CHAP. X.] CRETACEOUS PERIOD. 177 



appearance of having been formed in much shallower water. 

 Fossils are often abundant, G-asteropoda are not uncom- 

 mon, and include such genera as Turbo, Cerithium, Avellana, 

 Aporrhais, Natica, Crepidula, and Emarginula, the modern 

 representatives of which do not live in deep water, some 

 not ranging below 100 fathoms, and none lower than 150 

 fathoms. Moreover, some of the species are indistinguish- 

 able from those of the Chalk Marl nearly 400 feet below. 

 The rock also contains numerous grains of glauconite, and 

 includes layers of large green-coated nodules. 



This sudden recurrence of peculiarities which charac- 

 terize the Chalk Marl and Totternhoe Stone, associated 

 with a fauna of similar character, compels us to conclude 

 that the sea had again become shallower by the rise of a par^ 

 of its floor ; there is, however, no sign of strong current- 

 action or of the' proximity of land. The compact rock and 

 its contents seem to have accumulated slowly in very quiet 

 water under conditions which permitted a large number of 

 marine animals to migrate eastwards, and repopulate the 

 bed of the sea. 



This rock continues with little change as far westward 

 as the Chalk extends in England, but in Ireland there is 

 nothing which exactly corresponds to it, or to any part of 

 the Middle Chalk. Some of the fossils of this division 

 occur, however, in certain glauconitic sands and marls 

 which lie between the Cenomanien sandstones and base of 

 the Upper Chalk ; they vary from six to sixteen feet thick, 

 they contain large grains and occasionally pebbles of quartz, 

 and have evidently been formed in shallow water not far 

 from land. 



The Scotch deposits are still more abnormal, for the beds 

 corresponding to the Irish Cenomanien are succeeded by 

 white sandstones from 30 to 100 feet thick, without fossils, 

 but including a seam of coal, and thus evidently formed in 

 close proximity to land. 



