92 GEOLOGY. 



of sand and sandy marl, with concretions of limestone, 

 &c. The green sand is of a ferruginous nature, and con- 

 sists chiefly of silicate of iron. The depths of these 

 strata amounts to nearly 500 feet. 



CHALK. This is the uppermost of the Secondary se- 

 ries, and is 600 feet or more in depth. The chalk is 

 evidently a marine deposit, and was formed when Eu- 

 rope was covered with water ; and although it extends 

 over a considerable portion of the Continent, and is of 

 such exceeding thickness, it is, to a very considerable 

 amount, made up t)f animal matter; such as testacea, 

 echini, corals, &c. The time which must have been taken 

 for this deposit must have been immense, composed, as 

 it principally is, of countless millions of shells of once 

 living beings. The white coating of flints, found in the 

 chalk, is composed of the shells of infusoria. Life was 

 in these early times so universal, that it is difficult to 

 say what had been and what had not been organic mat- 

 ter. Tripoli, a well known polishing material, has been 

 found to be composed entirely of myriads of skeletons of 

 microscopic animalia. It is difficult to convey an idea 

 of the minuteness of these animalcula, but in the Tripoli 

 of Bilin, in Bohemia, it has been calculated that there 

 were 187,000,000 in a single grain. 



Various species of fossil fish, and gigantic reptiles, 

 are found in this formation ; but there is one species 

 peculiar to this group, the Mosasaurus. This animal was 

 between 30 and 40 feet long, its head was four feet in 

 length, and it had four large paddles like those of the 

 whale. 



TERTIARY SERIES. 



Names. . Subdivisions: 



t Plastic Clay, London Clay, 



T-, ) Hampshire Freshwater 



Eocene < c 



\ formation, 



\^ Bagshot Sands. 



.... . $ English Clay when not re- 



' ferred to the Pleiocene. 



C Newer Pleiocene, 

 Pleiocene ;' Older Pleiocene. 



