such as a river-basin. Taking merely the matter in mechanical suspension, and assuming that the proportion of it transported annually in the water of the Mississippi may be regarded as an average proportion for the rivers of Europe, we find that this continent, at the Mississippi rate of degradation, might be reduced to the sea-level in rather less than 6,000,000 years.

In pursuing their course over the land, running waters gradually deepen and widen the channels in which they flow, partly by chemically dissolving the rocks and partly by rubbing them down by the friction of the transported sand, gravel, and stones. When they have once chosen their channels, they usually keep to them, and the sinuous windings, at first determined by trifling inequalities on the surface of country across which the streams began to flow, are gradually deepened into picturesque gorges. In the excavation of such ravines, waterfalls play an important part by gradually receding up stream. River-channels, especially if cut deeply into the solid rock, remain as enduring monuments of the work of running water.

But still more important as geological records, because more frequent and covering a larger area, are the deposits which rivers leave as their memorials. Whatever checks the velocity of a current weakens its transporting power, and causes it to drop some of its sediment to the bottom. Accordingly, accumulations of sediment occur at the foot of torrent slopes, along the lower and more level ground, especially on the inner or concave side of the loops, over the flood-plains, and finally in the deltas formed where rivers enter lakes or the sea. In these various situations, thick stratified beds of silt, sand, and gravel may be formed, enclosing the remains of the plants and animals living on the land at the time. As a river deepens its channel, it leaves on either side alluvial terraces that mark successive flood-plains over which it has flowed.

CHAPTER IV

THE MEMORIALS LEFT BY LAKES

Fresh-water Lakes.-According to the law stated in last chapter, that when water is checked in its flow, it must drop some of its sediment, lakes are pre-eminently places for the deposition and accumulation of mineral matter. In their quiet depths, the debris worn away from the surface of the land is filtered out of the water and allowed to gather undisturbed upon the bottom. The tributary streams may enter a large lake swollen and muddy, but the escaping river is transparent. It is evident, therefore, that lakes must be continually silting up, and that when this process is complete, the site of a lake will be occupied by a series of deposits comprising a record of how the water was made to disappear.

To those who know the aspect of lakes only in fine weather, they may seem places where geological operations are at their very minimum of activity. The placid surface of the water ripples upon beaches of gravel or spits of sand; reeds and marshy plants grow out into the shallows; the few streamlets that tumble down from the surrounding hills furnish perhaps the only sounds that break the stillness, but their music and motion are at once hushed when they lose themselves in the lake. The scene might serve as the very emblem of perfectly undisturbed conditions of repose. But come back to this same scene during an autumn storm, when the mists have gathered all round the hills, and the rain, after pouring down for hours, has turned every gully into the track of a roaring torrent. Each tributary brook, hardly visible perhaps in drought, now rushes foaming and muddy from its dell and sweeps out into the lake. The large streams bear along on their swift brown currents trunks of trees, leaves, twigs, with now and then the carcase of some animal that has been drowned by the rising

flood. Hour after hour, from every side, these innumerable swollen waters bear their freights of gravel, sand, and mud into the lake. Hundreds or thousands of tons of sediment must thus be swept down during a single storm. When we multiply this result by the number of storms in a year and by the number of years in an ordinary human life, we need not be surprised to be told that even within the memory of the present generation, and still more within historic times, conspicuous changes have taken place in many lakes.

Filling up of Lakes.—In the Lake of Lucerne, for example, the River Reuss, which bears down the drainage of the huge mountains round the St. Gothard, deposits about 7,000,000 cubic feet of sediment every year. Since the year 1714 the Kander,

FIG. 14.-Alluvial terraces on the side of an emptied reservoir.

which drains the northern flanks of the centre of the Bernese Oberland, is said to have thrown into the lower end of the Lake of Thun such an amount of sediment as to form an area of 230 acres, now partly woodland, partly meadow and marsh. Since the time of the Romans, the Rhone has filled up the upper end of the Lake of Geneva to such an extent that a Roman harbour, still called Port Valais, is now nearly two miles from the edge of the lake, the intervening ground having been converted first into marshes and then into meadows and farms.

It is at the mouths of streams pouring into a lake that the process of filling up is most rapid and striking. But it may be detected at many other places round the margin. Instructive lessons on this subject may be learned at a reservoir formed by damming back the waters of a steep-sided valley, and liable to be sometimes nearly dry (Fig. 14). In such a situation, when the water is low, it may be noticed that a series of parallel lines runs all round the sides of the reservoir, and that these lines

consist of gravel, sand, or earth. Each of them marks a former level of the water, and they show that the reservoir was not drained off at once but intermittently, each pause in the diminution of level being marked by a line of sediment. It is easy to watch how these lines are formed along the present margin of the water. The loose debris from the bare slope above, partly by its own gravitation, partly by the wash of rain, slides down into the water. But as soon as it gets there, its further downward movement is

arrested.

By the ripple of the water it is gently moved up and down, but keeps on the whole just below the line to which the water reaches. So long as it is concealed under the water, its position and extent can hardly be realised. But as soon as the level of the reservoir sinks, the sediment is left as a marked shelf or terrace. In natural lakes, the same process is going on, though hardly recognisable, because concealed under the water. But if by any means a lake could be rapidly emptied, its former level would be marked by a shelf or alluvial terrace. In some cases, the barrier of a lake has been removed, and the sinking of the water has revealed the terrace. The famous "parallel roads" of Glen Roy, in the west of Scotland, are notable examples (Fig. 15).

The valleys in that region were anciently dammed up by large glaciers. The drainage accumulated behind the ice, filled up the valleys and converted them into a series of lakes or fresh-water "fjords." The former levels of these sheets of water and the successive stages of their diminution and disappearance are shown by the series of alluvial shelves known as "parallel roads." The highest of these is 1140 feet, the middle 1059 feet, and the lowest 847 feet above the level of the sea.

Thus, partly by the washing of detritus down from the adjoining slopes by rain, partly by the sediment carried into them by streams, and partly by the growth of marshy vegetation along their margins, lakes are visibly diminishing in size. In mountainous countries, every stage of this appearance may be observed (Fig. 16). Where the lakes are deep, the tongues of sediment or

FIG. 16.-Stages in the filling up of a lake. In A two streamlets are represented as pouring their "deltas" into a lake. In B they have filled the lake up, converting it into a meadow across which they wind on their way down the valley.

"deltas" which the streams push in front of them have not yet been able to advance far from the shore. In other cases, every tributary has built up an alluvial plain which grows outwards and along the coast, until it unites with those of its neighbours to form a nearly continuous belt of flat meadow and marsh round the lake. By degrees, as this belt increases in width, the lake narrows, until the whole tract is finally converted into an alluvial plain, through which the river and its tributaries wind on their way to lower levels. The successive flat meadow-like expansions of valleys among hills and mountains were probably in most cases originally lakes which have in this manner been gradually filled up.

Lake Deposits.-The bottoms of lakes must evidently contain many interesting relics. Dispersed through the shingle, sand, and mud that gather there, are the remains of plants and animals that lived on the surrounding land. Leaves, fruits, twigs,