borate deposits described below. Whatever their source, the borates are carried in solution by the waters of occasional rains to shallow basins, which become covered with temporary thin sheets of water or "playa lakes." Evaporation of these lakes leaves broad flats covered with the white salts. These may subsequently be covered with drifting sands and capillary action may cause the borates to work up through the sands, becoming mixed with them and efflorescing at the surface. One of the largest of the California deposits of this general class is that at Searles Lake, from which it has been proposed to recover borax along with the potash (pp. 113-114).
The deposits which at present constitute the principal source of domestic borax are not the playa deposits just described, but are masses of colemanite in Tertiary clays and limestones with interbedded basaltic flows. The principal deposits are in Death Valley and adjacent parts of California. The colemanite occurs in irregular milky-white layers or nodules, mingled with more or less gypsum. The deposits are believed to be of the replacement type, rather than ones formed contemporaneously with the sediments. Whether they are due to magmatic solutions carrying boric acid from the associated flows, or to surface waters carrying materials leached from other sediments, is not clear. The crude colemanite as mined carries an average of about 25 per cent B_2O_3; it is treated with soda in the manufacture of borax, or with sulphuric acid in making boric acid.
Boron is present in minute quantities in sea water. When such water evaporates, it becomes concentrated, along with the magnesium