globigerina appear to be identical 

 in the chalk and the modern Atlantic mud. Other 

 extremely minute organisms whose nature is doubtful, 

 called coccoliths and discoliths, are also found in both 

 formations, while there is a considerable general resem- 

 blance between the higher forms of life. Sir Wyville 

 Thomson tells us, that — ''' Sponges are abundant in both, 

 and the recent chalk-rnud has yielded a large number of 

 examples of the group porifcra xitrea, which find their 

 nearest representatives among the Ventriculites of the 

 white chalk. The echinoderm fauna of the deeper parts of 



restricted adjacent areas ; and the effect has been to bring each portion in 

 succession beneath the ocean but always bordered on one or both sides by 

 the remainder of the continent, from the denudation of whicli the deposits 

 are formed which, on the subsei|uent upheaval, become mountain ranges. 

 (Manual of Geology , 2nd Ed., p. 751.) 



