Bolson Plains of the Southwest. — Tight. 279 
great plains are some sixty or seventy miles long and twen- 
ty to thirty miles wide with the Sacramento mountains on 
the east rising to an average level of 6,000 or 7.000 feet 
above the plains and the San Andreas and Organs on the 
west and the Sierra Oscuras on the north rising to a some- 
what less elevation. The plains are very level or slightly 
depressed through the central axis and show a decided 
grade toward the south. In the upper part of this great 
plains valley are the white sands and the salt marshes of the 
ancient lake Otero basin recently described by Prof. C. L. 
Herrick, late of Socorro, New Mexico, in the September 
number of the Geologist 1904. At the northern end of the 
plains lies one of the most extensive lava flaws in New 
Mexico, surpassed only probably by the great northern lava 
flow in Ria Arriba and Taos counties, and in western 
Valencia county. That the deposits forming the floor of 
this great basin are very deep and composed almost entirely 
of fluviatile material has been demonstrated by numerous 
wells which have been sunk through various portions of the 
plain ranging in depth from a few feet to a well in the 
southern portion of the plain over 2,000 feet deep, which 
did not even at that extreme depth reveal the rock. 
North of the great lava flow lies the Chapedero mesa 
and still farther north of that are the Estancia plains (San- 
doval bolson of Hill). While it cannot be definitely assert- 
ed with the data in hand, there are many facts which would 
seem to indicate that the Estancia plains and the white 
sands plains represent a g reat north and south structural 
valley, more or less parallel to the Rio Grande valley, from 
which the ancient river which occupied it, was either 
diverted by the extensive lava flows or by the normal pro- 
cesses of aggradation, or, what also seems very probable, 
that the sediments of the great bolson plains in these great 
structural valley sections have reached such enormous 
thickness that the waters of the through flowing drainage 
are at present entirely subterranean. There are many facts 
in hand to prove that there is a subterranean drainage which 
passes out of the southern end of this great axial trough. 
In referring to the region of the Rio Pecos, Dr. Keyes 
says: "Of these the last two streams mentioned" (Rio 
