containing 26 per cent. of saline matter, of which 15 parts are magnesium chloride. This great lake is surrounded by lacustrine deposits, consisting of marls and shales with beds of rock-salt and gypsum, reaching to a height of 600 feet above the lake, and attesting its much greater size in former times. The cliffs of Jebel Usdum, at the south end of the sea, are described by Professor Hull, as consisting in the lower part of solid bluish rock-salt which is from 30 to 50 feet thick, and is capped by beds of marl, salt, and gypsum.1

Lakes fed by Mineral Springs.-We have hitherto only considered the chemical deposits produced by the evaporation and concentration of ordinary river waters when poured into lakes. But extensive deposits may be rapidly formed in lakes without great evaporation, if their waters are supplied by springs which contain a large amount of mineral matter in solution.

Such mineral springs are especially abundant in districts where volcanic action has been rife; they are often thermal, and generally impregnated with carbonic acid, so that they usually hold in solution large quantities of carbonate of lime. The carbonic acid escapes as a gas, and the calcareous matter is precipitated upon the lake-bottoms in the form of travertine.

In Sir C. Lyell's "Principles of Geology," a description is given of the lake of the Solfatara, between Rome and Tivoli. This is fed by a stream of warm water proceeding from a smaller lake above; and the water is so saturated with carbonic acid, that the escape of the gas gives it the appearance of being in a state of ebullition. Tufa and travertine are formed in the lake at a very rapid rate. The principal buildings of ancient and modern Rome are built of travertine obtained from the quarries of Ponte Lucano in the same district, where there has evidently been a lake at some remote period in which this deposit has been formed.

Travertine derives its other name of Tibur-stone from the town of Tivoli (ancient Tibur), which stands on an "Mount Seir and Western Palestine," Richard Bentley and Son, 1885, p. 131.

2 Vol. i. p. 404.

enormous mass of this rock. The walls of the chasm below the cascade of the river Anio disclose a magnificent section of tufa and travertine in horizontal beds for a depth of 400 or 500 feet. Lyell observes: "There can be little doubt that the whole of this deposit was formed in an extensive lake which existed at the close of the period of volcanic activity, by which the lavas and tuffs of the Roman territory were formed. The external configuration of the country has since been greatly changed, and the Anio now throws itself into a ravine excavated in the ancient travertine. Its waters give rise to masses of calcareous stone, scarcely, if at all, distinguishable from the older rock."

Lakes resulting from the Isolation of Sea-Water.-Whenever portions of sea-water are separated by any means from the open sea so as to form isolated lakes or lagoons, the solution quickly becomes concentrated by evaporation, and precipitation takes place.

Sea-water contains about 33 per cent. of solid matter in solution, that is to say, every 100 parts of sea-water contain 96 parts of pure water and 3 of mineral matter. In water from the English Channel the actual amount is 3.525, and in the Mediterranean 3·765.

When the solid matter is dried and analyzed it is found to consist of certain salts in the following proportions:

Chloride of sodium (common salt)

Chloride of magnesium

77.758

10.878

4.737

3.600

2.465

217

.345

100.000

The smallness of the quantity of carbonate of lime to be found in sea-water, compared with that in almost all rivers, is a point that will be discussed in a subsequent chapter. Another substance occurring in minute proportions in

rofessor Dittmar, "Challenger' Report

sea-water is silica. Forchhammer found it in all the specimens of sea-water which he analyzed, the mean proportion being 9 in 100,000 parts of water.

When a quantity of sea-water is isolated and evaporated the point of saturation for sulphate of lime is much sooner reached than that for chloride of sodium; the former requiring only 37 per cent. of the water to be removed, and the latter 93 per cent. Gypsum, therefore, must always be deposited before rock-salt, and it is possible for this deposition of gypsum to take place without the point of saturation for rock-salt being attained. This may be the reason why, though the sea contains twenty-one times as much sodium salt as it does gypsum, that the latter more frequently occurs as a mineral deposit than the former, though it is not often found in such massive beds.

A good instance of the formation of gypsum beds from the concentration of sea-water is described by Professor Dana, as occurring in the dried-up lagoon of a coral island. called Jarvis Island in the Pacific Ocean.' The flat surface of the central basin is covered with a deposit of guano, and underlying this is a stratum of sulphate of lime, frequently 2 feet thick, resting upon a bed of coral, sand, and shells. This deposit of gypsum is probably to be explained by the gradual elevation of the island, during which the lagoon waters were partially evaporated, but replenished from time to time by an influx from the sea, so that for a long time the condensation was not sufficient to precipitate chloride of sodium. Eventually, however, the whole was dried up, and salt was deposited, for around the lowest portion of the basin are incrustations of gypsum and common salt, ripple marks, and similar evidences of the gradually disappearing lake. Much of the salt may have been washed out by rain. Similar deposits of gypsum occur on many other elevated lagoons among the Pacific islands.

A good instance of the production of rock-salt by the evaporation of sea-water is presented by the Bitter Lakes of the Isthmus of Suez. Before the construction of the Suez Canal the surface of these lagoons was far below the

1 Dana's "Coral Reefs," 1875, p. 251.

level of the Red Sea, and the evaporation of their waters had produced a bank of salt 66,000,000 square metres (16,000 acres) in extent, composed of layers which were 5 to 25 centimetres (2 to 10 inches) in thickness. It would appear that the lakes had been inundated from time to time by the waters of the Red Sea, while in the intervals between these incursions the evaporation and concentration was sufficient to precipitate a layer of salt, and so in time the large deposit above mentioned was accumulated.

Similarly in the limans of Bessarabia, on the Black Sea, which dry up in summer, we have the formation of salt beds going on before our eyes.

One of the salinas of South America is thus described by Mr. Darwin:-"The mine consists of a hard stratum, between 2 and 3 feet thick, of the nitrate, mingled with a little of the sulphate of soda, and a good deal of common salt. It lies close beneath the surface, and follows for a length of 150 miles the margin of a grand basin or plain. This, from its outline, manifestly must once have been a lake, or more probably an inland arm of the sea, as may be inferred from the presence of iodic salts in the saline stratum. The surface of the plain is 3,300 feet above the Pacific."

IN

CHAPTER XIV.

MARINE DEPOSITS.

N describing the operations of a river, we have followed the course of the detritus transported by its current, and have seen, that though this is temporarily deposited at certain points along the river-valley, yet most of it is eventually moved on again and triturated into smaller and smaller particles. Sediment is thus continually brought down from higher to lower levels, and never finds a permanent resting-place until it is carried into a lake or into the sea. Even delta-mud is liable to removal, and from its mouth the river is constantly discharging a cloud of sediment, which is carried away by the tidal currents and spread over the sea-bottom.

Excepting, therefore, the small portion which is intercepted by lakes, the sea is the ultimate recipient of the materials carried down by rivers. To this is added the detritus which the sea-waves erode from the coast, and the whole is carried by the marine currents till circumstances determine its deposition. Some of it is carried along the coast and sorted by the action of waves and currents till it is thrown up and deposited in bays and inlets. The rest of it is carried out to sea, and dropped at a greater or less distance from land, according to the fineness of the sediment and the strength of the tidal current, but it is only the very finest material which is carried more than 100 miles from land.

We must also remember that besides this visible sediment there is much invisible material dissolved in the water, and that this latter also is poured into the sea.

The lime is used by millions of marine creatures in building up their shells or stony structures, and the silica is used in the same way by certain lowly animals and plants. On the death of these organisms the hard parts