STRONTIUM-90 ON THE EARTH’S SURFACE. II 363 sent southern-hemisphere sampling points. Sites in the northern hemi- sphere are labeled with the city name. The five sites with fallout of more than 60 mc/sq mile indicate that the soil values averaged about 10 to 15% greater than the deposition collected in the pots during the same period. These results are not surprising since dry fallout has been shown to be of about this order of magnitude and would be less efficiently sampled by the precipitation collectors. There is also evidence, especially during periods of high winds or snow, that precipita- tion collectors are not completely efficient in collecting all the precipitation falling on a given area. GLOBAL DEPOSITION The results of the 1963 and 1964 sampling are shown in Fig. 5. The isolines are drawn on the basis of the observed worldwide sampling data and mean precipitation patterns since precipitation is a principal mechanism for the deposition of "Sr on the surface of the earth. The individual data for the continental United States will be shown sepa- rately. The latitudinal distribution is shown in Fig. 6. Sites in the United States are indicated by crosses. The solid curve indicates the mean latitudinal values as determined from Fig. 5. Since samples are collected principally in or near populated or agricultural areas, normally regions with adequate rainfall, there is a tendency for the individual points to lie above the mean curve. The mean latitudinal values reflect interpolation that includes regions of sparse rainfall. The total global fallout indicated by the 1963 to 1964 sampling is 7.6 Mc of Sr with an estimated random uncertainty of less than +3 Mc. Table 1 shows the inventory of *Sr in late October 1963 anda comparison with the amount thought to be available for global fallout. In addition to the random uncertainties, there are several possible sources of systematic uncertainties, most of which would tend to make the true deposition higher than that estimated from the data. Among these are percolation of the "Sr to greater depths in the soil; the Table 1—INVENTORY OF Sr IN OCTOBER 1963 Megacuries* Worldwide deposition Estimated in atmosphere below 100,000 ft _ Local fallout 8 5 2 Total 15 Estimated total production 19 *Corrected for decay.