Erosion in Colorado Desert. 71 
still common, and several sections were found, possibly from the 
same tree, which measured about 150 centimeters (5 feet) in dia- 
meter. There are pebbled beds miles in extent, made up of agate, 
moss-agate, chalcedony, jasper, obsidian and fossil wood, with not 
so much as a spear of grass or bit of cactus between them. On the 
other hand, many of the mesas and plains are covered with sand 
and decomposed marls, which support a scanty growth of cactus, 
yucea, grease-wood and a few other forms of vegetation character- 
istic of arid regions. 
The bed of the Little Colorado River contains the only running 
water in this part of Arizona, and it “goes dry ” a large part of the 
year, a little water remaining in scattered pools, which are strongly 
alkaline. Some of the salt and alkali flats on the river-bottom sup- 
port a luxuriant growth of a singular fleshy plant belonging to the 
genus Salicornia, which at a little distance tooks like a leafless bush 
with green stems. During the rainy season, and whenever the 
river ‘‘runs,” the liquid which flows down its course is red alkaline 
mud, about the consistency of ordinary sirup. This is the case also 
with its tributaries, of which Moencopie Wash and Tenebito Wash 
are the only ones which cross the Painted Desert proper. 
The physical and climatic features of the Painted Desert are 
peculiar and striking, and result in the production of an environ- 
ment hostile alike to diurnal forms of animal life and to the person 
who traverses it- The explorer is impressed witb the unusual 
aspects of nature—the strange forms of the hills, the long ranges of 
red and yellow cliffs, the curiously buttressed and turreted buttes 
and mesas, the fantastic shapes of the rocks carved by the sand- 
blast, and rendered still more weird by the hazy atmosphere and 
steady glare of the southern sun, the sand-whirls moving swiftly 
across the desert, the extraordinary combination of colors exposed 
by erosion, the broad clayey flats whitened by patches of alkati 
and bare of vegetation, the abundance of fossil-wood, the extensive 
beds of shining pebbles, the unnatural appearance of the distant 
mountains sharply outlined against the yellow sky, the vast stretches 
of burning sand, the total absence of trees, the scarcity of water, 
the alluring mirage, the dearth of animal life, and the intense heat, 
from which there is no escape. 
The Plateau region of the interior of North America is noted for 
its scanty rain-fall, and the same may be said of Arizona as a 
whole. The annual precipitation and mean humidity are greatest 
on the high mountains and least on the low plains and deserts. 
The San Francisco Mountain has many times the rain-fall of the 
Little Colorado Desert, near by, and the quantity of aqueous vapor 
in the air is correspondingly higher. Evaporation is retarded by 
the clouds which frequently rest upon the summit, and by the 
dense spruce forests which protect the soil from the direct rays of 
tne sun, enabling it to retain enough moisture to permit the growth 
of plants requiring a humid atmusphere for their existence. 
There are two rainy seasons on the San Francisco Mountain 
plateau: one in summer, usually in July or August, the other in 
mid-winter. The summer rainy season is characterized by daily 
thunder-showers. As a rule, several such showers occur each day, 
and not infrequently several may be seen at the same time from 
any of the volcanic cones. The area covered by each is very small, 
its diameter rarely exceeding half, or even a quarter of a mile; 
