196 



LYELL'S ELEMENTS OF GEOLOGY. 



Different Character of Chalk in South of Europe. 



Fig. 172. 



and France to the countries bordering the Mediterranean, we 

 perceive, in the first place, that the chalk and green-sand in the 

 neighbourhood of London and Paris, form one great continuous 

 mass, the strait of Dover being a trifling interruption, a mere 

 valley with chalk cliffs on both sides. We then observe that the 

 main body of the chalk which surrounds Paris stretches from 

 Tours to near Poitiers, (see the annexed map (Fig. 172.), in 

 which the shaded part represents chalk.) 



Between Poitiers and La Ro- 

 chelle, the space marked A on the 

 map separates two regions of 

 chalk. This space is occupied by 

 the oolite and certain other forma- 

 tions older than the chalk, and has 

 been supposed by M. E. de Beau- 

 mont to have formed an island in 

 the cretaceous sea. South of this 

 space we again meet with a for- 

 mation which we at once recog- 

 nize by its mineral character to be 

 chalk, although there are some 

 places where the rock becomes 

 oolitic. The fossils also are upon 

 the whole very similar, although" 

 some new forms now begin to ap- 

 pear in abundance, which are rare 

 or wholly unknown further to the 

 north. Among these may be men- 

 tioned many Hippurites, Sphasru- 

 lites, and other members of that 



great family of mollusca called Rudistes by Lamarck, to which 

 nothing analogous has been discovered in the living creation. 

 Although very uncommon in England, one species of this family 

 has been discovered in our chalk. 



Fiff. 174. 



Fig. 175. 



Hippurites Mortoni, Mantell. Maidstone, Kent. 

 Diameter one-seventh of nat. size. 



