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 
coccoliths 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 Diademidae seem to approach Echinothuria.” 1 
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. II., p. 297. 
