liIPPURITE LIMESTONE. 
805 
superposition, mineral character, 
and fossils, the geologist has been 
enabled to refer to the same Cre¬ 
taceous period certain rocks in 
the north and south of Europe, 
which differ greatly both in their 
fossil contents and in their miner¬ 
al composition and structure. 
If we attempt to trace the cre¬ 
taceous deposits from England 
and France to the countries bor¬ 
dering the Mediterranean, we per¬ 
ceive, in the first place, that in 
the neighborhood of London and 
Paris they form one great contin¬ 
uous 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. 273, in which the shaded part repre¬ 
sents chalk). 
Between Poitiers and La Rochelle, the space marked A 
on the map separates two regions of chalk. This space is oc¬ 
cupied by the Oolite and certain other formations older than 
the Chalk and ISTeocomian, 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 rocks which we at 
once recognize to be cretaceous, partly from the chalky ma¬ 
trix and partly from the fossils being very similar to those 
Fig. 273. 
Fig. 274. 
a. Radiolitea radiosa, D’Orb. 
b. Upper valve of same. 
White chalk of France. 
Fig. 275. 
Radiolites folianeus, D’Orb. 
Syn. Sphcerulites agarici- 
formis, Blainv. 
White chalk of France. 
of the white chalk of the north : especially certain species of 
the genera Spatangus, Ananchytes, Cidarites, Nucula, Ostrea, 
