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,