163 



ISLAND LIFE. 



[part 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 be 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 tlie evidence of remote 

 Glacicd Epochs. — It may be supposed, that if earher 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. CroU 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 foimd 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 ah 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 



