MINERALOGICAL GEOLOGY 
407 
Vein 'Character of Colemanite Deposition. Direct evidence of 
the genetic character of colemanite, of course, lies in the deposits 
themselves. In natural outcrops relationships are obscure; and 
the mines present difficulties that are not favorable to detailed 
study of the character of the mineral. Judgment as to whether 
these deposits originated as veins or through desiccation involves 
a study of other more general considerations. Gale’s recent hy¬ 
pothesis is thus expressed: 
“Artificially colemanite is readily produced by reaction from 
other calcium borates in saturated alkali chloride solutions; but 
there appears to be no data concerning the possible formation of 
this mineral by direct action of boric acid and limestone. 
“It is natural to assume that free boric acid, which is a common 
constituent of many volcanic emanations, might react with lime¬ 
stone, and that by substitution of boric acid for carbonic acid the 
lime borate would be formed. Thus by a process of metasomatic 
replacement might be formed deposits of the typical irregular 
character of the known colemanite masses, roughly following the 
bedding of the original calcareous rock, or of the interbedded 
lenticular bodies of limestone. Therefore, in so far as the natural 
reactions are concerned the formation of colemanite as fissure and 
replacement vein deposits is well within the limits of possibility. 
“The limestone strata with which the principal colemanite ore- 
bodies are associated are evidently original in the sedimentary 
sequence. They are in part at least travertine-like, weathering in 
rough tufa-like surfaces, and are not compact like more typical 
limestones. Their character suggests the probability that they are 
chemical deposits, possibly of local extent, laid down in shallow 
waters, being similar to the travertine deposits now forming near 
springs or where ground-waters flow into ponds or saline lakes. 
The limestone masses are believed to be lenticular in form and to 
occur interbedded with shales at various horizon? within the flows 
of basaltic lava. They are not necessarily the product of desic¬ 
cation. 
“Lastly, an explanation of the source of the boric acid is to be 
sought. Naturally, a volcanic origin is suggested and often the 
association of colemanite with basaltic lava is certainly intimate. 
It is a common saying among prospectors that “borax” will not 
be found except near a ‘porphyry contact’ and in association with 
