30 G. K. GILBERT — DOMES AND DOME STRUCTURE" 



According to the other theory the structure originated subsequently 

 to the form, and was caused by some reaction from the surface. Visit- 

 ing the Sierra in the summer of 1903, I had these two theories in mind, 

 and sought for characters by which they might be tested. 



The dome structure appears not to extend downward and inward in- 

 definitely, but to be limited to a somewhat shallow zone. The oppor- 

 tunities 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 by several writers, 

 has been undercut in the development of the 

 glacial trough of Ten ay a creek, so that its 

 northwestern part has fallen away. The curved 

 plates are there seen (figure 1) to occupy a 

 very moderate depth, probably not more than 

 50 feet, while beneath them the rock is mass- 

 ive, except as vertical shear planes or joints 

 have developed parallel to the flat face. In 



Figure 1.— Section of the Half . . . . . -i -i ,1 e ±t 



Dome, showing the relation of the another instance the estimated depth of the 

 dome structure to the surf ace and zone of dome structure is about the same, and 



in a third instance about 100 feet. This down- 



The section is at right angles j t •, ,• /. ,1 , 



to the side shown in plate 2. ward limitation of the zone appears to me 



favorable to the second theory. If the struct- 

 ure were original, one would expect to find it continuing indefinitely 

 downward and inward. 



The structure is not restricted to domes. In some districts the walls 

 of canyons, the sides of ridges, and the bottoms of trough valleys are 

 characterized b} r partings approximately parallel to the surface. (See 

 plate 3, figures 1 and 2.) These partings are not ordinary joints, but 

 are distinguished by curvature, and their forms of curvature are always 

 adjusted to the general shapes of the topography. In the last respect 

 they differ greatly from the structures produced by folding of strata. 

 The curves of folded strata are diversely related to topographic features. 

 A syncline may be found in a valley or on a hilltop, and an anticline 

 may have either of these positions ; but in dome structure each anti- 

 cline coincides with a summit and each syncline with a valley. If the 

 dome structure were original, we should expect that it would often be 

 traversed discordantly by superposed drainage and dissection, and the 

 fact of its accordance with features of dissection is therefore unfavorable 

 to the theory that it is an original structure. 



