Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/256

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Mr. Webster on the Strata lying over the Chalk.

that seem to have had considerable influence upon organic life. The almost entire absence in the lower beds of those siliceous nodules that are so numerous in the upper one, and the remarkable differences in their animal remains, furnish sufficient reasons for this supposition.

Calcareous, argillaceous and siliceous matter, the whole or a part of which was in a state of solution, originally formed the mass of this formation. Of these the argillaceous seems to have subsided first, but more or less mixed with calcareous earth. The silex now occupies the upper division, but whether separated by the action of chemical affinities, or introduced subsequently, does not yet appear. The tranquil state of the ocean during this period may be inferred from the perfect preservation of the numerous delicate fossil organic bodies now found in the chalk.

An era of turbulence seems to have succeeded; during which however the depositions of the plastic clay and sand denote certain intervals of repose. The surface of the chalk already solidified was in a certain degree, though irregularly, subjected to the agency of water in motion; and other causes might have combined in destroying the original horizontality of its position. But where the vast bed of London clay subsided, the sea must have regained its former tranquility.

The numerous vestiges of vegetables as well as of animals to be found in this stratum, whose recent analogues are now seen only in tropical countries, involuntarily leads the mind to contemplate with wonder the altered condition of this portion of the globe. Have the laws which regulate the place and motions of this earth in the system of the universe been subjected to change? Are there in these any sources of irregularity or gradual alteration, the proofs of which can be detected? these are questions for astronomers.