SOMETHING ABOUT ROCK-SALT AND GYPSUM. 295 



The salt of the rocks is the residuum of the once univer 

 sal ocean. The reader will remember that reference has 

 already been made to the origin of salt lakes, like those of 

 Utah and the Caspian and Aral Seas. Such lakes are but 

 remnants of the last oceanic inundation. They occupy de 

 pressions in the terrestrial surface from which there is no 

 outlet. If, like Lake Superior, they had been drained to 

 the sea, the original saline waters would long since have 

 been replaced by fresh waters from the clouds. 



In consequence of the changed condition of the earth, 

 the amount of evaporation from the surfaces of these in 

 land seas has generally exceeded the contributions of fresh 

 water from the clouds. Their saltness has therefore been 

 intensified, and, in many cases, a deposit of crystallized salt 

 has been formed upon the bottom and around the shores. 

 Indeed, there have been salt lakes that are now extinct, in 

 consequence of the exhalation of their waters ; and in the 

 place of each remains a salt plain, the surface of which is 

 composed of salt and the other mineral constituents of the 

 ancient sea-water, variously intermingled with argillaceous 

 matter washed in from the surrounding country. The in 

 terior of the American continent furnishes abundant phe 

 nomena of this kind, stretching from Utah, through the 

 Great American Desert, to Mexico. Such products are 

 the residua of salt lakes which have evaporated since the 

 surface received its existing configuration. It will be re 

 membered, however, that extensive salt-beds exist in Ne 

 vada, which are derived from the teachings of the salifer- 

 ous strata of the mountains ; and it may be that some of 

 the ancient salt lakes of the region were supplied with salt 

 to some extent by contributions from similar sources. This, 

 nevertheless, would not prove that all salt lakes have been 

 similarly fed. Besides, if it should appear that they are, 

 we have still to account for the existence of strata of salt 



