But the spaces which in this manner the dry land has gradually or suddenly lost, or still loses, to the chafing ocean are largely compensated for in other places, by the vast accumulations of mud and sand, which so many rivers continually carry along with them into the sea. Thus at the mouths of the Nile, of the Ganges, and of the Mississippi, large alluvial plains have been deposited, which now form some of the most fruitful portions of the globe. The whole Delta of Egypt, Bengal, and Louisiana, have thus gradually emerged from the waters.
The volcanic powers, which once caused the highest mountain chains to rise from the glowing bosom of the earth, are still uninterruptedly active in changing its surface, and are gradually displacing the present boundaries of sea and land, upheaving some parts and causing others to subside.
On the coast of Sweden, it has been ascertained that iron rings fixed to rocks which formerly served for the fastening of boats are at present much too high. Flat cliffs on which, according to ancient documents, seals used to be clubbed while enjoying the warm sunbeam, are now quite out of the reach of these amphibious animals. In the years 1731, 1752, and 1755, marks were hewn in some conspicuous rocks, which after the lapse of half a century were found to have risen about two feet higher above the level of the sea. This phenomenon is confined to part of the coast, so that it is clearly the result of a local and slowly progressive upheaving.
Whilst a great part of Scandinavia is thus slowly but steadily rising, the shores of Chili have been found to rise convulsively under the influence of mighty volcanic shocks. Thus after the great earthquake of 1822, the whole coast, for the length of a hundred miles, was found to be three or four feet higher than before, and a further elevation was observed after the earthquake of Feb. 21st, 1835.
While to the north of Wolstenholme Sound, Kane remarked signs of elevation, a converse depression was observed as he proceeded southwards along the coast of Greenland, Esquimaux huts being seen washed by the sea. The axis of oscillation must be somewhere about 77° N. lat.
At Keeling Island, in the Indian Ocean, Mr. Darwin found evidence of subsidence. On every side of the lagoon, in which the water is as tranquil as in the most sheltered lake, old cocoa-nut trees were undermined and falling. The foundation-posts of a store-house on the beach, which the inhabitants had said stood seven years before just above high-water mark, were now daily washed by the tide. Earthquakes had been repeatedly remarked by the inhabitants, so that Darwin no longer doubted concerning the cause which made the trees to fall, and the store-house to be washed by the daily tide.
On the columns of the temple of Serapis, near Puzzuoli, the astonished naturalist sees holes scooped out by Pholades and Lithodomas, twenty-four feet above the present level of the sea. These animals are marine testacea, that have the power of burying themselves in stone, and cannot live beyond the reach of low-water. How then have they been able to scoop out those hieroglyphic marks so far above the level of their usual abodes? for surely marble originally defective was never used for the construction of so proud an edifice. Alternate depressions and elevations of the soil afford us the only key to the enigma. Earthquakes and oscillations, so frequent in that volcanic region, must first have lowered the temple into the sea, where it was acted upon by the sacrilegious molluscs, and then again their upheaving powers must have raised it to its present elevation. Thus, even the solid earth changes its features, and reminds us of the mutability of all created things.
There can be no doubt that, in consequence of the perpetual increase of alluvial deposits, and of the volcanic processes I have mentioned, the present boundaries of ocean must undergo great alterations in the course of centuries, and the general level of the sea must either rise or fall; but the evidence of history proves to us that, for the last 2000 years at least, there has been no notable change in this respect.
The baths hewn out in the rocks of Alexandria, and the stones of its harbour, have remained unaltered ever since the foundation of the city by the Macedonian conqueror; and the ancient port of Marseilles shows no more signs of a change of level than the old sea-walls of Cadiz. Thus, all the elevations and depressions that have occurred in the bed of ocean, or along its margin, and all the mud and sand that thousands of rivers continually carry along with them into the sea, have left its general level unaltered, at least within the historic ages. However great their effects may appear to the eye that confines itself to local changes, their influence, as far as the evidence of history reaches, has been but slight upon the immensity of the sea.
Geodesical operations have proved that the level of the ocean, with the exception of certain enclosed seas of limited extent, is everywhere the same. The accurate measurements of Corabœuf and Delcros show no perceptible difference between the level of the Channel and that of the Mediterranean. In the course of the operations for measuring the meridian in France, M. Delambre calculated the height of Rodez above the level of the Mediterranean at Barcelona, and its height above the ocean which washes the foot of the tower of Dunkirk, and found the difference to be equal to a fraction of a yard.