226 GEOLOGY. 
temperature of the surface, for any moist surface exposed to the dry airs of that region suffers 
a rapid reduction of temperature in consequence of the evaporation. 
The moist, cool air from springs or moist places is quickly perceived by the senses, especially 
as the greater part of the atmosphere is dry and warm. Observations at night therefore, with 
the hygroscope, and of the temperature of the ground in different depressions would probably 
lead to interesting results. 
DISTRIBUTION OF PLANTS ON THE SURFACE. 
Although the greater part of the surface of the Basin is without trees or shrubs of any mag- 
nitude, some portions of the slope, especially near the Cajon Pass, are rendered picturesque by 
a growth of cedars or junipers and the yucca or ““ Spanish bayonet tree.”” 
. VEGETATION OF THE SLOPE OF THE BASIN NEAR THE AJON PASS 
This plant grows to a great size, and forms a thick grove in some places, giving a tropical 
aspect to the landscape. It was observed in the channel leading to the Tejon Pass and to the 
Cañada de las Uvas,! and at various points around the margin of the Basin; also on the slopes 
flanking the Lost Mountains. Several of the trees near the Spanish trail were eighteen or 
twenty inches in diameter at the base, and twenty to twenty-five feet high. It generally rises 
in one straight trunk or column, from three to fifteen feet high ; but near the Cajon it branches 
out and attains a great breadth of top. Green leaves or spines are chiefly confined to the last 
two or three feet of tops of the plants, or at the ends of the short and thick limbs. The leaves 
! See View XI. 
