Vol. 6] 



Eakle: Neocolemanite. 



181 



series, especially carbonates and sulphates of lime and soda, make 

 up the deposit, and the well-known Searles Borax Lake in San 

 Bernardino County, with its many associated minerals, is a good 

 illustration of a desert formation. The Lang deposit, however, 

 is an exception, as the neocolemanite is practically unaccom- 

 panied by other minerals except howlite, which is a silico-cole- 

 manite, and some calcite. Waters emptying into the basin could 

 not have been charged with mixed alkali salts. 



It seems probable that the original site of the deposit was a 

 marsh containing marl and calc tufa with mud and considerable 

 organic growth, and that later waters charged with boracic acid 

 flowed into the basin and converted the carbonate of lime into 

 the borate. Some and perhaps the greater part of the argil- 

 laceous material which forms the shales was precipitated by the 

 decomposition of the impure limestone, together with organic 

 matter. The carbon dioxide set free may not wholly have 

 escaped, but possibly became occluded in the mud and later con- 

 verted into carbon. Most of the borate is of a blackish gray 

 color, due to impregnations of carbon along the cleavages and 

 fractures. The conversion of the limestone into the borate in 

 all probability took place before the overlying sandstones were 

 formed. The absence of soda compounds and the presence of 

 abundant plant life indicate that the lake or marsh was fresh 

 into which springs containing boric acid discharged. The deposit 

 later became submerged and the sandstones were laid down. 



The origin of the boric acid is presumably volcanic and the 

 springs probably issued from vents in the immediate vicinity of 

 the basin. The deposit is situated in a hilly district and is partly 

 surrounded by high masses of volcanic tuffs and rhyolites. The 

 subsequent tilting of the deposit was not accompanied by heat 

 or pressure sufficient to modify the borate materially, yet the 

 mineral shows lines of strain and columnar partings due to pres- 

 sure and shrinkage. The fissile shales owe their solidity to this 

 slight pressure, and carbonization to some extent was also the 

 result. The reduction of the occluded carbon dioxide may have 

 been brought about through the decomposition of organic 

 material deposited in the basin. There is, of course, the possi- 



