MINERALOGICAL GEOLOGY 
411 
the center. It is inconceivable that the cavities were originally 
in the clay, and that they were later filled in with colemanite. 
Where the ulexite is aggregated in more compact masses the cole¬ 
manite takes a more massive form, but the contraction of volume 
still allowed for a large number of drusy cavities such as are so 
abundant in the deposits. In some deposits, as at Lang, the 
pressure exerted was sufficient to close the cavities and compact 
the mass. The selenite, and perhaps some of the limestone also, 
resulted from the action of sulphated and carbonated waters upon 
the colemanite. 
This appears to be a clear summary statement of the Foshag 
argument for the playa formation of colemanite. 
K^yEs.^ 
Faulting of Colemanite Beds. There is one feature concerning 
the colemanite deposits of California and Nevada which merits 
especial notice as having critical yet hitherto unrecognized bearing 
upon the notion of possible metasomatic replacement of limestones 
by calcium borate, and which is a decisive test of the alleged vein 
character of colemanite deposits. This is the faulting of the 
colemanite beds themselves. This structural feature is one which 
is known to occur in all the borax fields; but it is not noted in 
Fig. 24. Faulting of Borate Beds in White Basin 
