THE 



GEOLOGICAL MAGAZINE. 



Ho. XXIIL— MAY, 1866. 



I. — On the Origin of Yalleys. 



By G. Poultjtt Sckope, Esq., M.P., F.E.S., F.G.S., etc. 



CONTROVEESY appears to be carried on between geologists 

 as to tbe causes of the excavation of valleys, and the degree 

 in which marine denudation on the one hand, or the eroding power 

 of rain and rivers on the other, have been concerned in the process. 

 To this controversy Mr. D. Mackintosh has contributed two papers, 

 in recent numbers of this Magazine.' I am desirous of recalling to 

 the recollection of its readers certain facts which demonstrate, beyond 

 the possibility of doubt, how vast is the error of those writers who, 

 like Mr. Mackintosh, speak of " the impotence of rain as a denuding 

 agent." ^ 



Few persons, I think, can have watched the effect of a single 

 heavy shower of rain, lasting perhaps less than an hour, on an 

 exposed surface of earth, sand, clay, or gravel, or even on a hard 

 road, without being struck by the wearing power of the direct fall of 

 the rain-drops, and the escape of the water towards the lowest ac- 

 cessible levels, carrying with it the solid particles it has abraded from 

 the surface. The general surface is more or less lowered, and grooves 

 and channels formed, opening out into miniature ravines and valleys. 

 And no one can doubt that the multiplication of such influences 

 through an indefinite period of time must produce an amount of 

 superficial denxidation well worthy of consideration by the geologist 

 as a possible cause of many, if not most, of the depressions observable 

 in the subaerial surfaces of the earth. 



Examples on a larger scale are among others found in the newly- 

 formed valleys in Georgia and Alabama, described by Sir Charles 

 Lyell (Principles, ed. 1843, p. 204), where within twenty years 

 ravines have been formed, in what was, previously to the destruction 

 of the forests, an even plain, some of them 65 feet in depth and 180 

 in width. 



1 Vol.iii., p. 63, No. 20, for February, 1866 ; andiJ.,p. 155, No. 22, for April, 1866. 

 2 Page 155, supra. 

 VOL. in. — NO. xsin. 13 



