Handbook of Paleontology 17 



portion of magnesia was increased. Magnesium carbon- 

 ate is also secondarily deposited from solutions in circu- 

 lating ground water. 



Rock salt and gypsum are also precipitates found in 

 many parts of the world and of great commercial value. 

 They are found in the Salina beds of central New York 

 where they are mined. The rock salt, a chloride of so- 

 dium, commonly occurs in a succession of beds, some- 

 times of great thickness, which are separated by gypsum, 

 anhydrite (sulphate of lime) which slowly changes to 

 gypsum by taking on water, limestone, dolomite or clay. 

 These deposits are due to the concentration of sea water 

 in basins cut off from the sea or in lagoons behind a bar 

 with an inlet permitting a continual supply of sea water. 

 Gypsum (hydrous sulphate of lime) is found beneath the 

 beds of salt because in the concentration of the sea water 

 it separates out of the solution first. Salt is also deposited 

 in lake basins in arid regions through evaporation, as in 

 Great Salt Lake, Utah; and in desert basins, such as 

 Salton Sink, salt deposits are formed from the salt dis- 

 seminated through the rocks in their rims. Gypsum is 

 formed secondarily by alteration of limestones by per- 

 colating waters charged with sulphuric acid. There are 

 large gypsum beds of this kind in New York State. There 

 are numerous other chemical precipitates, among them 

 the potash and borax salts, saltpeter and metallic deposits, 

 such as oxides of iron (limonite, hematite), but it is 

 unnecessary to discuss them here. 



Materials of organic origin that enter into the forma- 

 tion of rocks are of two kinds, organic precipitates and 

 organic tissues. Animals and plants take carbonate of 

 lime or silica from the water in which they live and in 

 which these minerals are held in solution and from this 



