In the basin of the Colorado are found in perfection all the extraordinary conditions that are needed to bring forth mammoth canyons. The headwaters of all the important tributaries are _invariably in the highest regions_ and at a long distance from their mouths, so that the flood waters have many miles of opportunity to run a race with the comparatively feeble erosive forces of desert lands. The main stream-courses are thus in the lower arid regions and in sedimentary formations, while their water-supply comes from far away. The deepest gorges, therefore, will be found where the rainfall is least, unless diminishing altitude interferes. Thus the greatest gorge of the whole basin, the Grand Canyon, is the one farthest from the sources of supply, and in the driest area, but one, of the whole drainage system. It ends abruptly with the termination of the high arid plateau which made it possible, but had this plateau extended farther, the Grand Canyon would also have extended a similar distance. It is plain then that the cutting of these canyons depends on the amount of water (snow is included) which may fall in the high mountains, the canyons themselves being in the drier districts. It is also clear that if, by some chance, the precipitation of the high sources should increase, the corrasion of the stream-beds in the canyons would likewise increase and outrun with still greater ease the erosion of their immediate surroundings. On the other hand, if the precipitation in the arid surroundings should increase, the wearing down of the side walls would for a time—till covered by débris and vegetation—go on more rapidly till, instead of Canyons of the Colorado River type, there would be deep, sharp valleys, or wide valleys, according to the amount of difference between the precipitation of the low lands and the high. Where the two were nearly the same, that is, a balance of precipitation,[6] the slopes might be rounded and verdure-clad, though this would depend on the _amount_ of precipitation. On lower Snake River a change seems to be going on. The former canyon-cliffs are covered by débris and vegetation, but in places the old dry cliff-lines can be discerned beneath like a skeleton. The precipitation there has not been great enough to destroy the old lines—only enough to mask them. [6] There could be a balance of precipitation and still very little snow- or rainfall, or they might be very great. Pinnacle in the Canyon the Chelly. Pinnacle in the Canyon the Chelly. About 1500 feet high. It is much wider from the side. Photograph by BEN WITTICK. Bad Lands on Black’s Fork of Green River. Bad Lands on Black’s Fork of Green River. Photograph by U.S. Geol. Survey.