172 ISLAND LIFE part i 



alternately and with several repetitions, within a space of 

 time which, geologically speaking, is very short indeed. 



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 the Evide7ice 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 diffi- 

 culty 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 it has been carefully measured for 

 several rivers, large and small, in diff*erent parts of the 

 world. The details of these measurements 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, on an average, 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 



