328 MINERALOGY 



mother brines having become saturated by the evaporation of an 

 enclosed arm of the sea. In some cases the supply of salt has been 

 added to by a periodical breaking in of the sea, or by the waves wash- 

 ing over the inclosing bar ; through these additions of salt, carried 

 in by each fresh supply of water which in turn is evaporated, salt 

 beds of enormous thickness have been made possible. Those at 

 Sperenberg, Germany, reach a thickness of 4000 feet, while the 

 average thickness of the New York beds is 75 feet, and those of 

 Michigan are in places 400 feet thick. Such beds of salt are in 

 course of formation at the present time, under practically the above 

 conditions, in some of the bays of the eastern shore of the Caspian 

 Sea. Other deposits may form by the simple concentration of an 

 inland sea, as the Salton Sea deposits of California. Both the 

 Great Salt Lake and the Dead Sea are highly concentrated brines. 

 The water of the Dead Sea contains 22.86 per cent, salt, while the 

 average sea water contains but 3.5 per cent. Such lakes are noth- 

 ing more than large evaporating dishes which deposit their salts 

 in more or less definite order when concentration has reached satura- 

 tion (see anhydrite), as the salt deposits of Stassfurt, Germany. 

 When such deposits of salt are protected from the solvent action of 

 percolating ground waters by an impervious stratum of clay, they 

 have been preserved through the ages ; however, many salt springs 

 exist which derive their salt from such embedded deposits. All 

 natural waters contain from one to ten parts of salt in a million 

 in solution. The origin of the sodium contained in all these 

 natural solutions and enormous deposits of salt have in large part 

 resulted from the decomposition of feldspars ; but the source of 

 such quantities of chlorine is more difficult of explanation, since 

 few primary minerals, as soldalite and apatite, contain chlorine, 

 and they only in small amounts. The two states, Michigan and 

 New York, produce three quarters of the thirty million barrels of 

 salt used annually in the United States, while fourteen states con- 

 tribute to the remaining quarter. 



SYLVITE 



Sylvite. KC1 ; Potassium chloride ; K = 52.4, Cl = 47.6 ; 

 Isometric; Type, Tesseral Holoaxial; Common forms, c (100), 

 o(lll); Twins rare; Cleavage, cubic perfect; Brittle, fracture 

 uneven ; H. = 2; G. = 1.97 ; Color, white, yellow, reddish, or blue ; 

 Streak, white; Luster, vitreous; Transparent to translucent; 

 n = 1.490. 



