212 



Sierra Club Bulletin. 



The dome structure appears not to extend downward 

 and inward indefinitely, but to be limited to a somewhat 

 shallow zone. The opportunities for observing this fact 

 of distribution are not numerous, and, so far as I am 

 aware, are found only on what are called half-domes — 

 that is, domes that have been pared away on one side so 

 as to exhibit the structure in section. The Half-Dome at 

 the head of Yosemite Valley, which has been described 



In another instance the estimated depth of the zone of dome 

 structure is about the same, and in a third instance about 

 one hundred feet. This downward limitation of the zone 

 appears to me favorable to the second theory. If the 

 structure were original, one would expect to find it con- 

 tinuing indefinitely downward and inward. 



The structure is not restricted to domes. In some dis- 

 tricts the walls of canons, the sides of ridges, and the 

 bottoms of trough valleys are characterized by partings 

 approximately parallel to the surface. (See plate XXXI, 

 figures I and 2.) These partings are not ordinary joints, 

 but are distinguished by curvature, and their forms of 



Figure i. — Section of the 

 Half - Dome, Showing the 

 Relation of the Dome 

 Structure to the Surface 

 AND TO Joints. 



(The section is at right 

 angles to the side shown in 

 Plate XXX .) 



by several writers, has been un- 

 dercut in the development of the 

 glacial trough of Tenaya Creek, 

 so that its northwestern part has 

 fallen away. The curved plates 

 are there seen (figure i) to 

 occupy a very moderate depth, 

 probably not more than fifty 

 feet, while beneath them the rock 

 is massive, except as vertical 

 shear planes or joints have de- 

 veloped parallel to the flat face. 



