192

FRESHWATER SHELLS IN THE PURBECK STONE.

which the Purbeck strata frequently contain, both in nodulesand m alternating layers, including the same fossil shells asthe limestone. This chert, having lain upon the hills for ages,exposed to the action of decomposing causes, has its surface con-siderably corroded; but the shells, being converted into calcedony,are harder than the chert, and consequently have resisted theeffects of these agents, and remain on the surface entire. Theseshells in general are minute; and several of them appeared to befreshwater shells, among which, a species of Planorbis, and theHelix vivipara Linn, or Cyclostoma Lam. were distinguishable.Some of them even still preserve their original nacre.

It was long ago observed by Woodward, in his History ofFossils, that the shells in the Purbeck marble consisted chieflyof the Helix vivipara; and it is rather surprising that this veryancient freshwater formation should not have excited more atten-tion. Beautiful impressions of fish are frequently met with bythe quarrymen between the laminae of the limestone; and I sawabundance of fragments of bones, some of which belonged tothe turtle. Complete fossil turtles have been found ; and, lately,one extremely perfect.*

Having now obtained a general idea of this coast, I was desirousof returning by land through the valleys of Smedmore andEncombe ; but the lateness of the hour, an clothe impossibility ofobtaining accommodations through the whole of this tract, com-pelled me to have recourse again to the boat.

I am, &c,

THOMAS WEBSTER-

* 1 his is now in the possession of Mr. Bullock, the proprietor of an interesting museum.