As a member of the party investigating the head district of the Tone River, the writer surveyed the area between the Sudagai Dam to Mt. Otonedake along the valley of the mentioned river, as shown in the geological map. The geological formations developed along the valley are as follows in descending order :Mesozoic Okutone Group4. Higashimatazawa formation : chiefly of shale and sandstone, associated with thin limestone at places. Molluscan fossils occur.3. Kengakurasawa shale shale predominating.2. Idozawa sandstone : an alternation of sandstone and shale predominating.1. Minagasawa shale : shale predominating.From the Higashimatazawa formation the writer collected some molluscs, which H. Fujimoto and Y. Yabe determined as Entomonotis cf. ochotica (KEYSERLING), and its variety var. eurachis (Teller) from Loc. 17 T, and Entomonotis tenuicostata KOBAYASHI and ICHIKAWA and its variety var. mabara KOBAYASHI and ICHIKAWA from Loc. 192.The evidence afforded by the fossils indicates that the Higashimatazawa formation is upper Triassic in age.