Lake Otero, X. Me.v. — Herrick. lyg 
the time of the throat early hasic (mostly andcsiticj flows and 
the succeeding acid eruptive period. 
There is much to lend plausibility to the theory that a river 
of considerable magnitude occupied the valley and fiovved 
south to enter what is now a part of the Rio Grande valley. 
At any rate, a change was introduced bv the basaltic overflows 
which produced a great sheet of '"mal pais" occupying the 
valley east of the Oscuros and possibly serving to cut off the 
superficial waters from the north. It may be that other 
causes had operated to occlude the original outlet to the south 
for it seems that a long period of lacustrine quiet had preceded 
this lava flow. Perhaps the condition of periodic rain-fall 
or arid status had assisted in silting up the outlet. 
lliere is room for much detailed <^.tudy in connection with 
all of these questions. At any rate, it may be regarded as 
certain that the great basin north of the Jarilla mountains and 
extending northward to east of the Oscuros was for a very 
long period covered by a salt lake in which gypsum, salt, and 
saline alkalis were deposited with Jitermittent regularity. 
Thus were formed the great saline beds which we venture to 
call the Otero marls. This formation has been penetrated bv 
wells to a depth of some 200 feet in many places and every- 
where reveals a succession of gypsum and saline beds inter- 
calated in gypsiferous marls. It is impossible to guess at the 
thickness of this formation. It may very possibly prove to 
be of Tertiary age, at least in part, as it seems to pass undis- 
turbed under the lava beds at the north end of the valley. The 
upper surface, where exposed, is a plane and the superposed 
sandy marls ( Tularosa Formation) are distinctly differen- 
tiated. 
Section across old Lake Oteru. 
The upper or Tularosa Beds are rarely over twenty-five 
feet thick and in many places contain fresh-water lacustrine 
