364 TRTNCIPLES OF STRATIGRAPHY 



tinct mineral species have so far been obtained from the deposits 

 (see Clarke-10 -.19^-200 : J'^t ) , mostly sulphates, borates, and carbon- 

 ates, with some chlorides, soda niter (NaNOg), and ammonium salts. 

 In the southern part of San Bernardino County, CaHf ornia, near Dag- 

 gett, occurs a solid bed of colemanite (CaoBi.O^i.sHoO) from 5 to 30 

 feet in thickness, highly crystalline, and interstratified with lake 

 sediments. At one end it is much mixed with sand, gypsum, and 

 clay, suggesting that it had been laid down at the edge of an evapor- 

 ating sheet of water. Colemanite was originally obtained from 

 Death A'alley Desert, California. The ]\Iohave Desert, on the 

 borders of California and Nevada, and the Atacama desert of South 

 America have in recent times been the chief sources of boron com- 

 pounds, the boron in these deposits having probably been supplied 

 by hot springs and solfataras of volcanic origin. In the South 

 American region, the principal mineral is ulexite (NaCaBgOg. 

 8H,0). 



In Tuscany deposits of borates are formed on an extensive scale 

 from emanations of fumaroles or jets of steam issuing from the 

 ground. The deposits concentrate in lagoons, forming orthoboric 

 acid or sassolite (H3BO3), ammonium borate or larderelHte 

 ((NHj2B«Oi3.4H,0) ; borocalcite or bechilite (CaB,0,.4H20), the 

 hydrous ferric borate; lagonite (Fe'''oB,,Oj.^.3H20), and the am- 

 monium compound boussingaultite ( (NH^)2Mg(SO^)o.6H20). In 

 the most concentrated of the lagoons the orthoboric acid (H3BO3) 

 amounted to 19.3 grams per liter. 



From Tibet borax deposits have been known since very early 

 time. In the lake plain of Pugha, in Ladakh, the deposits occur at 

 an elevation of 15,000 feet above the sea. The deposit covers an 

 area of about 2 square miles and has an average depth of 3 feet. It 

 is formed by hot springs which issue at this elevation with a tem- 

 perature ranging from 54° to 58° C. The deposit is impure, other 

 minerals, inclutiing gypsum, occurring with it. 



Deposits of Nitrates. 



Soda niter, or Chili saltpeter (NaN03), is found in the deposits 

 of Searles's Marsh and in various other parts of southern Califor- 

 nia, especially around Death Valley, and along the boundary between 

 Inyo and San Bernardino counties, forming beds associated with 

 the later Eocenic clays. In Searles's Marsh it is associated with the 

 borates, and the same holds true for the occurrences in the Atacama 

 and Tarapaca deserts, northern Chile. Here are found the largest 

 known deposits of nitrates in the world, the amount being officially 



