The quantity of salt held in solution in various parts of the Mediterranean differs widely, as the submarine ridges and shoals which divide it into separate basins do not permit its waters to mingle as freely as in the open ocean. Owing to the excess of evaporation, the quantity of salt is greater on the whole than in the Atlantic, and this is the case more particularly on the coast of Africa. But in the Black Sea it is far less, and near the mouths of some of the large rivers which enter that sea the water is almost fresh.[8] The temperature of the Mediterranean is affected by the same causes which produce its varying salineness, viz. the existence of shoals and banks, which separate it into distinct sub-basins. In the open ocean the currents convey to all latitudes large bodies of water, some of them heated by a tropical sun, others cooled by contact with the ice of the polar regions. But these layers of unequal density are regularly superimposed one upon the other, owing to the differences in their temperature: the warm water remains on the surface, whilst the cold water descends to the bottom. In the Mediterranean an analogous superimposition exists only to a depth of 110 fathoms, which is the depth of the Atlantic current, flowing into it through the Strait of Gibraltar. If a thermometer be lowered to a greater depth it will indicate no further decrease of temperature, and the immense body of water, remaining almost still at the bottom of the Mediterranean, has an equable temperature of about 56° F. Observations made at depths varying between 110 and 1,640 fathoms have always exhibited the same result. Professor Carpenter believes, however, that the abyssal waters of some of the volcanic regions have a somewhat higher temperature, which may be due to the presence of lava in a state of fusion. II.—ANIMAL LIFE. FISHERIES AND SALT PANS.