The movements of the sea-water fall into several categories. There is (1) a general circulation of sea-water, determined chiefly by three factors: differences in density in the sea-water, differences of level, and the general movements of the atmosphere; (2) periodic movements which are not primarily circulatory, brought about by the attraction of the sun and moon; and (3) aperiodic movements, due to occasional causes, such as earthquakes, volcanic explosions, landslides, etc., which determine local and temporary movements, often of exceptional strength. =Differences in density and their results.=—Differences in density result from differences in temperature and salinity. Temperature alone considered, water would be densest where it is coldest, namely in the polar regions. Differences in salinity result from differences in evaporation and from inequalities in the supply of fresh water. Evaporation alone considered, the sea-water should be densest where evaporation is greatest; but the equatorial region, where evaporation is greatest, is also a region where precipitation is heavy, and precipitation, by freshening the water, opposes the effect of great evaporation. The greatest differences in density due to the unequal supply of fresh water are to be found near the borders of continents, where the precipitation on the land is discharged into the sea. In the polar regions, the great supply of fresh water, especially during the season when the ice is melting, opposes the effect of the low temperature, so far as the density of the water is concerned. The result of the operation of these factors affecting the density of the sea-water is to insure a general circulation, directed to the end of equalizing the densities; and since the disturbing factors are constantly in operation, equilibrium is never established, and the movements of the water are perpetual. The pressure gradients resulting from differences of density are so slight that the resulting movements are scarcely more than a creep of the waters. In general they are far too slow to be of importance in gradational work; but the earth’s rotation deflects the creeping waters and tends to concentrate the equator-ward movement into currents on the east sides of the continents, and the pole-ward movement on the west sides. In favorable situations these currents may be competent to produce sensible mechanical results. Even where this is not the case the circulation helps to equalize the temperatures of the sea, and so of the air above and of the land about. Indirectly, therefore, the circulation of the ocean-waters affects every geological process which is sensitive to climate.