THE BROAD-SCLEROPHYLL VEGETATION OF CALIFORNIA. 83 



of the xerarch class. Three distinct lines of development have been 

 observed upon primary areas, corresponding with three common 

 modes of soil formation. These are the successions on rock surfaces, 

 on washes and alluvial fans, and on coastal dunes. In dealing with 

 such an extensive and varied region the treatment can not be com- 

 plete, and there is here a fruitful field for further study. 



Development on rock surfaces. — Though most of the chaparral- 

 covered mountains have a uniform, unbroken mantle of vegetation, 

 there are locally extensive areas where bare rock is being invaded. 

 Such successions are much the same in character wherever they occur. 

 The usual lichens and xerophytic mosses cover the surfaces and 

 their establishment is assisted by the abundant scaling of the rock 

 due to sudden temperature changes. Crevice plants as usual are 

 of supreme importance. Among the herbaceous species, Pellcea 

 mucronata and Selaginella bigelovii are frequent. The most impor- 

 tant pioneers are the chaparral shrubs themselves, the successional 

 stages thus being greatly telescoped. Adenostoma is perhaps most 

 important among these. Yucca whipplei is common in such places 

 in the southern part of the State. Dead remains of shrubs anchored 

 in the crevices show that permanent establishment may be a matter 

 of many generations. Gradually the rock disintegrates and the 

 chaparral shrubs increase in number. It is not uncommon, however, 

 to find a solid cover of chaparral concealing and growing entirely 

 from the crevices of a rock-layer whose surface is barely at all dis- 

 integrated. Thus, through a process long in point of time but com- 

 prising few stages, the climax community comes to control the bare 

 rock areas. 



Development on alluvial fans and washes. — Because of frequent 

 production of fresh surfaces due to erosion and deposition by periodic 

 torrents, there is excellent opportunity for the study of such succes- 

 sions. Alluvial fans and continuous piedmont slopes occur through- 

 out the State, but they are especially well developed in the Los 

 Angeles and San Bernardino Valleys, bordering the high ranges to 

 the north. The physiographic processes involved in their formation 

 are a complex combination of erosion and deposition. The periodic 

 torrents from the mountain canyons and ravines are continually 

 changing their courses, depositing material now here, now there, 

 cutting into deposits already made, and forming new surfaces through 

 both processes. If the cutting goes deep enough terraces are formed 

 whose surfaces are thereafter free from further direct disturbance. 



Certain ancient fans now in process of dissection show all stages 

 in the physiographic cycle. There is a very remarkable example 

 south of San Timoteo Canyon which may be viewed to excellent 

 advantage from Smiley Heights, near Redlands. V -shaped ravines, 

 cut sharply into the original smoothly sloping surface of the fan, 



