272 The American Geologist. 
November. 1905 
an aggradation plain of some sort has been mistaken for a 
degradation plain and called a peneplain. There are un- 
doubtedly many cases where the two types of plains look 
nearly alike and only a careful study would determine the 
true character. 
This superficial resemblance, which extends to various 
types of topographic forms, is a constant source of error. 
The most serious mistakes are made when two types of 
radically different origin and structure are classed together 
and made the basis for broad generalizations. 
When an author has described a certain type of topo- 
graphic form, presumably from a careful study of some 
definite region, and has given a name to that type, it can 
only lead to much confusion and error when the same name 
is used by another to describe a different form, or to attempt 
to modify the meaning of the term to fit other conditions. 
It is not intended at the present writing to discuss the 
uses and abuses of he term "bolson" as applied to inter- 
montane valleys but rather to take the term as originally 
described* and apply it to the study of the great valleys of 
the southwest and especially to New Mexico. 
To quote briefly: "Bolsons are generally floored with 
loose, unconsolidated sediments derived from the higher 
peripheral regions. Along the margins of these plains are 
talus hills and fans of boulders, and other wash-deposits 
.brought down by mountain freshets. The sediments of 
some of the bolsons may be of lacustral origin." "The 
bolson plains on the other hand," (as distinguished from 
plateau plains) "are newer and later topographic features, 
consisting of structural valleys between mountains or 
plateau plains, which have been partially filled with debris 
derived from the adjacent eminences." "The bolson plains 
are constructional detritus plains filling old structure 
troughs." 
It seems that this description is clear enough. The 
type form is not dependent upon the characters of the 
bordering mountains nor the character or structure of the 
deeper valley floor, nor is it especially concerned with the 
total thickness of the wash-deposits over the floor of the 
*Hill, Top. Atlas, U. S., folio 3, p. 8, 1900. 
