42 
The Colorado River 
being the chief land sculptors, it is evident that there will be a 
continual wearing down of the faces of the bounding cliffs. 
The softer beds will be cut away faster than the harder, and 
where these underlie the harder the latter will be undermined 
and fall. Every canyon is always 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 sedi¬ 
ment 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. 
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 be¬ 
low the level of their surroundings. The perpendicularity of 
the walls of these channels, or canyons as they are called, de- 
’ 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. 
