The constant agitation of the waters, and the declivity of the bottom, are no doubt the causes of this gradual and widely extended deposition. A soft mass of alluvial deposit, having its pores filled with water, and being subject to the vibrations of a superincumbent fluid, will yield to the pressure of that fluid on the side of the least resistance, that is, on the side toward the sea, and thus will be gradually extended more and more over the bottom. This will happen not only to the finer parts of the detritus, but even to the grosser, such as sand and gravel. For suppose that a body of gravel rests on a plane somewhat inclined, at the same time that it is covered with water to a considerable depth, that water being subject not only to moderate reciprocations, but also to such violent agitation as we see occasionally communicated to the waters of the ocean; the gravel, being rendered lighter by its immersion in the water, and on that account more moveable, will, when the undulations are considerable, be alternately heaved up and let down again. Now, at each time that it is heaved up, however small the space may be, it must be somewhat accelerated in its descent, and will hardly settle on the same point where it rested before. Thus it will gain a little ground at each undulation, and will slowly make its way towards the depths of the ocean, or to the lowest situation it can reach. This, as far as we may presume to

follow a progress which is not the subject of immediate observation, is one of the great means by which loose materials of every kind are transported to a great distance, and spread out in beds at the bottom of the ocean.

370. The lighter parts are more easily carried to great distances, being actually suspended in the water, by which they are very gradually and slowly deposited. A remarkable proof of this is furnished from an observation made by Lord Mulgrave, in his voyage to the North Pole. In the latitude of 65° nearly, and about 250 miles distant from the nearest land, which was the coast of Norway, he sounded with a line of 683 fathoms, or 4098 feet; and the lead, when it struck the ground, sunk in a soft blue clay to the depth of ten feet. * The tenuity and fineness of the mud, which allowed the lead to sink so deep into it, must have resulted from a deposition of the lighter kinds of earth, which being suspended in the water, had been carried to a great distance, and were now without doubt forming a regular stratum at the bottom of the sea.

371. The quantity of detritus brought down by the rivers, and distributed in this manner over the bottom of the sea, is so great, that several narrow seas have been thereby rendered sensibly shallower.

The Baltic has been computed to decrease in depth at the rate of forty inches in a hundred years. The Yellow Sea, which is a large gulf contained between the coast of China and the peninsula of Corea, receives so much mud from the great rivers that run into it, that it takes its colour, as well as its name, from that circumstance; and the European mariners, who have lately navigated it, observed, that the mud was drawn up by the ships, so as to be visible in their wake to a considerable distance. * Computations have been made of the time that it will require to fill up this gulf, and to withdraw it entirely from the dominion of the ocean: but the data are not sufficiently exact to afford any precise result, and are no doubt particularly defective from this cause, that much of the earth carried into the gulf by the rivers, must be carried out of it by the currents and tides, and the finer parts wafted probably to great distances in the Pacific Ocean. † The mere attempt, however, towards such a computation, shows how evident the progress of filling

• Staunton's Account of the Embassy to China, Vol. I. p.

448.

+ Pérouse, in sailing along the coast of China, from Formosa to the strait between Corea and Japan, though generally fifty or sixty leagues from the land, had soundings at the depth of forty-five fathoms, and sometimes at that of twenty-two. Atlas du Voyage de la Pérouse, No. 43.

up

is to every attentive observer; and, though it may not ascertain the measure, it sufficiently declares the reality of the operations, by which the waste of the present continents is made subservient to the formation of new land.

972. Sandbanks, such as abound in the German Ocean, to whatever they owe their origin, are cer tainly modified, and their form determined, by the tides and currents. Without the operation of these last, banks of loose sand and mud could hardly preserve their form, and remain intersected by many narrow channels. The formation of the banks on the coast of Holland, and even of the Dogger Bank itself, has been ascribed to the meeting of tides, by which a state of tranquillity is produced in the waters, and of consequence a more copious deposition of their mud. Even the great bank of Newfoundland seems to be determined in its extent by the action of the Gulf stream. In the North Sea, the current which sets out of the Baltic, has evidently determined the shape of the sandbanks opposite to the coast of Norway, and produced a circular sweep in them, of which it is impossible to mistake the cause.

In proof of the action here ascribed to the waters of the sea, in transporting materials to an unlimit ed extent, we may add the well known observation, that the stones brought up by the lead from the bottom of the sea, are generally round and polish

ed, hardly ever sharp and angular. This could never happen to stones that were not subject to perpetual attrition.

373. Currents are no doubt the great agents in diffusing the detritus of the land over the bottom of the sea. These have been long known to exist; but it is only since the later improvements in navigation, that they have been understood to constitute a system of great permanence, regularity, and extent, connected with the trade winds, and other circumstances in the natural history of the globe. The Gulf stream was many years since observed to transport the water, and the temperature of the tropical regions into the climates of the north; and we are indebted to the researches of Major Rennell, for the knowledge of a great system of currents, of which it is only a part. That geographer, who is so eminent for enriching the details of his science with the most interesting facts in history or in physics, has shown, that along the eastern coast of Africa, from about the mouth of the Red Sea, a current fifty leagues in breadth sets continually towards the southwest. * It doubles the Cape of Good Hope, runs from thence northwest, preserving on the whole the direction of the coast, but reaching so far into the ocean, that, about the parallel of St Helena, its breadth exceeds 1000 miles.

* Geography of Herodotus, p. 672.