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Sierra Club Bulletin. 



large areas the granite is divided by such joint systems into 

 angular blocks (plate XXXII, figure i ), and in these areas 

 the peculiar domes do not appear. I thought at one time 

 that the two types of partings might be correlated with 

 certain rock types, but this tentative generalization was 

 afterward completely disproved. There are at least three 

 prominent and broadly exposed types of granite in the 

 Sierra which exhibit dome structure, and each of these is 

 also characterized in some different locality by plane 

 joints. It is easy to understand that the existence of 

 either system of partings within the rock might, by facili- 

 tating the relief of strain, prevent the development of the 

 other system, so that their mutual exclusiveness gives no 

 indication of their relative age. But there is independent 

 reason for assigning a greater age to the plane-joint sys- 

 tems. The dome structure, being conditioned by surface 

 forms, is in each locality more recent than the topographic 

 features; but the topographic sculpture is superposed on 

 the systems of plane joints. Minor details of form show 

 the influence of joint structure, but features of the rank 

 of hill and valley are notably independent, their trends 

 making all angles with the strikes of joint systems. 



Joints and other division-planes are aids to erosion, 

 whether the process be subaerial or glacial. When in 

 ordinary jointing several sets of division-planes intersect 

 and the rock is separated into blocks, weathering and 

 transportation are both facilitated. In dome structure 

 there is but a single set of division-planes, and the broad 

 rock-plates are almost as resistant as a continuous mass. 

 It results that the granite masses divided only by dome 

 structure tend to survive general degradation, and often 

 to stand forth as prominent hills. 



