BORAX AND BORATES 363 



the bottom of these lakes consists of a lower bed 5 meters thick 

 with much sodium carl)onate in its composition and an upper layer 

 also 5 meters thick composed largely of sodium chloride. The 

 smaller lakes generally dry up, only salt encrustations remaining, 

 while the sand of the surrounding desert is underlain by a dark gray 

 clay containing gypsum and sodium salts. 



Borax and Borates. 



Borates are deposited by some lakes of which Borax Lake, Lake 

 County. California, is a typical example. The analysis of the water 

 is given on p. 159, F4, the mineral contents of the water being chiefly 

 sodium carbonate and sodium chloride, with borax (Sodium bi- 

 borate, Na^B^O.) next in importance. The bed of the lake is oc- 

 cupied by a large mass of crystallized borax of great purity. A 

 neighboring smaller lake, Hachiilchama, furnished a larger supply 

 of borax, which probably came from neighboring hot springs. 



Deposits of borates are forming in a number of "marshes" or 

 playa lakes of Nevada and California. Rhodes' marsh, Esmeralda 

 County, Nevada, has a central area of common salt surrounded by 

 a deposit of sodium sulphate, outside of which borax and ulexite 

 (NaCaB^Oo.SHgO) occur. According to M. R. Campbell (8:401) 

 these salts of similar lakes in California are leached from beds of 

 Tertiary sediments. Searles's marsh or borax lake, in San Berna- 

 dino County, California, has furnished the following succession 

 through borings (De Groot-15 :5J5; Clarke-io: ipp; 2j^) : 



Surface 



1. Salt and thenardite (Na2S04) 2 feet 



2. Clay and volcanic sand, with some hanksite: Na22K(S04)9 



(CO3) 2CI 4 feet 



3. Volcanic sand and black clay, with bunches of trona NasH- 



(COs).. 2H2O 8 feet 



4. Volcanic sand, containing glauberite Na^Ca (804)2; then- 



ardite, and a few crystals of hanksite 8 feet 



5. Solid trona overlain by a thin layer of very hard material 28 feet 



6. Mud, smelling of hydrogen sulphide and containing layers 



of glauberite, soda, and hanksite 20 feet 



7. Clay, mi.xed with volcanic sand and permeated with hydro- 



gen sulphide 230+ feet 



Borax is chiefly found in the top crust or crystallized in the water 

 which sometimes accumulates in the depressions of the bed (Clarke- 

 10). The layer is reproduced by slow degrees, through capillary 

 action, which brings up the soluble salts from below. Some 20 dis- 



