RELATION OP STRUCTURE TO TOPOGRAPHY 31 



Where the granite is divided by a solitary joint into distinct masses, the 

 dome structure of each mass is independent of the structure developed in 

 its neighbor (figure 1). The curves of the dome structure do not cross the 

 joint plane, and are thus shown to be newer than the joint. This phenom- 

 enon is not favorable to the view that the structure is original. 



These considerations, as they were developed gradually in the field, led 

 me to abandon altogether the hypothesis that the structure was developed 

 either in the original constitution of the granite or at some early stage in 

 its history, and to adopt the alternative view that it followed the produc- 

 tion of the principal topographic features and was in some way condi- 

 tioned by the surface forms. 



Relation of Dome Structure to Plane Jointing 



The dome structure appears to have been developed only in massive 

 rock ; that is to say, it is not found in rock which is divided by systems 

 of parallel plane joints. Through large areas the granite is divided by 

 such joint systems into angular blocks (plate 4, figure 1), 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 char- 

 acterized in some different locality by plane joints. It is easy to under- 

 stand that the existence of either system of partings within the rock 

 might, by facilitating 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 systems. The dome structure, being con- 

 ditioned by surface forms, is in each locality more recent than the topo- 

 graphic 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 pro- 

 cess 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 



