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.