- 49 - (pycnocline). The deep sea comprises approximately 95 per cent of the ocean waters. The principal current systems of the surface water in the northern hemisphere circulate in the ocean basins in a clockwise direction. In the southern hemisphere the cir- culation is counterclockwise. The waters move slowly, the average speed of the surface currents in the open ocean being one-half tome mile per hour, move much more slowly. layer, but the deeper currents Below the relatively thin surface often called the stirred layer, the waters are stratified into a series of layers of increasing density and slow movement, with little mixing between layers. The direction of flow of the deepest layers may be in counter direction to the flow of the surface current. With these conditions the exchange of deep water and surface water can be expected to be a slow process. At the present time the age of deep ocean water in the Atlantic is being determined by the c14 age-dating method. From the best informa-~ tion now at hand, the time required for the replacement of Atlantic bottom water with surface water is in the range of 200 to 500 years, but in the Pacific the time may be as great at 1,000 years (Schaefer 1958). An exception to this