MINERALOGICAL GEOLOGY 
401 
are not, as is commonly assumed, areas of constant and extensive 
deposition. They are notably tracts of tremendous erosion and 
removal. The fine blown sediments which are entrapped by the 
thin sheets of water which spread over such areas for a few weeks 
or a few days a year, are soon dried and are frequently entirely 
blown away in a single desert gale. Should any borate be depos¬ 
ited on such bottom muds it would more than likely be speedily 
deflated ani scattered rather than concentrated year after year. 
In general, playa tracts are areas of characteristic desert denuda¬ 
tion rather than intermontane repositories of prodigious soil ac- 
cummulation. 
Ke^yEs. 
Contemporary Formation of Commercial Borates. When some¬ 
thing over a decade ago the geological setting of the remarkable 
borate deposits of Death Valley, California, were described their 
principal formation was tentatively ascribed to normal but suc¬ 
cessive precipitations of enclosed basins or bittern lakes. This 
possibility was based partly upon the theoretical behavior of com¬ 
plex salt solutions during progressive evaporation, but mainly upon 
the observed stratigraphic and lithologic features displayed by the 
colemanite deposits themselves, as observed in the field. It was 
also assumed that solfataric action, as noted in the Ash Spring 
near by, was doubtless an important contributory factor. 
The opinion expressed was finally challenged on the assumption 
that the chief borate mineral, calcium borate, or colemanite, 
(CagBgOii -I" 5 aq.), being in the form of distinct seams and dis¬ 
posed nearly vertically was essentially a vein formation, boracic 
acid having acted upon limestone, thereby driving out the carbonic 
acid. Previously, this possible replacement process had been en¬ 
tertained in connection with the consideration of the Death Valley 
beds but it was soon discarded because of the utter lack of sub¬ 
stantiating evidence. 
Colemanite crystal is not the only phase of the formation of 
boracic acid in nature. Half a dozen other forms occur in the 
same region. Nof is the genesis of the borate salts all the same. 
Probably colemanite is unique in its formation. Unquestionably 
it does not come strictly under the ordinary laws of salts deposi¬ 
tion from the waters of bittern lakes, as laid down by Van’t Hoff, 
