Sivut kuvina

erect on their bases amid the ruin, to have occurred, on the same coast, since the erection of the Temple of Serapis at Pozzuoli. Little more than a century before the occurrence of the quiet, but very appreciable, volcanic displacement of 1858, a portion of the bank of the Tagus, comprising a quay thronged with the inhabitants of Lisbon, went down like a stone to an as yet unsounded depth-so at least they assert on the spot-being severed from the undisturbed portion of the city as if it had been cut in twain by a knife.

It may seem almost paradoxical to assert that the present century is witness of a process which, if continued for a sufficient length of time, will convert the basin of the Mediterranean into a vast river valley, in which marsh and lagoon will gradually be warped up into cultivated soil, and through which the waters of the Nile and the Atbara, receiving as affluents the Danube, the Po, the Rhone, the Tiber, and other tributaries, will be poured into the Atlantic. Yet nothing is more certain than that the causes now in daily operation are adequate to effect this great physical change, provided that no geological convulsion intervenes during the period required for its completion. Not only so, but the data which have been collected and are in course of collection by hydrographers, geographers, and engineers are becoming so numerous and exact, that it may he possible, before long, to assign the period within which this obliteration of the inland sea would be effected. As the first assumption, however-that of the uninterrupted continuance of the actual geological order-is one of such unwarranted magnitude, it would be little more than scientific trifling to complete the calculation. It is much more to the point to inquire how far we can ascertain, either from historic records or by the methods of the surveyor, the annual amount of delta formation that is actually taking place in the Mediterranean. In some instances we have recently been provided with careful measurements of flow and of deposit. In other cases we have indications, more or less reliable, of the condition of the littoral in the neighbourhood of the great river mouths at given dates. Herodotus supplies us with important landmarks showing the growth of the delta of the Nile, which have never yet been either understood or thoroughly investigated. M. Lenthéric has given us much valuable information as to the growth of the delta of the Rhone. Admiral Spratt 'has prepared charts exhibiting the advance of the shallow banks in the delta of the Kilia, the northernmost branch of the Danube, between 1830 and 1856. From Venice comes information that the silting up of the lagoons, which Sir John Rennie, in 1819, predicted would ensue if certain precautionary measures were neglected, has made rapid progress since the Austrian engineers departed from the wiser plans of their Italian predecessors. It remains to be seen whether the information as yet accessible is sufficient to allow us to arrive at any approach to the definition of a law that would be applicable, under various cases, to the determination of the secular growth of the deltas of the rivers flowing into the tideless waters of the inland seas of Europe.

The conversion of the bed of the Mediterranean into a cultivable river valley, vast as the change may appear to the imagination, is, after all, but a special example of that steady, silent, unintermitting, and therefore mighty change that is in progress over the greater portion of the surface of our globe.

The physical powers of nature, the rifting energy of frost, the parching and crumbling effect of heat, the mechanical friction of rain,

the chemical action of the atmosphere, are all engaged in a mighty and combined effort to reduce the surface of the planet to its true mathematical form of a spheroid of rotation. So certain, however slow, is the result of the incessant action, that it is only to the counterbalancing effects of geological convulsions--or at least of upheavals which deserve that title by their magnitude, whatever be the rapidity with which they may have been effected—that we can attribute the fact that our globe is not now in the condition of a solid nucleus, surrounded everywhere with a watery envelope. Almost all that we can observe of the steady operation of natural causes is tending to reduce the earth to that condition. Inorganic nature hastens, we will not say to destruction, but to that obliteration of the features of individuality, which would result in the destruction of terrestrial life. The toil of man, feeble and puny as are its results when compared with those of the great agencies of nature, tends in some cases rapidly to hasten, in others slightly to delay, the assimilating process. The great conservative element which resists the erosive force of atmospheric and of aqueous degradation, is the vigorous energy of vegetation. By absorbing and distributing the mountain rainfall; by clothing and protecting the banks of rivers; by arresting the deportation of the sandy banks of the sea by the waves; and by forming a barrier to the destructive march of the sand dunes in the track of prevailing winds; forest trees, marsh and aquatic plants, creeping knot grasses, and sociallygrowing pines effect more for the maintenance of the actual condition of the dry land than any other, or than all other agencies. By mining, quarrying, draining, and similar works, 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 ef 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 flột, 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

« EdellinenJatka »