DELESSE SEAS OF THE OLD WORLD. 7
gradually under the sea, forming a vast terrace, which is covered
with sand ; this is notably the case in the Gulf of Gabes, where the
sand extends to a distance of more than 200 kilometres from the
shore.
In the Mediterranean, certain submarine rocks reappear in the neighbourhood of the coasts, particularly when these are mountainous. Clay extends over large surfaces in the archipelago, in the Gulf of Syrte, in the south and to the east of Malta, in the Adriatic, round Italy, round the Baleares, and to the east of Spain.
Although the Mediterranean contains a great abundance of mollusks, the deposits rich in shell-remains do not cover vast surfaces, — a circumstance which is apparently due to the fact of its shores being generally steep.
The Baltic is an inland sea, very shallow when compared with the seas of the south of Europe. Submarine rocks constitute a large portion of the bed of the Baltic, especially along the coasts of Sweden and Finland, as well as in the Gulf of Livonia. In the archipelago of Aland they even show a connexion with the granitic rocks which form the promontories of Stockholm and Finland. Clay occurs in nearly all the eastern Baltic, and it even extends over large surfaces there. It must no doubt be attributed to the argillaceous beds of the schistose Silurian series ; for this formation is strongly developed on the neighbouring shores, more especially in Sweden and Russia. Pebbles also form interrupted zones, which appear to range nearly parallel to the coast of Sweden ; their mean depth is about 50 metres, but towards the north it is much greater, so that the sea cannot now displace them. They indicate, therefore, an unconsolidated deposit anterior to the existing epoch, and probably an ancient beach of the Baltic.
Silt fills up several distinct basins ; it follows at some distance the indentations of the coast-line, and again recedes round the islands. It fills up the central portions of the Baltic and of the Gulf of Bothnia, but not always the deepest parts.
Sand forms large margins on the shores of the Baltic ; it occupies also vast submarine surfaces, particularly off the coasts of Pomerania and Courland, in the Gulfs of Livonia and Finland, in the archipelago of Aland, and in the Gulf of Bothnia. The abundance of sand in the Baltic may be attributed to the circumstance that this sea is not deep, and that it receives numerous torrential rivers which are frequently swollen by melting snows, descending from Finland or the Scandinavian Alps after having swept over granitic rocks ; it is owing especially to the fact that the rivers of Scandinavia, Russia, and the north of Germany descending into the Baltic flow through hydrographic basins covered by the diluvium of the north of Europe, which is essentially sandy. Mollusca are rare in the Baltic on account of the very slight saltness of its waters.
Let us now pass to the ocean, leaving the seas of France and of the British Isles, which have been previously studied.
The ocean is of great depth along the Iberian peninsula, and at a short distance from its banks. Submarine rocks indicate the con-
