256 
IOWA ACADEMY OF SCIENCES. 
In the Llano Estacado the remnant of the great plain 
contains 50,000 square miles. The bolson plains are al- 
ready beginning to give way to erosion agencies. In the 
valley of the Rio Grande nearly all traces of the old plain 
are already destroyed. The displaced intermontane basins, 
]ike the Jornada del Muerto, which adjoin the long Rio 
Grande valley are beginning to be deeply dissected wher- 
ever the great river touches the borders. 
In its broader features the surface of New Mexico may 
be regarded as a ribbed tableland. Both 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, Jor- 
nada, 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 
Indian Territory and thence its waters find their way to 
the Mississippi. Rio Pecos trends southeastwardly, enter- 
ing Texas near the southeast corner of New Mexico. 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 physiographic features of basin valleys 
which the great mountain-born streams traverse and of 
those which are not so occupied, quickly demonstrates that 
the bolson owes its existence merely to lack of erosion 
agencies. The Rio Grande no doubt at first passed through 
a series of bolsons identical with those at present found on 
either side of its present valley. It has cut down its 
channel often 2,000 feet below the surface of the ancient 
bolsons. Within the valley nearly all traces of the bolson 
characters are now lost. No waters are received by the 
great stream after it emerges from the Rockies. The work 
of this river has been confined to cutting its canyon. 
Little additional work of side streams has imposed upon it. 
