Vol.1] Hall. — Boianicol Survey of San Jacinto Mountain. 43 



very low altitudes. But opposed to this factor is that of the 

 warm air currents rising from the Colorado Desert. The lower 

 edge of the timber belt, which furnishes a good indication of 

 the results of the sti'uggle, is seen to be extremely sinuous on 

 these slopes, running well out on the protected sides of all 

 ridges and spurs, but immediately retreating to higher altitudes 

 wherever it comes around on those sides exposed to the desert 

 winds. This would seem to indicate that the lower limits of this 

 belt are influenced not so much by the slowly ascending air 

 currents as by the hot winds, since the former would tend to 

 equalize the temperature over all that region, while the latter 

 strike as hot, drying blasts, on all exposed areas. 



While the only factor to be considered in the case of the ideal 

 mountain, as pictured in the opening section of this chapter, 

 was that of altitude, it is now seen that in actual cases many 

 other factors demand our consideration, and it is interesting to 

 notice that these may be combined in such a manner that the 

 effect of those acting in one dii-ection will be neutralized by the 

 effect of those acting in an opposite direction, the resultant thus 

 being the same as though altitude were the only acting factor. 

 But the slightest increase in the value of any one factor will 

 immediately destroy the equilibrium and manifest itself in its 

 effect on plant distribution. In this connection we may compare 

 the conditions prevailing on tlie west side of San Jacinto Moun- 

 tain with those of the east side. On the former the desert winds 

 have but little influence, while slope exposure carries the lower 

 edge of the timber belt up to an average altitude of 5000 feet; 

 on the latter slope exposure maybe considered as negative, since 

 the slope is such that during the hottest part of the day the 

 sun's rays strike it at only a very ol^licjue angle, but desert 

 winds here exert a pronounced influence, carrying the life zones 

 even higher than did slope exposure on the west side, the lower 

 edge of the timber belt being at 6000 feet altitude east of 

 Tahquitz Valley and considerably higher on the northeast "side. 

 At a few places, however, a new factor comes in. This is the 

 effect of cold streams, which, opposing that of desert winds, 

 carries the timber belt some 500 feet lower along the creeks than 

 it is on the drier slopes; but even after the north slope and the 



