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slowly or more rapidly, according to circumstances, from higher to lower levels. In temperate latitudes, where the rainfall is distributed over the year, this transference of material is not so conspicuous as in countries where the rainfall is crowded into a short season. Even in our own country, however, one may observe how in gently undulating tracts rain washes the finer particles down the slopes and spreads them over the hollows. After exceptionally heavy or longcontinued rain this process becomes intensified—fine mud, silt, sand, and grit are swept into the brooks and streams, and the swollen rivers run discoloured to the sea. Similar floods often result from the melting of snow in spring. During such floods our rivers are generally more turbid than when they are swollen merely by heavy or continuous rain. When thaw ensues weathered rock-surfaces crumble down, while superficial accumulations of disintegrated materials become more or less saturated by melting snow. To such a degree is this soaking sometimes carried, that the whole surface of sloping fields may be set in motion. The soils creep, slide, and occasionally flow. Not infrequently also the subsoils and disintegrated rock-surfaces on steep inclinations collapse and slide into the valleys. Everyone, in short, is familiar with the fact that flooded rivers are invariably muddy, and that the mud or silt which discolours them has been abstracted from the land.
In temperate lands of small extent like England the rivers are under ordinary conditions somewhat clear. But in continental tracts the larger rivers are always more or less turbid. This is due to many causes. Some rivers, for example, head in glaciers, and are thus clouded at their very origin. Others, again, cross several degrees of latitude, and traverse different climatic regions. Hence it will rarely happen that snow is not melting or rain falling in some part of a great drainage-area. Many rivers, again, after escaping from the mountains, flow through countries the superficial formations of which are readily undermined and washed away, and thus the main stream and its affluents become clouded with sediment. It is in tropical and subtropical latitudes, of course, that the most destructive effects of rain are witnessed. During the wet season the rivers of such regions discharge enormous volumes of mud-laden water.
We may conclude, then, that under the influence of atmospheric agents rocks are everywhere decomposed and disintegrated; and, further, that there is a universal transference from higher to lower levels of the materials thus set free. Now and again, it is true, there may be long pauses in the journey—-the materials may linger in hollows and depressions. Eventually, however, they are again put in motion, and by direct or circuitous route, as the case may be, find their way into the rivers, and finally come to rest in the ocean. The river-systems of the world, then, are the lines along which the waste products of the land are carried seawards. But rivers are much more than mere transporters of sediment. Just as in desert lands wind employs disintegrated rock-material as a sand-blast, so rivers use their stones, grit, and sand . as tools with which to rasp, file, and undermine the rocks over which they flow. In this way their channels are gradually deepened and widened. Some of the transported material is held in solution, part is carried in mechanical suspension, and another portion is pushed and rolled forward on the bed. It is the solid ingredients, of course, that act as eroding agents. While much of the finer sediment finds its way into the drainage-system by the agency of rain and melting snow, the coarser materials are derived chiefly from the destruction of the rocks that underlie or overhang the course of a river and its feeders. I n temperate and northern latitudes natural springs and frost are responsible for much of the rock ddbris which cumbers the beds of streams, but much also is dislodged by the undermining action of the water itself. Rock-fragments when first introduced are more or-less angular, but as they travel down stream <they often break up into smaller pieces along natural cracks or joints, and the sharp corners and edges of these get worn away by mutual attrition, and by rasping on the rocky bed. In this manner the several portions gradually become smoothed and rounded—the process of abrasion resulting necessarily in the production of grit, sand, silt, etc. Thus in a typical river-course, consisting of mountain-track, valley-track, and plaintrack, we note a progressive change in the character of the sediments as the river as ^followed from its
source to the sea. In the mountain-track, where the course is steep and usually in a rocky channel, angular and subangular fragments abound, and the detritus generally is coarse. In the valley-track, the inclination of which is gentle, well-rounded gravel, with grit and sand, predominate, the latter becoming more plentiful as the plain-track is approached. In the plain-track the prevailing sediments are fine sand and silt.
The amount of material removed by a river depends on the volume of the water, the velocity of the current, and the geological character of the drainagearea. Thus, the larger the river, other things being equal, the greater the burden of sediment. Again, a rapid current transports material more effectively than a gentler stream, while rivers that flow through lands whose rocks are readily eroded carry more sediment than rivers of equal volume and velocity traversing regions of more resistant rocks. Should a lake interrupt the current of a river, all the gravel, sand, and mud may be intercepted, and the stream will then issue clear and pellucid at the lower end of the lake, as the Rhone does at Geneva. The lake, in short, acts as a settling reservoir. By and by, however, the lacustrine hollow becomes silted up and converted into an alluvial flat, through which the silt-laden water winds its way towards the ocean. Reaching that bourn, the current of the river is arrested, and its sediment thrown down. Should no strong tidal current sweep the coast, removing sediment as it arrives, the sea becomes silted up in the same way as the lake, and in time a delta is formed. The growth of the latter necessarily depends partly on the activity of the river and partly upon the depth of the estuary and the action of waves and tidal currents. But if nothing interrupted the growth of a delta— were all the materials brought down by a river to accumulate at its mouth—it is obvious that the rate of increase of a delta would enable us to form an estimate of the rate at which the drainage-area of the river was being eroded. It is certain, however, that such conditions never obtain. Even in the quietest estuaries much of the sediment is carried away by the sea. The rate of delta-growth must be exceeded by that of fluviatile transport.
Geologists, however, have adopted another method of estimating the loss sustained by the land. They can measure the amount of material held in solution, and of solid matter carried in suspension and rolled forward on the bed of a river. As might have been expected, the amount varies with the season of the year in each individual river, while different rivers yield very different results. But even in the case of the least active streams the transported material is much more considerable than might have been supposed. Hence one need not wonder that in spite of obstacles the deltas of many rivers advance seawards more or less rapidly. The delta of the Rhone, for example, pushes forward at the rate of about 50 feet annually, while that of the Po increases by more than