Bolson Plains of the Southwest.— Tight. 283 
plains is in accordance with the writer's views upon the 
same subject, and present the conditions which prevail 
throughout the western plateau and great basins region 
during a portion of Tertiary time. 
That there is certainly a great similarity in the method 
of formation of the Tertiary deposits over the Llano Esta- 
cado and great bolsons of New Mexico, and the basin region 
there is no question. The same attitude of the land under 
uniform climatic conditions produced throughout the entire 
southwestetrn country more or less uniform phenomena of 
erosion and aggradation on the pre-existing land forms, but 
that the term bolson is to be applied to any and all portions 
of these deposits wherever found is to be much questioned. 
The same conditions which spread out a great sheet of 
gravels over the surface of the Cretaceous on the Llano Es- 
tacado produced an extensive filling in all the great moun- 
tain-bordered basins and erosion valleys of the plateau re- 
gion. 
Xor can we class as bolsons such plains of recent origin 
as occur along the Rio Grande, which have been described 
by Dr. Herrick, in the Geologist, vol. 33, June 1904, as 
clino -plains. 
If we are to consider the section of the Rio Grande be- 
tween Bernalillo and Socorro, the Jornado del Muerto, and 
the Mesilla valley as typical bolsons as described by Mr. 
Hill, and are also to consider the Roswell basin of the Pecos 
as a typical bolson, it seems certain that the Roswell bolson, 
so classified, must be of a much more recent origin than 
those of the Rio Grande, for the Rio Grande deposits seem 
to be correlated in time with the great Tertiary deposits 
over the surface of the Llano Estacado, while the deposits 
occupying the Roswell basin have been made since the 
erosion of that basin out of the Tertiary and Cretaceous 
formations of the Llano Estacado. Again if we are to con- 
fine our definition of the term bolson to plains formed with- 
in the structural valleys (using this latter term in a very 
broad sense )then we must exclude the Roswell valley from 
the class of bolsons. for the writer is satisfied that the sec- 
tion of the Pecos included within the borders of New Mex- 
ico is whollv an erosion form. 
