would do it in double that time. In the earlier part of its history,
when the rainfall was doubtless greater, and the river fuller, the
erosion must have been much more rapid than it is at present. The
widening of the canon was doubtless a slower process than the
downward cutting. But, as I have said, the downward cutting would
tend to check itself from age to age, while the widening process
would go steadily forward. Hence, when we look into the great abyss,
we have only to remember the enormous length of time that the aerial
and subaerial forces have been at work to account for it.

Two forces, or kinds of forces, have worked together in excavating
the canon: the river, which is the primary factor, and the meteoric
forces, which may be called the secondary, as they follow in the
wake of the former. The river starts the gash downward, then the
aerial forces begin to eat into the sides. Acting alone, the river
would cut a trench its own width, and were the rocks through which
it saws one homogeneous mass, or of uniform texture and hardness,
the width of the trench would probably have been very uniform and
much less than it is now. The condition that has contributed to its
great width is the heterogeneity of the different formations--some
hard and some soft. The softer bands, of course, introduce the
element of weakness. They decay and crumble the more rapidly, and
thus undermine the harder bands overlying them, which, by reason of
their vertical fractures, break off and fall to the bottom, where
they are exposed to the action of floods and are sooner or later
ground up in the river's powerful maw. Hence the recession of the
banks of the canon has gone steadily on with the downward cutting of
the river. Where the rock is homogeneous, as it is in the inner
chasm of the dark gneiss, the widening process seems to have gone on
much more slowly. Geologists account for the great width of the main