GEOLOGIC FORMATIONS OF NEW YORK 173 



pods were represented in great development by the orthoceras 

 in the Lower Silurian seas, so were they represented by the am- 

 monite in the Jurassic. The orthoceras disappeared after the 

 Triassic age. 



The smaller mollusks were also abundant and began to assume 

 more nearly the features of those which occur at the present day. 

 At this time the oyster made its appearance. 



CRETACEOUS SYSTEM 



The Jurassic system was succeeded by the Cretaceous. This 

 received its name in Europe from the chalk formation, which in 

 England and France is very prominent, being several hun- 

 dred feet thick. The chalk is a limestone which has not been 

 consolidated. If it had been exposed to the same agencies as 

 the Palaeozoic limestones it would probably like them have been 

 consolidated to form a hard rock. A large part of the chalk 

 consists of skeletons and shells of foraminifera, some of the 

 species being found in the ocean at the present day. With these 

 foraminifera, which are mostlv calcareous, are the remains of other 

 minute animals called polycystines which are silicious and also 

 the spicules of sponges. These, by some chemical action, have 

 been gathered together and consolidated into nodules of flint 

 which is a variety of quartz similar in composition to the horn- 

 stone of the Corniferous and other limestones. Hornstone is also 

 called chert and has furnished the material for most of the North 

 American Indian arrow-heads which are commonly called flint 

 arrow-heads. As a' matter of fact the true flint does not occur 

 in America and technically American flint arrow-heads are 

 made of chert or hornstone. It is not impossible, however, that 

 early traders from England may have supplied our Indians with 

 flint from Europe. 



In America there is but little chalk, although the Cretaceous 

 system is largely developed. It extends from the Gulf of Mexico 

 to the Arctic ocean in a belt 200 miles wide. On the Atlantic 

 coast Cretaceous deposits are found beneath the Tertiary and 

 consist chiefly of sand and clay. The clays which occur on Long 



