Bolsoii Plai)is and Their Existence. — Keyes. i6i 
on the other hand, are newer and later topographic features, 
consisting of structural valleys between mountains or jjlatiau 
plains, which have been partially filled with debris derived 
from the adjacent eminences. The plateau plains are usually 
destructional stratum plains. The bolson plains are construc- 
tional detritus plains filling old structural troughs."'* 
It has been recently shownt that the structure of the bolson 
plain is not nearly so simple as the description just quoted 
would intlicate ; that they are not, according to the strict phys- 
iogra])hic usage of the term, structural valleys ; that the bol- 
son j)lains are not necessarily any newer and later than some 
of the i)lateau plains which overlook them ; that the construc- 
tional detrital covering is no more important than that of the 
plateau plain ! and that the wash-deposits brought down from 
the ])eripheral area arc relatively of small importance. 
On the other hand, it has been clearly demonstrated that 
the rockbed surface of the bolson plain, that part beneath the 
thin detrital covering, is a planation surface, a surface worn 
out on the bevelled edges of the consolidated and indurated 
sedimentaries. 
The bolson plains" of New Mexico, for example, are found 
only in that part of the region which belongs to the geo- 
graphic subdivision known as the basin region. This includes 
the southern two-thirds of Xew ^Mexico, or the portion lying 
south of the Rocky mountains, which abruptly terminate lOO 
miles south of the Colorado line. Southward, from this lati- 
tude, the bolson plains occur — long, level strips of plains coun- 
try, separated from one another by high, but narrow mountain 
ranges. Far beyond the New Mexican boundaries the same 
tvpe of physiography prevails nearly so far as the citv of 
Mexico. 
The peculiar alternation of narrow mountain ranges and 
broad plains present?; many features which are not easily un- 
derstood until the country both to the eastward and to the 
westward is taken into account. In both directions from the 
central highland the '"perse" character of the basin plains is 
soon lost. 
The different plains become confluent and more contin- 
uous, and the mountain ran ges more disconnected and finalk 
• Hll.Lf Top. Atlas C. S., foli.o 3. p. 8. 190(). 
1 Anier.Jour. Sci., (*), vol. xvi, p. 207, 1903. 
