entrance of a fresh body of sea water; from the gradual evaporation of which, the formation of the upper bed of rock-salt took place; and there being then no further admission of sea water to the plain, the superincumbent strata of clay and marl were successively deposited in the order in which they at present appear.
This in s general sketch of the probable mode of formation of the Cheshire sock-salt; but as it would seem very doubtful whether any single accumulation of sea water could contain the materials of depositions possessing so great a thickness, the theory might perhaps he successfully modified, by supposing the barrier before noticed, to have had such an elevation in the progressive stages of the deposition of the salt, as to allow the very frequent ingress of sea water into the basin. Admitting this idea, we must suppose that the formation of the strata of indurated clay between the beds of rock-salt took place, either during some intermission of these overflowings, or when there was a great predominance of this earth in the water, from which the depositions were made. It seems probable too that the veins of salt intersecting these strata were formed rather by the penetration of water holding salt in solution, from the upper bed of rock-salt, than by a direct deposition from the waters of the sea, With respect to the sources of the clay, combined with the substance of the rock-salt, or found in intermediate and superincumbent beds, little doubt can exist that it has been derived from the decomposition of more ancient rocks, of the situation and precise characters of which no vestiges now remain.
This general idea of the formation of the Cheshire rock-salt derives confirmation from the fact that, with the exception of the sulphate of magnesia, the same earthy salts occur together with the muriate of soda in these strata, as are met with in the waters of the sea. The circumstance of the beds decreasing in thickness as they
