104 



University of California Publications. 



[Geology 



exercised at most but a very minor directive control. The posi- 

 tion of the joints in Lakeview Hill is well shown in plate 21b, 

 but no precise general statements can yet be made concerning 

 their position throughout the region. The strongest jointage 

 seems to be that in a north-south direction, with two other sets 

 at right angles, the latter varying in position at different places. 

 This variation seems to be due to the excessive displacement of 

 crust blocks. The joints appear to be due to stresses set up in 

 the mass when at great depth, becoming planes of actual dis- 

 ruption upon the removal of pressure of superincumbent rock 

 under the activity of atmospheric forces. On the old erosional 

 surface the schists are much more minutely fractured than the 

 granite. This is due probably to the greater changes in stress 

 they have experienced. 



Weathering. — Upon the formation of the many fault-scarps 

 of the region the weathering of the rocks can have had no con- 

 trol. But an investigation of the mature erosional features pres- 

 ent upon the summits of the various ridges necessitates a knowl- 

 edge of differential weathering of the older rocks. The results of 

 atmospheric attack upon the younger rocks are of a much less 

 order of magnitude than those of diastrophism. The drainage 

 lines are almost invariably consequent. Of the Bedrock Complex 

 the granodiorite, with the exception of the contact zone and the 

 few small areas noted for the variations in the crystalline mass, 

 presents a quite uniform resistance to disintegration and erosion. 

 The fault-scarps show fresh rock within a few feet of the sur- 

 face, except at the buried bases or upon the older planes of dis- 

 location. On the ancient surface of erosion disintegration has 

 taken place to considerable depth and fresh rock in place fails 

 to show. Such level areas show many boulders of disintegration 

 lying upon a base of coarse granite sand and "rotten" rock. 

 The only spots in the granite that project above the general 

 level are those characterized by a large number of pegmatite 

 dikes. Such spots, however, are elevated but a few feet, as the 

 crumbling of the normal rock removes the support from thin peg- 

 matite intrusions and allows them to break down. But the schists 

 also form part of the surface of erosion yet remaining. When 

 present the ancient peneplain passes from the one rock to the 



