Fig. 222.
Perna mulleti. Desh. in Leym.
- a. Exterior.
- b. Hinge of upper valve.
In the south of England, during the accumulation of the lower greensand above described, the bed of the sea appears to have been continually sinking, from the commencement of the period, when the freshwater Wealden beds were submerged, to the deposition of those strata on which the gault immediately reposes.
Pebbles of quartzose sandstone, jasper, and flinty slate, together with grains of chlorite and mica, speak plainly of the nature of the pre-existing rocks, from the wearing down of which the greensand beds were derived. The land, consisting of such rocks, was doubtless submerged before the origin of the white chalk, as corals can only multiply in the clear waters of the sea in spaces to which no mud or sand are conveyed by currents.
HIPPURITE LIMESTONE.
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 neighbourhood 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. 223.], in which the shaded part represents chalk).