348 PRINCIPLES OF STRATIGRAPHY 



Both are notably absent from the open ocean and the ordinary type 

 of mediterranean. In the marginal portions of the more enclosed 

 mediterraneans in the arid belts of the earth such deposits often 

 abound. Examples of these are found on the borders of the Black 

 Sea, to a moderate extent on the borders of the Red Sea, and in 

 the Rann of Cutch on the west coast of India. 



Experiments of Usiglio. The precipitation of salt on the evapo- 

 ration of sea water was studied by Usiglio at Cette (52) on the 

 Mediterranean. He began with 5 liters of water, which at 12.5° C. 

 had a density of 1.0258 and a salinity of 38.45 permille, and by evap- 

 oration in the air obtained the results given in the accompanying 

 table, reduced to grams per liter of volume. No salt was deposited 

 until the water had been reduced by evaporation to nearly one-half 

 its volume (53.3 per cent.) and a density of 1.0506,* when slight de- 

 posits consisting chiefly of CaCOg and ferric oxide were made. 

 When, on further evaporation, the density had increased to 1.1304, 

 hydrated calcium sulphate (gypsum) began to deposit, this contin- 

 uing until a density of 1.2627 was reached. During the change in 

 the density from 1.13 to 1.208, 84.34 per cent, of all the CaSO^ was 

 precipitated as gypsum. After this NaCl was deposited mixed with 

 a little magnesium chloride and sulphate, and this continued until 

 the density was 1.2627, when all the sulphate of lime had been de- 

 posited together with 78.23 per cent, of the total quantity of NaCl. 

 After this 12.38 per cent, of the total NaCl was precipitated without 

 any gypsum. At a density of 1.2444 a little sodium bromide had 

 begun to deposit. Thus between densities 1.0506 and 1.208, only 

 0.1583 per cent, of the total solids had been deposited, and this 

 chiefly as lime sulphate and carbonate and as iron, but between 

 densities 1.221 and 1.3 18 the following mixture was precipitated: 



Calcium sulphate o. 2828 0/00 



Magnesium sulphate o . 6242 0/00 



Magnesium chloride o . 1532 0/00 



Sodium chloride 27 . 1074 0/00 



Sodium bromide o . 2224 0/00 



Total 28 . 3900 0/00 



Of this 28.3900 permille of total solids, 27.1074 permille, or 

 about 95 per cent., was sodium chloride. The volume had now 

 been reduced to 1.62 per cent, of the original volume of 5 liters 



* The values here given are taken from the table. Slightly different values for 

 intermediate densities are often given. (See the article on Salt in Encyclopaedia 

 Britannica, also Clarke-io.) 



