Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/239

From Wikisource
Jump to navigation Jump to search
This page has been validated.
229
Mr. Webster on the Strata lying over the Chalk.

described it agrees most nearly with its corresponding formation in the Paris basin, with this difference however, that none of ours is siliceous; I shall therefore, in the succeeding comparison, confine myself to the calcareous part.

The external characters of this in both countries are sufficiently different from every other known rock to render them distinguishable even without the shells. That of France is described as white, or yellowish; sometimes as tender and friable as chalk or marl, and sometimes very hard, compact and solid, with a fine grain and conchoidal fracture. In the latter case it breaks into sharp fragments like flint, and cannot be worked as stone; sometimes it will even admit of being polished as marble. It is also frequently filled with infiltrations of calcareous spar.

This description corresponds very nearly with the freshwater of the Isle of Wight, and an examination of the specimens from both places leaves no room to doubt of the similarity of the strata.

The fossil shells which I found in the upper freshwater formation, and which have been described by Mr. Parkinson, are the following.

Names given by Lamarck. Linnean names.
Planorbis, much resembling that which Brongniart says approaches to P. cornu Helix planorbis
Planorbis, two other species
Planorbis, much resembling P. prevostinus
Ampullaria Helix
Cyclostoma Helix
Limneus longiscatus Helix
────── acuminatus