Domes and Dome Structure. 217 



ment of division-planes approximately parallel to the 

 surface. 



While it is possible that all these processes are con- 

 cerned in the production of the structure, I regard it as 

 more probable that some one cause is dominant. The 

 data at hand seem to me not to warrant a confident selec- 

 tion from the three suggested, but if the truth lies among 

 them, there should be little difficulty in obtaining addi- 

 tional facts of crucial character. Certain domes, some of 

 which I saw. at a distance, are supposed to be outside the 

 area of Pleistocene glaciation. If they exhibit the char- 

 acteristic structure, and are really extraglacial, their char- 

 acters can not plausibly be ascribed to secular changes of 

 cHmate. It should be possible to determine the relation 

 of weathering to the structure by petrographic study of 

 outer and inner layers at such a locality as that shown in 

 plate XXXI, figure i, where glacial erosion has exposed 

 a fresh section. 



Explanation of Rounding. — The view in plate XXXI, 

 figure I, was selected as an illustration of dome structure 

 because the plates and partings of the structure are there 

 shown in natural section. In the making of that section 

 the dominant erosional process was glacial attrition or 

 grinding. While this process has been of great impor- 

 tance in the sculpture of the higher parts of the Sierra, it 

 is probably second in rank to glacial plucking or quarry- 

 ing ; and glacial degradation as a whole has been small in 

 comparison with subaerial degradation. In glacial pluck- 

 ing and in most phases of subaerial erosion the most active 

 attack on rock traversed by dome structure is by way of 

 the partings, and the broad outer faces of the granite 



