CHAP. VI.] GEOGRAPHICAL AND GEOLOGICAL CHANGES. 87 



And, as subsidence will always be accompanied by deposition, 

 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 

 alternate subsidences and elevations, each of moderate amount. 



Supposed Oceanic Formations ; — the Origin of Chalk, — ^There 

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

 known to geologists correspond exactly to the deposits 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 correspond 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 

 portion of both deposits, more especially the pelagic Fora- 

 minifera, 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 

 cocfcoliths and discoliths, are also found in both formations, 

 while there is a considerable general resemblance between the 

 higher forms of life. Sir Wyville Thomson tells us, that — 

 "Sponges are abundant in both, and the recent chalk-mud 

 has yielded a large number of examples of the group porifera 

 vitrea^ which find their nearest representatives among the 

 Ventriculites of the white chalk. The echinoderm fauna of 

 the deeper parts of the Atlantic basin is very characteristic, 

 and yields an assemblage of forms which represent in a remark- 

 able degree the corresponding group in the white chalk. Species 

 of the genus Cidaris are numerous; some remarkable flexible 

 forms of the Diademidse seem to approach Echinothuria." ^ 

 Now as some explanation of the origin of chalk had long been 

 desired by geologists, it is not surprising that the amount of 

 resemblance shown to exist between it and some kinds of 

 1 Nature, Vol. IL, p. 297. 



