DILATATION FROM UNLOADING 33 



stress became operative. Pari passu with this release of expansive stress 

 there was cooling, and the effect of the cooling was to diminish expan- 

 sive stress; and the result may have been complicated by other stress 

 factors. So long as the pressure of superjacent material was great, the 

 equilibrium of stresses was approximately adjusted by flowage ; but as 

 the descending surface of degradation approached the granite, flowage 

 diminished, and it ultimately ceased. The final adjustment was by 

 change of volume, the change being contraction if lowering of tempera- 

 ture was a more important factor than relief from load, and expansion 

 if relief from load was the more important factor. In the latter case 

 (which I regard as the more probable) the parts of the granite succes- 

 sively exposed at the surface were in a condition of potential expansion, 

 or tensile strain, and that strain would be relieved by the separation of 

 layers through the development of division planes approximately par- 

 allel to the surface. 



While it is possible that all these processes are concerned in the pro- 

 duction 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 con- 

 fident selection from the three suggested, but if the truth lies among 

 them, there should be little difficulty in obtaining additional 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 characteristic structure, and are really extraglacial, their 

 characters can not plausibly be ascribed to secular changes of climate. 

 It should be possible to determine the relation of weathering to the 

 structure by petrographic study of outer and inner laj^ers at such a 

 locality as that shown in plate 3, figure 1, where glacial erosion has 

 exposed a fresh section. 



Explanation of Rounding 



The view in plate 3, figure 1, 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 importance in the sculpture of the higher parts of the 

 Sierra, it is probably second in rank to glacial plucking or quarrying ; 

 and glacial degradation as a whole has been small in comparison with 

 subaerial degradation. In glacial plucking and in most phases of sub- 

 aerial 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 plates 

 are comparatively unaffected. The removal of the rock is essentially 



