MINERALOGICAL GEOLOGY 
403 
and sands of the desert valleys, folded faulted and upturned as 
they now are, and yielding marine fossils, are still spoken of as 
fresh-water lake deposits. Because of this fact the reasons for 
believing that many of them were actually marine beds, and that 
the waters in which they were laid down were once connected 
with the Pacific Ocean were recently set forth to such length that 
the discussion was regarded as an argument for the origin of the 
desert salines solely from evaporated sea waters. 
Tertiaries these beds surely are. They occur in isolated patches 
of greater or less extent, which stretch out from the Ocean to 
Death Valley and the Virgen River at the Utah line. At some 
time or other these remnantal areas were perhaps connected, al¬ 
though not throughout the entire epoch of their deposition. They 
enclose numerous lava flows comprising a characteristic olivine 
trap. Some of the layers are possibly of playa origin. Some are 
undoubtedly old sand-dunes. Many are manifestly bittern lake 
beds. Clearly homologous throughout their range their affinities 
are with salt-water rather than with fresh-water. 
The history of the salts depositions in these beds is only slightly 
touched upon as yet. The genesis of the borate ores contained 
in this thick sequence, for it is upward of a mile in thickness in 
some localities, is doubtless varied. Some of the salts beds are 
unquestionably typical bittern lake products. Some may be re¬ 
placements of calcareous layers. Others of severely limited ex¬ 
tent may be transformations in old playa muds. Associated with 
the borate-bearing bodies are sometimes porous, fluffy and highly 
calcareous lenses several feet in thickness, beds of rock salt, and 
extensive gypsum layers. Certain it is, then, that the origin of 
the borate mineral cannot all be ascribed to a single process. 
KeyDs. 
Marine Origin of Boraciferous Terranes. Genetic basis of bor¬ 
ate ore localization finds essential control in the nature of the 
strata which enclose the deposits. Derivation of the borate meas¬ 
ures, conditions of their deposition, and the evidence of their 
geological age, give ample clue to the geneology of the enclosed 
ores. 
Custom of regarding the intermontane valleys of the Great 
Basin as the sites of former vast bodies of fresh interior waters is 
