THE CHALK FORMATION. 61 



shells and plants in some localities ; the total 

 thickness amounts to several hundred feet. The 

 fossils are almost entirely of extinct species. 



III. The chalk formation. — The white cal- 

 careous rock called chalk is well known ; but in 

 the nomenclature of Geology, the term Chalk for- 

 mation comprises not only the limestone that 

 extends over so large an area in the south-east 

 of England, but also numerous beds of sand, 

 sandstone, claj^, and limestone, that are very dis- 

 similar in their appearance and chemical characters, 

 yet so far correspond in the nature of their 

 organic remains, as to show that the entire group 

 was formed during the same geological epoch. In 

 other words, that the sea and land, and their in- 

 habitants, underwent no essential change during 

 the period in which the entire series was deposited : 

 it therefore constitutes, in geological language, but 

 one formation. 



The organic remains of the Chalk are essentially 

 marine ; but in some localities drifted wood and 

 plants, waterworn bones and teeth of terrestrial 

 reptiles, and other spoils of the land, are found 

 associated with the shells, corals, and fishes of the 

 cretaceous ocean ; and these have evidently been 

 transported by rivers and streams into the bed of 

 the sea. The fossils are, with but very few excep- 



