CHAP. VI.] GEOGRAPHICAL AND GEOLOGICAL CHANGES. 93 



abundance 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 organ- 

 isms 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 Polythalamia, 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 num- 

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

 free corals, Alcyonidse, 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 Cre- 

 taceous times. No fact is more certain than the considerable 

 break, indicative of a great lapse of time, intervening between 

 the Cretaceous and Tertiary formations. A few deposits of 

 intermediate age have indeed been found, but these have been 

 generally allocated either with the Chalk or the Eocene, leaving 

 the gap almost as pronounced as before. Now, what does this 

 gap mean ? It implies that when the deposition of the various 

 Cretaceous beds of Europe came to an end they were raised 

 above the sea-level and subject to extensive denudation, and 

 that for a long but unknown period no extensive portion of 

 what is now European land was below the sea-level. It was 

 only when this period terminated that large areas in 

 several parts of Europe became submerged and received the 

 earliest Tertiary deposits known as Eocene. If, therefore, 

 Geological Magazine, 1871, p. 426, 



