SOME RELATIONS OP THE OLDER AND YOUNGER TECTONICS OF THE 
GREAT BASIN REGION. 
BY CHARLES R. KEYES. 
{A'bstract.) 
The Basin ranges of western America stand for a distinctive type of moun- 
tain-building. Their characteristic structures are regarded as unique because 
of their extreme simplicity and because their genesis is ascribable to normal 
faulting on a gigantic scale. Of late years many of the earlier notions concern- 
ing the formation of these mountain ranges are being called into question; and 
the structural problems themselves appear to be farther than ever from satis- 
factory solution. As landscape features it now seems probable that instead of 
being regarded as strictly structural phenomena the present mountains are to 
be considered mainly remnantal erosional effects produced under the peculiar 
conditions of an arid climate during the progress of general desert-leveling. 
The desert region of the West includes several extensive regions other than 
the Great Basin, which is in reality only a minor part. The other districts 
are known as the Colorado dome, or plateau, the Californian gulf basin, and the 
Mexican tableland. In the present connection the history of opinion regarding 
the tectonics of the desert region need not be referred to. 
The results of recent investigation may be summed up as follows: 
1. The folded structures displayed in some of the Basin ranges, perhaps 
most of them, belong mainly to an ancient system of tectonics (Triassic or 
Jurassic), in no way genetically connected with the rearing of the existing 
mountains. This is true not only of the Great Basin region, but of the other 
grand divisions of the desert region. 
2. The tectonic effects discernible in the landscape are only the most recent 
simple faultings; all others have been more or less completely mastered by 
general desert-leveling. 
3. The discovery of extensive thrust-planes in various ranges, such as have 
been described as occurring in the Caballos mountains, may have far-reaching 
significance, and may demand an entirely new scheme of origin for the majority 
of the desert ranges, at least for their initiation. 
4. The rearing of the present mountains and the greater effects of mountain 
sculpture in the desert region is to be ascribed largely to the differential effects 
of general desert-leveling, that is, mainly to powerful and constant eolian in- 
fiuences, under conditions of aridity, in which water plays a very minor role. 
5. The chief effects of ordinary tectonics have been largely to produce 
alternate belts of resistant and non-resistant rocks and to accentuate them in 
the topographic expression of the region. 
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