272 



STRUCTURE COMMON TO ALL ROCKS. 



Fig. 245. — Dotted lines show material carried away by erosion. 



cliffs are circular, producing thus titanic amphitheatres, 100 or more 

 miles across, with cliff-benches 1,000 to 2,000 feet high.* How slow the 

 lifting of the strata in this region must have been is shown by the fact 



that the Green 



~Z'Z.'~\~~~'~~ ~~~„Iir~ River runs against 



the slope of the 

 strata, cutting its 

 canon deeper to 

 the edge of the 

 cliffs, as shown in 

 Fig. 244. Evidently the strata were lifted athwart the course of the 

 river, but so slowly that the river cut as fast as the strata lifted. 



Migration of Divides. — If the slopes on the two sides of a divide 

 are equal, the position of the divide remains stationary ; but if one 

 slope be steeper than the other, then by the greater erosion on the 

 steeper slope the divide will move steadily toward the gentler slope. 

 Thus, the rivers on the steeper slope continually increase their drainage 

 areas by appropriating from the other side. Examples of this may be 

 found in nearly all mountains, but especially in those of the monoclinal 

 type, and in all ridges, but especially in the case of hog-backs (Eigs. 

 240, 241). The recession of plateau cliffs is only an extreme case under 

 this law. 



5. Metamorphic and Granitic Rocks. — Sculptural forms in these 

 are usually irregular, and can not be reduced to a simple law. But, in 

 some cases, peculiar forms are traceable to peculiar structure. Thus, 

 for example : in and about the Yosemite Valley two kinds of forms are 

 found, viz., (a) perpendicular cliffs and towers and spires of the valley 

 itself ; and (b) rounded domes abundant in the high region about the 

 valley. The one is the result of a rough, imperfect vertical cleavage of 

 the rock ; the other of a concentric structure on a huge scale, usually 

 undetectable in the sound rock, 



but brought out by weathering. 

 This is shown in the diagram 

 (Fig. 246). 



6. The Kind of Agent. — It 

 is probable that the nature of 

 the erosive agent, whether as 

 rain and rivers or as snoiv and glaciers, also determines peculiar scen- 

 ic forms. It is probable that the former tend more to rounded sum- 

 mits and ridges and V-shaped gorges, the latter to sharp summits 

 {aiguilles) and comb-like divides and broad U-shaped valleys. 



Fig. 246. — Ideal Section showing Dome-structure. 

 Dotted line above shows original surface. 



* Dutton — High Plateaus of Utah, p. 19. 



