CHAP. VI GEOGRAPHICAL AND GEOLOGICAL CHANGES 95 



of the denudation of its shores and islands would form the 

 various sandstones, marls, and clays, which would be 

 deposited almost wholly within a few miles of its coasts ; 

 while the great central sea, perhaps at no time more than 

 a few thousand feet deep and often much less, would 

 receive only the impalpable mud of the coral-reefs and the 

 constantly falling tests of Foraminifera. These would 

 imbed and preserve for us the numerous echinoderms, 

 sponges, and moUusca, which lived upon the bottom, the 

 fishes and turtles which swam, in its waters, and some- 

 times the winged reptiles that flew overhead. The abun- 

 dance of ammonites, and other cephalopods, in the chalk, 

 is another indication that the water in which they lived 

 was not very deep, since Dr. S. P. Woodward thinks that 

 these organisms were limited to a depth of about thirty 

 fathoms. 



The best example of the modern formation of chalk is 

 perhaps to be found on the coasts of sub-tropical North 

 America, as described in the following passage : — 



" The observations of Pourtales show that the steep 

 banks of Bahama are covered with soft white lime mud. 

 The lime-bottom, which consists almost entirely of Poly- 

 thalamia, covers in greater depths the entire channel of 

 Florida. This formation extends without interruption 

 over the whole bed of the Gulf Stream in the Gulf of 

 Mexico, and is continued along the Atlantic coast of 

 America. The commonest genera met with in this 

 deposit are Globigerina, Rotalia cultrata in large numbers, 

 several Textilarise, Marginulinse, &c. Beside these, small 

 free corals, Alcyonidae, Ophiurse, Mollusca, Crustacea, small 

 fishes, &c., are found living in these depths. The whole 

 sea-bottom appears to be covered with a vast deposit of 

 white chalk still in formation.'' ^ 



There is yet another consideration which seems to have 

 been altogether overlooked by those who suppose that a 

 deep and open island-studded ocean occupied the place of 

 Europe in Cretaceous times. No fact is more certain than 

 the considerable break, indicative of a great lapse of time, 

 intervening between the Cretaceous and Tertiary for- 

 ^ Geological Magazine, 1871, p. 426. 



