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.