when these deep-cut tributary canyons carried perennial streams, and
when the volume of the Colorado itself was many times greater,
possessing a multiplied corrasive power, while the adjacent areas
were about as arid as now. At such a time, perhaps, the Colorado
performed the main work of the inner gorge, the Kanab, and similar
affluents, their deep now rather evenly graded canyons. Such an
increase of volume, if we suppose the aridity to remain as now, could
have come about only by an increase of precipitation on the mountain
summits. During the Glacial Epoch, the Rocky Mountain summits were
considerably glaciated, the amount varying according to altitude and
latitude. The general topography of the Colorado River was about as
it is to-day, and the rainfall in the valleys probably nearly the
same, or at least only a little greater. In other words, the
conditions were those of to-day intensified. In summer, then, the
amount of water seeking outlet by these drainage channels to the sea
was enormously multiplied, and the corrasive power was
correspondingly augmented. When the ice caps finally began to
permanently diminish, the summer floods were doubtless terrific. The
waters of the Colorado now rise in the Grand Canyon, on the melting
of the snows in the distant mountains, from forty to one hundred
feet; the rise must then have amounted to from one hundred to four
hundred or more. The Kanab heads in two very high regions--the Pink
Cliffs and the Kaibab. Though probably not high enough to be heavily
glaciated they were high enough to receive an increased snowfall and
to hold it, or a portion of it, over from one year to another. Thus
the canyons having their origin on these high regions would be given
perennial streams, with torrential floods each summer, compared with
which anything that now comes down the Kanab would be a mere rivulet.
The summit of the Kaibab is covered with peculiar pocket-like basins
having no apparent outlets. These were possibly glacial sinks,