According to the latest reckoning (E. Kossinna, 1921), the areas and mean depths of the four parts of the Mediterranean are : The total volume of the basin is calculated as 4,238,00o cu.km. (E. Kossinna, 1921).
very great part of the bottom of the Mediter ranean is covered with blue muds, frequently with a yellow upper layer containing 50-60% of carbonate of lime, chiefly shells of pelagic Foraminifera. In many parts, particularly in the eastern basin south of Crete, in depths varying between 30o and 3,000 metres, hard calcareous crusts from -I in. to 3 in. in thickness are met with. Their origin is not yet explained. The fine mud of the Nile, which is directed in a northerly and north-easterly direction by the off-shore currents, should go still further north to the deeps off the Syrian coast ; it is very poor in chalk content.
In any month of the year the superficial tem perature of the Mediterranean is highest in the south-east towards the Levant, and lowest in the Gulf of Lions, the north Adriatic and in the north of the Aegean sea. Thus we observe a temperature of 12-13° C in February at the Riviera, at Port Said 17° C; in August, Riviera 2o-24° and Port Said
Below a level of 200 metres the waters seem to hold an even temperature, independent of the depth, of about 13° C in the western basin and about 14° in the eastern. In these "homothermic" deep layers, the salt-content is likewise constant, namely, 33.4/o, in the western area and
in the eastern. In the upper levels of the sea east ward from Gibraltar along the Algerian coast, there is only about
of salt content; here, with the surface currents com ing into the sea from the ocean, is also Atlantic water weak in salt. A second area with little salt-content in the upper waters is the north Adriatic. In all other parts of the Mediterranean, the upper waters are rich in salt, with over
in the Levant basin over
while in the dry and hot summer period, evaporation is considerable.
There is little definite circulation of surface water within the Mediterranean itself. In the straits joining it with the Atlantic and the Black sea the fresher surface waters of these seas flow inwards to assist in making good the loss by evapo ration at the surface of the Mediterranean, and in both cases dense water makes its way outwards along the bottom of the channels, the outflowing currents being less in volume and delivery than the inflowing. Elsewhere local surface currents are developed, either
drifts due to the direct action of the winds, or streams produced by wind action heaping water up against the land ; but these no where rise to the dignity of a distinct current system.
The Mediterranean exerts a considerable influence on the climate of the lands whose shores it washes. In the winter months areas of low atmospheric pressure are formed over the individual basins, and over European lands and North Africa areas of high atmospheric pressure are formed. From these causes spring the annual cyclonal depressions from west to east in the Mediterranean; they bring with the changing winds the winter rains of this zone. But in summer there is, with a moderately high barometer, a proportionate fall in atmospheric pressure from north-west to south-east, and steady, dry, northwest-north-north easterly winds blow below a cloudless sky ; this is the Etesien of the ancient Greeks. In Malta there is an annual rainfall of 517 mm., of which 445 mm. fall during the period October to March, leaving 72 mm. only for the months April-September; June and July are entirely dry months. The Mediterranean has character istic winds, e.g., the hot and dry Sirocco (q.v.) of Sicily (particu larly in the early months), the hot and humid Sirocco of the Adriatic (in winter), both blowing from the south-east and south. There is the Bora, a cold wind in the territories of Istria and Dalmatia. The Mistral comes from the north (from the estuary of the Rhone). The Gulf of Lions (and, in general, the whole of the north-western part of the Mediterranean) is notorious for its terrible winter storms.
The air-temperature is usually, in winter, between io° and 12° C, reaching, however, in the height of summer (in July and August) tropical heat, viz., 25° to 27° C.