Page:A Treatise on Geology, volume 2.djvu/35

 CHAP. VI. like so many hammers on the rocks, ploughing long channels on their surface, or whirling round and round in deep pits, especially beneath a fall, or where the current breaks into eddies over an uneven floor of stone. This is admirably seen at Stenkrith Bridge in Westmoreland, under the waterfalls about Blair Athol, and in North Wales, and, indeed, very commonly. Not infrequently, on mountain sides or tops, far from any stream or channel, phenomena somewhat similar occur, sometimes the effect of rain, sometimes, we may suppose, the remaining evidence of the former passage of running water, when the levels of the country were differently adjusted.

As the slopes are greatest in the upper parts of valleys (generally), and gradually flatten towards the sea, it is commonly observed, that, from all the upper parts of these valleys, rivers abstract large quantities of the finer matter, and in times of inundation, not a little of the coarser fragments of rocks; much of this is deposited in the lower ground, where the current is more tranquil, and generally (unless the river be very deep) slower. We must, indeed, suppose, that every where some wearing effect on its bed ard sides is produced by every river, even to its mouth; but this effect grows almost insensible far from the high ground which gives birth to the streams; and long ere we approach the estuary, the wide flat meadows, which fill the whole breadth of the valley for miles in length, show what a mass of materials has been drifted away from the higher ground. Finally, where the tides and freshes meet, the sediment of both is disposed to drop; and some rivers may be viewed as sending little or no sediment to the sea.

Thus the whole effect of drainage, including all the preliminary influences of the atmosphere, rain, springs, &c., is to waste the high ground, and to raise the low; to smooth the original ruggedness of the valley in which it flows, by removing prominences and filling up hollows; and notwithstanding the length of years that rivers have flowed, they have, in general, net yet