chasm when compared with the depth, on the theory that the forces
that work laterally have been more continuously active than has the
force that cuts downward. There is convincing evidence that the
whole region has been many times lifted up since the cutting began,
so that the river has had its active and passive stages. As its
channel approached the sea level, its current would be much less
rapid, and the downward cutting would practically cease, till the
section was elevated again. But all the time the forces working
laterally would be at work without interruption, and would thus gain
on their checked brethren of the river bottom.

There is probably another explanation of what we see here. Apart
from the mechanical weathering of the rocks as a result of the arid
climate, wherein rapid and often extreme changes of temperature take
place, causing the surface of the rocks to flake or scale off, there
has doubtless been unusual chemical weathering, and this has been
largely brought about by the element of iron that all these rocks
possess. Their many brilliant colors are imparted to them by the
various compounds of iron which enter into their composition. And
iron, though the symbol of hardness and strength, is an element of
weakness in rocks, as it causes them to oxidize or disintegrate more
rapidly. In the marble canon, where apparently the rock contains no
iron, the lateral erosion has been very little, though the river has
cut a trench as deep as it has in other parts of its course.

How often I thought during those days at the canon of the geology of
my native hills amid the Catskills, which show the effects of
denudation as much older than that shown here as this is older than
the washout in the road by this morning's shower! The old red
sandstone in which I hoed corn as a farm-boy dates back to Middle