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THE BED-ROCK SURFACE AND THE CHANNELS. 339 
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 smoothéd 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 
flow 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 débris, unless there are ocean currents in 
