336 



HIPPURITE LIMESTONE 



[Ch. XVII, 



If the passage seem at present to be somewhat sudden from the flora 

 of the Lower to that of the Upper Cretaceous period, the abruptness 

 of the change will probably disappear when we are better acquainted 

 with the fossil vegetation of the Lower Greensand, and with that of 

 the Gault and Upper Greensand 



HIPPURITE LIMESTONE. 



Fig. 326. 



Difference between the chalk of the North and South of Europe. — 

 By 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. 326, in which the shaded 

 part represents chalk). 



Between Poitiers and La Rochelle, 

 the space marked A on the map sepa- 

 rates 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 are, upon 

 the whole, very similar; especially 

 certain species of the genera Spatan- 

 gus, Ananchytes, Cidarites, Nucula, 

 Ostrea, Gryphcea (Exogyra), Pecten, 

 Plagiostoma (Lima), Trigonia, Catil- 

 lus (Inoceramus), and Terebratula* 

 But Ammonites, as M. d'Archiac observes, of which so many species 



t l^f^ 



* D'Archiac, Sur la Form. Cretacee de S.-O. de la France, Mem. de la Soc. Geol 

 de France, torn. ii. 



