ORIGIN OF ROCK-SALT. 423 



water evaporated deposits first gypsum, then salt : also, that these de- 

 posits of salts and gypsum alternate annually with sediments of sand 

 and clay — the salt or gypsum deposit representing the dry season, and 

 the mechanical deposits representing the season of floods. It is, there- 

 fore, natural to look in this direction for an explanation of salt and 

 gypsum deposits — to think that salt -basins are dried-up salt-lakes. 

 But the immense thickness of the beds plainly shows that there must 

 have been important modifications of this process. It is plain that 

 the alternations of salt and sedimentary deposit were not annual but 

 secular. 



The conditions under which salt-measures were formed are not cer- 

 tainly known, but most probably they are a dry climate, a. low coast, 

 with bordering salt lagoons or bays, partly cut off from the sea by 

 bars, subject to intense evaporation, and resupplied with salt-water by 

 tides or by winds, or perhaps at longer intervals by crust-movements. 

 It is easy to imagine how salt-measures with their alternation of gyp- 

 sum, salt, and sediments, may thus have been formed. We have ex- 

 amples of the process now going on, on the east shore of the Caspian 

 Sea, where salt has been depositing for ages,* even though the water 

 of the Caspian is much fresher than that of the ocean. On the other 

 hand, it seems to us that the recent observations of Gilbert and Rus- 

 sell on the deposits of the great dried-up lakes Lahontan and Bonne- 

 ville of the Basin region (p. 566) throw much light on this subject, and 

 that in the phenomena of these deposits we probably have at least an 

 additional method in which salt-measures may be formed. There is 

 abundant evidence that these lakes have filled and dried up and left 

 beds of salt, more than once, and that at each refilling the lake com- 

 menced as a fresh lake. The process was briefly as follows : The great 

 lake, at first fresh, gradually became saline and finally dried away, 

 leaving a thick bed of salt. This salt-bed was then covered by the 

 washing in of fine, impervious clay, and thus protected from re-solution 

 when the lake reformed. This process was repeated in the case of Lake 

 Lahontan three times — that is, there are now, beneath the salt-lakes of 

 this region, two beds of salt separated by clay, and the third deposit is 

 now forming. Salt-beds are now reached and worked in many places 

 of this region by penetrating the fine clay which marks the places of 

 the old lakes, f 



In the deposits of salt-lakes or saturated lagoons we would not ex- 

 pect to find many animal remains, but the tracks of animals along their 



* Ochsetiius, Proceedings of the Academy of Natural Sciences, Philadelphia, 1888, 

 p. 181. 



f Gilbert, Lake Bonneville, Wheeler's Survey, vol. iii; American Journal of Science, 

 vol. xxxi, p. 354, 1886 ; Russell, Lake Lahontan, Monograph of United States Geological 

 Survey, vol. xi. 



