428 SPECIAL GEOLOGY. 



There is historical evidence of their having been wrought by 

 the Britons, and for 2000 years they have never failed. In 

 the year 1725, it having been ascertained that the strata 

 at Northwich were perforated to a greater depth than those 

 at Droitwich, the gypsum-beds of that place were broken 

 through, when springs of much stronger brine poured into 

 the pits, and by yielding a more valuable supply, greatly 

 enhanced the productiveness of the works. The bed of 

 gypsum lies about forty or fifty feet below the surface ; it 

 is a hundred and fifty feet thick, and on being perforated 

 the stream of brine is reached, which is about two feet in 

 depth, and rests on a bed of rock-salt. The brine rises 

 through the perforation, and is conveyed into iron boilers, 

 for the purpose of evaporation. Brine-springs are produced 

 by streams of water which have flowed over masses of rock- 

 salt; but the origin of that mineral substance is by no 

 means so easy of explanation. It constitutes a problem 

 which the present state of our knowledge does not 

 enable us to solve. Three theories have been proposed to 

 account for its origin : the first ascribing it to the evapo- 

 ration of sea-water, and considering it to have been deposited 

 in the bed of a sea ; the second referring it to similar depo- 

 sitions from a salt-water lake ; and the third attributing it 

 to volcanic agency. The first two propositions are liable to 

 the objection of the great depth of water requisite for pro- 

 ducing masses of rock-salt forty yards in thickness ; or a 

 mountain of salt six hundred feet in height, and twelve 

 hundred in breadth at its base, such as is known to exist 

 at Cardona, in Spain. The most conclusive proof in favour 

 of the volcanic theory consists in the perfect freedom of the 

 rock-salt from extraneous matter ; whereas if it had been 

 deposited by salt-water, it would have presented some im- 

 bedded substances indicative of its sedimentary origin. 

 The volcanic theory is strengthened by the facts that chlo- 

 ride of sodium is of common occurrence among the ejections 

 of volcanoes, and that salt-springs rise to the surface from 

 granitic rocks ; whence it is inferred that the sources which 

 supply the salt may lie as deep as those substances which 

 yield the materials for modern lavas, and which have fur- 

 nished the elements for the ancient trap-rocks and basalts 

 in the early history of the earth. 



