snow could ever break them up and wear them down so that they would become
like the rounded hills of the Appalachian Mountain chain, yet this is what
will happen unless nature's ways suddenly change to something which they
are not now. A visitor to the Grand Cañon of the Colorado sees a
magnificent chasm over a mile in depth and two hundred miles long which
has actually been carved through layer after layer of solid rock by the
rushing torrents of the river. Perhaps it is easier to estimate the
geological effects of a river in such a case as Niagara. Here we find a
deep gorge below the famous falls, which runs for twenty miles or so to
open out into Lake Ontario. The water passing over the brim of the falls
wears away the edge at a rate which varies somewhat according to the
harder or softer consistency of the rocks, but which, since 1843, has
averaged about 104 inches a year. Knowing this rate, the length of the
gorge, and the character of the rocky walls already carved out, the length
of time necessary for its production can be safely estimated. It is about
30,000 to 40,000 years, not a long period when the whole history of the
earth is taken into account. A similar length of time is indicated for the
recession of the Falls of St. Anthony, of the Mississippi River, an
agreement that is of much interest, for it proves that the two rivers
began to make their respective cuttings when the great ice-sheet receded
to the north at the end of the Glacial epoch.

What has become of the masses washed away during the formation of these
gorges? As gravel and mud and silt the detritus has been carried to the
still waters of the lower levels, to be laid down and later solidified
into sandstone and slate and shale. All over the continents these things
are going on, and indefatigable forces are at work that slowly but surely
shear from the surface almost immeasurable quantities of earth and rock to
be transported far away. In some instances it is possible to find out just
how much effect is produced in a given period of time, especially in the