S^ Mr. H. Holland on the Cheshire Rock-salt District, 



plain, In situations and with appearances very similar to those in 

 "which it occurs above the rock-salt. 



Interior character of 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 Northwich ; 

 I shall now mention more particularly the appearances exhibited in 

 their internal structure, in relation to which some interesting obser- 

 vations occur. 



The fineness or purity of the rock Is a circumstance very import- 

 ant to the Interests of the mining proprietor, and in this point consi- 

 derable 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 crys- 

 talline concretions of muriate of soda, variously disposed, some- 

 times 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 



