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IOWA ACADEMY OF SCIENCES. 
CERTAIN BASIN FEATURES OF THE HIGH 
PLATEAU REGION OF SOUTHWESTERN 
UNITED STATES. 
BY CHARLES R. KEYES. 
[Abstract.] 
In the Basin region of Western America there exists a 
remarkable type of intermontane valleys to which the 
Spanish name Bolson has been given. The geological sub- 
structure of these plains has been recently described at 
some length.* In the present connection some additional 
features are discussed. 
The bolson plains of New Mexico, for example, are found 
only in that part of the region which belongs to the 
geographic subdivision known as the basin region. This 
includes the southern two-thirds of New Mexico, or the 
portion lying south of the Rocky mountains, which abruptly 
terminate 100 miles south of the Colorado line. South- 
ward, from this latitude, the bolson plains occur — long 
level strips of plains country, separated from one another 
by high but narrow mountain ranges. Far beyond the 
New Mexican boundaries the same type of physiography 
prevails, nearly as far as the city of Mexico. 
The peculiar alternation of narrow mountain ranges and 
broad plains presents many features which are not easily 
understood 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 ranges more disconnected and 
* Am. Jour. Sci., (4), Vol. XV, pp. 207-212, 1908. 
