. Elements of geology : a text-book for colleges and for the general reader. Geology. 272 STRUCTURE COMMON TO ALL 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 th
. Elements of geology : a text-book for colleges and for the general reader. Geology. 272 STRUCTURE COMMON TO ALL 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.
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