Colorado and Mohave Deserts. 291 
ward into Mexico and which are there confined to the arid interior plateau of 
that country. The course ‘of migration has been perhaps through the Mexican 
states of Chihuahua and Sonora via, the southern part of the Great Basin, 
into Death Valley and the Mohave Desert‘). 
The Desert of the Little Colorado, or Painted Desert, is a deep 
basin on top of the great Colorado Plateau. From its topographic position, 
the only possible channel through which the fauna and flora of the Painted 
Desert could have reached this territory during existing climatic conditions is 
by way of the Grand Canyon of the Colorado. The inference is that the life 
of this desert is derived from the deserts of western Arizona, and that it came 
by the round-about way of the Grand Canyon of the Colorado. It behooves 
us, therefore, to examine briefly the elements of the flora of this great de- 
pression (6000 feet in depth). This canyon has ledges, terraces and mesas, 
barren crags and grassy slopes, lofty mountains and deep valleys, cool hill- 
sides clad in forests of balsam fir, and hot bottoms filled with subtropic thickets; 
it has arid stretches of sand bearing a scattered growth of cactus and yucca, 
marshes and springs, that never become dry and are hidden by the verdure 
of a multitude of plants requiring a moisture-laden atmosphere for their existence. 
Descending from the plateau level to the bottom of the canyon a succession 
of belts is encountered equivalent to those stretching from the coniferous 
forest of northern Canada to the cactus plains of Mexico. They result from 
the combined effects of altitude and slope exposure, the effects of the latter 
being here manifested in an unusual degree. Where the walls of the canyon 
face north, or northeast, the uppermost tree-belt consists of Pseudotsuga 
Douglasii and Abies concolor. Below this is a belt of pines, Pinus ponderosa, 
succeeded in turn by a belt of junipers and pihon, usually more or less mingled 
with pines. Immediately below the pifion belt is a belt which corresponds 
in the main to the Desert of the Little Colorado; but since it has humid as 
well as arid areas, forms of vegetation unknown on the desert interrupt its 
stretches of cactuses, yuccas and greasewood. Still lower down another belt 
is encountered which may be recognized by the presence of huge cactuses, 
arborescent opuntias, agaves and many plants characteristic of the Lower 
Colorado and Gila regions?) together with subtropic humid forms and a certain & 
Percentage of species not found elsewhere. 
Situated at the western edge of the painted Desert stands San Francisco 
Mountain, a peak, the flora of which demands a passing notice, rising as 
it does in the southern part of the Colorado Plateau to an elevation of 
12,794 feet (3900 m). Two trees Picea Engelmanni, Pinus aristata reach timber 
line and become stunted. They are accompanied by a number of hardy little 
plants which attain their maximum development here: 
ee BER 
I) Covıtıe, F. V.: Botany of the Death Valley Expedition. Contributions U. S. National 
Herbarium IV (1893); 31—33. 
z 2) MERRIAM, C. Hart: Results of a biological Survey of the San Francisco Mt. Region and 
esert of the Iittle Colorado, Arizona. North American Fauna No. 3, 1890: 33- 
19* 
