OcT., 1899.) EFFECTS OF STEEP SLOPES, 51 
side. Hence if the altitudes to which glaciers descend on the various 
slopes be accepted as indicating the course of a sinuous line of equal 
temperature, it follows that the difference in temperature dependent on 
the angle and conditions of slope exposure, as measured by the glaciers, 
is equivalent to a difference of upward of 2,000 feet in altitude. But 
this is doubtless excessive and due in part to local influences. 
EFFECTS OF STEEP SLOPES. 
Steep slopes, particularly those that face the southwest and west, 
exaggerate the ettects of slope exposure. Those that face the hot after- 
noon sun at nearly a right angle receive the greatest quantity of heat, 
but this alone is not sufficient to account for the very extraordinary 
degree to which the fauna and flora are sometimes affected. When itis 
remembered that the hottest ordinary slopes carry up the zones only 
3800 to 1,000 feet, one is startled to find that on some favorable steep 
slopes they are pushed up more than 2,000 feet above their normal 
limits. The explanation did not occur to me until, in discussing the 
matter with the geologist, G. K. Gilbert, he suggested the diurnal ascend- 
ing current as the missing factor. 
It is well knuwn that in ordinary cal weather the air currents on 
mountain sides and in deep canyons ascend by day and descend by 
night. Theascending currents are warm, the descending currents cold. 
The night current, being in the main free from local influences that 
affect its temperature, must exert an essentially equal effect on all sides 
of a mountain; but the temperature of the ascending day current, being 
constantly exposed to and in fact created by the influence of the sun, 
must vary enormously on different slopes. The activity and eftective- 
ness of this current increases with the steepness of the slope and the 
directness of its exposure to the afternoon sun. Hence the hottest 
normal slopes—those that face the sun at nearly a right angle during 
the hottest part of the day—are rendered still more potent by increased 
steepness, the direct exposure to the sun keeping up the supply of heat 
while the steepness of the slope accelerates the rate of movement of the 
diurnal ascending current, carrying the heated air upward a very great 
distance before it has time to be cooled by the general temperature of 
the stratum it penetrates. Thus it is that species characteristic of the 
Transition zone on Shasta—species which on normal southwesterly 
slopes attain their upper limits at an altitude of 5,500 to 5,700 feet—are 
in favorable places enabled to live at elevations of 7,900 and even 8,000 
feet, considerably more than 2,000 feet above their normal upper limits. 
The steep slopes of Diller Canyon furnish instructive illustrations of 
the effects of these ascending hot-air currents. Here, on the hot stony 
pumice slopes, such distinctive Transition zone species as Arctostaphylos 
patula, Kunzia tridentata, Ceanothus velutinus, and Chrysothamnus-ocet- 
dentalis flourish among the Shasta firs and white-bark pines at an alti- 
tude of nearly 8,000 feet in the belt where the Canadian and Hudsonian 
zones overlap, and more than 2,000 feet above the extreme upper limit 
of their normal distribution on uncomplicated hot southwesterly slopes. 
