OF COMMON SALT. 



155 



sometimes found crystallised in lenticular, or ring-shaped 

 masr es, in the midst of inferior salt ; this is not a usual sedi- 

 mentary arrangement ; it is a difficulty. But, when we 

 consider that we have to deal with crystalline masses of 

 unexampled extent, and with crystalline forces which are 

 imperfectly understood, this crystal, within a crystal, has 

 nothing startling about it. It exemplifies, in crystal masses, 

 a form of natural selection which we see illustrated by 

 concretionary nodules, elsewhere, and by the lenticular 

 gypsum crystals which are found in salt marshes. 



Again, veins of rock salt, at various angles, are found 

 running through the stone, or indurated clay beds, associated 

 with rock salt in some mines. At Cheshire, for instance, 

 such veins occur in the flag beneath the salt. From the 

 nature of these stones, it is not difficult to suppose that they 

 may have been traversed by cracks and fissures, into which 

 brine would percolate, and crystallise. The salt is crystallised, 

 and not fused in the veins, as it would be, if of igneous origin $ 

 and very perfect cubic crystals are often found in such 

 situations. 



These difficulties, which are of little account in themselves 

 and easily explained away, lead up to the igneous theory 

 of the origin of rock salt. The presence of sodic chloride 

 in hot springs is taken as evidence in the same direction ; as 

 is also the presence of common salt in the eruptive matter 

 of Mount Vesuvius. All we know about hot springs, is, 

 that the water comes from a distant and perhaps warm 

 region, though it may receive its warmth in transit, and that 

 the water takes up in solution more or less of the soluble 

 salts it comes in oontact with. Bock salt deposits are 

 abundant enough in most parts of the world to account 

 for the presence of salt in such springs. Hot springs 

 are very common in the salt regions of China and Burmah. 

 Perhaps the association of coal and mineral oil, with salt, may 



