Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/290

Rh 2276 been tossed over waterfalls and rapids in its journey. He ascertained also that sand with a. mean diameter of grain of {5 mm. will ﬂoat in feebly agitated water; so that all sand of finer grain must remain angular. The same observer has noticed that sand composed of grains with amean diameter of min., and carried along by water moving at a rate of 1 metre per second, gets rounded, and loses about -17%,-6 of its weight in every kilometre travelled. The effects of abrasion upon the loose materials on a river bed are only a portion of the erosive work performed by the stream. Where the bottom is covered with a layer of debris only the upper portion of which is pushed onward by the current, the solid rock of the river channel is there protected from waste. But this protection is only local, and is apt to be swept away from time to time by violent ﬂoods. In those parts of a river channel where the current is strong enough to keep gravel and boulders moving along, these loose materials rub down the rocky bottom over which they are driven. As the shape and declivity of the channel vary constantly from point to point, with at the same time frequent changes in the nature of its rocks, this erosive action is liable to continual modiﬁcations. While there is a gene- ral abrasion of the whole bottom over which loose detritus is rolled, the erosion goes on most briskly in the numerous hollows and grooves along which chieﬂy these loose materials travel. Wherever an eddy occurs in which gravel is kept in gyration, erosion is much increased. The stones in their movement excavate a hole in the channel, while, as they themselves are reduced to sand and mud, or are swept out by the force of the current, their places are taken by fresh stones brought down by the stream. Such pot—Izoles, as they are termed, vary in size from mere cup—like depressions to huge cauldrons or pools. As they often coalesce by the giving way of the intervening walls between two or more of them they materially increase the deepening of the river bed. The shape of a river channel at any given point in its course depends mainly upon the nature and structure of the material out of which it has been eroded. One of the most characteristic features of streams, whether large or small, is the tendency to wind in serpentine curves when the angle of declivity is low and the general surface of the country tolerably level. This peculiarity may be observed in every stream which traverses a ﬂat alluvial plain. Some slight weakness in one of its banks enables the current to cut away a portion of the bank at that point. By degrees a concavity is formed, whence the water is deﬂected to the opposite side, there to break with increased force against the bank. Gradually a similar concavity is cut out on that side, and so, bending alternately from one side to the other, the stream is led to describe a most sinuous course across the plain. By this process, however, while the co11rse is greatly lengthened, the velocity of the current pro- portionately diminishes, until it may, before quitting the plain, become a lazy, creeping stream, which in England is bordered perhaps with sedges and willows. Such meandering courses are most frequent in soft alluvial plains, but they may also be found in solid rock if the original form of the surface was tolerably ﬂat. The windings of the gorges of the Moselle and Rhine through the table-land between Treves, Mainz, and the Siebengebirge form a notable illustration. Abrupt changes in the geological structure or lithological character of the rocks of a river-channel may give rise to waterfalls. In many cases waterfalls have originated in lines of escarpment over which the water at first found its way, or in the same geological arrangement of hard and soft rocks by which the escarpments themselves have been produced. In the ease of the falls of Niagara, for example, the stream may have fallen over the Queenstown cliff when the river first sought its way to the sea. But GEOLOGY [111. 1)v.‘...uc.- L. much more probably the escarpment and waterfall began to arise simultaneously and from the same geological structure. As the escarpment grew in height, it receded from its start- ing point. The river-ravine likewise crept backward, but at a more rapid rate, and the result has been that at present the clitf worn down by atmospheric causes stands at Queenstown, while the ravine extends 7 miles further inland, witha width of from ‘.200 to 400 yards, and a depth of from 200 to 300 feet. In this as in other cases the waterfall has cut its way backward up the course of its stream, and will continue to do so as long as the structure of the gorge continues as it is now——a thick bed or beds of limestone resting horizontally upon soft shales. The softer strata at the base are undermined, and slice after slice is cut off from the cliff over which the cataract pours. It has been estimated that at their present rate of recession the Niagara Falls must have taken about 35,000 years to cut their way backward and excavate the gorge between their present position and Queenstown. In other cases waterfalls have been produced by the existence of a harder and more resist- ing band or barrier of rock crossing the course of the stream, as, for instance, where the rocks have been cut by an intru- sive dyke or mass of basalt. In these and all other cases the removal of the ha rdermass destroys the waterfall, which, after passing into a series of rapids, is ﬁnally lost in the general abrasion of the river-channel. The n1ost marvellous river gorges in the world are those of the Colorado region in North America. The rivers there ﬂow in ravines thousands of feet deep and hundreds of miles long, through vast table- lands of nearly horizontal strata. The Grand Caﬁon (ravine) of the Colorado river is 300 miles long, and in some places more than 6000 feet in perpendicular depth. The country is hardly to be crossed except by birds, so profoundly has it been trenched by these numerous gorges. Yet the whole of this excavation has been effected by the erosive action of the streams themselves. In the excavation of a ravine, whether by the recession of a waterfall or of a series of rapids, the action of the river is more rapid than that of the atmospheric agents. The sides of the ravine consequently retain their vertical char- acter. But where, from the nature of the ground, the denuding action of rain, frost, and general weathering is more rapid than that of the river, a wider and opener valley is hollowed out, through which the river ﬂows, and from which it carries away the materials discharged into it from the surrounding slopes by the rain and brooks. 3. Reproductive Power.——Every body of water which when in motion carries along sediment drops it when at rest. The moment a current has its rapidity checked it is deprived of some of its carrying power, and begins to lose hold upon its sediment, which tends more and more to sink and halt on the bottom the slower the motion of the water. In the course of every brook and river there are frequent checks to the current. If these are examined, they will usually be found to be each marked by a more or less con- spicuous deposit of sediment. We may notice seven dif- ferent situations in which stream deposits or alluvium may be accumulated. ((1.) At the foot of Jllountain Slopes.—When a r11nnel or torrent descendsa steep declivity it tears down the soil and rocks, cutting a deep gash out of the side of the mountain. On reaching the level ground at the base of the slope the water, abruptly checked in its velocity, at once drops its coarser sediment, which gathers in a fan-shaped pile or cone, with the apex pointing up the water co11rse. Huge accumu- lations of boulders and shingle may thus be seen at the foot of such torrents,—the water flowing through them often in several channels which re-unite in the plain beyond. (1).) In River-becls.—This is characteristically shown in many of the rivers of Britain, by the accumulation of a bed