nS 0018933 oxide, “calcium hydroxide and calcium carbonate formed by the decarbonation, Hydration and recarbonation processes going on in the firebafl and subsequently. The majority of them were white or transparent but some were yellow or brown. Many of the flaky aggregates were observed to disintegrate spontaneously into smaller particles within a few hours after collection. In addition to these primary types, a fourth kind was noted con- sisting of small black spheres of calcium iron oxide (2CaO+ Fe 03), These were usually observed adhering to the surfaces of the large grains but occasionally were found isolated (Reference 12). For detonations over ocean surfaces the fallout collected consisted of droplets of salt slurry 50-300 microns in diameter. These contained about 80% salt, 18% water and 2% insoluble solids by volume, The major part of the radioactivity was found in the insoluble solids portion. The fallout deposited at more distant points has not been as well characterized but is believed to be composed of minute spheres formed by condensation of the environmental material from the vapor plus a very fine, unfused dust swept up into the cloud from the area around the shot point (Reference 14), The availability of the radioactivity in the fallout for assimilation into the biosphere depends to a large extent on its solubility in aqueous SAN BRUNO FRC 18 oo —s