IOWA ACADEMY OF SCIENCES. 
257 
A still grander example of its kind is found in the Colorado 
River of the West. Its great valley, so far below the level 
of the table land, exists merely because the drainage-way 
has its source in a region of abundant moisture. 
Between the Rio Pecos and Rio Grande valleys at the 
i south end of the Rockies there is a small mountain stream,, 
the Rio Galisteo, which crosses the Estancia bolson, and 
which soon falls into the Rio Grande. This little stream 
has carved out a remarkable valley. It is an illustration 
of how wonderfully effective is even a small, often dry,, 
rivulet in corrading the high plains. 
The bolson plains may be considered as sections of an 
upraised peneplaned surface in its earliest infancy, at a 
stage in which they are as yet untouched by stream action. 
k They could not exist under present hyprometric conditions 
except in an arid region, which snow-fed perennial rivers 
do not traverse. The bolsons are only apparently lake-like 
basins. They have a marked slope in at least one direction 
of their major axis, as in the case of the Jornada del 
Muerto, where the slant is twenty feet to the mile and 
greater than the gradient of the parallel Rio Grande. Had 
the latter stream entered at Santa Fe the Estancia- Huerco, 
or Estancia-Jornada line of bolsons instead of the line to 
the westward, a vast canyon would have occupied these 
basins. 
The bolson plains of central New Mexico hang high 
above the great channels of the Rio Pecos and Rio Grande 
on either side. Were the rainfall of the region sufficient 
to produce perennial streams the plains would soon be as 
deeply carved out as the great adjoining valleys of the 
rivers just mentioned. 
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