T. M. Reade—Denudation of the two Americas. 297 
sidered as applied to the whole world a very fair approxi- 
mation 
Let us pause to consider the meaning of all these figures, for 
unless they have a meaning which the mind and imagination 
can seize upon, the wearisome labors of collecting the data and 
making the computations were well nigh wasted. 
First, as regards the Mississippi, of which we possess the most 
relfable particulars.—I have shown that the estimate of the rate 
of denudation of its basin must be increased in round figures 
from 7 tO z57 Of a foot per annum® in consequence of the 
solid matter which is removed in solution. Is it not a striking 
an observant reasoner as a Geikie ora Croll. Thus we arrive at 
the first and not unimportant result which I promised from 
quantitative examination. Now mark, it is not that geologists 
Take up any text-book or manual and will find a chapter 
devoted to it and the whole process correctly explained, never- 
theless the quantity of matter r s not realized and 
never could have been except through laborious calculations. 
That being done it is easy to see how these great results occur. 
Examine the hardest rock and you will find it weathered, you 
will find it coated over with a crust, of a thickness varying 
with the time its surface has been exposed. This crust is com- 
tome of the constituents of the rock that remain after part have 
een removed by chemical action.t Examine the waste talus 
from some of the old quarries at Penmaenmawr and you will 
see that atmospheric agents have in the space of 30 years per- 
ceptibly affected a felstone rock that seems at first sight abso- 
lutely indestructible.t How much more then must they affect 
rocks of a more friable and soluble nature. I have shown that 
mrp ofa foot per annum is removed from the surface of Eng- 
land and Walesin a soluble form every year,$ say =, of an 
inch, so that in 30 years it would amount to #, of an inch. 
* This calculation as before explained takes no account of matters pushed along 
the bottom, they have not been determined with much accuracy and it is probable 
as Mr. A. Tylor has suggested there is more than has been estimated. This would 
Water, bu r. Al 
prove that the solvents of the rocks are largely organic acids existing in decaying 
vegetable matter. ‘On the geological action of the Humous Acids.” Proc. of the 
merican Assoe, for the Advancement of Science, Saratoga meeting, 18 9 
This stone is largely used for making “ setts” for street paving, sold under 
Ps name of ‘* Welsh granite setts,” and found to be the most lasting material for 
€ 
purpose. 
§ Chemical Denudation. 
