VOLCANIC FOKCE OPPOSED TO PUNNING WATER. 115 
scend from mountains of great height. The Mississippi is 
also more likely to afford us a fair test of ordinary denuda¬ 
tion, because, unlike the St. Lawrence and its tributaries, 
there are no great lakes in which the fluviatile sediment is 
thrown down and arrested in its way to the sea. In striking 
a general average we have to remember that there are large* 
deserts in which there is scarcely any rainfall, and tracts 
which are as rainless as parts of Peru, and these must not be 
neglected as counterbalancing others, in the tropics, where 
the quantity of rain is in excess. If then, argues Mr. Geikie, 
we assume that the Mississippi is lowering the surface of the 
great basin which it drains at the rate of 1 foot in 6000 years, 
10 feet in 60,000 years, 100 feet in 600,000 years, and 1000 
feet in 6,000,000 years, it would not require more than about 
4,500,000 years to wear away the whole of the Korth Amer¬ 
ican continent if its mean height is correctly estimated by 
Humboldt at 748 feet. And if the mean height of all the 
land now above the sea throughout the globe is 1000 feet, 
as some geographers believe, it would only require six mil¬ 
lion years to subject a mass of rock equal in volume to the 
whole of the land to the action of subaerial denudation. It 
may be objected that the annual waste is partial, and not 
equally derived from the general surface of the country, in¬ 
asmuch as plains, water-sheds, and level ground at all heights 
remain comparatively unaltered; but this, as Mr. Geikie has 
well pointed out, does not affect our estimate of the sum to¬ 
tal of denudation. The amount remains the same, and if we 
allow too little for the loss from the surface of table-lands 
w^e only increase the proportion of the loss sustained by the 
sides and bottoms of the valleys, and vice versa,^ 
Antagonism of Volcanic Force to the Levelling Power of 
Eunning Water. —In all these estimates it is assumed that 
the entire quantity of land above the sea-level remains on 
an average undiminished in spite of annual waste. Were it 
otherwise the subaerial denudation would be continually less¬ 
ened by the diminution of the height and dimensions of the 
land exposed to waste. Unfortunately we have as yet no 
accurate data enabling us to measure the action of that force 
by which the inequalities of the surface of the earth’s crust 
may be restored, and the height of the continents and depth 
of the seas made to continue unimpaired. I stated in 1830 
in the “ Principles of Geology,”f that running water and vol¬ 
canic action are two antagonistic forces; the one laboring 
* Trans. Geol. Soc. Glasgow., vol. iii., p. 169. 
t 1st ed., chap, x., p. 167, 1830; see also 10th ed., vol. i., chap, xv., p. 
327. 1867. 
