266 BOTANICAL GAZETTE [APRIL 
joints are horizontal the peaks have a terraced appearance, as on 
the south flank of Pyramid Peak. The sandy slopes formed from 
the crumbling granite are very pervious to water, and on their 
higher reaches only such plants as have especial advantages in 
obtaining a requisite amount of water can survive. At their bases 
one finds a distinctly mesophytic association, for the water absorbed 
above oozes out below and may be of amount sufficient to cause 
marsh conditions. On the east side of Angora Peak is such a sand 
slope whereon the plants become progressively more mesophytic 
as the foot is neared. Since the soil is the same from top to bot- 
tom, except for the increasing amount of organic material present, 
such a slope seems to be a direct, though imperfect, gauge for the 
water factor in plant life. 
ere the granite is massive the process of rock decay is en- 
tirely different. While the factors concerned (temperature and 
moisture) are the same, their action is largely neutralized by the 
lack of rock fissuring; rock decay becomes almost entirely a matter 
of exfoliation. Huge slabs become broken from the surface and 
by their position protect the rock beneath; only as the slabs are 
slowly broken and slide from place can the process continue. In 
the granite deserts of Desolation Valley, Rubicon Valley, and 
Donner Pass, this protecting action of the exfoliated slabs may be 
particularly well seen. Here the only spots capable of bearing 4 
flora are the small depressions between the glaciated ridges with 
soil formed in large part of wind blown granite dust. Over the 
greater surface plant life is impossible save for crustose lichens and 
a few hardy crevice plants, which have settled in the fissures about 
the borders of the slabs. 
Limestone seems to be lacking in the district, though so abun- 
dant and important a rock base of the Basin ranges. 
The rocks derived from extrusive lavas are mainly andesites and 
basalts. The principal vents, from which these flows issued, lie 
on the crest of the so-called Great Western Divide. From Round 
Top Peak, some 30 miles south of Lake Tahoe, there is a succession 
of ancient volcanoes terminating in Castle Peak, northwest of 
Truckee. The chief center for these flows seems to have bee 
Mt. Mildred (2), 10 miles west of Lake Tahoe. These irruptives 
