Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/64

 52 plain, in situations and with appearances. very similar to those in which it occurs above the rock-salt.

Having stated the several facts which regard the extent, thickness, and other general. characters of the beds of rock-salt at Northwich; I shall now mention more particularly the appearances exhibited in their internal structure, in relation to which some interesting observations occur.

The fineness or purity of the rock is a circumstance very important to the interests of the mining proprietor, and in this point considerable varieties appear in different parts of the strata. The great body of the rock-salt, both in the upper and lower stratum, is composed of crystals of muriate of soda, intimately mixed with certain proportions of clay and oxide of iron, giving to the mass a red or reddish-brown tinge; and in addition to these constituent parts, contains likewise certain earthy salts, the sulphate of lime, and the muriates of lime and magnesia, but these in small proportion. In every part, however, of this compound rock, we find separate crystalline concretions of muriate of soda, variously disposed, sometimes occurring distinctly in the cubical form; in other places in masses of larger size, and irregularly shaped. The colour of these concretions, which are of the foliated species of fossil salt, is usually a greyish or milk-white; they are always translucent, and often attain a considerable degree of transparency. It would appear that they contain the muriate of soda in its purest form; the sulphate of lime in specimens of this kind being scarcely distinguishable by the delicate tests applied to its discovery.

This finer rock-salt occurs not only in separate concretions, but also in veins intersecting the coarser mass, and in the rims or borders