in the direction of the currents and eddies, have been enough to turn a stream now to one side now to another, until it has assumed its present meandering course. How easily this may be done can be instructively observed on a roadway or other bare surface of ground. When quite dry and smooth, hardly any depressions in which water would flow might be detected on such a surface. But after a heavy shower of rain, runnels of muddy water will be seen coursing down the slope in serpentine channels that at once recall the winding rivers of a great drainage-system. The slightest differences of level have been enough to turn the water from side to side. A mere pebble or projecting heap of earth or tuft of grass has sufficed to cause a bend. The water, though always descending, has only been able to reach the bottom by keeping the lowest levels, and turning from right to left as these guided it.

When a river has once taken its course and has begun to excavate its channel, only some great disturbance, such as an earthquake or volcanic eruption, can turn it out of that course. If its original pathway has been a winding one, it goes on digging out its bed which, with all its bends, gradually sinks below the level of the surrounding country. The deep and picturesque gorge in which the Moselle winds from Trèves to Coblenz has in this way been slowly eroded out of the undulating tableland across which the river originally flowed.

In another and most characteristic way, the shape of the ground and the nature and arrangement of the rocks over which they flow, materially influence rivers in the forms into which they carve their channels. The Rhone and the Niagara, for instance, though filtered by the lakes through which they flow, do not run far before plunging into deep ravines. Obviously such ravines cannot have been dug out by the same process of mechanical attrition whereby river-channels in general are eroded. Yet the frequency of gorges in river scenery shows that they cannot be due to any exceptional operation. They may generally be accounted for by some arrangement of rocks wherein a bed of harder material is underlain by one more easily removable. Where a stream, after flowing over the upper bed, encounters the decomposable bed below, it eats away the latter more rapidly. The overlying hard rock is thus undermined, and, as its support is destroyed, slice after slice is cut away from it. The waterfall which this kind of structure produces continues to eat its way backward or up the course of the stream, so long as the necessary conditions are maintained of hard rocks lying upon soft. Any change of structure which would bring the

hard rocks down to the bed of the channel, and remove the soft rocks from the action of the current and the dash of the spray, would gradually destroy the waterfall. It is obvious that, by cutting its way backward, a waterfall excavates a ravine.

The renowned Falls of Niagara supply a striking illustration of the process now described. The vast body of water which issues from Lake Erie, after flowing through a level country for a few miles, rushes down its rapids and then plunges over a precipice of solid limestone. Beneath this hard rock lies a band of comparatively easily eroded shale. As the water loosens and removes the lower rock, large portions of the face of the precipice behind the Falls are from time to time precipitated into the boiling flood below. The waterfall is thus slowly prolonging the ravine below the Falls. The magnificent gorge in which the Niagara, after its tumultuous descent, flows sullenly to Lake Ontario is not less than 7 miles long, from 200 to 400 yards wide, and from 200 to 300 feet deep. There is no reason to doubt that this chasm has been entirely dug out by the gradual recession of the Falls from the cliffs at Queenstown, over which the river at first poured. We may form some conception of the amount of rock thus removed from the estimate that it would make a rampart about 12 feet high and 6 feet thick extending right round the whole globe at the equator. Still more gigantic are the gorges or cañons of the Colorado and its tributaries in Western America. The Grand Cañon of the Colorado is 300 miles long, and in some places more than 6000 feet deep (Fig. 9). The country traversed by it is a network of profound ravines, at the bottom of which the streams flow that have eroded them out of the table-land.

ii. DEPOSITION OF MATERIALS BY RUNNING WATER. Permanent Records of River-Action.-If, then, all the streams on the surface of the globe are engaged in the double task of digging out their channels and carrying away the loose materials that arise from the decomposition of the surface of the land, let us ask ourselves what memorials of these operations they leave behind them. In what form do the running waters of the land inscribe their annals in geological history? If these waters could suddenly be dried up all over the earth, how could we tell what changes they had once worked upon the surface of the land? Can we detect the traces of ancient rivers where there are no rivers now?

From what has been said in this lesson it will be evident that

in answer to such questions as these, we may affirm that one unmistakable evidence of the former presence of rivers is to be found in the channels which they have eroded. The gorges, rocky defiles, pot-holes, and water-worn rocks which mark the pathway of a stream would long remain as striking memorials of the work of running water. In districts, now dry and barren, such as large regions in the Levant, there are abundant channels (wadies) now seldom or never occupied by a stream, but which were evidently at one time the beds of active torrents.

Alluvium. But more universal testimony to the work of running water is to be found in the deposits which it has accumulated. To these deposits the general name of alluvium has been given. Spreading out on either side, sometimes far beyond the limits of the ordinary or modern channels, these deposits, even when worn into fragmentary patches, retain their clear record of the operations of the river. Let us in imagination follow the course of a river from the mountains to the sea, and mark as we go the circumstances under which the accumulation of sediment takes place.

The more

The power possessed by running water to carry forward sediment depends mainly upon the velocity of the current. rapidly a stream flows, the more sediment can it transport, and the larger are the blocks which it can move. The velocity is regulated chiefly by the angle of slope; the greater the declivity, the higher the velocity and the larger the capacity of the stream to carry down debris. Any cause, therefore, which lessens the velocity of a current diminishes its carrying power. If water, bearing along gravel, sand, or mud, is checked in its flow, some of these materials will drop and remain at rest on the bottom. In the course of every stream, various conditions arise whereby the velocity of the current is reduced. One of the most obvious of these is a diminution in the slope of the channel. Another is the union of a rapid tributary with a more gently flowing stream. A third is the junction of a stream with the still waters of a lake (see p. 42) or with the sea. In these circumstances, the flow of the water being checked, the sediment at once begins to fall to the bottom.

Tracing now the progress of a river, for illustrations of this law of deposition, we find that among the mountains where the river takes its rise, the torrents that rush down the declivities have torn out of them such vast quantities of soil and rock as to seam them with deep clefts and gullies. Where each of these rapid streamlets reaches the valley below, its rapidity of motion is at once

lessened, and with this slackening of speed and consequent loss of carrying power, there is an accompanying deposit of detritus. Blocks of rock, angular rubbish, rounded shingle, sand, and earth are thrown down in the form of a cone of which the apex starts from the bottom of the gully and the base spreads out over the plain (Fig. 10).

Such cones vary in dimensions according to the

size of the torrent and the comparative ease with which the rocks of the mountain-side can be loosened and removed. Some of them, thrown down by the transient runnels of the last sudden rain-storm, may not be more than a few cubic yards in bulk. But on the skirts of mountainous regions they may grow into masses hundreds of feet thick and many miles in diameter. The valleys in a range of mountains afford many striking examples of these alluvial cones or fans, as they are called. Where the tributary