Captain Smyth, truly inferred that an under-current of such denser water, flowing outward, if of equal breadth and depth with the current near the surface, would carry out as much salt below as is brought in above, although it moved with less than one-fourth part of the velocity, and would thus prevent a perpetual increase of saltness in the Mediterranean beyond that existing in the Atlantic. It was also remarked by others, that the result would be the same if, the swiftness being equal, the inferior current had only a fourth of the volume of the superior. At the same time there appeared reason to conclude that this great specific gravity was only acquired by water at immense depths; for two specimens of the water, taken at the distance of some hundred miles from the Straits, and at depths of 400, and even 450 fathoms, were found by Dr. Wollaston not to exceed in density that of many ordinary samples of sea-water. Such being the case, we can now prove that the vast amount of salt brought into the Mediterranean does not pass out again by the Straits. For it appears by Captain Smyth's soundings, which Dr. Wollaston had not seen, that between the Capes of Trafalgar and Spartel, which are twenty-two miles apart, and where the Straits are shallowest, the deepest part, which is on the side of Cape Spartel, is only 220 fathoms. It is therefore evident that if water sinks in certain parts of the Mediterranean, in consequence of the increase of its specific gravity, to greater depths than 220 fathoms, it can never flow out again into the Atlantic, since it must be stopped by the submarine barrier which crosses the shallowest part of the Straits of Gibraltar.

The idea of the existence of a counter-current, at a certain depth, first originated in the following circumstance. M. De l'Aigle, commander of a privateer called the Phenix, of Marseilles, gave chase to a Dutch merchant-ship, near Ceuta Point, and coming up with her in the middle of the gut, between Tariffa and Tangier, gave her one broadside, which directly sunk her. A few days after, the sunk ship, with her cargo of brandy and oil, was cast ashore near Tangier, which is at least four leagues to the westward of the place where she went down, and directly against the strength of the central current. * This fact, however, affords no evidence of an undercurrent, because the ship, when it approached the coast, would necessarily be within the influence of a lateral current, which, running westward twice every twenty-four hours, might have brought back the vessel to Tangier.

What, then, becomes of the excess of salt?-for this is an enquiry of the highest geological interest. The Rhone, the Po, and many hundred minor streams and springs, pour annually into the Mediterranean large quantities of carbonate of lime, together with iron, magnesia, silica, alumina, sulphur, and other mineral ingredients, in a state of chemical solution. To explain why the influx of this matter does not alter the composition of this sea has never been regarded as a difficulty; for it is known that calcareous rocks are forming in the delta of the Rhone, in the Adriatic, on the coast of Asia Minor, and in other localities. Precipitation is acknowledged to be the means whereby the surplus mineral matter is disposed of, after the consumption of a certain portion in the secretions of testacea, zoophytes, and other marine animals. But * Phil. Trans., 1724.

before muriate of soda can, in like manner, be precipitated, the whole Mediterranean ought, it is said, to become as much saturated with salt as Lake Aral, the Dead Sea, or the brine-springs of Cheshire.

It is undoubtedly true, in regard to small bodies of water, that every particle must be fully saturated with muriate of soda before a single crystal of salt can be formed; such is probably the case in all natural salterns; such for example as those described by travellers as occurring on the western borders of the Black Sea, where extensive marshes are said to be covered by thin films of salt after a rapid evaporation of sea-water. The salt étangs of the Rhone, where salt has sometimes been precipitated in considerable abundance, have been already mentioned. But whether it be necessary that every part of a sea of enormous depth should be fully saturated before any precipitate can take place is a question of some difficulty. In the narrowest part of the Straits of Gibraltar, where they are about nine miles broad, between the Isle of Tariffa and Alcanzar Point, the depth varies from 160 to 500 fathoms: but between Gibraltar and Ceuta, Captain Smyth sounded to the enormous depth of 950 fathoms; where he found a gravelly bottom, with fragments of broken shells. Saussure sounded to the depth of two thousand feet, within a few yards of the shore, at Nice, and M. Bérard has lately fathomed to the depth of more than six thousand feet in several places without reaching the bottom.*

The central abysses of this sea are in all likelihood at least as deep as the Alps are high, and, as at the depth of seven hundred fathoms only, water has been found to contain a proportion of salt four times greater than at the surface, we may presume that the excess of salt may be much greater at the depth of two or three miles. After evaporation, the surface water becomes impregnated with a slight excess of salt, and its specific gravity being thus increased, it instantly falls to the bottom, while lighter water rises to the top, or flows in laterally, being always supplied by rivers and the current from the Atlantic. The heavier fluid, when it arrives at the bottom, cannot stop if it can gain access to any lower part of the bed of the sea, not previously occupied by water of the same density. In this manner the bottom of the nethernmost submarine abysses must annually receive new supplies of brine, while the water at the surface, being incessantly renewed by rivers and the current from the ocean, can never become saturated.

How far this accumulation of brine can extend before the inferior strata will part with any of their salt, and what difference in such a chemical process the immense pressure of the incumbent ocean might occasion, are questions which cannot be answered in the present state of science. There is also another curious topic of speculation; what changes may be effected by volcanic heat, so active in many parts of the bottom of the Mediterranean. A submarine hotspring or stufa would give rise to a new set of phenomena. But perhaps it may be said that their effect would only be to cause ascending and descending currents, and thereby to promote the intermixture of the upper and lower waters of the sea. A solfatara, or rent through which inflammable gases are continually escaping, might certainly convert sea-water into steam; and in this case salt would be precipitated in the space from which the steam was expelled. Additional supplies of water might then find their way into the fissure, being injected into every pore of the rock by the vast pressure of the incumbent ocean. If, by a repetition of this process, the cavity was filled with salt, other crystals of the same mineral would more easily be formed from a solution, and might then spread along the bottom of the sea. Yet even in this case it should seem that the fluid must first be fully saturated with muriate of soda.

In regard to the probable origin of those continuous masses of rock-salt which we find in Poland, Hungary, Transylvania, and Spain, geologists have entertained very different opinions; but the theory which has obtained most favour in later times attributes them not to precipitation from an aqueous menstruum, but to sublimation from volcanic exhalations rising from below, which insinuate themselves into rents and vacuities, caused by the fracture and decomposition of rocks.

The straits of Gibraltar are said to become gradually wider by the wearing down of the cliffs on each side at many points; and the current sets along the coast of Africa so as to cause considerable inroads in various parts, particularly near Carthage. Near the Canopic mouth of the Nile, at Aboukir, the coast was greatly devastated in the year 1784, when a small island was nearly consumed. By a series of similar operations, the old site of the cities of Nicopolis, Taposiris, Parva, and Canopus, have become a sand-bank.*

Sand Hills.-It frequently happens, where the sea

* Clarke's Travels in Europe, Asia, and Africa, vol. iii.