To produce canyons like those of the Colorado, peculiar and unusual conditions are necessary. There must exist a vast region lying high above sea-level. This region must be arid. Out of it must rise separated mountain masses to such heights that they shall be well watered. These most elevated regions alone having abundant rain and snowfall, torrential streams are generated and poured down upon the arid wastes, where they persistently scour their beds, ploughing deep channels below the level of their surroundings. The perpendicularity of the walls of these channels, or canyons as they are called, depends on the volume and continuity of the flowing stream, on the aridity of the country through which they are cut, and on the rock-formation. A fierce and continuous torrent, where the rainfall is at the minimum, will so speedily outrival the forces of erosion that the canyon will have vertical walls. An example is seen in those frequent “mud” canyons found in arid regions, where some brook, having its source in highlands, cuts a channel through clay or dry earth with vertical sides, that stand for years. As long as the surface of the adjacent lands is undisturbed, it acts like a roof, throwing off the water that falls upon it into the main stream.[4] Thus the foundations of these walls are not assailed from _behind_, which is their weakest point. If the land surface is broken up, permitting the rains to soak in and saturate the clay or earth, the whole mass becomes softened and will speedily fall and slide out into the canyon.[5] The sides of all canyons in an arid region are more or less protected in the same way. That is, the rains fall suddenly, rarely continuously for any length of time, and are collected and conducted away immediately, not having a chance to enter the ground. Homogeneous sandstone preserves its perpendicularity better than other rocks, one reason being that it does not invite percolation, and usually offers, for a considerable distance on each side of the canyon, barren and impervious surfaces to the rains. Where strata rest on exposed softer beds, these are undermined from the front, and in this way recession is brought about. [4] Just as wheat flour getting wet on the surface protects the portion below from dampness. The rainfall is often so slight, also, that a surface is unchanged for years. I once saw some wagon tracks that were made by our party _three_ years before. From peculiar circumstances I was able to identify them. [5] Robert Brewster Stanton explained this very clearly in his investigations for the Canadian Pacific Railway into the causes of land-slides on that line. Looking down upon Glen Canyon. Looking down upon Glen Canyon. Cut through homogeneous sandstone. Photograph by J.K. HILLERS, U.S. Colo. Riv. Exp.