In their general extent and position, it is likely that ocean basins have been permanent since the waters were gathered together. The study of deep-sea observations does not support the view that, by alternate rising and sinking of the earth's crust, oceans and continents have successively occupied the same areas.

All known chemical elements have been found in the waters of the ocean, and if there are any that are unknown, they will also be found there. Although the total amount of salts contained in sea water varies in different parts of the ocean, the existing proportions set forth in the following table, according to Dittmar's

DIAGRAM MATIC SECTION SHOWING THE AVERAGE CONTOUR OF THE LITHOSPHERE, BASED UPON THE PERCENTAGES OF THE AREAS BETWEEN THE CONTOURLINES ABOVE AND BELOW SEA-LEVEL.

analysis, are practically the same in all parts and at all depths of

The dissolved substances are present mainly as ions. Of the elements in solution, sodium and chlorine seem to be the only ones that are not withdrawn by organisms and organic processes,

AND 50°S.

50°

7-8 cm

6-7ccm

Om

and they may therefore have been uniformly on the increase since the beginning of geological time. This conclusion has been made use of in the calculations which have assigned 100,000,000 years as the age of the ocean.

The average value of the salinity of the surface waters of the ocean is 35 parts in a thousand parts of water. The factors affecting its variation are evaporation, dilution, and the action of winds. In the Red Sea, where the rate of evaporation is high and the rainfall small, the salinity rises to 39, while, under the opposite conditions of the China Sea, it falls to 33. In the borders of the frigid zones, as well as in the doldrums of the tropics, the salinity is reduced, and in the trade-wind regions increased. In general, there is a small decrease of salinity with increasing depth in the open ocean down to about 1000 fathoms, and below that a tendency to increase is observable.

The gases of the atmosphere are everywhere contained in sea water, but the proportions in which they are present are different in the atmosphere and the ocean, because of the varying capacity of water to dissolve them. Dry air contains about 21 per cent of oxygen, but air extracted from sea water contains about 34 per cent. The gas content of sea water is chiefly made up of nitrogen and oxygen, together with small quantities of free carbonic acid and traces of argon and neon.

While the nitrogen suffers little or no diminution, the oxygen is continually being used up by the processes of marine organisms, so that, if it were not being constantly dissolved at the surface and distributed by the general circulation, the proportion of oxygen in the depths would be reduced below the amount necessary to support the animal life which now abounds there. The distribution of oxygen in the Atlantic Ocean is shown in the preceding diagram. The rate of solubility varies with the temperature in such a manner that more gas is absorbed at low than at high temperature. Since the rate of solubility is greater at low than at high temperatures, the polar waters are found to contain as high as 24 parts of oxygen and nitrogen per thousand parts of water while tropical waters contain as low as 12 parts.

It is in respect of temperature that the ocean exhibits many of its most significant characteristics. Over 80 per cent of its