average of
one foot:

The Mississippi removes one foot in 6,000 years.
,, Ganges ,, ,, 2,358 ,,
,, Hoang Ho ,, ,, 1,464 ,,
,, Rhone ,, ,, 1,528 ,,
,, Danube ,, ,, 6,846 ,,
,, Po ,, ,, 729 ,,
,, Nith ,, ,, 4,723 ,,

Here we see an intelligible relation between the character of the river
basin and the amount of denudation. The Mississippi has a large portion of
its basin in an arid country, and its sources are either in forest-clad
plateaux or in mountains free from glaciers and with a scanty rainfall. The
Danube flows through Eastern Europe where the rainfall is considerably less
than in the west, while comparatively few of its tributaries rise among the
loftiest Alps. The proportionate amounts of denudation being then what we
might expect, and as all are probably under rather than over the truth, we
may safely take the average of them all as representing an amount of
denudation which, if not true for the whole land surface of the globe, will
certainly be so for a very considerable proportion of it. This average is
almost exactly one foot in three thousand years.[34] The mean altitude of
the several {105} continents has been recently estimated by Mr. John
Murray,[35] to be as follows: Europe 939 feet, Asia 3,189 feet, Africa 2020
feet, North America 1,888 feet, and South America 2,078 feet. At the rate
of denudation above given, it results that, were no other forces at work,
Europe would be planed down to the sea-level in about two million eight
hundred thousand years; while if we take a somewhat slower rate for North
America, that continent might last about four or five million years.[36]
This also implies that the mean height of these continents would have been
about double what it is now three million and five million years ago
respectively: and as we have no reason to suppose this to have been the
case, we are led to infer the constant action of that upheaving force which
the presence of sedimentary formations even on the highest mountains also
demonstrates.

We have already discussed the unequal rate of denudation on hills, valleys,
and lowlands, in connection with the evidence of remote glacial epochs (p.
173); what we have now to consider is, what becomes of all this denuded
matter, and how far the known rate of denudation affords us a measure of
the rate of d