1908] on Ice and its Natural History. 271 



the rock debris which it finds in front of it in the valley. This 

 debris consists partly of matter which has been brought down by the 

 glacier at some previous time when its dimensions were greater, and 

 partly from the general wasting of rock on the faces of the moun- 

 tains, which form the sides of the valley. This is the source of the 

 detritus which gravitates surely into the valley, whether it be occupied 

 by glacier or river. The function of the river also is mainly distribu- 

 tiVe : it has very little share in the duty of procuring or shaping the 

 pebbles in its bed. The stones in the river-bed are furnished by the 

 chemical process of weathering on the rocks above. This loosens 

 and separates, and, in many cases, rounds the fragments in situ. 

 Gravity does the rest. It often helps the disintegration by fracturing 

 blocks, the support of which has been sapped by weathering along 

 the joints, and in all cases it brings the fragments down into the 

 valley so soon as they have lost their support. The river cleans the 

 stones that arrive in its bed. When a flood comes, it pushes on a 

 certain quantity of them, and during each flood the fragments suffer 

 some little attrition. But the greater the flood the more rapidly are 

 the pebbles hurried on towards the region where the stream becomes 

 a depositing rather than a moving agency. No single pebble can be 

 exposed to the wearing action of the flood over a greater distance 

 than that from the spot where it fell from the mountain into the 

 valley to the mouth or beginning of the delta of the stream ; the 

 motion is always downwards, always in the same direction, and takes 

 place only in floods. The low water of the stream rather protects 

 than wastes the stones. 



The real region of mechanical erosion and attrition is the 

 sea-shore. In comparison with it, every other is insigniflcant. The 

 power available and spent over it is enormous. This is provided by the 

 winds whicli blow over the opposing ocean. Their energy is ac- 

 cumulated in the form of undulations by the waves which they generate, 

 and it is carried without sensible dissipation by the waves until 

 tliey meet the sliallow water of the coast, where it is discharged 

 as breakers. These are the symbol of the conversion of the 

 potential energy of waves into the kinetic energy of currents. They 

 sweep the pebbles up the beach, and both return together by their 

 own weight. The work of that wave has been done ; but with the 

 extinction of one wave another follows, and so on for ever. 



The great potential energy residing in rocks which occupy an 

 elevated position, the imminence of its conversion into kinetic energy, 

 by any, even the least decay of the material, and the far-reaching 

 effects which the conversion can produce, have not received adequate 

 appreciation. 



A rock precipice is a seat of weathering, with gravity always at 

 its foot. When weathering has produced decay, and decay has re- 

 moved support, gravity claims the fragment as its own, and the result 

 is a talus. 



