and disappearance are shown by the series of alluvial shelves known as parallel roads. The highest of these is 1140 feet, the middle terrace 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 disappearance may be observed (Fig. 16). Where the lakes

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.

are deep, the tongues of sediment or "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.

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, branches, and trunks embedded in the silt may preserve for an indefinite period their record of the vegetation of the time. The wings or wing-cases of insects, the shells of land-snails, the bones of birds and mammals, carried down into the depths of a lake and entombed in the silt there will remain as a chronicle of the kind of animals that haunted the surrounding hills and valleys.

The layers of gravel, sand, and silt laid down on the floor of a lake do not essentially differ from these deposited in the terraces of a river. But they ought generally to be finer in grain, and the proportion of silt, mud, or clay among them, especially away from the margin of the lake, must usually be greater than in the alluvium of a river. They are, no doubt, further distinguished by the greater abundance of the remains of plants and animals preserved in them.

But lakes likewise serve as receptacles for a series of deposits which are peculiar to them, and which consequently have much interest and importance as they furnish a ready means of detecting the sites of lakes that have long disappeared. The molluscs of lacustrine waters are quite distinct from the snails of the adjoining shores. The shells of these animals gather on the bottoms of some lakes in such numbers as to form there a deposit of the white crumbling marl, already referred to on p. 6. If nothing occurs to interrupt this deposit, it may grow to be many feet or yards in thickness. The shells in the upper parts are quite fresh, some of the animals having only recently died; but they become more and more decayed below until, towards the bottom of the deposit, the marl passes into a more compact chalk-like substance in which few or no shells may be recognisable (Fig. 17). On the sites of lakes that have been

FIG. 17. -Piece of shell-marl containing shells of Limnæa peregra.

naturally filled up or artificially drained, such marl has been extensively dug as a manure for land. Besides the shells from the decay of which it is chiefly formed, it sometimes yields the bones of deer, oxen, and other animals, whose carcases must originally have sunk to the bottom of the lake and been there gradually covered up in the growing mass of marl. Many examples of these marl-deposits are to be found among the drained lakes of Scotland and Ireland.

Yet another peculiar accumulation is met with on the bottom of some lakes, particularly in Sweden. In the neighbourhood of banks of reeds and on the sloping shallows of the larger lakes, a deposit of hydrated peroxide of iron takes place, in the form of concretions varying in size from small grains like gunpowder up to cakes measuring 6 inches across. The iron is no doubt dissolved out of the rocks of the neighbourhood by water containing organic acids or carbonic acid. In this condition, it is liable to be oxidised on exposure, and can then no longer be retained in solution. It is accordingly precipitated to the bottom where it collects in grains which by successive additions to their surface become pellets, balls, or cakes. Possibly some of the microscopic plants (diatoms) which abound on the bottoms of the lakes may facilitate the accumulation of the iron by abstracting this substance from the water and depositing it inside their siliceous coverings. Beds of concretionary brown ironstone are formed in Sweden from 10 to 200 yards long, 5 to 15 yards broad, and from 8 to 30 inches thick. During winter when the lakes are frozen over, the iron is raked up from the bottom through holes made for the purpose in the ice, and is largely used for the manufacture of iron in the Swedish furnaces. When the iron has been removed, it begins to form again, and instances are known where, after the supply had been completely exhausted, beds several inches in thickness were formed again in twenty-six years.

The salt-lakes of desert regions present a wholly peculiar series of deposits. These sheets of water have no outlet ; yet there is reason to believe that most of them were at first fresh, and discharged their outflow like ordinary lakes. Owing to geological changes of level and of climate, they have long ceased to overflow. The water that runs into them, instead of escaping by a river, is evaporated back into the air. But the various mineral salts carried by it in solution from rocks and soils are not evaporated also. They remain behind in the lakes, which are consequently becoming gradually salter. Among the salts thus introduced, common salt (sodium-chloride) and gypsum (calcium-sulphate) are two of the most important. These substances, as the water evaporates in the shallows, bays, and pools, are precipitated to the bottom where they form solid layers of salt and gypsum. The latter substance begins to be thrown down when 37 per cent of the water containing it has been evaporated. The sodium-chloride does not appear until 93 per cent of the water has disappeared. In the order of deposit, therefore, gypsum comes before the salt. Some bitter lakes contain sodium-carbonate, in others magnesium-chloride is abundant. The Dead Sea, the Great Salt Lake of Utah, and many other salt lakes and inland seas furnish interesting evidence of the way in which they have gradually changed. In their upper terraces, 1000 feet or more above the present level of the water, fresh-water shells occur, showing that these basins were at first fresh. But their valley-bottoms are now crusted with gypsum and salt, and their waters are almost wholly devoid of life. Such conditions as these help us to understand how great deposits of gypsum and rock-salt were formed in England, Germany, and many other regions where the climate would not now permit of any such condensation of the water (chapter xxi.)

Summary. The records inscribed by lakes in geological history consist of layers of various kinds of sediment. These deposits may form mere shelves or terraces along the margin of the water which, if drained off, will leave them as evidence of its former levels. By the long-continued operations of rain, brooks, and rivers, continually bringing down sediment, lakes are gradually filled up with alluvium, and finally become flat meadow-land with tributary streams winding through it. The deposits that thus replace the lacustrine water consist mainly of sand or gravel near shore, while finer silt occupies the site of the deeper water. They may also include beds of marl formed of fresh-water shells,