314 Probable Origin and Age of the Sun. [July, 
know the probable rate at which the denudation took place. 
If we had a means of forming even the roughest estimate of 
the probable average rate of sub-aerial denudation during past 
ages, we should be enabled thereby to assign approximately 
an inferior limit to the age of the stratified rocks. We 
could then tell, at least, whether the amount of sub-aerial 
denudation known to have been effected during past geolo- 
gical ages could have been accomplished within 20 million 
years or not, and this is about all with which we are at pre- 
sent concerned. And if it can be proved that a period of 
20 millions of years is much too short to account for the 
amount of denudation known to have taken place, then it is 
certain that the gravitation theory cannot explain the origin 
and source of the sun’s heat. 
A very simple and obvious method of determining the 
present mean rate of sub-aerial denudation was pointed out 
several years ago,* viz., that the rate of denudation must be 
equal to the rate at which the materials are carried off the 
land into the sea. But the rate at which the materials are 
thus abstracted is measured by the rate at which sediment is 
carried down by our rivers. Consequently, in order to deter- 
mine the present rate of sub-aerial denudation, we have only 
to ascertain the quantity of sediment annually carried down 
by the river systems. 
Very accurate measurements have been made of the 
quantity of sediment carried down into the Gulf of Mexico 
by the River Mississippi, and it is found to amount to 
7,474,000,000 cubic feet. The area drained by the river is 
1,224,000 square miles. Now 7,474,000,000 cubic feet re- 
moved from 1,224,000 square miles of surface is equal to 
1 -4566th of a foot off the surface per annum, or 1 foot in 
4566 years. The specific gravity of the sediment is taken 
at i*g, and that of the rock at 2*5 ; consequently the amount 
removed is equal to 1 foot of rock in about 6000 years. For 
many reasons there are few rivers better adapted for affording 
us a fair average of the rate of sub-aerial denudation than 
the Mississippi. In reference to the above I may here quote 
the words of Sir Charles Lyell : — “There seems,” he says, 
“ no danger of our over-rating the mean rate of waste by 
selecting the Mississippi as our example, for that river drains 
a country equal to more than half the continent of Europe, 
extends through 20 degrees of latitude, and therefore through 
regions enjoying a great variety of climate, and some of its 
tributaries descend from mountains of great height. The 
* Phil. Mag., May, 1868; Feb., 1867. Climate and Time, chap. 20. See 
also Trans. Geol. Soc. of Glasgow, vol. iii. 
