PAET IV 



THE TEANSPOBTATION AND RBDBPOSITION OF 



BOCK DEBRIS 



It rarely happens that more than a comparatively small pro- 

 portion of the products of disintegration and decomposition are 

 left to accumulate on the site of the parent rock. In most in- 

 stances a very considerable proportion, in some instances all, of 

 the debris is removed immediately, or soon after its formation, 

 and deposited elsewhere. A portion of this material is removed 

 in solution, as has already been described. A still larger portion 

 is transported mechanically, and it is to a discussion of the 

 method of this transportation that a few pages may now be 

 devoted with proJBit. 



The chief agencies involved in this transportation are grav- 

 ity, water, in either a solid or liquid form, and the wind. Un- 

 doubtedly the major part of the work is done by water, but as 

 the wind^s action is so frequently overlooked, and as, moreover, 

 the results thus produced are of more than ordinary interest 

 from the present standpoint, it may perhaps be well to dwell 

 upon this branch of the subject with considerable detail. 



(1) Action of Gravity. — Gravity, especially when aided by 

 the lifting power of frost, may locally exert no insignificant 

 influence. The tremendous power of landslides, or avalanches, 

 has, owing to their devastating effects, been impressed upon 

 us from the beginnings of written history. There are, how- 

 ever, other results, due to similar causes, but which, operating 

 on an almost microscopic scale, are wholly overlooked by the 

 ordinary observer, and the full meaning of which can be dis- 

 covered only when the results of years are taken into account. 

 Professor W. C. Kerr, in 1881, described^ the manner in which 

 the superficial cap of soil from the decomposition of micaceous 

 and hornblendic gneisses near Philadelphia had crept down 

 the inclined surface on which it rested, and the gradual attenu- 

 ation of the bands of variously colored debris of which it was 



^Am. Jour, of Science, 3d Series, Vol, XXI, p. S45. 



274 



