Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/70

 58 plains which are so surrounded by hills of secondary formation, as to leave only a narrow egress for the waters collected on their surface. This structure of the plain constituting the salt district of Cheshire, I have particularly described; and, regarded in its general character, it leads strongly to the conclusion that the waters of the Sea must, at some former period, have occupied the lower parts at least of the basin thus formed, which at that time had a level eighty or one hundred yards lower than the one now appearing. To account for the great depositions of salt in the lower parts of this basin, it is necessary to suppose that some barrier must have been afterwards interposed to prevent the free communication of the waters of the sea with these those collected, and the general course of the streams, the position of the beds of rock-salt, and the contractions in the valley of the Weaver, which appear below Northwich at Anderton and Frodsham, point out with some distinctness the place where these obstructions may probably have occurred.

To explain the appearance of the strata of indurated clay, intermediate between the beds of salt, we must suppose that the obstruction still continued, when the deposition of salt from the waters first confined, had nearly ceased; and that at this period, the deposition of clay, which had hitherto been going on in conjunction with that of the salt, proceeded in a great measure alone; the salt which remained in the water being merely sufficient to form small veins in its substance. When these strata had been deposited to a thickness often or eleven yards, it would appear that the barrier preventing the access of the sea to the basin or plain, was again so far removed as to allow the