Vol.1] Hall. — Botanical Survey of San Jacinto Mountain. 37 



By reference to the map of life zones it will be seen that a 

 long- arm of the Upper Sonoran Zone, corresponding to the chap- 

 arral belt, enters the main body of the Transition Zone, or Yel- 

 low Pine belt, from the sontheast. This chaparral area is a 

 moderately steep slope facing the southwest, while in the 

 more nearly level valleys below are open groves of Yellow Pine, 

 the crest of the ridge above being also forested with this and 

 other conifers. The raising of the chaparral belt is here due 

 almost entirely to slope exposure, the slope being such as to be 

 nearly at right angles to the incident rays of the sun during the 

 hottest part of the day, while other factors scarcely enter in. 



In some of these cases we find that slope exposure not only 



carries an arm of one zone well up into a higher one l)ut that 



this intruding arm may be entirely cut off from the main body of 



the belt to which it belongs. A striking example of this is seen 



on Chalk Hill, a prominent south slope crossed by the stage road 



just before entering Strawbeny Valley, where we have a case of 



a Sonoran, or foot-hill, flora being carried far above its normal 



position on the mountain and completely surrounded by a flora 



consisting of Transition species, thus producing a veritable 



Sonoran island (see plates vii and viii). The elevation of this 



slope ranges from 4400 to 5400 feet, and yet the flora is strictly 



that of the Upper Sonoran Zone, the most prominent species 



being Arctostaphylos tomentosa, Adenostoma fasciculatum, A. 



.<<parsi folium, Gastilleia foliolosa, Mimulus hrevipes, Diplacus 



longiflorus and CryptantJie Jonesii, all of which are characteristic 



of the lower foot-hills. The Transition Zone species from above 



creep down along the creeks and in the shade of low ridges to 



unite with a large transition area at the foot of Chalk Hill. 



The presence of this transition flora is explained in part by the 



comparatively level surface and in part by the presence of a ridge 



which, rising to the south, intercepts the sun's rays during a 



part of the afternoon. This ridge supports on the slope facing 



the north, and thus opposite Chalk Hill, a vegetation consisting 



of such characteristically transition species as the Coulter and 



Yellow Pines. In the valley between this ridge and Chalk Hill 



we find the transition area mentioned above sending out arms 



