At about the same time, the outer surfaces of the calcium hydroxide icles must have been carbonated by the carbon dioxide of the atmos- phere. It is well known from observations on the bardening of plaster (calcium hydroxide) that the outer layer of the plaster is slowly con- verted to calcium carbonate in the presence of moist air, The fallout particles were exposed to moist tropical air several days before their shipment to this Laboratory, A study of the thin sections plainly showed the progressive carbonation of the calcium hydroxide which was, however, confined to a surface layer usually not exceeding 100 4. in thickness, Any calcium carbonate formed in this manner would probably have the calcite structure as this is the stable form at low temperatures, The X-ray diffraction analyses showed the presence of both calcite and ara~ gonite (unaltered coral) in the fallout particles. However, the amoumt of calcite Was much greater than the amount of aragonite indicating that most of the calcium carbonate in the fallout particles was of this secondary origin. It seems probable that moet of the fallout particles were formed from discrete grains of coral rather than by the agglomeration of pulverized materials. This is evidenced by the homogeneity of the particle in texture and composition, by the angular shape of the particle and by the occurrence in some particles of a central core of unaltered coral sur- rounded by a layer of calcium hydroxide. A few of the particles, however, showed definite signs of being formed by accretion. They had spherical or sub-epherical shapes and were not homogeneous but were formed of agglomerations of crystalline grains and the radioactivity was distributed irregularly throughout the particle. Some of the particles were not close enough to the fireball to be decarbonated and remained wmaltered except for collecting a surface coating of radioactivity.