408 
MINERALOGICAL GEOLOGY 
limestone. It is true that the larger ore-bodies mined are all in 
close relation with some part of the basaltic lava-flow rock in 
place, generally within a few feet of it. The borate-bearing beds 
follow the outcrop of the basalt flows. The question in what way 
boric acid originating with the extrusion of these flows could 
have caused reactions and mineral deposition soon or long after the 
period of the lava extrusion remains for further investigation. 
“Evidence favoring the hypothesis of a desiccated saline lake 
to explain the origin of the colemanite has little td support it be¬ 
yond general assumptions. The actual character of the deposits 
themselves indicates rather a vein type of formation. The gyp¬ 
sum which has been pointed to as a desiccation deposit related to 
the colemanite is also of vein character. Other salines which 
would naturally be expected in desiccation deposits resulting from 
natural salines solutions are not found in association with the 
colemanite. Those who have supported the desiccation hypothesis 
have offered no explanation of the reaction which might produce 
colemanite in such massive deposits as a product of water evapor¬ 
ation, while on the contrary, its formation from limestone in 
veins by replacement of carbonic acid with boric acid is a natural 
working hypothesis that deserves experimental investigation. The 
relations of the deposits to basalt lava flows indicate the probable 
origin of the boric acid at the time of the extrusion of these lavas, 
although it may also be necessary to assume that this acid con¬ 
tinued to find its way into solution of the circulating ground- 
waters long after the period of the lava extrusions.” 
Keyes. 
Vein Attitude of Colemanite Beds. The thick sedimentary suc¬ 
cession which carries the colemanite crystal evidently belongs 
partly, if not entirely, to Eocene deposition. This is indicated 
also by the tremendous disturbance which' the strata have mani¬ 
festly undergone. In all of the known borate districts the beds 
are more or less highly inclined; some are vertical; none are still 
horizontal. Since numerous surface flows of basalt of the olivine 
variety are also involved the genetic significance is obvious. More¬ 
over, the sedimentaries are very old compared with the other 
consolidated deposits of the desert; and they perhaps long ante¬ 
date the desert itself. 
