i6o The American Geologist. September, i904 
currents near the margin after the epeirogenic depression in the 
lowan stage of glaciation restored a warm temperate cHmate 
upon the ice border. 
In the next paper of this series I wish to present my detail- 
ed observations of the drift around the southern part of Puget 
sound, where the proportion of modified drift, stratified gravel, 
sand, and fine silt, far exceeds the till deposits. 
BOLSON PLAINS AND THE CONDITIONS OF 
THEIR EXISTENCE." 
By Charles R. Keyes, S(jcorro, New Mexico. 
Bolson plains are the most characteristic features of the 
Mexican tableland. They are peculiar to the arid regions. 
They could not exist in the same country in which the}- now 
do, under conditions of copious rainfall. While they are 
among the most novel of physical features and while much 
has been made of them as forming a distinct type of geo- 
graphic relief they hardly deserve, from the viewpoint of gen- 
etic geography and geology, the attention that has been given 
them. 
Bolsons, as their name signifies (Spanish, a purse), are 
broad, level valleys, depressed slightly in the center, and bor- 
dered bv mountains. The writer who introduced the name 
into American geography thus describes them. 
"These plains, or 'basins," as they are sometimes called, 
are largely structural in origin. Bolsons are generally floored 
with loose, unconsolidated sediments derived from the higher 
peripheral region. Along the margins of these plains are 
talus hills and fans of bowlders, and other wash-deposits 
brought down l\v mountain freshets. The sediments of some 
of the bolsons may be of lacustral origin. 
"It is essential, in both the. geographic and the geologic 
discussion, to bear in mind the distinction between bolson 
plains and plateau plains. The plateau plains and the moun- 
tains are genetically related, the strata composing the one be- 
ing bent onto or flexing out into the other. The bolson plains. 
* Read before the Iowa Academv ol Sciences, Iowa City Meeting^ April 1; 
1904. 
