406 
MINERALOGICAL GEOLOGY 
ditions. In the great succession of terranes there are several 
marked unconformities; marine beds might easily alternate with 
fresh-water strata. Moreover, there are, overlying the Tertic 
deposits in many places, thick clay beds of much later geologic age; 
the two series often can hardly be distinguished from each other. 
There is, indeed, nothing in the quotations which one could 
regard as undoubtable testimony of the fresh-water character of 
all of the Tertiaries of the region; and it is not probable that any 
of the authors cited originally intended to give that impression. 
The keenest discrimination is absolutely necessary in generalizing 
concerning the Tertic deposits of the Great Basin. There is no 
region of like size on the face of the globe in which more con¬ 
fusion exists regarding the exact correlation of geologic deposits. 
For this reason, if for no other, differentiation of the strictly ma¬ 
rine beds from the fresh-water deposits is of the greatest import¬ 
ance to the prospector as well as the miner of borates. 
In reality, the application of the term “lake-beds” to all of the 
Tertic deposits of Nevada and California is misleading. Notwith¬ 
standing the fact that this name is widely used in the reports of 
the Federal Government and in private publications, many of the 
so-called lake-beds are as far from being such as are now known 
to be the vast “Fresh-water Tertiaries” of the Great Plains, be¬ 
tween the Mississippi River and the Rocky Mountain front. Pos¬ 
itive evidence that any of the Tertic clays in which borates are 
now mined are fresh-water deposits is as yet wholly wanting. 
The Tertic clays which contain the borate minerals are in all 
of their physical characters strikingly like both the loess and the 
arid adobe soils. Both of these formations are now generally 
regarded as largely eolian in origin. Present view concerning the 
Tertic clays in question is that they are derived from desert dusts 
blown into the contiguous waters of arms of the ocean, like the 
Californian Gulf of today, or perhaps in some cases saline bodies 
of water which finally became nearly completely desiccated. The 
geographic distribution and limits of the workable deposits of the 
borates amply bear testimony of this. 
If there are really any fresh-water deposits among the boracifer- 
ous Tertiaries they should be by all means clearly differentiated 
from those which are without question marine beds. 
Keye;s 
