Mr. H. Holland on the Cheshire Rock-salt District. 53 



of the polyhedral figures which will afterwards be mentioned. Its 

 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 circum- 

 stance 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 evi- 

 dently 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 proportion of earth as large as in the upper 

 stratum : at this depth, however, a greatly increased degree of purity 

 appears, which is continued for five or six yards further down, when 

 the proportion of earthy admixture again becomes as large as before. 



It is invariably this purer portion of the lower bed which is at 

 present worked in the Northwich mines, and the rock-salt obtained 

 from it, being principally exported to the Baltic, obtains the name of 

 Prussia Rock. The extent of the cavity formed by the workings 

 varies in different mines ; the average depth may probably be taken 

 at about sixteen feet. In some of the pits, where pillars six or eight 

 yards square form the supports of the mine, the appearance of the 

 cavity is singularly striking, and the brilliancy of the effect is greatly 

 increased, if the mine be illuminated by candles fixed to the side of 

 the rock. The scene so formed, would almost appear to realize the 

 magic palaces of the eastern poets. Some of the pits are worked in 

 aisles or streets, but the choice here is wholly arbitrary. The me- 

 thods employed in working out the rock-salt offer nothing worthy 

 of notice. The operation of blasting is applied to the separation of 

 large masses from the body of the rock, and these are afterwards 

 broken down by the mechanical implements in common use. The 

 present number of mines is eleven or twelve, from which there are 



