1 877.] 
Probable Origin and Age of the Sun. 315 
Mississippi is also more likely to afford us a fair test of or- 
dinary denudation, because, unlike the St. Lawrence and its 
tributaries, there are no great lakes in which the fluviatile 
sediment is thrown down and arrested on its way to the 
sea.” * 
Rough estimates have been made of the sediment carried 
down by some eight or ten European rivers ; and although 
those estimates cannot be depended upon as being anything 
like accurate, still they show that it is extremely probable 
that the European continent is being denuded at about the 
same rate as the American. 
I think we may safely assume, without the risk of any 
great error, that the average rate of sub-aerial denudation 
during past geological ages did not differ much from the 
present. The rate at which a country is lowered by 
sub-aerial denudation is determinedf not so much by the 
character of its rocks as by the sedimentary carrying power 
of its river systems. And this again depends mainly upon 
the amount of rain-fall, the slope of the ground, and the 
character of the soil and vegetation covering the surface 
of the country. And in respect of these we have no reason 
to believe that the present is materially different from the 
past. No doubt the average rain-fall during some past 
epochs might have been greater than at present, but there is 
just as little reason to doubt that during other epochs it 
might have been less than now. We may therefore conclude 
that about one foot of rock removed from the general surface 
of the country in 6coo years may be regarded as not very 
far from the average rate of denudation during past ages. 
But some of the cases we have given of great denuda- 
tion refer to comparatively small areas, and others to beds 
which form anticlinal axes, and which, as is well known, 
denude more rapidly than either synclinal or horizontal beds. 
We shall therefore — to prevent the possibility of over-esti- 
mating the length of time necessary to effedt the required 
amount of denudation — assume the rate to have been double 
the above, or equal to one foot in 3000 years. 
To lower the country one mile by denudation would there- 
fore require, according to the above rate, about 15 million 
years ; but we have seen that a thickness of rock more than 
equal to that must have been swept away since the Carbon- 
iferous period. For even during the Carboniferous period 
itself more than a mile in thickness of strata in many places 
was removed. Again, there can be no doubt whatever that 
* Student’s Manual of Geology, p. 91 (second edition), 
f See Climate and Time, p. 334. 
