GEOLOGY: C. KEYES 
37 
lofty ranges and border a region of moister climate the action of running 
waters is more nearly normal than in the Great Basin proper. Yet, in 
the interior of the latter the same phenomena are also well displayed. 
Around the southern rim of one remarkable desiccated Las Vegas, in 
southern Nevada, in the low Spring, Newberry and Eldorado ranges, 
the bajada is frequently dissected and terraced in a singular manner. 
So extensive is it that it may be clearly distinguished even at a dis- 
tance of 10 or a dozen miles. Dissection and terracing are also every- 
where apparent around the region of the excessively dry basins of 
Death valley, the Armagosa plains and the Mojave desert. . 
In this connection there is one circumstance which not only A. C. 
Trowbridge and C. L. Baker appear to have overlooked, but likewise 
F. L. Hess, J. E. Spurr, G. K. Gilbert, H. W. Fairbanks and other 
earlier writers. In the descriptions of the terrestrial deposits the 
origin of the latter is ascribed entirely to water-action. No account 
seemingly is taken of possible assistance of wind-action in piling up 
locally these masses of debris. Photographs of the region, which the 
authors named reproduce, display unmistakable signs of plenty of wind- 
work. On this point direct personal observation is even more con- 
clusive. In the building up of the so-called alluvial fans and of the 
bajada when composed of fine materials the winds appear to be the 
controlling power. Arroyo-waters seem mainly to be merely modify- 
ing agents, supplying some coarse rock-waste from the mountains, but 
largely locally turning back the materials brought in up-grade from the 
lowland plains. The effects simulate the alluvial fans of humid regions; 
but they are not by any means their exact counterpart. 
After the lower reaches of the canyons, immediately before they 
debouch upon the plains, become over-filled in the course of a few 
months or a few years, with the wind-driven sands and dusts and are 
eolicly aggraded they are readily dissected and even terraced by the 
first appearance of heavy storm-waters on the mountains. For accom- 
plishing these results the time-element is certainly not so interminably 
long as has been commonly supposed. It is not necessary to stretch it 
back to the Glacial period, and far beyond. It is not to be gauged by 
tens of millenia. Its span is to be measured not even by years, but by 
months or weeks. It is known to have been limited by a single rising 
and setting of the sun. 
The local dissection and terracing of the so-called alluvial fans in arid 
regions may have a significance still broader than that commonly 
ascribed to it. In the explanation of these phenomena we may have the 
key to the specific method by which general leveling and lowering of 
