The ocean must be considered as a unit in which all parts and manifestations are interrelated and interdependent. Although subject to periodic variations, the conditions can, in the broader sense, be considered as stationary, where the tendency of mass movements to modify the distribution of conditions is balanced by the effects of external agencies (sun and atmosphere) and the internal processes of turbulent mixing. The distribution of conditions in the ocean can be understood only when the processes operative therein and the effects of external agencies can be evaluated.

External agencies tend to bring about regional differences in conditions and motion in the sea. The effects of the external agencies depend upon the properties of the water and the degree of topographic isolation of the zone under consideration. The stability, that is the increase of density with depth, also plays an important role in determining the distribution of conditions. The physical properties of sea water depend upon the three independent variables: temperature, salinity and pressure. Conditions at and near the surface are determined by the external agencies, mixing and the general circulation. Conditions at subsurface depths are determined by the mixing and the origin and history of the water. The thermal conductivity, diffusion and transfer of momentum (viscosity) associate with the turbulent motion in the ocean are of the greatest importance in the maintenance of the distribution of conditions.

The peculiar environment existing in basins and the nature of the sediments laid down in such areas are to be attributed to the degree of isolation of the water and the effects of the local external agencies.

Movements in the ocean can be considered as turbulent motion or wave motion when, in a given element of space and interval of time, there is no net transport of water. Currents exist where there is a certain uniformity of velocity and a net transport in a definite direction. Currents arise from the effects of solar radiation, the stresses exerted by the winds on the surface of the sea, and the horizontal movements associated with the tides. To understand the three general types of currents,--"convection" currents, currents due to wind action, and tidal currents,--the forces concerned must be evaluated. Convection currents may be computed when the distribution of temperature and salinity is known. Tidal currents may be computed when the characteristics of the tides are known. Currents et up and maintained by the wind are not yet entirely susceptible to quantitative treatment. In areas of divergence the upwelling of subsurface water, brought about by the wind, has a marked effect upon the physical conditions and the biological productivity.

The types of sediments laid down in any area depend largely upon the proximity to land, the nature of the source materials, the biological conditions, which in turn depend upon the nature of the local environment, and the turbulence and transport, which affect the distribution of the sediments. In order to evaluate the processes of sorting and transportation, the amount of turbulence and the velocity distribution near the bottom must be known.