Nohi rhyolite, a representative product of the Cretaceous silicic volcanism in Central Japan, was studied in its eastern marginal part, both stratigraphically and volcanologically. Stratigraphic succession of the rhyolite in the studied area is shown in the following table. Nohi rhyolite in this area are roughly divided, stratigraphically, into the lower group (stages III and IV) and the upper group (stage V) by a marked structural disharmony, although the latter is subordinate in amount. Volcanic products of stages I and II are distributed in the westerly neighboring area. The lower group constitutes a large-scale volcanic pile of two or three thousands meters thick and is composed of densely welded rhyolite to rhyodacite tuffs, intercalating some lacustrine deposits. It shows rather complicated structure formed by the basin-forming movement and the block movement. It is in contact with the fractured zone of the Permian geosynclinal strata in the east by a NW-SE trending fault. The upper group is distributed mainly in the above-mentioned fracutured zone, overlying the Permian strate with a marked unconformity. Welded tuff sheets composing the upper group are rather thinner than the ones of the lower group and show nearly horizontal structure. Senzawa welded tuff sheet, the lowermost one of the group, is characterized by the abundance of the accidental rock fragments and also by the intercalation of three or more beds of breccias mostly composed of angular, unsorted Paleozoic rock fragments. The mechanism of emplacement of these breccias is discussed in some detail and they are inferred to have been formed by singular pyroclastic flow deposits due to a kind of explosive eruption. The welded tuffs of the upper group is intruded by the adamellite porphyries with a NW-SE trending elongation. The last-stage volcanism in this area can be pictured in such a scheme that it is initiated by the explosive phreatic eruption, followed by the intermittently repeated eruption of the pyroclastic flows and terminated by the intrusion of adamellite porphyries of co-magmatic nature with the erupted materials.

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In the studied area, a large number of minor faults ranging from several millimeters to a few meters in displacements are developed. They are classified into four or five fault systems on the basis of parallelism of strikes, mode of occurrence and successive relationship. Among them, the most predominant one-the main minor fault system -is that of the normal faults with the strikes of N-S to NNE-SSW and dips toward E or W. From the analytical study of these faults, it is considered that they were formed during middle to late Pleistocene under the stress field where σ_1, the axis of maximum (tensional) principal stress, lies nearly horizontal and in east and west direction. On the other hand, the major faults of normal sence, whose range in displacement is from several meters to a handred meter, are also developed well in the present area and have been represented in detailed geological survey. They are almost in paralled to the above main minor fault system, and therefore, both are considered to be formed under the same stress field. Of these normal, major- and minor-faults, the ones which dip to east tend to be more frequently developed than those dipping to west, and the greater the fault displacement, the more marked the tendency. The frequency of occurrence of minor faults seems to become high in inverse proportion to the magnitude of displacement. These phenomena may suggest such mechanism that the minor faults were formed originally as a conjugate set, and then rather one of the conjugate pair dipping to east grew to become a large fault, and thus the present structure stepping down to east has been constituted.

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