INTRODUCTION, CLIMATE. 17 
valleys there is every gradation. The mountains are cut by gorges 
which in the more elevated ranges are deep canyons. The streams 
descend from the mountains to the flat valleys on alluvial fans and 
commonly even the stream channel disappears before the playa in 
the center of the valley is reached. 
The valleys are inclosed basins which slope rather steeply next to the 
mountains, but decrease in grade rapidly, the central portion being 
a flat in which the eye can see no differences in relief. This level 
bottom is in most cases a playa. The borders of any given valley 
are, as a rule, approximately equal in elevation, although the desert 
gravels extend to greater elevations on the higher inclosing moun- 
tains. In such cases, while the slopes are of approximately equal 
descent, that from the higher mountains is much longer, and in con- 
sequence the playa is nearer the lower hills. Near the mountains 
the alluvial slopes are scored by numerous drainage lines, and hills 
protrude through their surface. The inclosed basins send bays and 
arms of detrital wash into the mountains and in places two opposed 
arms meet and form a strait of alluvial material. Every hill within 
the area- furnishes each year material with which it is slowly being- 
overridden by the constantly growing flat valleys. These valleys at 
one time may have been united to one another so that they formed a 
single drainage system, flowing to the south. Whether this was the 
case. or not, it is evident that they are formed in three w r ays — by the 
union across a valley of opposed alluvial fans, as is exemplified by 
the small inclosed valleys of Grapevine Canyon ; by the outflow of a 
lava barrier, such as that between Sarcobatus Flat and the inclosed 
valley to the north; or by orogenic movements, of which the Bullfrog 
Hills between Sarcobatus Flat and the Amargosa Desert are, in part 
at least, an example. 
CLIMATE. 
The area has an arid climate, with hot, dry summers and, except in 
the higher mountains, mild winters. The temperature in summer 
often rises above 100° F., the intensity of the heat, however, being 
mollified by the dryness of the atmosphere. Eecords kept at Haw- 
thorne, Sodaville, and Palmetto, Nev., and quoted by Turner" show 
that the average annual precipitation to the north of the area varies 
from 3 J inches in the valleys to 15 inches on the highest mountains. 
These figures are probably both somewhat high for the area under 
■(Consideration. The precipitation is largely concentrated in cloud- 
bursts and 4 or 5 inches of rain may pour clown in a few hours. 
« Turner, II. W., Silver Peak folio ; unpublished. 
Bull. 308—07 m 2 
