Mr. H. Holland on the Cheshire Rock-salt District. 59 



entrance of a fresh body of sea water ; from the gradual evaporation 

 of which, the formation of the upper bed of rock-salt took place ; 

 and there being then no further admission of sea water to the plain, 

 the superincumbent strata of clay and marl were successively depo- 

 sited in the order in which they at present appear. 



This is a general sketch of the probable mode of formation of the 

 Cheshire rock-salt ; but as it would seem very doubtful whether any 

 single accumulation of sea water could contain the materials of depo- 

 sitions possessing so great a thickness, the theory might perhaps be 

 successfully modified, by supposing the barrier before noticed, to 

 have had such an elevation in the progressive stages of the deposition 

 of the salt, as to allow the very frequent ingress of sea water into 

 the basin. Admitting this idea, we must suppose that the formation 

 of the strata of indurated clay between the beds of rock-salt took 

 place, either during some intermission of these overflowings, or when 

 there was a great predominance of this earth in the water, from 

 which the depositions were made. It seems probable too that the 

 veins of salt intersecting these strata were formed rather by the pene- 

 tration of water holding salt in solution, from the upper bed of 

 rock-salt, than by a direct deposition from the waters of the sea. 

 With respect to the sources of the clay, combined with the substance 

 of the rock-salt, or found in intermediate and superincumbent beds, 

 little doubt can exist that it has been derived from the decomposition 

 of more ancient rocks, of the situation and precise characters of 

 which no vestiges now remain. 



This general idea of the formation of the Cheshire rock-salt de- 

 rives confirmation from the fact that, with the exception of the sul- 

 phate of magnesia, the same earthy salts occur together with the 

 muriate of soda in these strata, as are met with in the waters of the 

 sea. The circumstance of the beds decreasing in thickness as they 



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