SOMETHING ABOUT ROCK-SALT AND GYPSUM. 297 



ate at the bottom of the Coal-measures — to be copiously 

 saturated with brine ; and in the lower peninsula of Mich- 

 igan, the Marshall sandstones at the bottom of the Carbon- 

 iferous system are a reservoir of saline accumulations. 

 Still lower, the American geologist finds the Salina group 

 of the Upper Silurian system the source of supplies of brine 

 throughout a wide extent of territory. 



The attempt has been made to explain the existence of 

 saliferous and gypsiferous deposits by reference to chem- 

 ical reactions transpiring subsequently to the solidification 

 and upheaval of the strata ; but I am led to regard the 

 presence of sulphuric acid and other chemical constituents 

 of gypseous and saliferous formations as products of the 

 decomposition of previously existing gypsum and salt, 

 rather than the agents employed in the present generation 

 of them. 



The body of water in which the saliferous materials ac- 

 cumulated may have been a bay or sea having imperfect 

 communication with the ocean. Under ordinary circum- 

 stances, the evaporation from the surface of the bay would 

 exceed the supply from atmospheric sources, and there 

 must arise, consequently, a gradual influx of sea-water from 

 the ocean. The bay-water would finally reach such a state 

 of condensation as to begin to precipitate its least soluble 

 constituents. These would be mingled with the ordinary 

 sediments and debris of saline waters. This process con 

 tinuing, the condensation would reach, in succession, those 

 stages at which peroxyd of iron, gypsum, common salt, and 

 Epsom salts would be crystallized and deposited around 

 the shores and bottom of the bay, and mingled with the ar- 

 gillaceous mud brought in by the influx of surface waters. 

 These substances are all constituents of sea-water. In 

 course of time, the bottom of the bay may have been con- 

 verted into dry land through the course of continental ele- 



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