Abstract: The gabbroic mass described in the present paper, is situated along the Sano Valley, southern Kanto, and its geologic age is probablly Miocene. The petrographic features of this mass are described here, with special reference to the anorthite contents (An%) of plagioclase in some rocks of the mass. The study of plagioclase is summarized in the An composition frequency diagram proposed by C. Kim (1962). In this diagram, it seems likely that the maximum peaks of the frequency curve show stable environments in the lithological evolution of a rock series. The conclusions derived from the above studies are as follows. 1. The mass consists of three groups; i) gabbroic rocks (norite, pyroxene gabbro, pyroxene porphyrite, uralite porphyrite, hornblende gabbro), ii) dioritic rocks (diorite, quartz diorite) and iii) albite porphyrite. 2. Various parts of gabbroic rocks and of dioritic rocks grade gradually to each other, while albite porphyrite forms another by sheared zones. 3. From the frequency curves of An% of plagioclases, it is noted that frequency curves of plaigoclase of norites and of quartz diorites are the most stable, while those of hornblende gabbros are the most unstable. The gabbros and diorites were subjected to different tectonic movements and the formation of hornblendes in pyroxene gabbro occurred at the stage between the above two tectonic movements. Coarse-medium grained plagioclases in norite, and fine grained plagioclase in quartz diorite show the most stable frequency curves. 4. The evolutional succession of the mass is as follows: i) intrusion of pyroxene porphyrite, ii) emplacement of norite, iii) transformation of the two rocks facies into uralite porphyrite and hornblende gabbro, iv) shearing movement, followed by formation of diorites, and v) formation of albite porphyrite along the sheared zones around the diorites.