xix. ORIGIN OF VALLEYS. 489 



before,, it would in a remarkable degree tend to break down the 

 weakened roof and convert the channels into rugged valleys. 



Let us view this interesting subject under a third aspect: let 

 the same tract A, B, <?, D be supposed to sink gradually, as all 

 geologists admit to have happened in very many cases and over 

 very large spaces. The effect will, obviously, be felt successively 

 on the lines of successive sea-level, where the oceanic battery is 

 strongest; first at 10, then at 9, 8, &c., till the highest point is 

 reached by the water. 



Thus the whole surface will be in some degree wasted, and the 

 waste will be greatest on the lines of weakness, so that by this 

 process valleys must be occasioned, much as in the case of the land 

 rising. If the land neither rises nor falls, only the sea-coast line 

 and the sea-bed to a small depth are worn away. This effect is 

 quite different on different coasts, especially by reason of pebbles 

 and fragments drifted over the bed, by which it is ground down, 

 often to a nearly level surface. 



The origin of valleys, then, dates from the first elevation of 

 land above the sea; and nature is still engaged putting her last 

 touches to landscape by the daily action of heat and cold, rain 

 and rivers, snows and glaciers. All parts of the surface have been 

 wasted, all high parts are still undergoing abatement and waste ; 

 the most coherent parts, those capable of most resistance, suffer 

 the least or rather the slowest decay, and often remain as monu- 

 ments to mark how great has been the mass of earth removed from 

 around them. In this point of view the Matterhorn stands up 

 a melancholy pyramid among the crests of the Alps, like a broken 

 column surrounded by the ruins of an Eastern city, but with this 

 difference Man ' makes a solitude and calls it peace V while 

 Nature converts slow decay into perpetual beauty, and gives the 

 spoils of the Alps to fertilize the valleys of Switzerland. 



Applying this theory to the higher valleys of the Thames, and 

 taking up first the general view of atmospheric action on a surface 

 which had previously been rough hewn and broadly shaped by 

 internal movement and wave action; we may speedily be satisfied 

 that the uppermost of the Cotswold streams are still undergoing 

 modification of their principal surface-features from the ordinary 



f Tacitus, Vit. Agric. 



