urface is unchanged for years. I once saw some wagon tracks that were
made by our party three years before. From peculiar circumstances I was
able to identify them.

** Robert Brewster Stanton explained this very clearly in his
investigations for the Canadian Pacific Railway into the causes of
land-slides on that line.

In the basin of the Colorado are found in perfection all the
extraordinary conditions that are needed to bring forth mammoth canyons.
The headwaters of all the important tributaries are INVARIABLY IN THE
HIGHEST REGIONS and at a long distance from their mouths, so that the
flood waters have many miles of opportunity to run a race with
the comparatively feeble erosive forces of desert lands. The main
stream-courses are thus in the lower arid regions and in sedimentary
formations, while their water-supply comes from far away. The deepest
gorges, therefore, will be found where the rainfall is least, unless
diminishing altitude interferes. Thus the greatest gorge of the whole
basin, the Grand Canyon, is the one farthest from the sources of supply,
and in the driest area, but one, of the whole drainage system. It ends
abruptly with the termination of the high arid plateau which made it
possible, but had this plateau extended farther, the Grand Canyon would
also have extended a similar distance. It is plain then that the cutting
of these canyons depends on the amount of water (snow is included) which
may fall in the high mountains, the canyons themselves being in
the drier districts. It is also clear that if, by some chance, the
precipitation of the high sources should increase, the corrasion of the
stream-beds in the canyons would likewise increase and outrun with still
greater ease the erosion of their immediate surroundings. On the other
hand, if the precipitation in the arid surroundings should increase, the
wearing down of the side walls would for a time--till covered by debris
and vegetation--go on more rapidly till, instead of Canyons of the
Colorado River type, there would be deep, sharp valleys, or wide
valleys, according to the amount of difference between the precipitation
of the low lands and the high. Where the two were nearly the same,
that is, a balance of precipitation,* the slopes might be rounded and
verdure-clad, though this would depend on the AMOUNT of precipitation.
On lower Snake River a change seems to be going on. The former
canyon-cliffs are covered by debris and vegetation,