VEGETATION OF THE SLOPE OF THE BASIN. 227 
lower down become dry and yellow, and then hang downwards over the trunk, overlapping like 
shingles. Every leaf is as sharp as a thorn at the end, and is very stiff. 
The unequal distribution of plants upon the surface is worthy of notice, and is distinctly ob- 
servable along the Spanish trail, which traverses the long slope from the Cajon to the Mojave 
river. In descending this slope a succession of belts or zones are passed, in each of which some 
particular plant or group of plants predominates, and determines the aspect of the surface. 
At the summit of the Pass, the dwarf oaks and deciduous shrubs are the most abundant. They 
are succeeded by a belt of low, dwarf cedars, which spread out over a large surface of ground, 
but seldom attain a height of over fifteen feet. Larrea Mexicana is found mingled with them. 
Lower down on the slope the cedar is replaced by the peculiar yucca; and towards the foot of the 
slope, at the Mojave, this gradually disappears, and the surface supports a growth of low thorny 
shrubs, which are almost without leaves, and seem to consist of a mere aggregation of thorns. 
These plants exist in great variety, and may be found among the other plants on almost all 
parts of the slope. At the foot of the slope, along the river, we find cotton-woods, willows, and 
the mezquit. 
There is not any sufficiently well-marked difference in the chemical constitution of the soil at 
the parts of the slope occupied by different plants to authorize the supposition that their unequal 
distribution may be thus explained. The only variation appears to be, that the upper portions 
of the slope are of coarser materials than the lower portions. The explanation of this phenom- 
enon of distribution cf the plants in successive belts or zones is doubtless to be found in the 
climatic conditions which exist at the different altitudes. The variation of the humidity of the 
atmosphere at the different elevations is probably the principal modifying cause. 
