16S 
ISLAND LIFE. 
[fart I. 
Let us then inquire first into the character of the evidence 
we should expect to find of such changes of climate, if they 
have occurred ; we shall then he in a better position to estimate 
at its proper value the evidence that actually exists, and, after 
giving it due weight, to arrive at some conclusion as to the 
theory that best explains and harmonises it. 
Effects of Denudation in destroying the evidence of remote 
Glacial Epochs . — It may be supposed, that if earlier glacial 
epochs than the last did really occur, we ought to meet with 
some evidence of the fact corresponding to that which has 
satisfied us of the extensive recent glaciation of the northern 
hemisphere ; but Dr. Croll and other writers have ably argued 
that no such evidence is likely to be found. It is now generally 
admitted that .sub-aerial denudation is a much more powerful 
agent in lowering and modifying the surface of a country than 
was formerly supposed. It has in fact been proved to be so 
powerful that the difficulty now felt is, not to account for the 
denudation which can be proved to have occurred, but to explain 
the apparent persistence of superficial features which ought long 
ago to have been destroyed. 
A proof of the lowering and eating away of the land-surface 
which every one can understand, is to be found in the quantity 
of solid matter carried down to the sea and to low grounds by 
rivers. This is capable of pretty accurate measurement, and 
has been so measured for several rivers, large and small, in 
different parts of the world. The details of these measure- 
ments will be given in a future chapter, and it is only necessary 
here to state that the average of them all gives us this result — 
that one foot must be taken off the entire surface of the land 
each 3,000 years in order to produce the amount of sediment 
and matter in solution which is actually carried into the sea. 
To give an idea of the limits of variation in different rivers it 
may be mentioned that the Mississippi is one which denudes 
its valley at a slow rate, taking 6,000 years to remove one foot; 
while the Po is the most rapid, taking only 729 years to do the 
same work in its valley. The cause of this difference is very 
easy to understand. A large part of the area of the Mississippi 
