48 



NORTH AMERICAN FAUNA. 



[NO. 16. 



the atmosphere, would be "reatest at noon, wlum the snn's position 

 with reference to tlie 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 i)art of the afternoon, 

 the hour varying- with the locality. Ilence slopes that face the sun 

 most nearly at a right angle at tlie time of day when the atmosphere 

 is hottest are naturally the hottest slojies — those that carry the zones 

 highest; while conversely, sloj)es 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. Conseiiuently 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 : 



Flo. 28. — Diagram showing average hourly niarcli 

 of temperature. 



The influence of slo])e exposure on the faunas and floras of moun- 

 tainous regions is i)rofound. INIeasured by a scale of altitudes, it 

 amounts on ordinary sloi)es to nearly a thousand feet, and on steep 

 sloi)es is still more marked. Thus on mountains it is usual for plants 

 and animals of i)articular 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 sloi)es often belong to lower zones than those of ecpial 

 elevations on the cool east slopes. 



Shasta affoi'ds innumerable examples of tlie cll'ccts of s]o])c exi)osure, 

 both simple and complicated by canyons. 



An excellent illustration of the latter is t(k he found in .Mud (-reek 



