duced, inasmuch as great changes are known to have been there effected during the historical period.

The advance of these dunes is described as irresistible, and at a rate of 60 and 72 feet per annum. They force before them lakes of fresh water, formed by the rains, which cannot find a passage into the sea in the shape of streams. Forests, cultivated lands, and houses disappear beneath them. Many villages noticed in the middle ages have been covered, and a few years since it was stated, that in the department of the Landes alone, ten villages were threatened with destruction. "One of these villages, named Mimisan, has been," said Cuvier, "striving for 20 years against them; and one sand-hill, more than 60 feet high, may be said to be seen advancing. In 1802, the lakes invaded five fine farms belonging to St. Julien; they have since covered a Roman causeway, which led from Bordeaux to Bayonne, and which was seen about 40 years since, when the waters were low. The Adour, which was once known to flow by Vieux Boucaut, and to fall into the sea at Cape Breton, is now turned aside more than a thousand toises."

There are few extended lines of coast which will not afford opportunities for the observation of sand-hills, and their mode of accumulation and change, for strong winds acting upon even a comparatively exposed surface, soon produce a marked alteration of their form. Successive accumulations, shown by the remains of surface vegetation grown during times where it could partially establish itself, are cut away and heaped up into other hillocks, new matter derived from the sea being added to the general mass. At times, a strong off-shore wind forces sand back to the sea, acting not only on the sand-hills over which it blows, but also on the dried surface of the sands bared between high and low tide, these still more easily carried seaward when left dry for a longer time, between the highest lines of neap and spring tides.

As the sand commonly found in sand-hills is not usually borne high in mechanical suspension by the winds, such districts will not long have engaged attention before the power of running water, even of small streams, if their courses be unobstructed and fairly rapid, will be seen to prevent the extension of blown sands. The sand drifted, falling into the streams, is carried onwards by these waters, and is thus prevented from traversing them.† Sand

*Cuvier, Dis. sur les Revolutions du Globe. A thousand toises is about 6,400 English feet, or somewhat less than a mile and one quarter.

† Good examples of this fact may be observed on the coast of Cornwall. The Perran Sands are thus bounded for nearly two miles between Treamble and Holy

drifts are sometimes also found stopped by the flow of tidal waters in and out of lagoons. Of this kind, the accumulation of sand at the northern side of a spit of land, terminated by sand-hills, near Tramore, on the eastern coast of Ireland, may be considered as a good example.

As having a geological bearing, the observer would do well to direct his attention to the manner in which the remains of vegetable and animal life, both terrestrial and marine, become mingled in sand-hills. Portions of seaweeds will frequently be found blown, when dry, amid the terrestrial vegetation of the sand-hills; and the shells of the helices, which are often found in multitudes in such situations get mingled with marine shells, or their fragments.

In some situations, the sand-hills are largely composed of comminuted shells, ground to that state by the breakers; and in such cases, consolidation of parts of them may be observable, having the hardness of many sandstones. The carbonate of lime of the shells becomes acted upon by the carbonic acid in the rain waters, with additions from decomposing vegetation, when plants have established themselves on the surface of the sand, and a final deposit of the carbonate of lime, thus held in solution, agglutinates the grains of sand together. Indurated sands of this kind are sufficiently hard, occasionally, to be employed for building purposes.*

Well Bay. Much land is stated to have been covered by drifts from the Perran Sands, in consequence of a small stream having been covered by mining operations near Gear.

* The consolidated calcareous sand of New Quay, Cornwall, has been long used as a building stone. Not only is the neighbouring church of Crantoch built of this modern sandstone, but very ancient stone coffins have also been discovered, composed of the same consolidated sand, in the adjoining churchyard. The grains are so firmly cemented in this New Quay sandstone, that where it graduates into a kind of conglomerate, pebbles of quartz and hard sandstone are generally broken through by a blow on the compound rock.

CHAPTER V.

DISTRIBUTION AND DEPOSIT OF SEDIMENT IN TIDELESS SEAS.-DEPOSITS OF THE NILE.OF THE PO AND RHONE.-CONTEMPORANEOUS DEPOSITS OF GRAVEL, SAND, AND MUD.-DEPOSIT OF VOLCANIC ASHES AND LAPILLI.DEPOSITS IN THE BLACK SEA AND THE BALTIC.-GULF OF MEXICO AND MISSISSIPPI.

As tideless seas might be considered as mere salt-water lakes, the distribution and deposit of detritus in them would, as a whole, resemble that of fresh-water lakes, particularly of those attaining the magnitude of the great North American lakes, but for the difference in the relative specific gravities of their waters. Slight attention to the overflow of rivers swollen by rains, and charged with mechanically-suspended matter, into the sea, will show that the discoloured waters of the rivers, instead of falling beneath the waters into which they flow, as is seen at the higher part of the lake of Geneva, and numerous other lakes, proceed seawards on the surface of the sea waters, and often to considerable distances. The cause is simply that, though discoloured by the detrital matter held in mechanical suspension, these river waters are still specifically lighter than the sea waters into which they flow.

The distances to which the river waters sometimes flow seaward, transporting fine detrital matter, parting with it gradually, must, when the great rivers of the world become full and turbid, be often very considerable. Colonel Sabine has stated, that at three hundred miles distant from the mouth of the Amazons, discoloured water, supposed to come from that river, was found, with a specific gravity of 10204, floating above the sea water, of which the specific gravity was 1.0262, the depth of the lighter water being estimated at 126 feet. It would be well that observers should direct their attention to such facts, for their accumulation would tend much to show us the extent to which fine sedimentary matter may be thus borne beyond the action of tides and coast

currents.* As much matter may be thus distributed in chemical solution, valuable information might also be collected as to the kind and quantity of substances so held in solution.

From the varied depths near its shores, the Mediterranean affords us a good example of the deposits effected in seas which are commonly termed tideless. The great rivers which discharge themselves into it, such as the Nile, Po, and Rhone, now transport little sedimentary matter that is not finely comminuted, and of easy mechanical suspension. The Nile, which has been estimated to deliver a body of water annually into the Mediterranean about 350 times that which flows out of the Thames, beginning to rise in June, attaining its maximum height in August, and then falling until the next May, must thrust forward, from its periodical rise and fall, fine sedimentary matter with great regularity, tending thus to produce consecutive layers or beds of mud and clay of considerable uniform thickness and character, in those situations where modifying conditions do not interfere. Part of the fine matter brought down from the interior in mechanical suspension is deposited on the lower grounds traversed by the Nile; and it has been calculated that the surface of Upper Egypt has, in this manner, been raised more than six feet since the commencement of the Christian era. The fine matter not so deposited, passing with the river waters seaward, is necessarily borne furthest outwards when the greatest force of the river water prevails, namely, in August of each year.

The matter thus borne seaward may be kept a greater or less time mechanically suspended, according to the agitation of the surface by winds, but, as a whole, there must be an average area over which it is thrown down; the greatest distance of the deposit from the mouths of the Nile being attained in August, though the greatest thickness of a year's deposit will be nearer the land. As the river mouths advance, these sheets of fine sediment would be expected to extend further scaward, overlapping each other.

Where the surface of the sea cuts the slightly-inclined plane of sedimentary matter, partly in the sea, and partly on the land, the

* Very little practice would enable those who may have opportunities of making such observations to ascertain the amount of matter mechanically suspended in waters of this kind. If the scales be not very delicate, by pouring a large volume of the water through a filter, previously weighed, such an approximation to the truth may be obtained as might be useful. As previously observed (p. 28), mere evaporation of the water would give not only the matter in mechanical suspension, but that also in chemical solution.

breakers separate the finer from the coarser substances, keeping the former easily in mechanical suspension, and removing them from the shore outwards. The result is, an arenaceous boundary, with banks so formed as to include lagoons, such as are seen in the accompanying sketch of the delta of the Nile (fig. 56), at Lakes Mareotis, Bourlos, and Menzaleh.

These lakes gradually fill up, the shore advances, and so, even supposing the same relative level of sea and land not to be altered through a long succession of ages, the bed of the Mediterranean becomes more shallow in that region, and a mass of matter, such, for the most part, as would eventually form clay, is accumulated; the upper portion sandy from the action of the breakers upon the level of the sea, and from the sifting action, so to speak, of the waves further seaward, at depths where that influence could be felt.

From the periodical character of the rise of water in the Nile, the equivalent periodical deposits might even be marked by bands or layers extending to distances bearing a relation to the amount of transporting power of the river waters, so that coarser particles could be carried further and over more extended areas at one time than at another. The general deposit, however, gradually advancing seaward, successive annual accumulations would, as a whole, overlap each other.

When we regard the Po and Rhone, we have not the same very marked periodical rise of their waters; though no doubt, taken as

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