Bolson Plains and Their Existence. — Keyes. 163 
Rio Grande nearly all traces of the old plain are already de- 
stroyed. Tlie displaced intermontane basins, like the Jornada 
del Muerto, which adjoin the long Rio Grande valley, are be- 
ginning to be deeply dissected wherever the great liver 
touches the borders. 
In its broader features the surface of New Mexico may 
be regarded as a ribbed tableland. Broad north and south 
valleys alternate with long narrow more or less continuous 
mountain ridges. The most important of the long basin plains 
and valleys are the Pecos, Huerco, Estancia, Jornada, Rio 
Grande, San Augustine, and Mimbres. 
Over such a surface from the southern end of the Rockies 
three great streams diverge. These are the Canadian river, 
the Rio Pecos, and the Rio Grande. The first of these after 
leaving the mountains flows eastward to the Arkansas in In- 
dian Territory and thence its waters find their way to the 
Mississippi. Rio Pecos trends southeastwardly, entering Tex- 
as near the southeast corner of New ^^lexico. From the San 
Luis valley in Colorado the Rio Grande flows slightly west 
of south to El Paso. Of these the last two streams mentioned 
flow in broad valleys between lines of block mountains. 
Comparison of the physiographic features of basin valleys 
which the great mountain-born streams traverse with those 
which are not so occupied, quickly demonstrates that the bol- 
son owes its existence merely to lack of erosion agencies. The 
Rio Grande no doubt at first passed through a series of bol- 
sons identical with those at present found on either side of its 
present valley. It has cut down its channel often 2000 feet be- 
low the surface of the ancient bolsons. Within the valley 
nearly all traces of the bolson characters are now lost. Xo 
waters are received by the great stream after it emerges from 
the Rockies. The work of this river has been confined to cut- 
ting its canyon. Little additional work of side streams has 
been imposed upon it. A still grander example of its kind is 
found in the Colorado river of the A\'est. Its great valley, so 
far below the level of the tableland, 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 south 
end of the Rockies there is a small mountain stream, the Rio 
Galisteo, which crosses the Estancia bolson, and which soon 
