196 REPORT OF NATIONAL MUSEUM, 1899. 



it can be distinguished by its crystalline form, that of a combination 

 of cube and octahedron (Specimen No. 40223, U.S.N.M. See fig. 4, 

 p. 203), and more biting taste. Owing to its ready solubility it is 

 rarely found in a state of nature. Bischofite, the chloride of mag- 

 nesium (Specimen No. 62428, U.S.N.M.) is still more soluble and 

 practically unknown except in crystals artificially produced. 



Origin and occurrences. — Sodium in the form of chloride, to which 

 is commonly given the simple name of salt, is one of the most widely 

 disseminated of natural substances, and not infrequently occurs in 

 large masses interstratified with other rocks of the earth's crust in 

 such a manner as to constitute a true rock mass. 



The geological history of these beds of rock salt is as follows: 



No terrestrial waters are absolutely pure, but all hold in solution 

 more or less mineral matter which has been taken up from the rocks 

 and soils with which they have come in contact. The nature of these 

 impurities depends on the nature of the formations permeated and 

 their relative solubility. Numerous analyses of river waters have 

 shown that the substances mentioned below, though sometimes exist- 

 ing as mere traces, are almost invariably present; these are sodium, 

 potassium, magnesium, silicon, aluminimi, and iron, which exist mostly 

 in the form of carbonates, oxides, sulphates, and chlorides. 



When a stream bearing these substances in solution flows into a lake 

 with no outlet, as the Great Salt Lake or the Dead Sea, the water is 

 returned to the atmosphere by evaporation, while the impurities 

 remain. In this way the water gradually becomes charged more and 

 more heavily with mineral matter, until the point of saturation is 

 reached and further concentration is impossible without precipitation. 

 When such precipitation of mineral matters takes place, it is in the 

 inverse order of their solubilities; that is, those substances which are 

 least soluble will, under like conditions of temperature, be first precipi- 

 tated. Hence a water containing the ingredients before mentioned on 

 being subjected to complete evaporation would deposit its load in the 

 following order: (1) Carbonates of lime and magnesia in the form of 

 limestones, marls, and dolomites; (2) sulphate of lime in the form of 

 anhydrite and gypsum; (3) chloride of sodium, or common salt; and 

 these followed in regular order by the sulphates of magnesia and soda 

 (Epsom salt and Glauber's salt) and the chlorides of potassium and 

 magnesium. These last are, however, so readily deliquescent that they 

 are rarely found crystallized out in a state of nature as above noted. 



It rarely happens, however, that nature's processes are sufiiciently 

 regular and uninterrupted to allow a complete precipitation of the 

 pure salts as above outlined. During periods of flood suspended silt 

 may be poured into the inclosed basin to finally settle, forming thus 

 alternating beds of saliferous clay or marl. 



Such having been the method of formation, it is scarcely necessary 



