2J2 fee and its Natural History 



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 

 insignificant. The power available and spent over it is 

 enormous. It is provided by the energy of the winds which 

 blow over the opposing ocean, and are accumulated in the 

 form of undulations by the waves which they generate. This 

 energy is carried without sensible dissipation by the waves 

 until they meet the shallow water of the coast, and there it 

 is discharged in the form of 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 ex- 

 tinction 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, 



