172 



ROCK-SALT AND SALT SPRINGS. 



In sea-water a large portion of muriate and sulphate of magnesia 

 is found, which gives it that bitter nauseous taste, distinct from its 

 saltness. This difference in the composition of sea-water and of 

 rock-salt, might seem to indicate that rock-salt was not, as some sup- 

 pose, produced by the evaporation of sea-water; but if it were 

 formed in detached lakes, it is possible that the waters of these 

 lakes did not contain precisely the same salts in solution, as those of 

 the sea. We know that the waters of some of the salt lakes, exist- 

 ing at present, differ in their contents from sea-water. If, however, 

 the evaporation were very slow, the salt of the ocean would separate 

 from all its impurities by crystallization ; these impurities, being more 

 deliquescent, might be washed away. 



It may deserve notice, that few, if any, remains of marine or other 

 organized bodies are found in the beds accompanying the rock-salt 

 of Cheshire. In the Polish salt mines, bivalve shells and the claws 

 of crabs are met with in the upper strata of marl ; and vegetable im- 

 pressions in the bed covering the lower salt, at the depth of two hun- 

 dred and twenty five yards from the surface. But some of these 

 mines are now believed to occur in tertiary formations. 



The salt formation at Droitwich in Worcestershire, appears to be 

 surrounded by the same kind of red sand rock, and covered with 

 similar beds of gypsum and marl, to that of Cheshire. Here the 

 rock-salt, though its existence has been proved by boring, is no where 

 worked. The salt is procured by evaporating the water, which is 

 nearly saturated with it. 



Salt springs rise in some of the coal strata, adjacent to the red marl 

 and sandstone : in all probability the brine is infillered from that for- 

 mation, into the basset edges of the strata overlying coal. There 

 are salt springs in some of the coal mines in Northumberland, Der- 

 byshire, and Yorkshire ; and a spring of brine rises in the river Wear, 

 in the county of Durham. 



Brine springs, containing from five to six per cent, of salt, rise in 

 the coal mines near Ashby-de-la-Zouch in Leicestershire, at the 

 depth of two hundred and twenty five yards under the surface. A 

 weaker brine also rises in the upper strata : it springs through fissures 

 in the coal, attended with a hissing noise, occasioned by the emission 

 of hydrogen gas. 



I examined these mines, belonging to the Earl of Moira, in the 

 summer of 1812: they are situated at Ashby Wolds, in the very 

 centre of England ;^ and what may appear remarkable in this situ- 

 ation, they are worked one hundred and forty yards below the level 

 of the sea, which is ascertained from the levels of the canal that 

 passes by the pits. Had this circumstance been known, before the 

 attention of geologists was directed to the structure of the earth's 



* Baths and hotels are now erected there for the accommodation of visiters: 

 they are called the Moira Baths, near Ashby-de-la-Zouch. 



