widening at its top and sides, through the action of rain, frost, and wind, as well as deepening through the action of its flowing stream.  Erosion is this power which carves away the cliffs, and corrasion the one which saws at the bottom, the latter term, in geological nomenclature, meaning the cutting power of running water.* This cutting power varies according to the declivity and the amount of sediment carried in suspension.  It is plain that a stream having great declivity will be able to carry more sediment than one having little, and in a barren country would always be highly charged with sand, which would cut and scour the bed of the channel like a grindstone.  As Dutton says, a river cuts, however, only its own width, the rest of a canyon being the “work of the forces of erosion, the wind, frost, and rain.  That is why we have canyons.  The powers of erosion are far slower than those of corrasion, especially in an arid region, because they are intermittent.  Where rocks take a polish, as in Marble Canyon, the scouring and polishing work of corrasion is seen in the shining bright surface as far as the water rises.  This all belongs to the romance of the Water-gods, those marvellous land sculptors.

* The introduction of this subject may seem unnecessary to the general reader, but no just comprehension of this river can be reached without some knowledge of the forces creating its chasms.

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.* 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.**