clay a few inches in thickness. Much, however, it is evident, must depend upon the number of the beds thus disposed, but this I do not find any where noticed. The earthy saline contents of the foreign rock-salt very exactly resemble those of the Cheshire; the gypsum existing in much larger proportion than the other earthy salts, and appearing in considerable masses, both distinctly, and in mixture with the beds of clay. It is an important fact, however, that sea-shells and other marine exuviæ are found in these beds of clay and gypsum; a circumstance which, as I before stated, never occurs in the Cheshire mines. It would seem that the portion of oxide of iron combined with the clay in the substance of the English rock-salt does not exist in the mineral as found abroad, or at least in a proportion not so considerable.
The comparative commercial value of the English and Polish mines is best ascertained by the fact that many thousand tons of rock-salt are annually sent from Cheshire to the parts of the Prussian coast most nearly adjacent to the salt-mines; independently of the large supplies of the English manufactured white salt which are exported to the same country.
Considerations on the Origin of the Cheshire Rock-salt.
With respect to the theory of the formation of rock-salt, as applicable particularly to that of Cheshire, I shall not venture to say much, and that little will be of a general nature. Though it must be acknowledged that there are some difficulties connected with the supposition, little doubt can exist of the general fact, that the beds of this mineral have been formed by deposition from the waters of the sea. Such an opinion acquires much probability from the situation in which these beds usually occur; occupying the vallies and lower parts of
