270 The American Geologist. May, 1908- 
rocks being everywhere tlioiovghly cracked up, and the frag- 
ments although about in their original position are easily dis- 
placed. When a cloudburst occurs, the rain runs off in tor- 
rents, and sweeps hefore 1 large quantities of this fractured 
material and when the clondbursts are of sufficient size, they 
will carry boulders many tons in weight far out on to the 
plains. There is, therefore, no difficulty in accounting for the 
formation of the alluvial fans, but the time that must be allotted 
to the formation if we suj-ose the precipitation to have been 
no greater in the early Pleistocene than at present, would be 
enormous. It is quite certain, however, that in earlier Pleisto- 
cenc time, the y:recipitation was much greater than at present. 
It is probable, therefore, that the larger part of these detrital 
slopes were formed during the first half of the Pleistocene. 
This would harmonize with the record in the Sierra Nevada, 
where the larger part of Pleistocene time was required for the 
excavation of the canyons. This early Pleistocene period of 
erosion may be termed ihe Sierran period, and the larger de- 
trital slopes of the Great Bzsin would then be referable to this 
period, The detrital fans of the early Pleistocene often attain 
a thickness of 2vo feet or more, as in the “wash” on the west 
side of the Silver Peak range which is followed by the road ffom 
Fish Lake valley to Cave Springs. The older detritus has 
undergone elevation at many points, as can be seen on the west 
side of Fish Lake valley, along the base of White mountain, 
and on the west of the Silver Peak range, particularly to the 
north of Fish lake— Siiver Peak road, there being here hills 
1,000 feet in elevation zbove the valley composed chiefly of 
beds of Pleistocene gravei and detritus, which have been tilted, 
while the detrital beds south of this road have merely under- 
gone elevation. There are detrital masses high up in some of 
the ranges, as on the north slope of the Palmetto mountains 
on the south slope of the Pilot mountains, and in Chilcoot pass 
by the road from Silver Peak to the South Klondyke quartz 
inine. These detrital masses must have originally occupied de- 
pressions, and suggest recent elevation of the ranges on whose 
fianks they lie. 
To the early or middle Pleistocene must be relegated the 
lake Lakontan terraccs of the north part of the county, best 
secn on the west side ef Walker lake. 
