brought the surface slowly downward. We can
imagine the ceaseless corrosion due to the ground water, bringing
about a constant though slow descent of the whole surface. Again and
again the streams, swinging to and fro under the guidance of the
underlying rock, or from the obstacles which the _debris_ they carried
imposed upon them, have crossed the surface. Now and then perhaps the
wearing was intensified by glacial action, for an ice sheet often cuts
with a speed many times as great as that which fluid water can
accomplish. On the whole, this exercise of the constructive
imagination in conceiving the history of a river valley is one of the
most enlarging tasks which the geologist can undertake.

Where in a river valley there are many lateral streams, and especially
where the process of solution carried on by the underground waters is
most effective, as compared with erosive work done in the bed of the
main river, we commonly find the valley sloping gently toward its
centre, the rivers having but slight steeps near their banks. On the
other hand, where, as occasionally happens, a considerable stream fed
by the rain and snow fall in its torrent section courses for a great
distance over high, arid plains, on which the ground water and the
tributaries do but little work, the basin may slope with very slight
declivity to the river margins, and there descend to great depths,
forming very deep gorges, of which the Colorado Canon is the most
perfect type. As instances of these contrasted conditions, we may
take, on the one hand, the upper Mississippi, where the grades toward
the main stream are gentle and the valley gorge but slightly
exhibited; on the other, the above-mentioned Colorado, which bears a
great tide of waters drawn from the high and relatively rainy region
of the Rocky Mountains across the vast plateau lying in an almost
rainless country. In this section nearly all the down-wearing has been
brought about in the direct path of the stream, which has worn the
elevated plain into a deep gorge during the slow uprising of the
table-land to its present height. In this way a defile nearly a mile
in depth has been created in a prevailingly rather flat country. This
gorge has embranchments where the few great tributaries have done like
work, but, on the whole, this river flows in an almost unbroken
channel, the excavation of which has been due to its swift,
pebble-bearing waters.

The tendency of a newly formed river is