94 
divide between the depositional basins to its west and east, and also, 
as at present, warmer waters existed in the eastern basin than in the 
western, 
In the vicinity of the road, these tilted Tertiary sediments have 
been truneated by stream action. A fairly plane surface was cut across 
their edges, and on this have been deposited water-laid sands and gravels 
over a broad area. They here form the old valley floor which the pres- 
ent streams are dissecting. They are probably Quaternary in age. 
These gravels which often carry rather large boulders, are made up 
of a variety of rock types evidently derived from the mountains. Of 
particular interest is the large quantity of voleanie pebbles of various 
types, for on the road down the mountains one may not notice voleanic 
rocks either in the canyon walls or in the creek detritus. But volcanic 
rocks do occur along the mountain front to the north and south, and near 
the eastern edge of the high mountain area, hills were observed with 
flat-lying rocks that seemed in part at least to be voleanic. These vol- 
canic rocks, both along the range front and on the summit region, must 
have been much more widely spread before the erosion pericd that pre- 
duced the gravels of the valley, and streams that were formerly flowing 
in lava beds have removed the voleanic covering from the surface and 
are now cutting into granite. 
On the surface of these desert gravel beds le many scattered peb- 
bles and boulders that show beautifully the effect of desert polish, etch- 
ing 
typical desert ‘‘varnish’’ was not observed. 
and sometimes of the development of a surface film, although the 
Weathering and Erosion.—In the journey across the mountains 
the weathering forms of the rocks, especially the granitic rocks, were 
constantly before us, and occasionally we had the opportunity of ob- 
serving the effects more closely in road cuts or stream banks. They 
were typical of arid conditions. Jointage, spalling, mechanieal disin- 
tegration are the controlling features. Decomposition, soil formation 
are relatively slight. 
Rock exposures are numerous on the hillsides and even the flatter 
elevated slopes. Samples of the rocks taken from these surface ex- 
posures commonly show the minerals to be lustrous and but little altered 
in composition, but the rock may crumble in the fingers or be easily 
broken up with a hammer. 
Here and there a surface is seen covered with large rounded rock 
masses, often averaging several feet or even yards in diameter, and the 
impression may be given of glacial erraties seattered over the surface. 
But a closer examination shows these to be boulders of disintegration 
formed in place, though sometimes slipped, rolled, or washed to a lower 
