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.** 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.

* Just as wheat flour getting wet on the surface protects the portion
below from dampness. The rainfall is often so slight, also, that a
s