the Cheshire Rock Salt District. 345 



a proportion of earth as large as in the upper stratum : at 

 this depth, however, a grtatly increased degree of purity 

 appears, which ij continued for five or six yards further 

 down, when the proportion of earthy admixture again be- 

 comes as large as before. 



It is invariably this purer portion of the lower bed which 

 is at preseni. worked in the Northwich mines, and the rock- 

 salt obtained from it, being principally exported to the 

 Baltic, oi)tams the name oi Prussian Rock. The extent of 

 the cavity formed by the workings vari-es 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 raised, on 

 an annual average, fifty or sixty thousand tons of rock-salt. 

 The greater part of this quantity is exported to Ireland and 

 the iialtic: the remainder is employed in the Cheshire 

 district in the manufacture of white salt by solution and 

 subsequent evaporation. 



It is very doubtful whether in any instance the body of 

 rock-salt can be considered as stratified, or disposed in 

 distinct layers. A perpendicular section does sometimes 

 indeed present irregular appearances of this kind, and more 

 especially in the purer part of the lower bed ; but the great 

 body of the rock offers to the eye merely a confused red 

 mass, varied here and there by the occurrence of the cry- 

 stalline portions of salt. 



One of the most striking facts connected with the in- 

 ternal structure of the Northwich rock-salt, is the ap- 

 pearance observable on the surface of an horizontal sec- 

 tion of the rock, as viewed in any of the mines. On this 

 surface may be traced various figures, more or less distinctly 

 marked, and differing considerably in the forms which they 

 assume ; some appearing nearly circular, others perfectly 

 pentagonal, and others again having au irregular polyhe- 



Vol. 39. No. IC9. Alay 1812. Z dral 



