422 MESOZOIC ERA— AGE OF REPTILES. 



beds Triassic and Jurassic ; the celebrated Polish beds at Cracow are 

 Tertiary. 



Mode of Occurrence. — Salt occurs in immense heels of pure rock-salt, 

 or else impregnating strata. It is obtained by direct mining, or else by 

 boiling down the saline waters either of natural springs or of artesian 

 wells sunk into the salt-bearing strata. The further explanation of its 

 mode of occurrence is best and most concisely given by comparing it 

 with coal. 



1. Like coal, it occurs in isolated basins, but these are far more 

 limited than the great coal-fields. 2. Like coal, it is interstratified 

 with sands and clays, the whole series repeated often many times. In 

 Galicia, for example, there are found seven salt-beds in the same sec- 

 tion. In the Kansas Trias there are seven beds. Like coal, also, each 

 bed is usually underlaid with clay. 3. But it differs from coal in the 

 great thickness of the beds. In Canada the salt-bed is 100 feet thick 

 (Gibson).* In Cheshire, England, there are two beds, one 100 feet, the 

 other 90 feet thick, separated by 30 feet of shale. At Stassfurt, a salt- 

 bed has been penetrated 1,000 feet, and the bottom not yet reached. f 

 The Berlin salt- well is 4,172 feet deep, and, except the upper 292 feet, 

 penetrates solid salt. J 4. Eecollecting the somewhat limited extent of 

 basins, it is evident that salt-beds thin out far more rapidly than coal. 

 The English salt-beds thin out fifteen feet per mile. Coal, therefore, 

 lies in extensive sheets, salt in lenticular masses. 5. Coal has its char- 

 acteristic valuable accompaniment in iron-beds, salt in beds of gypsum. 

 Thus, as coal-measures consist of repetitions of sands, clays, occasional 

 limestones, with valuable beds of coal and iron-ore many times repeated, 

 so salt-measures consist of sands, clays, and occasional limestones, with 

 valuable beds of salt and gypsum many times repeated. Gypsum-beds 

 are often entirely separate from salt-beds, but each salt-bed is apt to be 

 underlaid by gypsum. 6, While coal-measures are remarkable for the 

 abundance of organic remains, both vegetable and animal, salt-meas- 

 ures are equally remarkable for extreme poverty in this respect. The 

 presence of these remains in the one case, and their absence in the 

 other, are the cause of the difference in the color of the sandstones. 

 Coal-measure sandstones are white or gray, being leached of their 

 oxide of iron by organic matter. Salt-measure sandstones are usually 

 red, the iron being diffused as coloring-matter. 



Theory of Accumulation. — We have already seen (p. 80) that salt- 

 lakes are evaporated residues of river-water or sea-water in dry cli- 

 mates, and are now, most of them, depositing salt; also, that sea- 



* American Journal of Science, vol. v, p. 362, 1873. 

 f Bischof, Chemical Geology, vol. i, p. 383. 

 \ Nature, vol. xv, p. 240, 1877. 



