A dome structure formed of successive pyroclastic accumlation of so-called “green tuff effusion” is the main geologic constituent of the Shakotan Peninsula, southwestern Hokkaido. Igneous effusion and intrusion held in the district are carried wholly concordant to this dome structure; intrusive bodies are prefered to the central part of the dome and younger effusives are arranged to the peripheral part. Further, prominent mineralized zones closely allied to this geologic structure are known in this volcanic field. They are arranged to the cross direction of the axis of the peninsula. With them, interesting zonal distribution of ore deposits such arrangement as (1) Mn (Pb, Zn) deposits disposed to the central part of the dome succeeded by (2)|Cu, Pb, Zn (Au, Ag) deposits, (3) FeS₂ deposits, and (4) BaSO₄ deposits to the outer parts, are well ascertained.
Of these deposits, some peculiar features of ore mineral are mentioned. They are unusual occurrences of pyrrhotite and molybdenite in the (1) Mn (Pb, Zn) deposits, realgar and orpiment in the (2) Cu, Pb, Zn) (Au, Ag) deposits and stibnite in the (4) BaSO₄ deposits. The trace elements associated with these deposits have also peculiar behaviours. High concentration of W in the rhodochrosite from the (1) Mn (Pb, Zn) deposits, and Bi, Mo, Sn in the sulphide minerals from the (2) Cu, Pb, Zn (Au, Ag) deposits are their prominent features.
Several varieties of wall rock alteration are mentioned with these ore deposits. Although sezicite-quartz and montmorillonite-kaolinite-quartz associations are the common feature of wall rock alteration of them, carbon-itization around the (1) Mn (Pb, Zn) deposits, occurrence of alunite, diaspore or jarosite with the (3) FeS₂ deposits and the (4) BaSO₄ deposits are the peculiar feature of this ore field. Their peculiarity is well serve for the prospecting researches.

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K-A dating on seventeen samples of granitic rocks, one pegmatitic vein in hornblende gabbro and one epidote-amphibolite from the Abukuma massif were carried out. As a result, maximum and minimum values of granitic rocks show 107×10₆ years and 87×10₆ years respectively, and these ages of emplacement of granitic rocks are broadly divided into two periods 90×10₆ and 100×10₆ years. On the other hand, formerly the ages of intrusion of granitic rocks in the Abukuma massif have been geologically divided into two main periods younger and older. But the results of K-A dating of this time do not accord well with the former geological observations, and even the reverse cases are recognized indicating the minor time difference of these emplacements. It is clarified that the granitic rocks of the Abukuma massif are emplaced in later Cretaceous and younger than that of Kitakami massif about 20×10₆ years on the average. K-A dating on metamorphic rock from the northern Abukuma by muscovite minerals shows 300×10₆ years; this indicates that the metamor-phism would have occurred older than Carboniferous age at least.

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The Tanzawa mountainland consists mainly of a large amount of marine pyroclastics of the Miocene age. The central part of this area is occupied by a quartz diorite mass accompanied with various kinds of metamorphics around and within it. The present paper gives the general geology of the Tanzawa mountainland and its geological meaning in the Japanese islands.
The metamorphics and the plutonics of the Tanzawa mountainland compose a plutonic zone, and the non-metamorphosed rocks of Neogene Misaka series are tectonically discordant with this plutonic zone. The quartz diorite mass including gabbroic parts is genetically related to some metamorphics. The metamorphics consist of green schist-amphibolite facies in the western contact with the plutonic mass, and hornblende hornfelsbiotite chlorite hornfels fades at the eastern contact. And both show an increasing of metamorphic grade towards the pultonic mass. Some high grade metamorphics and the other metamorphics are distinguished each other in their field occurrence and the petrographic observations. The low grade metamorphics and non-metamorphosed rocks are often separated by large faults and/or basic intrusions.
It is pointed out that this mountainland covers not only a part of the Fossa Magna region and the Green Tuff area, but also the Shimanto geosynclinal area of the Mesozoic age. In comparison with ultrabasic and acidic plutonic rocks of the post Mesozoic age, this quartz diorite mass probably belongs to the basic intrusive zone with E-W direction of post Mesozoic igneous activities.

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