o'T., 1899.] EFFECTS OF STEEP SLOPES. 51 



side. Hence if tLie altitudes to whicii glaciers descend on the various 

 slopes be accepted as indicating tlie 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 e(iuivaleiit to a difference of upward of 2,(H)0 feet in altitude. I>ut 

 this is doubtless excessive and due in part to local influences. 



EFFECTS OF STEEP SLOPES. 



^5teep slopes, jjarticulaily those that face the southwest and west, 

 exaggerate the effects of slope exposure. Those that face the hot after- 

 noon sun at nearly a riglit angle receive the greatest (juantity of heat, 

 but this alone is not sufiicient to account for the very extraordinarj^ 

 degree to which the fauna and flora are sometimes affected. When it is 

 remembered that the hottest ordinary slopes carry up the zones only 

 800 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 w ith the geologist, G. K . (lilbert, he suggested the diurnal ascend- 

 ing current as the missing factor. 



It is well known that in ordinary calm weather the air currents on 

 mountain sides and in deep canyons ascend by day and descend by 

 night. The ascending currents are warm, the descending currents cold. 

 The night current, being in the main free from local influences that 

 afl'ect 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 diflerent slopes. The activity and eflective- 

 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 supjDly of heat 

 while the steepness of the sloi)e 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 ett'ects of these ascending hot-air currents. Here, on the hot stony 

 pumice slopes, such distinctive Transition zone species as Arctostaphylos 

 patula, Kunzid tri<lentat(i, Ceaiwthns velutinn.s, and Chrysofhamnus-occi- 

 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. 



