chap. x. Formation of Valleys. 301 



agent, and that its chief tendency is to widen the 

 valleys; whilst torrents and rivers tend to deepen them, 

 and to remove the wreck of the sea's destroying action. 

 As the waves have more power, the more open and ex- 

 posed the space may be, so will they always tend to 

 widen more and more the mouths of valleys compared 

 with their upper parts : hence, doubtless, it is, that 

 most valleys expand at their mouths, — that part, at 

 which the rivers flowing in them, generally have the 

 least wearing power. 



When reflecting on the action of the sea on the 

 land at former levels, the effect of the great waves, 

 which generally accompany earthquakes, must not be 

 overlooked : few years pass without a severe earthquake 

 occurring on some part of the west coast of South 

 America ; and the waves thus caused have great power. 

 At Concepcion, after the shock of 1835, I saw large 

 slabs of sandstone, one of which was six feet long, three 

 in breadth, and two in thickness, thrown high up on 

 the beach ; and from the nature of the marine animals 

 still adhering to it, it must have been torn up from a 

 considerable depth. On the other hand, at Callao, the 

 recoil-wave of the earthquake of 1746 carried great 

 masses of brickwork, between three and four feet square, 

 some way out seaward. During the course of ages, the 

 effect thus produced at each successive level, cannot 

 have been small ; and in some of the tertiary deposits 

 on this line of coast, I observed great boulders of 

 granite and other neighbouring rocks, embedded in 

 fine sedimentary layers, the transportal of which, except 

 by the means of earthquake-waves, always appeared to 

 me inexplicable. 



