282 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



faulting' described by Becker as common along these fis- 

 sures or joint planes, although there are sheared zones in 

 the granite areas where the surfaces of movement show 

 slickensides, and unquestionable faults accompanied by 

 crushing and shearing are occasionally found. Dr. Becker 

 writes : — 



The granite and other granular rocks are intersected by fissures at short 

 intervals. Sometimes these appear to be without any regularity; but much 

 more often they are manifestly grouped in systems. A few square miles will 

 be particularly characterized by horizontal fissures, and here the granite 

 mountains will appear terraced. In such cases the vertical or diagonal fis- 

 sures are always present, but are less prominent than the horizontal partings. 

 In other areas the granite will be intersected by diagonal fissures usually dip- 

 ping at about 45° and striking between northwest and north. These dip in 

 both directions and divide the mass into horizontal columns. Much more 

 frequent than any other fissure systems are vertical partings, and these are 

 remarkably uniform in strike, almost always running either nearly north-north- 

 west or at right angles to this direction. Though the fissures in the granite never 

 gape and are usually only cracks, they are not mere partings or joints^ but 

 true fault planes. * * * In innumerable cases they show excellent slick- 

 ensides and very often the amount of dislocation can be demonstrated. The 

 'faulting has been attended by intense compression, so that at some points the 

 rock for an inch or two from the principal plane of motion has been sheared 

 to a mass resembling slate. This is comparatively rare, but it is often mani- 

 fest that, for some distance from a pronounced fissure, lines of weakness 

 have been developed in the rocks at intervals of a quarter of an inch, or 

 thereabouts. These lines grow less distinct as the distance from the fissure 

 increases, and disappear a foot or two from the fissure. 



Effect of Irregular Distribution. — A few notes may next be made with 

 reference to the effects of the fissure systems on the course of events in the 

 part of the Sierra here discussed. As has been mentioned, the distribution 

 of fissures is not uniform. In many places one system or the other is highly 

 and almost exclusively developed. Sometimes the rock is divided into very 

 regular prisms of indefinite length, and again these are cut by horizontal 

 partings into rectangular blocks. Finally, there are areas in which the mass 

 consists of polyhedral fragments. 



The rate at which decomposition and erosion will take place clearly 

 depends upon the frequency of the fissures of a system and the number of 

 fissure systems developed in a given locality, for both erosion and disinte- 

 gration vary with the amount of surface exposed per unit volume. Thus, 

 where the granite is shattered into fragments of small size, disintegration will 



1 1 refer here particularly to the systematic faulting described by Becker, in which 

 the upthrow is always on the east side, which, if true, would cause in the total a marked 

 elevation of the crest of the range. 



2 In a more recent paper. Bull. Geol. Soc. Amer., Vol. IV, 1893, pp. 13-90, Dr. Becker 

 (p. 73) comes to the conclusion that jointing and faulting are concomitant. 



V 



