34 where variable levels of activity with depth would mask any diurnal-nocturnal variation that might exist. The change of relative radioactivity of the water with derfth is variable but appears to fall into regional patterns. It is likely that at the time of the Marsh survey those regions containing the highest relative activity levels in the deeper layers were regions primarily affected by fallout, while those regions having the bulk of the radioactive materials in the surface layer were regions to which the radioactive materials in solution had been carried by surface currents. It would follow that by the time of the survey vertical mixing had not taken an important part in redistribution of the radioactive materials; possible exceptions may occur in the immediate vicinity of the Mershall and Marianas Islands. This interpretation which is based on the assumption that most of the radioactive material went into either a true solution or a suspension of very small colloidal particles soon after fallout is supported by the fact that most of the activi- ty passes through a Millipore filter (Table 3). However, dif- ferences in specific gravity of the particles would result in the same type of distribution. It is possible that relatively higher values near the surface for stations to the westward are accounted for, in part, by fallout of lower specific gravity than that of fallout nearer the test site. The particles of lower specific gravity would be assumed to have gone into com- plete solution while still near the surface.