Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/362

 observable, nor is the least dulness of the delicate lines and embossments of the crusts, or of the spines of the echini, to be detected.

That the deposition of chalk and of flint was sometimes alternate, and even, as it is expressed by Messrs. Cuvier and Brongniart, periodical, appears from the seams or strata of flinty nodules, and particularly from the widely extended flat or tabular flinty depositions interposed between the chalk.

But that the chalk was permeated by the silex at some distance of time after the deposition of the former, seems also to be proved by the state of the fossils of this stratum. There does not appear to be a single instance in which the animal remains are impregnated with silex. On the contrary, the substance of all these fossils has become calcareous spar, and their cavities have been filled with flint; thus plainly evincing that sufficient time must have elapsed for the crystallization of the calcareous spar, previously to the infiltration of the flint.

It may not be improper to remark, that in no instance does the flint, although in contact with the calcareous spar, appear to have become mixed with it. The reverse of this is the case with the chalk, since this latter may be seen in almost every degree of union with the flint; from being blended with its substance, to being merely united with its surface, and forming the white coat of the flint. It has been, without doubt, from certain appearances resulting from this union, that M. Carrosi and others have been led to believe in the change of lime to flint.

There can be hardly any hesitation in agreeing with Mr. Jameson, that the most probable explanation of the formation of imbedded flint is that which was first proposed by Werner, “that during the deposition of chalk, air was evolved, which in endeavouring to escape, formed irregular cavities, that were afterwards filled up, by