BOCK-SALT OF CHESHIRE. 



171 



three inches, and might be mistaken for strata, but they are irregular 

 and of limited extent. In Nottinghamshire, the fibrous gypsum on 

 the banks of the Trent is often beautifully white and translucent, and 

 is accompanied with scales of chlorite, exactly similar to what 1 have 

 observed in the beds of gypsum in the Valais, in Switzerland. The 

 white fibrous gypsum is employed by the paper-makers to whiten 

 writing-paper and add to its weight. 



Massive gypsum is granular : it occurs in irregular beds and blocks, 

 in the red marl, and is evidently a local formation. Anhydrous gyp- 

 sum is occasionally met with in Nottinghamshire. Gypsum is asso- 

 ciated with rock-salt, wherever the latter mineral is found. It is now 

 discovered, that the gypsum in the Alps, when uncovered in its na- 

 tive beds, is always anhydrous. Common gypsum contains 21 per 

 cent, of water. Anhydrous gypsum is entirely free from water, and 

 is much harder and heavier than common gypsum. Should it prove 

 a general fact, that the gypsum associated with rock-salt is always ori- 

 ginally anhydrous, it might tend to elucidate the formation of both 

 minerals; a subject which will be referred to, after describing some 

 of the principal repositories of rock-salt. 



Many repositories of rock-salt are situated near the feet of moun- 

 tain ranges, and were probably deposited originally in salt-water 

 lakes : beds of rock-salt are now found at the bottom of some of the 

 salt lakes in Africa. But though many salt formations are in com- 

 paratively low situations, there are others that occur at great alti- 

 tudes, both in the Alps and the Cordilleras. In England, the prin- 

 cipal beds of rock-salt are situated at a little distance from the west- 

 ern side of the range of hills, which separate the rivers that flow into 

 the eastern and the western seas. 



The rock-sah of Cheshire cannot properly be said to lie in or un- 

 der the red sand rock before described, but is surrounded by it, and 

 probably rests upon it ; but as the lowest bed of salt has not been sunk 

 through, this cannot be yet ascertained. The upper bed of rock- 

 salt in that county is about forty-two yards below the surface : it is 

 twenty-six yards thick, and is separated from the lower bed of salt, 

 by a stratum of argillaceous stone ten yards thick. The lower salt 

 has been sunk into forty yards. The upper bed was discovered 

 about a hundred and forty years since, in searching for coal. Rock- 

 salt at Northwich, extends, in a direction from N. E. to S. W., one 

 mile and a half ; its further extent in this direction has not been as- 

 certained : its breadth is about fourteen hundred yards. In another 

 part of Cheshire, three beds of rock-salt have been found. The up- 

 permost is four feet, the second twelve feet, and the lowest has been 

 sunk into twenty-five yards, but is not cut through. Besides the 

 beds of rock-salt, numerous brine springs, containing more than 25 

 per cent, of salt, rise in that county. The transparent specimens of 

 rock-salt are nearly free from foreign impurities, and contain scarce- 

 ly any water of crystallization. 



