man aids in the great operation of the degradation of the exposed portion of the surface of the earth. By his breakwaters, dykes, dams, quays, and other engineering labours, he endeavours to arrest the conversion of dry land into sea. But the accumulated efforts of the human race, since the first traces of their abode upon earth, have effected less change in the condition. of the countries they inhabit, than has been wrought by the greedy or petulant haste of a single generation through the clearance and destruction of forests. It is only by the aid of the vegetable kingdom that man can contribute, in any appreciable degree, to the maintenance of the present condition of the surface of the earth. It is by his wanton inroads on the great conservative power of vegetation, that he most efficiently hastens the degradation of the soil.

But little more than the third part of the superficies of our planet is estimated by Humboldt to be uncovered by the waters of the ocean. The extreme height attained by the mountain ranges is less than the lowest depths measured by the plumb line. Indeed, the mean height of the continents above the level of the sea has been estimated at only one-fifteenth part of the mean depth of the ocean. The mean elevation of Europe is estimated at 636 feet above the sea. The dry land, then, if gradually degraded and carried into the sea, would not only find ample room for deposit beneath the water, but would-if the assumed proportions are any way near the mark—fail to raise the surface of the latter by more than 50 feet, or to increase the length of the mean diameter of the globe by much. more than one four hundred-thousandth part, to say nothing of the counterbalancing loss of diameter occasioned by the degradation of the highlands. The more lofty are the mountain ranges, the more powerfully do they attract that rainfall which acts on them with the slow pertinacity of the file; the more rapid are the torrents that furrow their slopes, and bear rocks and boulders to the plains below, the more copious and irresistible are the floods that pound boulders into gravel, and gravel into sand, and finally veil the evidence of their toil under a blank mantle of mud. The annual rainfall of the world has been estimated by French men of science at very nearly 50 inches (actually 1.5 metres) over the entire surface of the globe. As regards the proportion of the areas of land and water, it may appear at first sight that the ocean receives on its surface two-thirds of the total quantity of rain. But when we come to study the result of actual observation, we find that the attractive power of the mountain ranges on the water borne aloft in the atmosphere is so great as to go far to

compensate for the comparatively limited area of the dry land. Thus we find that in India, along the west coast of the peninsula, from the seashore to the summits of the Ghauts, and again from the mouth of the Irawadi along the east coast of the Bay of Bengal, up the valley of the Bramaputra, and along the skirts of the Himalaya, there exist wide belts of rainfall of more than 100 inches in annual depth. In special localities this large downpour is more than doubled. Even in the Lake district of England, where atmospheric phenomena are very far from attaining a tropical intensity, an annual rainfall of 244 inches has been actually measured at the Stye Head Pass. On the other hand, over a very great portion of the surface of the ocean there does not exist any more attraction for rainfall than is found to be exerted by the rainless districts of Asia, of Arabia, or of Africa. The estimate above quoted of average rainfall has been taken from calculations as to heat and evaporation, rather than from measurement of rain or rivers. But such phenomena as are presented by the Atbara, the Uruguay, and indeed by most of the great torrential rivers of the world, are conclusive as to the fact of the immense concentration of rainfall that occurs on the most prominent mountain rangesthat is to say, on those very portions of the earth's surface that now project most sensibly above the mean level of the sea. While the rainfall in the British islands (notwithstanding the extraordinary instance above cited) does not exceed from 24 to 60 inches, according to the zone of country, that of Dutch Guiana is stated at 229 inches, that of Brazil at 276; that of the Western Ghauts, at an elevation of between 4,000 and 5,000 feet, at 302; and that of the Khasia Mountains in Bengal at 600 inches: no less than 30 inches being mentioned by Dr. Hooker as having there fallen in 24 hours.

The general profile of the rivers of the world, whatever be their variation in length, in volume, and in regularity or sinuosity of course, has been described as approaching a parabolic curve. For the precipitous descents over which the vertically falling rain is at first hurried, the violent action of the mountain torrents has earned the name of the zone of erosion. As torrents blend and calm into rivers, and as the slopes of the hills become more gradual, the channels of discharge assume greater regularity. The shingle and gravels into which the rocky fragments, borne down from the steeper portions of the watercourses, have become broken and ground, have a tendency to form in beds and shoals whenever the accidentation of the ground deflects the course of the stream. After any mountain storm, or long-continued rainy season, rivers, in this part

of their course, are apt to keep up a perpetual movement of the loose materials of their bed. A substantial shoal of gravel will disappear, to be replaced, later on, by contributions from the same source to which the deposit was in the first instance to be traced. The course of the Po, in the neighbourhood of Turin, is a characteristic example of this portion of the system of an important river. In our own island the River Towey, in Carmarthenshire, may be cited as an appropriate parallel, respect being had to the inferior volume of the stream. This second portion of the general system of rivers has been called the zone of compensation.

By no sharp and defined change, but by gradual diminution of the inclination of the river-bed, the zone of compensation passes into the zone of deposit. It is in this zone that a river finally loses its individuality, and that its waters mingle with the sea. In some cases, where the low land stretches to a considerable distance from the foot of the hills, the course of the rivers through the zone of deposit is slow, tortuous, and comparatively feeble; the fall being almost imperceptible until the spot is reached where the descending current first becomes sensible of the opposing action of the tide. When this limit is attained, the further course of the river is such as to fall into one or other of two very distinct categories or groups.

The essential difference of these groups depends on the question whether the river discharges its waters into a tidal or a tideless basin. These terms are used absolutely in the first instance, as denoting a marked difference of condition. In practice, however, the limit is less rigid. Tidal seas rise with very different velocity and height of flood on various parts of their coasts, as is to be seen in our own island. Thus the tide,

which, in the Thames, may have a rise at springs of from 20 to 24 feet, hardly exceeds the fourth of that rise in the roads of Yarmouth; while in the Wye, above its confluence with the Severn, the equinoctial spring tides are said to have reached the height of 72 feet. A like enormous rise is said to occur in the Bay of Fundy. On the occasion of the floating of the bridge built to carry the South Wales Railway over the Wye at Chepstow the actual rise of the tide was about 60 feet. The remarkable funnel-shaped mouth of the Severn is no doubt one main cause of this piling up of the incoming wave. In the Seine, which also has a funnel-shaped mouth, the phenomenon of the bore, barre, or flôt, which is a rare occurrence on the Severn, and is unknown in most rivers, is due to a like cause. When a strong wind drives the rising tide into either of these broad estuaries, the impetus gained by the wave is such that

as the course narrows the water is heaped up on itself by its own momentum, and rushes up the channel as a vertical wall, coped by a crest of tumbling water, spray and foam, canopied very often by driving storm, rain, or sleet, and rising as much as 12 or 14 feet above the surface of the descending rivers.

Between such a tidal estuary as that of the Seine or of the Severn, and the placid expanse of the almost tideless Tyrrhenian Sea, into which the Tiber discharges its waters at Ostia, the contrast is extreme. It is not, however, one that can be taken as typical. Each great river has features of its own, as well as features that are common to its class. Thus, while the level of the Mediterranean, as a rule, does not vary more than from 24 to 30 inches, under the influence of winds and of currents, it is more tempestuous on certain portions of its coasts; and is said to have an actual tide, when the wind is northerly, rising as much as from 30 to 55 inches at the head of the Gulf of Venice.

Bearing in mind that either term is used rather as denoting the central idea of a group than as a rigid definition applicable to any member of that group, we return to the statement that rivers, in the third and last division of their systems, may be distinguished as they fall into either tidal or tideless waters. It is the latter class of rivers alone that properly presents the phenomenon known by the term delta. The word, as is well known, is taken from the similarity of the triangular islets formed at the embouchure of the Nile, to the form of the Greek capital delta; a similarity which is more apparent in the case of the early Hellenic or of the Phoenician alphabet than in the more regular modern form-in which indeed the idea is altogether lost, except in the capital letter. The action of rivers that fall into tidal seas generally differs from that of the delta-forming streams, inasmuch as the force of the reflux of the tide, aiding the torrent of the river, is ordinarily enough to maintain a deep and navigable channel. In other words, rivers that flow into tideless seas are apt to deposit the solid matter which they bring down in banks and islands at the spot where the regular movement of the water is first checked by the opposition of the sea. Tidal rivers, on the contrary, send their deposits more freely forth, to be deposited over the general bed of the ocean.

There is one point with reference to the action of tideless rivers which has been disputed by some writers, but which seems to be established by indubitable evidence, at all events, in the best observed cases, which are those of the Rhone and of the Nile. This is the permanent fixity of the point of

diramation. Regarding the extremely tortuous and irregular course of the channels of such rivers as we have named; the absence of any rocky or artificial bridles to determine their divergence; and the habit, which they all share, of varying the position of their channels apparently at will, and certainly under the influence of comparatively slight causes; it might be anticipated that the point of diramation, or the landward apex of the delta, would shift its position as it became left inland by the accumulation of material at the base of the islet, and move either up or down the stream. Such an anticipation, however, is not supported by observation. This fact tends to show that the point of diramation is not casually decided. In the Nile, at the present time, islands occur above the point of bifurcation. The existence of an island betokens the actual division of the stream by some obstacle; and thus shows that there was a facility offered for permanent division at that point, of which the river refused to avail itself. It therefore may be held, with some confidence, that the position of the point of diramation—that is to say, the commencement of the formation of the delta-depends chiefly upon level. It is at a certain point in its descending curve that the river first meets that silent but sensible opposition which is offered to its movement by the sea. This point will be at, or near to, the spot where the level of the surface of the river at low water is the same as that of the mean surface of

the sea. As this level remains unchanged-we are now speaking only of historic times-so does the point where the tendency to deposit first undergoes material facilitation also remain unchanged. In other words, the apex of the delta is a fixed point, irrespective, to some degree, of the volume or velocity of the rivers. This view, if established by more exhaustive observation, may perhaps hereafter take rank as a primary law of the formation of river deltas.

The establishment of this law (which we will now assume as hypothetical) tends to explain how it is that rivers of such different character as the Nile, the Danube, and the Rhone present such remarkable similarities in the matter of delta formation. It cannot be owing to mere chance that each of these rivers, which originally poured their undivided streams into the sea, should have not only diramated but split into seven streams, subsequently choking up one after another of their channels; and again pouring the main body of their waters, in two cases through two, and in the third through three, main mouths or outlets. Indeed, the whole course of the formation of what the French hydrographers call the