252 



HIPPURITE LIMESTONE. 



[Ch. XVIL 



HIPPURITE LIMESTONE. 



Fig. 292. 



Difference hetioeen the chalk of the north and south of Europe. — B} 

 the aid of the three tests of relative age, namely, superposition, mineral 

 character, and fossils, the geologist has been enabled to refer to the same 

 Cretaceous period certain rocks in the north and south of Europe, which 

 differ greatly, both in their fossil contents and in their mineral composition 

 and structure. 



If we attempt to trace the cretaceous deposits from England and 

 France to the countries bordering the Mediterranean, we perceive, in the 

 first place, that the chalk and greensand in the neighborhood of London 

 and Paris form one great continuous mass, the Straits 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. 292, in 

 which the shaded part represents chalk). 



Between Poitiers and La Rochelie, the 

 space marked A on the map separates two 

 regions of chalk. This space is occupied by 

 the Oolite and certain other formations older 

 than the Chalk, and has been supposed by 

 M. E. de Beaumont to have formed an 

 island in the cretaceous sea. South of this 

 space we again meet with a formation 

 which we at once recognize by its mineral 

 character to be chalk, although there are 

 some places where the rock becomes 

 oolitic. The fossils aie, upon the whole, 

 very similar ; especially certain species of 

 the genera Spatangus^ Ananchjtes^ Cida- 

 rites, JV^ucitla, Ostrea^ Gryphoea [Exogyra)^ 

 Pecten, Plagiostoma (Lima), Trigonia, 

 Catillus [Inoceramus), and l^rehratula!^ 

 But Ammonites^ as M. d'Archiac observes, 

 of which so many species are met with in 

 the chalk of the north of France, are scarcely ever found in the southern 

 region ; while the genera Ilamite, Twrilite, and Scajjhite, and perhaps 

 Belemnite^ are entirely wanting. 



On the other hand, certain forms are common in the south which are 

 rare or wholly unknown in the north of France. Among these may be 

 mentioned many H'lppurites^ Sphceridites, and other members of that 

 great family of moUusca called Rudistes by Lamarck, to which nothing 

 analogous has been discovered in the living creation, but which is 

 quite characteristic of rocks of the Cretaceous era in the south of 



* D'Arcliiac, sur la Form, Cretacee du S. 0. de la France, Mem. de la Soc. Geol 

 ie France, torn. ii. 



