ORIGINAL STREAMS IN GENERAL DESERT-LEVELING 271 
through deflation, they finally become local rain-provokers of 
some small influence. During the period of arid youth the streams 
developed on the mountain slopes become slowly larger and larger, 
and longer and longer, until as the region is about to pass into its 
maturity, they attain their maximum size and efficiency. The 
mountains are now at their loftiest, their sides are steepest, the 
intermont plain encroaches deepest into them, the moisture gathered 
about them is greater in amount than at any time before or than 
will be afterward, the mountain watercourses reach their greatest 
extension, notwithstanding the facts that they carry relatively 
little water, are intermittent in character, and their lower reaches 
seldom pass beyond the foot of the ranges. Instead of being head- 
water remnants of extensive stream-systems which have long since 
withered away under the arid climate, as is a necessary consequence 
of the adapted normal-cycle hypothesis, they must be regarded as 
original streams coming into being as the differential effects of 
regional deflation become more and more pronounced. With the 
advancement of physiographic maturity these streams must 
begin to wither, and as senile relief approaches, they must, with 
few possible exceptions, undergo complete obliteration. 
It is the custom to regard all water-action upon the desert 
ranges as normal stream-erosion in the process of dissecting recently 
upraised orographic blocks. This hypothesis seems to fall at once 
when it is considered that the major faulting of the mountain- 
blocks, is as already stated, mainly very ancient, and not modern 
as has been so long assumed. Certain effects of general deflation 
have greatly aided in imparting to the mountain sides the infantile 
aspects of the stream-work. As recently suggested’ the locus of 
maximum lateral deflation in the desert ranges is at their base, 
where plain sharply meets mountain without the intervention of 
foot-hills. The hard mountain-rock is encroached upon at the 
level of the general plains-surface as the sea gnaws away a line of 
its bordering cliffs, until, in many instances, the surface of the 
intermont plain extends into the mountain-blocks distances of 
several miles. 
t Bull. Geol. Soc. America, XXI (1910), 543, 
2 Science, N.S., XXIX (1909), 753. 
