152 LAKE DEPOSITS 



cipitated, which deposition continues steadily as concentration 

 proceeds, but at an advanced stage the salt is mingled with the 

 sulphate of magnesia (MgS0 4 ), should that be present. The 

 highly soluble salts, such as the chlorides of magnesium and cal- 

 cium (MgCl, CaCl), remain in solution until the water is com- 

 pletely evaporated to dryness, hence they are rarely found in beds 

 of rock salt. 



Various circumstances may change the order of precipitation 

 just given. In seasons of high water the flooded rivers dilute the 

 waters of the lake, checking the chemical precipitation and, at the 

 same time, increasing the mechanical deposition ; thus beds of 

 sand and mud are thrown down upon the beds of gypsum and 

 salt, alternating with them, as the influx of fresh water or evapora- 

 tion predominates. Changes of temperature also have an effect 

 upon the order of precipitation. Thus, in cold weather, Salt Lake 

 washes up on its shores quantities of sulphate of soda (Na 2 S0 4 ), 

 which is formed at low temperatures by the double decomposition 

 of NaCl and MgS0 4 . 



Besides the chemical deposits already mentioned, others occur 

 on a smaller scale. On the western side of the Great Basin, in 

 Nevada, California, and Oregon, are several lakes which contain 

 large proportions of carbonate of soda, and in some of them the 

 concentration is sufficiently advanced to cause precipitation. 



Much the most abundant of the chemical deposits made in salt 

 lakes are gypsum and rock salt, and the enormous scale on which 

 the latter was formed in past ages of the world's history is demon- 

 strated by the vast bodies of rock salt which are found embedded 

 in the rocks in so many parts of the world. Near Berlin, at 

 Sperenberg, an artesian well was sunk through such a deposit for 

 nearly 4000 feet, without reaching the bottom. In various regions 

 of the United States, notably in New York and Kansas, large 

 bodies of salt are found, but not on such a scale as in Europe. 



It should be noted that the chemical deposits made in salt lakes 

 are crystalline and at the same time stratified. This association 

 is not the usual one, as stratified rocks are ordinarily not crystal- 

 line, and crystalline rocks are mostly unstratified. 



