344 A Sketch of the Natural History of 



I may remark that jrypsum has been met with in several 

 other parts of the Cheshire plain, in situations and with 

 appearances very similar to those in which it occurs above 

 the rock-salt. 



Interior Character oj" the Beds of Rock' Salt. 



Having stated the several facts which regard the extent, 

 thickness, and other general characters of the beds of rock- 

 salt at Norihwich ; I shall now mention more particularly 

 the appearances exhibited in their internal structure, in re- 

 lation 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 mu- 

 riate 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 

 Tock, we find separate crystalline concretions of muriate ot 

 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 

 grayish- 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 concre- 

 tions, but aho in veins intersecting the coarser mass, and 

 in the rims or borders of the polyhedral figures which will 

 afterwards be mentioned. lis proportion varies both in 

 the two great beds of rock, and likewise in different parts 

 of the same bed ; and it is a regard to this circumstance 

 which determines the situation and extent of the workings 

 in the several mines. In the upper bed this variety is less 

 considerable than in the lower : but here the substance of 

 the rock-salt is evidently purer three or four yards above 

 the lower surface than in other parts of the same stratum, 

 and continues so for about four feet. In the lower bed, 

 the first twenty or twenty-five yards passed through contain 



a pro- 



