140 GEOLOGY. 



ing away of rocks by flowing water. This erosive pow- 

 er of water, spoken of in 186, has produced some of 

 the most immense changes that have taken place in the 

 earth. You have seen what vast quantities of material 

 it supplies to be carried down by the rivers into lakes 

 and seas. The hills and mountains, though made by 

 foldings of strata and upheavals, generally received their 

 final shaping from denudation. Though erosion by wa- 

 ter is comparatively a slow process, yet, acting through 

 long periods, it produces great effects. It is supposed 

 that the Appalachian Mountains have lost from this 

 cause as much material as is now contained in them, or 

 even more, and the same can be said of some other 

 ranges of mountains. Indeed, in the changes of the 

 long ages occupied by the earth's foundation, the great 

 majority of the rocks have been built up from materials 

 furnished by denudation from pre-existing rocks, and 

 often denudation and reconstruction have succeeded 

 each other many times over in the case of the same ma- 

 terial. This same succession is seen to a considerable 

 extent even now. 



225. Faults -with Denudation. Denudation has often 

 united with faults in producing great changes. This 

 may be illustrated by Fig. 68, representing a section of 

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Fig. 68. 



a coal-field in England of several miles in extent. There 

 are four divisions, A, B, C, and D, made by faults which 

 have put the strata in them at different relative depths. 

 Thus, in A, the bed of coal, X, is 900 feet from the snr- 



