50% below the average values found for natural gamma radiation levels (7 ur/hr). San Francisco Bay Area The survey in this area was undertaken in cooperation with the University of California Lawrence Radiation Labora- tory (UCRL) at Berkeley. Soil samples were taken at our survey locations and analyzed spectrometrically by UCRL scientists for K49, Th232, and v238 concentrations.“ Their results were most helpful in assessing the validity of our methods of analysis, Two specific Locations were of particular interest. The first was a fern patch near Bonny Doon, California, where we obtained our largest K49 dose rate contribution (over 8 ur/hr), corresponding to a soil potassium concentration of about 5%. This measurement was | very helpful in verifying our K*9 calibrations. The second was an asbestos mine near Copperopolis, California. This location was. unique in that there appeared to be almost no (<1 ur/hr) gamma radiation from natural sources in the serpentine bedrock itself. This enabled us to obtain a spectrum which was due primarily to cosmic radiation (see Figures 3 and 4). The measurements obtained at this location were especially useful in substantiating the cosmic ray calibration of the ionization chamber as well as in verifying the assumption of negligible cosmic ray contribution to the total spectrum "energy" (spectrum "energy" due to cosmic sources is equivalent to a gamma contribution of ~O.2 pr/fhr at sea level). It should be noted that the 1963 measurement was not taken at exactly the same location as the 1962 measurement. The larger U238 series dose rate in 1962 may have been due partly to radon-222 daughters in the air or on the surface of the ground. Fallout levels in this area were fairly low, ranging from 0-1.5 ur/hr during both the 1962 and 1963 surveys. This might be explained by the fact that both surveys were carried out after several months of very little rainfall. A number of measurements were made between San Francisco and Denver at elevations greater than 6000 feet, (Fruitland, Rawlins, Laramie, Wyo.). These altitudes correspond to cosmic ray dese rates of about twice the sea level value and thus any significant errors in the cosmic ray calibration