Chemical and mineralogical changes of volcanic ash beds which were erupted from the Volcano Ontake around Ina district, Nagano Prefecture by weathering were examined by means of the chemical analysis, X-ray powder diffraction, differential thermal analysis and grain size distribution analysis. In the Middle Loam unit, clay mineral formed by weathering was mainly allophane in the pumice beds of Pm-II, II', III, whereas it was mainly halloysite 10Å in the Pm-I and the other loam beds. In the Younger Loam unit, clay mineral formed was mainly allophane and halloysite 10Å was not observed. By weathering the compositions of SiO2, CaO, MgO, Na2O and K2O decreased, whereas those of 2O3 and ΣFe2O3 increased by the relative condensation, and H2O increased by the formation of clay minerals. It was found in the chemical composition changes of SiO2, Al2O3 and ΣFe2O3 that SiO2 and ΣFe2O3 leached and Al2O3 relatively condensed in the lower part of the outcrop, whereas SiO2 leached and Al2O3 and ΣFe2O3 relatively condensed in the middle and upper parts. These chemical changes were understood by assuming the existences of reducing, intermediate and oxidizing zones by the reaction of underground water and/or by considering the changes of the chemical compositions of the host rocks from rhvolitic to andesitic.
