Vol. n Hall. — Botanical Survey of San Jacinio Mountain. 9 



this becomes six to ten iuclies deep, but it is usually much less 

 as it is destroyed whenever passed over by fire. This layer of 

 organic matter is of importance not only as a source of humus, 

 and therefore of nitrogen, but also because of its service in 

 catching and retaining the water from snow and rains, thus 

 reducing the amount of run- off. 



Since the temperature will be considered in connection with 

 the factors affecting plant distribution, and since reliable data 

 concerning this subject are lacking, no thermometrical records 

 ever having been kept on the mountain, the topic is not here 

 discussed. 



As the amount of precipitation is largely controlled by the 

 temperature, we find that the higher portions of the mountain 

 are much more freciuently visited by showers and snow storms 

 than the lower, and that the west side is less arid than the 

 eastern. In the foot-hill region the rainfall is quite limited in 

 amount and restricted almost entirely to the winter months, but 

 throughout the belt' of coniferous forest at middle altitudes the 

 precipitation during the fall and s])ring is much more generous, 

 besides which there are occasional thundershowers during the 

 summer and heavy snowfall in winter, the snow often reaching 

 a depth of three or four feet in Strawberry Valley. Where the 

 slopes are covered with chaparral the water is well conserved, 

 being compelled to percolate slowly through the soil, which is 

 held in position hx the numerous roots and acts as a water 

 sponge. Of similar service but less efficient is the layer of 

 decaying vegetable matter in the forested portions; but, wher- 

 ever the slopes are unusually steep, and also where the forests 

 have been removed, the amount of run- off is greatly increased, 

 as is indicated by the numerous ravines and gullies on these 

 areas. , 



On the upper parts of the mountain the rainfall is abundant 

 and it has been reported that snow sometimes falls to a depth of 

 fifteen feet on the higher summits. All the slopes above 9000 

 feet altitude, with the exception of the most exposed, are covered 

 with snow from December to April, while in the steep canons 

 which descend to the north of the main peak it remains through- 

 out the year. This abundant precipitation at the high altitudes 



BOT.— 2. 



