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Tridymite occurs in the vicinity of Kusatsu Spa as extremely fine crys tals in opaline silica which has replaced two pyroxene andesite. Specific gravity: 2.2•}0.1, optical properties: n1•†1.478, n2•...1.480, 2V ?? 0, elongation negative, showing straight but wavy extinction. Chemical composition: SiO2 96.89, TiO2 0.86, A12O3 0.71, Fe203 tr., FeO none, MnO none, MgO none, CaO none, Na20 0.72, K20 0.16, H20(+) 0.12, H20(-) 0.08, P2O5 tr., total 99.54%. The d. t. a. curve shows two endothermic peaks, one at 128•Ž. and the other at 155•Ž.; they are attributed to the transition from the low to the middle form and from the middle to the high form respectively. Many sharp X-ray powder diffraction lines were confirmed, most of which have not been reported in previous papers. Under the microscope, noticeable tex ture of the silicified andesite was observed, suggesting that the tridymite has gradually crystallized out of opaline silica: some of its crystals are extremely small and appear to grade into opaline silica. Introduction Along the river Yugawa running through Kusatsu Spa, silicified rocks are distributed over considerable area. The writer discovered that some of them are largely composed of opaline silica and tridy mite. Any report concerning such a mode of occurrence of tridymite has not yet been published*). The purpose of this paper is to pro* Just after this paper had been completed , H. Minato reported the similar occurrence of tridymite associated with hydrated halloysite at the Joshin Mine, Gumma Prefecture (Advances in Clay Science, 3, 264 (1961), in Japanese.)

TRIDYMITE CRYSTALS IN OPALINE SILICA FROM KUSATSU, GUMMA PREFECTURE