i8o The American Geologist. September, 1904 
shells. It would appear, therefore, that the line between the 
upper and lower beds marks a period of transition to a time 
when more water and more sediments entered the lake from 
the sides. The lake was divided during this period, probably 
near its close, into two parts which may have been connected, 
as now, by a narrow strait. The division is apparent to-day 
as a bank rising some twenty-five to forty feet above the beds 
of the residual lakes and is nearly parallel and adjacent to 
the north boundary of Dofia Ana county where it crosses 
the valley. 
The sandy marl of this period is usually quite fine and 
may be gypsiferous. It has apparently been derived from 
the Cretaceous sandstone and shale and the soft gypsiferous 
shales and sandstones of the Permian. .This points to the 
probability that the change from the lower to the upper forma- 
tion may have been due to the lava flow which penetrated the 
border of the lake and doubtless laid these formations open 
to fresh erosion. If .these intrusions were of the same age as 
the flow covering the northern part of the valley the latter 
may at the same 'tiriie- have -served to rut oft connection with 
saline districts to the north. 
In the absence of more minute investigation it is useless 
to speculate as to the entire course of the original river valley 
but it is obvious that even the present drainage area tributary 
to the great salt basin 'is sufficient to supply an enormous 
amount of water which must either find an underground out- 
let or return to the atmosphere through evaporation. The 
amount lost in the latter way is reduced by the fact that the 
waters tend to bury themselves under the upper or Tularosa 
formation. A comparatively small part of even the local 
flood waters remains upon those playas which represent the 
last remnants of the original lake Otero. The streams 
flowing from the lateral canons soon lose themselves under 
the sandy strata occupying the middle of the basin. Water 
may be found by penetrating the Otero beds at any point 
and' frequently near the top of that formation. All such 
waters are strongly impregnated with common salt and alka- 
line salts and with gypsum but different strata in the same 
well may afford water very dissimilar in point of salinity. 
