THE BED-ROCK SURFACE AND THE CHANNELS. 



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grounds for believing that the uplift of great mountain-ranges has usually, if 

 not always, been the result of forces which, from their very nature, could 

 not have done otherwise than tear the surface of the region in which they 

 operated into forms of utter raggedness. An inspection of the position of 

 the stratified and eruptive masses as exhibited on the flanks and along the 

 axis of any great mountain-chain, such as the Alps or the Himalayas, either 

 on the spot or as represented in faithfully drawn geological sections, will 

 show at once that such a crushing, folding, and overturning of the rock 

 masses could not have taken place under the surface without leaving their 



impress upon it. 



The study of the numerous sections with which we have been furnished 



by the great mining operations, especially for coal, which have been carried 

 on in various parts of the world, shows us that in many instances great dis- 

 placements of the rocky strata have taken place, where at present no cor- 

 responding effects are visible on the surface. It cannot be denied that, in 

 such cases, erosive agencies must have smoothed down the previously much 

 broken surface, and there can be little doubt that the sea has been the chief 

 agent in such work as this. The larger the body of water which acts through 

 its movements on the surface, the greater the tendency to reduce projecting 

 masses to the common level. The sea, smooths over the surface, rivers fur- 

 row it, — that is, in those portions of their courses where they flow with 

 sufficient rapidity to act as erosive agents ; where, on the other hand, they 

 How over an almost level surface, they may cease to erode entirely, and 

 merely deposit all of the material which they have brought down from higher 

 regions, excepting perhaps the very finest. 



With the excavation of mountain valleys the ocean has had little to do ; 

 they are originated by orographic causes, and chiefly worked out in detail by 

 the action of running water. The form of the cross-section of any eroded de- 

 pression will, of course, vary at different points along its line; and will, in all 

 probability, have varied considerably, at any one point, at different periods. 



To recapitulate : the following is a condensed statement of the circum- 

 stances and conditions connected with the work of the erosive agents. 



First, the steeper the slope, the more rapid the erosion, provided this be 

 subaerial. The same cause is effective in the Case of marine erosion ; but the 

 latter will soon be brought to a stoppage, unless effectively aided by other 

 conditions. If the ocean waves beat against a high cliff, this will gradually 

 become protected by a fringe of debris, unless there are ocean currents in 



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