48 NORTH AMERICAN FAUNA. [No. 16. 
the atmosphere, would be greatest at noon, when the sun’s position 
with reference to the earth is most nearly vertical. But, as everyone 
knows, the atmosphere becomes heated slowly and does not attain its 
highest temperature until the middle or latter part of the afternoon, 
the hour varying with the locality. Hence slopes that face the sun 
most nearly at a right angle at the time of day when the atmosphere 
is hottest are naturally the hottest slopes—those that carry the zones 
highest; while conversely, slopes that face the opposite direction are 
naturally the coldest slopes—those that depress the life zones lowest. 
At Portland, Oregon (about 275 miles north of Shasta), the hottest 
part of the day in summer is a little after 4 o’clock in the afternoon, 
at which time the sun is nearly due west. Consequently the hottest 
uncomplicated slopes are those that face west or a little south of west. 
The accompanying diagram shows the actual mean hourly march of 
atmospheric temperature at Portland, Oregon, for June, July, and 
August: 
& 
AM. 8 PM. 
‘el3]aTslet7 [8 [9 poly hele Te [sla] 5617 [619 bow (2) 
78. i 
77 I i 
76 \ 
7S 
Pad 
[3t |] | N 
[zt Ty ty vy 
vhf iF 
if it \ 
ia 
al N 
/ 
ca 
[63 AUGUST 
AUGUST {62| vi IN 
67] “| yur 
uty [60 
59) 
58 
57, Vs 
lest ! JUNE 
JUNE |55| aT: 
d 7 T == 
13 T i 
2 2 1 May 
Mar A I i 
i is iN 
a. 
4 
a APRIL 
APRIL 4, 
a = { 
is | 
Fig, 28.—Diagram showing average hourly march 
of temperature. 
The influence of slope exposure on the faunas and floras of moun- 
tainous regions is profound. Measured by a scale of altitudes, it 
amounts on ordinary slopes to nearly a thousand feet, and on steep 
slopes is still more marked. Thus on mountains it is usual for plants 
and animals of particular species to occur on warm southwesterly 
slopes at elevations 800 to 1,000 feet higher than on cool northeasterly 
slopes. Similarly on north and south ridges, the faunas and floras of 
the warm west slopes often belong to lower zones than those of equal 
elevations on the cool east slopes. 
Shasta affords innumerable examples of the effects of slope exposure, 
both simple and complicated by canyons. 
An excellent illustration of the latter is to be found in Mud Creek 
