Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/260

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Mr. Webster on the Strata lying over the Chalk.

with marine bodies. The thickness of the beds of stone varies from 6 to 12 feet. They contain river shells, bones, sculls of animals, kernels of fruits, ears of corn, &c. These strata extend quite under the city so the borders of the Unstrutt, near to which are seen strata of alabaster and limestone, the detritus of which has probably given origin to these beds. It is to be remarked that no remains of marine animals are found above the clay where the ancient beds commence.”

A very interesting account has been given by Von Buch of a freshwater formation in Locle in the district of Jura. It is contained in a high inclosed valley surrounded by mountains of white compact limestone; and consists of various alternating beds of marly limestone whitish and somewhat friable, bituminous shale, coal, hornstone of a smoke grey colour, and of a fine splintery or imperfectly conchoidal fracture, and containing crystals of quartz; also of opal of a brownish black colour, glistening lustre, and perfect conchoidal fracture. Both the limestone and hornstone contain freshwater shells, among which may be distinguished the helix cornea and bivalves.

From these and other accounts it appears probable that these freshwater formations were purely local, and there appears no necessity for supposing that the others were any thing more than local deposits in a former state of the earth.

I am aware that such formations have lately been traced on the shores of the Baltic, in the south of France, Spain, Germany, and Silesia; but this can only prove that fresh water lakes were in former times as at present very numerous and often extensive.

One of the most striking differences between these ancient lakes and modern ones, as has been remarked by Messrs. Cuvier and Brongniart, is the property possessed by the former of forming siliceous strata.