THE BOTANY OF THE CUYAMACA MOUNTAINS. 91 
railroad surveyors to be actually over two hundred feet below the 
level of the sea in its central parts. Looking off from the moun- 
tains in that direction, we see an immense, sandy level plain, 
bounded by distant rugged mountains toward the northeast, but 
illimitable toward the southeast, except by the dim misty horizon. 
Not a tree nor a trace of greén vegetation relieves the eye, and we 
gladly turn away from it to the verdant hills above the summit of 
the wall from which we have been looking eastward. 
The base of the range is therefore about fifty-five miles wide, 
with a slope of five thousand four hundred feet in thirty miles to- 
ward the west, and of the same amount in only fifteen miles on the 
east. It is singular that all the water falling east of the high 
. peaks finds its way around them and runs to the west. 
VEGETATION. — Being thus the western rim of the desert, the 
dryest portion of the United States, where the rainfall (as meas- 
ured at Fort Yuma) averages only about two inches annually, we 
may expect the mountains to partake, in some degree, of the 
sterility of the desert itself. But their vicinity to the Pacific 
Ocean, that exhaustless reservoir from which most of the rains of 
the western slope are derived, produces a fair amount of rainfall 
in winter, and at the same time increases the dryness of the desert, 
by intercepting this precipitation. At the same time the summer 
rains of Mexico and Arizona are to some extent poured out upon 
the eastern slope of the mountains between four thousand and six 
thousand feet elevation, thus failing to reach the coast, though they 
ean be seen frequently from San Diego falling as thunder-storms 
upon the mountain tops, and very rarely pass over the -lower 
passes northward to the Los Angeles plains. 
Consequently the highest ridge is thickly clothed with trees, and 
although they end at four thousand feet on the east slope, they 
-extend down the western, gradually thinning out, to the edges of 
the mésa, and thence along the banks of the rivers nearly to the 
sea. The lower mountains, and parts of the mésa are covered 
with shrubs but scarcely dense enough to hide the sterile rocks and 
= Near the sea, herbage of various kinds, but thin and of little 
value as pasture, covers the surface; improving, however, where 
ploughing has loosened the soil, packed almost to the hard- 
ness of bricks by two centuries of eattle-grazing and by the arid 
climate. A narrow belt of shrubby oaks (Adenostema and 
E 
a 
