Bolson Plains of the Southwest — Tight. 275 
place over the floor of the great structural valley, the pro- 
duction of the bolson does not materially modify the topo- 
graphic features, but when the great structural valley has 
been deeply eroded and then subsequently filled the produc- 
tion of the bolson does make a decided difference in the 
topographic relations of valley and mountains. We see 
then, that the bolson plain finds its proper place in the 
series of constructional plains derived from fluviatile actions, 
and the lowest member of the series is represented by the 
flood plain of the river, in which the material is almost 
wholly derived from the longitudinal action of the stream; 
while the bolson plains are at the opposite end of the series, 
in which the longitudinal stream may contribute largely in 
some stages of the building of the plain but that the pre- 
dominant factor is the derivation of material from the bor- 
dering hights by torrential action of temporary streams and 
rivulets which are produced largely by rainfall. 
This in general is the writer's understanding of the use 
of the term bolson, as applied to many of the great inter- 
montane plains of the southwestern plateau and great basin 
region. Whether the writer is correct or not in his inter- 
pretation of the use of this term, it is certain that under any 
definition of the term there is found in this region a large 
number of valley plains having very diverse characters and 
very diverse origins. The writer cannot therefore agree 
with Dr. Chas. R. Keyes of Socorro, in his discussion of 
the bolson plains of New Mexico in the American Geologist 
for September, 1904. Dr. Keyes has grouped under this 
term such plains as the Jornada del Muerto, San Augustine, 
Estancia, Mimbers and the great plains along the Rio 
Grande, Rio Pecos and ■ Canadian rivers. He has seemed 
to correlate these plains, with others, with the great Llano 
Estacada of Texas and other great plains to the southwest. 
If Dr. Keyes' correlations are correct, it does not seem to 
the writer that these great intermontane plains of the New 
Mexico region can be by any definition classed as bolson 
plains and many of them certainly conform to that defini- 
tion. In view of this difference of opinion it seems advisa- 
ble to call attention to some of the features of some of these 
various plains in detail. It is well understood that the 
