70 ORIGIN OF VALLEYS. [Ch. "VI 



from large areas. That water has, in this case, been the denuding agent, 

 we may infer from the fact that the rocks have yielded according to their 

 different degrees of hardness ; the hard trap of the Wrekin, for example, 

 and other hills, having resisted more than the softer shale and sandstone, 

 so as now to stand out in bold relief.* 



Origin of valleys. — Many of the earlier geologists, and Dr. Hutton 

 among them, taught that " rivers have in general hollowed out their val- 

 leys." This is no doubt true of rivulets and torrents which are the feeders 

 of the larger streams, and which, descending over rapid slopes, are most 

 subject to temporary increase and diminution in the volume of their 

 waters. It must also be admitted that the quantity of mud, sand, and 

 pebbles constituting many a modern delta is so considerable, as to prove 

 that a very large part of the inequalities now existing on the earth's 

 surface are due to fluviatile action ; but the principal valleys in almost 

 every great hydrographical basin in the world, are of a shape and magni- 

 tude which imply that they have been due to other causes besides the 

 mere excavating power of rivers. 



Some geologists have imagined that a deluge, or succession of deluges, 

 may have been the chief denuding agency, and they have speculated on a 

 series of enormous waves raised by the instantaneous upthrow of continents 

 or mountain chains out of the sea. But even were we disposed to grant 

 such sudden upheavals of the floor of the ocean, and to assume that great 

 waves would be the consequence of each convulsion, it is not easy to ex- 

 plain the observed phenomena by the aid of so gratuitous an hypothesis. 



On the other hand, a machinery of a totally different kind seems capa- 

 ble of giving rise to effects of the required magnitude. It has now been 

 ascertained that the rising and sinking of extensive portions of the earth's 

 crust, whether insensibly or by a repetition of sudden shocks, is part of 

 the actual course of nature, and we may easily comprehend how the 

 land may have been exposed during these movements to abrasion by the 

 waves of the sea. In the same manner as a mountain mass may, in the 

 course of ages, be formed by sedimentary deposition, layer after layer, so 

 masses equally voluminous may in time waste away by inches ; as, for 

 example, if beds of incoherent materials are raised slowly in an open sea 

 where a strong current prevails. It is well known that some of these 

 oceanic currents have a breadth of 200 miles, and that they sometimes 

 run for a thousand miles or more in one direction, retaining a considera- 

 ble velocity even at the depth of several hundred feet. Under these cir- 

 cumstances, the flowing waters may have power to clear away each 

 stratum of incoherent materials as it rises and approaches the surface, 

 where the waves exert the greatest force ; and in this manner a volu- 

 minous deposit may be entirely swept away, so that, in the absence of 

 faults, no evidence may remain of the denuding operation. It may in- 

 deed be affirmed that the signs of waste will usually be least obvious 

 where the destruction has been most complete ; for the annihilation 



* Prestwich, Geol. Trans, second series, vol. v. pp. 452, 473. 



