34 8 A Sketch of the Natural History of 



exuvi.^ are found in these beds of clay and gypsum ; a 

 circumstance which, as I before staled, never occurs in the 

 Cheshire mines. It would seem that the portion of oxide 

 of iron combined with the clay in the substance of the 

 Eughsh rock-sali does not exist in the mineral as found 

 abroad, or at least in a pro[ioriion not so considerable. 



The comparative commercial value of the English and 

 Polish mines is best asc.-rtained by the fact that many thou- 

 sand tons of rock-salt are annually sent from Cheshire to 

 the pans of the Prussian coast most nearly adjacent to the 

 salt-mines; independently of the large supplies of the 

 English manufactured white salt which'are exported to the 

 same country. 



Coris'ideratiovs on the Origin of the Cheshire Rock-Salf. 



With respect to the theory of the formation of rock- 

 salt, as applicable particularly to that of Cheshire, I shall 

 not venture to say much, and that little will be of a general 

 nature. Though it must lie acknowledged that there are 

 some difficulties connected with the supposition, little 

 doubt can exist of the general fact, that the beds of this 

 mineral have been formed by deposition from the waters of 

 the sea. Such an opinion acquires much probability from 

 the situation in which these beds usually occur; occupying 

 the valleys and lower parts of plains which are so sur- 

 rounded by hills of second'ary formation, as to leave only a 

 narrow egress for the waters collected on liieir surface. 

 This structure of the plain constituting the salt district of 

 Cheshire, I have particularly described ; and, regarded in 

 its general character, it leads strongly lo the conclusion 

 that the waters of the sea must, at some former period, 

 have occupied the lower parts at least of the basin thus 

 formed, which at that time had a level eighty or one hun- 

 dred yards lower than the one now appearing*. To ac- 

 count for the great depositions of salt in the lower parts of 

 this iiasin, it is necessary to suppose that some barrier must 

 have been afterwards interposed to prevent the free com- 

 munication ot the waters of the sea with those thus col- 

 lected; and the general course of the streams, the position 

 of the beds of rock salt, and the contractions in the valley 

 of tht Weaver, which appear below Northwich at Anderton 

 and Frodsham, point out w ith some distinctness the place 

 where these obstructions may probably have occurred. 



* This general character of the Cheshire salt district was remarked to me 

 by my friend Sir John Stanley, in reference to the formation of the rock- 

 salt; on which subject he fiblijjed me bv some verv interesting observations, 

 Vvhlch are inserted in the Cheshire Report. 



To 



