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Show PBEFATORY NOTE. ix River Mountains on the north; in the Park Mountains on the east, and a number of tributaries come from the west. In their courses through the plateaus they run in canons. These canons are profound gorges corraded by the streams themselves. The "country rock" of the region is composed of sedimentary beds, nearly horizontal, as already stated. The region is also excessively arid, but the mountains that stand on the rim of the basin precipitate a large proportion of moisture, and in this manner streams of comparatively large volume head in the mountains, run through the plateaus and descend rapidly to the level of the sea, while the country through which they pass is very meagerly supplied with moisture. Under these conditions the profound gorges have been cut, as the process of canon cutting is more rapid than the lateral degradation of the country. In this manner every river runs in a deep gorge, and these canons further serve to divide the region into plateaus. The division is completed by lines of cliffs. These cliffs are bold escarpments hundreds and thousands of feet in altitude-grand steps by which the region is terraced. As the rivers corrade their channels more rapidly than general degradation is carried on, the stratigraphic conditions of the horizontal beds play a very important part in the method of degradation. Here degradation by surface erosion is less and degradation by sapping greater, and thus the walls of the canons retreat slowly in a series- of steps by this sapping process. Softer beds easily yield to atmospheric agencies, while harder beds resist and stand in bold escarpments. Thus by faults and monoclinal flexures, by deep canons, and by lines of cliffs the surface is cut into a great number of plateaus. In addition to the Plateaus proper, there are mountains due to upheaval and degradation. The more important of these are the Zufii Range, to the south, and the Uinta Range, far to the north. The Uinta Range is carved from a broad upheaval having an east and west axis. On either flank of the upheaval there is a line or zone of maximum displacement where the upheaval is by flexure or by faulting. Between these zones there.is a gentle flexure either way to the axis. Thus the upheaval is in part by general flexure from the axis as an anticlinal, and in part by faulting and monoclinal flexure, as in the Kaibab structure.. Again there are small areas which are |