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THE ULTIMATE SOURCE 



berg, and at other places. And it may be said, too, of the 

 great majority of rock salt formations, that the true bottom 

 has never been explored. In many cases the bottom of the 

 rock salt has not been reached ; in many others, as in Cheshire, 

 where rock salt strata alternate with strata of marl, the 

 exploration is given up as soon as inferior salt is tapped 

 in deep workings, or, as soon as marl is found underlying it 

 at unprofitable depths. The lowest stratum of rock salt 

 overlying the gypsum is generally so inferior, and mixed up 

 with gypserous marls and shell limestone, as at Wimpfen, that 

 where these things are done for profit, exploration is at once 

 abandoned. It is most probable that gypsum does underlie 

 the salt oftener than is suspected, but as will be shown in the 

 next para, it is not essential to the aqueous theory that it 

 should be so. 



The way in which gypsum comes to overlie the salt deposit 

 is this. Gypsum attains its maximum of solubility when the 

 density of the sea brine equals 4 C Beaume, or specific gravity 

 1,033. If the volume of ordinary sea water be taken as 1,000, its 

 volume, after it hasbeen evaporated to the density specified, will 

 be about 850. If there be sufficient gypsum to form a saturated 

 solution, any further diminution in volume, accompanied as it 

 must be by increase of density, tells on the contained gypsum, 

 a portion of which becomes insoluble and separates. This 

 deposition of gypsum from sea brine continues from volume 

 850 until the volume of the sea water is reduced to 100, or to 

 one-tenth of its original bulk, at which point hardly a trace 

 of gypsum remains. At this point the deposition of common 

 salt begins. We have now to consider the shape of the sea 

 basin in which the deposit is taking place. Without going 

 into mathematical figures, it is sufficient to observe that it has 

 a slope from the circumference to the centre, and that as the 

 volume of the sea diminishes from 1,000 to 100, its waters 

 naturally gravitate towards the centre or deepest part into 



