26 STUDIES, SCIENTIFIC AND SOCIAL chap. 



from the rock-falls that actually occur every year, but 

 from the numerous places where great flakes or jutting 

 blocks can be observed in every stage of detachment from 

 the parent rock. These fallen masses, however large, are 

 at once subject to fresh causes of decay. Almost all their 

 surfaces are exposed to atmospheric action or to expan- 

 sion and contraction by heat and cold. Every crack and 

 cranny is seized upon by vegetation — first the lowly herb, 

 then the shrub, later the tree, whose roots penetrate the 

 minutest fissure, eat away the surface, or even split off 

 portions by the power of growth. And though in the life 

 of a man a block may seem unchanged, in a few thousand 

 years it may have entirely disappeared ; and such a lapse 

 of time probably bears a less proportion to the period 

 occupied by the valley's formation, than does a single 

 hour to the life of a man. 



It has now, I think, been shown that the two remark- 

 able valleys here described do not owe their exceptional 

 physical features to any catastrophic or unusual mode of 

 origin. Every characteristic they possess is fully ex- 

 plained by that simple theory of earth sculpture by 

 atmospheric agency which has been found applicable to 

 the solution of similar problems in all other parts of the 

 world. This theory does not, of course, imply that 

 subterranean movements have no part in determining the 

 direction or hastening the excavation of some valleys, but 

 only that they have in no case produced the valleys them- 

 selves. Many examples can be pointed out in which 

 valleys follow for a certain distance lines of fault, of the 

 junction of different strata, or of the fractured summit of 

 an anticlinal ; but the explanation of these cases is, pro- 

 bably, that during elevation above the sea, wave-action 

 produced slight hollows along these several lines of weak- 

 ness, and that the hollows thus formed were occupied by 

 the primitive rivulets as their line of least resistance 

 when flowing towards the ocean. But these cases are 

 very few as compared with those of valleys which pay no 

 regard whatever to the geological features of the under- 

 crust, but which cross over faults and outcrops, and break 

 through transverse hills and mountain ranges, as if the 



