DAVIS: MOUNTAIN RANGES OF THE GREAT BASIN. iG 
fault block, owing its relief to gradual and long-continued displacement, 
whose later movements are clearly recorded in the form of its base. Al- 
though the observations on which this statement is made were made only 
from a passing train, they are believed to be fully deserving of credit. 
It should be understood, however, that they apply only to the northern 
part of the range. The passage from observation to explanation may be 
stated as follows : — 
The first feature to be noted is the block-like appearance of the mass, 
especially as indicated by its basal outline. The base line is relatively 
well defined, of very small irregularity and of moderate curvature; the 
basal mass is continuous but for the narrow ravines that divide it into 
full-bodied spurs. 
In the second place, attention should be given to the contrast of the 
heavy mountain mass and the broad piedmont plain. The lower slopes 
of the mountain are strong ; they change rather abruptly into the broad 
alluvial plain that stretches away unbroken for several miles. The de- 
pression, floored by the piedmont plain and drained by Humboldt river, 
is five or ten miles wide between Shoshone range on the south and Sho- 
shone mesa on the north, and does not give the impression of being a 
normal trunk valley, eroded in a once continuous rock mass; for if it 
were of such origin the branch valleys by which the mountain is drained 
ought to be of correspondingly advanced development with broad-open 
floors; while as a matter of fact the branch valleys are narrow to their 
mouths at the mountain base. Moreover, in the neighborhood of Pali- 
sade, twenty-five or thirty miles further east, the Humboldt river has 
what appears to be a perfectly normal valley ; a narrow canyon cut in 
lavas. The broad plain and the narrow canyon cannot both be parts of an 
undisturbed normally eroded valley ; and as the narrow canyon is mani- 
festly of river origin, the broad depression must be otherwise explained. 
The depression might at first sight be regarded as a down-warped part 
of a normal valley, heavily aggraded with alluvium ; but this supposition 
is untenable, because the alluvium does not invade the ravines on the 
mountain flank as it certainly should if the ravines had been carved 
with respect to a now-buried trunk valley. Some other origin than 
erosion must therefore be discovered for the depression alongside of 
the mountain. 
Differential movement or faulting, the only other conceivable origin 
of the depression — the supposition that the mountain rocks were origi- 
nally deposited only on their present limited area need not be considered 
— is not only permissible by its appropriateness to the outline of the 
