salt manufacture will be sufficiently obvious from the statement, that besides the salt made for home consumption, which annually amounts to more than 16,000 tons, the average of the quantity sent to Liverpool for exportation has not been less than 140,000 tons.
General situation, thickness, &c. of the beds of rock-salt.
Though springs impregnated with salt occur in several parts of the Cheshire plain, it may be remarked that the rock-salt itself has only been worked into near the banks of the Weaver and its tributary streams. It was first discovered at Marbury near Northwich, about one hundred and forty years ago, in searching for coal. This bed of rock was the only one worked for more than a century, when, in the same neighbourhood, a second and inferior stratum was met with, separated by a bed of indurated clay from the one previously known. This lower stratum was ascertained to possess at a certain depth a great degree of purity and freedom from earthy admixture; on which account, and from the local advantages of Northwich for exportation, the fossil salt is now worked only in the vicinity of this place.
This local limitation of the mines precludes the possibility of many comparative remarks which might be interesting to the geologist; and in giving a particular description of the rock-salt formation, I must confine myself in great measure to the facts which present themselves in the neighbourhood of Northwich, explaining first the circumstances of general position, &c. and then entering into the more minute particulars of the mines which have been sunk into these important strata.
The rock-salt of Northwich occurs, as I have just mentioned, in two great strata or beds, lying nearly horizontally, but on different levels, and separated, the superincumbent from the subjacent stratum,
