410 
MINERALOGICAL GEOLOGY 
Formation of Borates in Desert Playas. Under the ordinary 
climatic conditions prevailing in arid lands bittern lakes appear to 
throw down borate crystal in the form of the soda-lime mineral 
ulexite (NaCaBgOg -f- 8 aq.), rather than of the pure lime variety 
known as colemanite (Ca2B60ii +11 aq.). This deposition is in 
accordance with the recent experiments of Van’t Hoff, who also 
endeavors to show that in the subsequent history in order to ac¬ 
complish the splitting up of the ulexite into its component calcium 
borate and sodium borate the letter must be removed as fast as 
it is formed. 
In the closed basins of desert playas, with their clay floors, re¬ 
moval of the sodium borate cannot take place to any appreciable 
extent either by surface or underground drainage, so that the 
borate accumulations must consist wholly of ulexite and borax 
(^ 3 . 2 ^ 40 ^ + 10 aq.). When such deposits are later covered 
over and uplifted sufficiently to allow free drainage from the 
beds to take place, the percolation of sodium chloride solutions 
gradually converts the ulexite into colemanite and other members 
of the colemanite series. 
In support of this hypothesis are four lines of evidence: (1) 
total absence of colemanite and all members of the colemanite 
series from all playa deposits; (2) occurrence of ulexite in large 
quantities in some colemanite deposits, as at Lang, California, 
where it lies for the most part near the foot-wall; (3) bedded 
character of the deposits; (4) structural features of the deposits, 
especially the nodular and geodal form of a large portion of the 
colemanite. 
' Regarding the last mentioned feature it is to be especially noted 
that nodular and geodal masses of colemanite are the prevailing 
deposits at Calico and in some parts of Death Valley and these 
masses are embedded in the clays. This suggests LeConte’s 
description of the occurrence of the “cotton-balls” in the clays 
of Rhodes Marsh. 
If the hypothesis of the deverivation of colemanite from ulexite 
be accepted, there is a ready explanation of this type of ore. The 
ulexite embedded in the clays is acted upon by salt solutions. The 
light, fluffy “cotton-balls” are converted into the more or less 
compact colemanite, giving rise to the more or less spherical 
geodes, and allowing the free crystallization of the colemanite in 
