Neogene Tertiary composed of Atsumi, Kamigo, Mogami and Shonai Groups are developed on the basement of pre-Tertiary granites in the Shonai basin of Yamagata, Japan. Mogami Group which is composed of Aosawa, Kusanagi, Kitamata, Tateyama, Maruyama and Kannonji Formations, has been studied from the petroleum geological viewpoints in order to examine its potentiality of oil and natural gas.
Six routes in mountaineous area of the eastern part of Shonai basin have been surveyed in detail and two hundreds of mudstones and siltstones in Mogami Group were collected. Mineral composition of these samples were analyzed by X-ray diffractometer using zinc oxides as internal standard sample. From these data, the writers settled the following five mineral assemblages. These assemblages include A₁ (montmorillonite-illite-poor chlorite-quartz-analcite and/or heulandite-calcite-dolomite zone), A₂ (poor montmorillonite-illite-poor chlorite-cristobalite-clinoptilolite-calcite-dolomite zone), A₃ (montmorillonite-illite-poor chlorite-cristobalite-clinoptilolite-calcite zone), B₁ (montmorillonite-illite-chlorite-kaolinite-cristobalite-clinoptilolite zone), and B₂ (montmorillonite-illite-poor chlorite-kaolinite-plagioclase-clinoptilolite zone). Stratigraphically, assemblage A₁ may be equivalent to Aosawa Formation; A₂ to Kusanagi and lower part of Kitamata F.; A₃ to upper part of Kitamata, Tateyama and Maruyama F.; B₁ to lower part of Kannonji F.; and B₂ to upper part of Kannonji F. (Jozenji Facies).
Significance of these mineral assemblages were discussed from the sedimentary petrological points of view. As a result, it is clear that these assemblages are quite suitable for the analysis of depositional environments, source materials and diagenesis in argillaceous rocks of Mogami Group.
The mineral assemblages in argillaceous rocks of the Mogami group at five wells drilled in the Shonai plain have been studied. Settled five mineral assemblages are somewhat different from those of mountaineous area. However, these assemblages may clear the stratigraphic problems between Aosawa and Kusanagi Formations which have been identified by lithological and paleontological data only.

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The Myoken-zan granodioritic complex is a small stock-like mass formed by Late Mesozoic to Early Tertiary acidic igneous activity. It can be devided into seven rock types according to the field occurrences and petrographic features. They are:
Boundary between seven rock types are almost sharp, but partly indistinct. The mode of occurrence of the rock types (1) to (5) is like a concentric ellipse. Mineral associations of orthopyroxene with clinopyroxene, orthopyroxene with biotite and ore mineral, clinopyroxene with hornblende and so on are often observed in the rock types (1) to (5).
Plagioclases in the rock types (1) to (5) show wide range of chemical composition and complicated zonal structure, the cores of which are high in anothite content (An_69-84). Plagioclases in the rock types (6) and (7), on the other hand, show narrow range of chemical composition and simple zonal structure, the cores of which are low in anothite content (An_44-58) than that of the former five. They show scattered ordering degree, and partly high-temperature type exists.
Bulk chemical composition was determined on sixteen rocks including three aplites. Variation trends of the rock types (1) to (5) on the D. I.-oxides diagram, AFM diagram and Or-Ab-An system vary lineally. Rock type (6) on the Or-Ab-An system, rock type (7) AFM diagram and Or-Ab-An system, fall total FeO and Ab side respectively.
North body is explained in terms of magmatic differentiation of an intermediate magma. Five fractions formed in magma reservoir intruded successively in the shallow levels of the crust. South body was derived from another magmas.

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Coexisting biotites and hornblendes from the rocks of the Tabito plutonic complex were analysed chemically. The Mg-Fe⁺² distribution coefficients in the two mafic minerals, ranging from 0.74 to 0.93, are directly proportional to the Al^IV contents in the hornbendes. The relation is universal in the talc-alkaline granitic rocks of various districts.

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