Bolson Plains ot the South west. — Tight. 28 1 
Concerning the bolsons of the Rio Mimbres or Ante- 
lope plains I have little data at hand except that it is well 
known that underneath the Antelope plains there is a large 
supply of subterranean water contained in deeply buried 
river gravels. It has never been my pleasure to visit the 
San Augustine plains, therefore I cannot speak authorita- 
tively concerning this extensive bolson. 
With this brief statement concerning some of the physi- 
ographic and structural features of New Mexico it seems- 
to the writer that Dr. Keyes is not justified in classifying as 
a common physiographic type the great plains of the Llano 
Estacado to the eastward of New Mexico and the typical 
bolsons which occur within its borders. In the judgment 
of the writer it would not even be possible to place the 
border plains of the Rio Grande and of the Rio Pecos in the 
class of bolsons, and certainly such plains as the Hueco r 
Mimbres, the Estancia and the Jornado can bear no relation 
whatever to the great plateau plain through which the 
Colorado river has cut its grand canyon. 
From the data in hand it appears to the writer that in 
New Mexico and much of the great basins region where the 
bolson plains form an important physiographic type, there 
is a common history of origin. The whole region has been 
at some time at a very much higher level than at present 
and subjected to such erosion that the great structural val- 
leys of the entire region were worn out several thousand 
feet m depth. Every feature of origin seems to point with 
unmistakable finger to a time of such erosion, under atmos- 
pheric conditions of heavy participation, with a much higher 
elevation of the plateau than at the present time. There 
certainly was a time when the carrying capacity of the axial 
streams of all the valleys was much in excess of the loads 
of material furnished to them by their lateral tributaries and 
by torrential action of the characteristic method of precipi- 
tation of the semi-arid region, resulting in the supply of en- 
ormous quantities of material from the steep mountain 
slopes into the valleys in such quantities that the larger 
streams were vastly overtaxed and the period of aggradation 
was inaugurated. This period continued until the deposit 
in these ancient valleys accumulated to thousands of feet 
