The exposure of quartz monzonite examined near Stevens Creek is of middle Tertiary age, possibly Eocene (Mertie, 1937). Granite on the opposite side (northwest bank) of the Yukon River, however, was mapped by Eakin (1916, pl. 2) as Paleozoic, probably Devonian or Carboniferous. No appreciable radioactivity was detected in either body. DEVONIAN Or carboniFEROUS DEPOSITS A very large area of greenstone of Devonian or Carboniferous age that is composed largely of altered mafic flows and tuffs, diabase, and impure limestone, is exposed along the Yukon River from about Fort Hamlin to Rampart (Eakin, 1916, pl. 2). No abnormal amount of radioactivity was detected in the greenstone, and readings only slightly above background were noted in areas of movement where the rock is crushed and has a shaly appearance. Granitic intrusive rocks of Devonian age (?) (Eakin, 1916) were examined in an area near the rapids about midway between Rampart and Tanana on the Yukon River (fig. 17). The radioactivity was about the same as for the monzonite that crops out along the Yukon River opposite Stevens Creek; higher readings were noted on the finer grained variety of monzonite. DEVONIAN AND PRE-DEVONIAN DEPOSITS Dark tough schistose metamorphic rocks of pre-Devonian age crop out in sections along the Yukon River upstream from Stevens Creek. Readings slightly above background were obtained on the metamorphic rocks examined in the field, but results obtained from laboratory analyses show no appreciable radioactivity. PROSPECT Also examined was the argentiferous galena prospect reported by Eakin (1916, p. 82) on Quartz Creek (fig. 17). He noted that most of the veins occur as small stringers cutting limestone. Radiometric tests were made around the adit, tailings, dumps at prospect pits, on ore in a box at the cabin, and on the nearby hills, but no anomalies were noted. TANANA TO RUBY QUATERNARY DEPOSITS Alluvial deposits of Quaternary age constitute most of the exposures in the banks of the Yukon River between Tanana and Ruby. No appreciable radioactivity was detected. CRETACEOUS DEPOSITS Sedimentary rocks of Cretaceous age are exposed in the Melozitna River canyon (fig. 17). Radiometric tests on conglomerate, sandstone, quartzose grit, black shale, and limestone in this area indicated about 3 times the radioactivity of other rock types tested farther upstream on the Yukon River. The grit gave the maximum reading in the field, but tests made in the laboratory show a low radioactivity content (maximum of 0.017 percent eU) for all the various rock types examined (table 7). DEVONIAN OR CARBONIFEROUS DEPOSITS The gneissic intrusive rocks that crop out along the right bank of the Yukon River a few miles below Birches were examined (fig. 17). The gneiss is probably of Devonian age (Eakin, 1916, pl. 2) and can be divided into a light-colored coarse-grained rock and a darkcolored fine-grained somewhat schistose gneiss. The radioactivity of both type of rock was only slightly higher than that of the metamorphic complex tested in the immediate area (table 7). A gneissic tourmaline granite found as talus below Birches was not abnormally radioactive. Downstream from Ruby for a distance of about 11⁄2 miles, the schist and metamorphic rocks are intruded by granitic dikes. The age of the dikes is not known and is listed here tentatively as Devonian or Carboniferous. None of the dikes is more than 10 feet thick. The radioactivity of the dikes is slightly above the background, about 0.004 percent eU. DEVONIAN AND PRE-DEVONIAN DEPOSITS The schist and metamorphic complex of Devonian and pre-Devonian age that occurs near Ruby and Kokrines (fig. 17) was examined, but no anomalies were detected. 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