10 University of California Puhlications in Botany [Vol. 9 



along the lines of fracture, becoming divided to a varying depth into 

 more or less cubical masses. If the rise is abrupt, these in time fall 

 from place and become aggregated into talus slopes at the foot of the 

 cliff. In many places such talus slopes rise nearly to the summits and 

 support a characteristic vegetation. Where the granodiorite is massive 

 weathering is mainly a process of exfoliation producing rounded sum- 

 mits ;^^ this is the origin of the many dome-like summits found in 

 the higher mountains. Lambert's Dome, on the upper Tuolumne, 

 Fresno Dome near Mt. Raymond, and Ralston Dome near Lake Tahoe 

 (plates 6, 7), with surfaces bare of all vegetation except crustose 

 lichens, are typical of many Sierran summits. Though theoretically 

 the granodiorite should disintegrate faster than the granite because 

 of its larger content of mica and soda-lime feldspar, the coarser texture 

 of the granite makes this rock the weaker. This relationship of 

 texture to weathering depends upon one important character of alpine 

 rock decay : the dominance of mechanical over chemical forces at high 

 altitudes. Solution plays a subordinate part because of the relative 

 deficiency of water and carbon dioxide and of the lower temperature. 

 On the other hand, temperature changes in the higher mountains are 

 both of considerable range and frequent in time ; the consequent 

 expansion and contraction quickly shatters a macrocrystalline rock. 

 Where water can penetrate the rock, as between mica foils, the dis- 

 ruptive freezing becomes especially effective. 



The rocks derived from extrusive magmas — rhyolites, andesites, 

 basalts — show the effect of these stresses ; areas of andesite are char- 

 acterized by heaps of conchoidal sharply angular stones riven by freez- 

 ing from the parent rock and often cast for a distance of two or three 

 feet.^° Such debris frequently form a true shingle, the fragments 

 overlapping each other and excluding all vegetation formed of higher 

 plants, as on the east slope of Mt. Tallac. 



Topographic form depends upon three factors : the character of 

 the rocks, their position or structure, and the subsequent changes 

 induced by weathering and erosion. The characters of the high moun- 

 tain rock species in the Sierra have been indicated ; since the mass 

 of the sedimentaries as compared to that of the magmatic rocks is so 

 small, the Sierra Nevada is properly considered as a range composed 

 of igneous rock. This description is all the more true when reference 

 is made to the region considered in this report. As a consequence 

 the high mountain region is practically devoid of that t.ype of struc- 

 ture produced by bedded rocks; nevertheless its structure is one of 

 the most interesting features of the geologj^ of the Sierra. 



