278 The American Geologist. November, 1905 
Marcial, which deflected the course of the Rio Grande far 
to the west of its old valley through the Elephant Butte 
canyon and west of the Sierra de los Caballos, reaching its 
eld course again just a little north of Las Cruces. The 
Jornada del Muerto lying between the San Andreas and the 
Sierra de los Caballos is undoubtedly the old valley of the 
Rio Grande, from which the river was diverted at the time 
of maximum aggradation and at the time of the great San 
Marcial lava flow. There is every reason to believe, from a 
careful study of the history of the Rio Grande that a cross 
section at the Jornada del Muerto is comparable in its 
history to a cross section of the river at Albuquerque, where 
the mesa deposits are known to be at least a thousand feet 
in depth. It is therefore evident that the plain of Jornada 
is in no way genetically related to the Llano Estacado, ex- 
cept in so far as concerns the Tertiary deposits of the latter. 
That the great mesas bordering the Rio Grande are wholly 
of fluviatile origin is further shown from the topographic 
characteristics in the vicinity of El Paso where the river 
runs through a narrow rock channel between the Franklin 
mountains and the range to the southwest. 
Some few miles above El Paso, and on the west of the 
Franklin mountains are preserved other extensive remnants 
of the old gravel and talus plains which extend out from 
the canyons of the Franklin mountains at a level of 300 or 
400 feet above the Rio Grande. Several miles to the west 
across the immediate channel of the Rio Grande are seen 
the opposite exposures of the same beds. Whether it was 
by the blocking of the old channel with another lava flow 
farther to the west or by the normal process of excessive 
aggradation, that the Rio Grande was forced through the 
narrow mountain pass at El Paso is yet undetermined, but 
that it is superposed in its present position upon an ancient 
col at El Paso is certain. 
The second instance to which attention is directed is 
the great basin in New Mexico lying between the San An- 
dreas and the Sacramento mountains, known as the White 
sands plain or the Hueco bolson.* This certainly is a typical 
bolson as the writer understands the use of the term. These 
* Hill, U. S. Folio, No. 3, p. 9. 
