In the northern Kanto district belonging to the Fossa Magna there are distributed Neogene Tertiary volcanics and sediments of the so-called Green Tuff series, Cretaceous granite, highly metamorphosed Paleozoic to Mesozoic formations, and some masses of Tertiary plutonics. The Tanigawadake plutonic complex, one of these Tertiary plutonics, has been reported to consist of quartz diorite with a large xenolith-like hornfels of probably Paleozoic formation. From the petrological study on this complex, the following conclusions are obtained.
1. Two geological units are recongnized in the northern Kanto district; basement complex of Paleozoic and Mesozoic formations and Cretaceous granite in the north west part and Neogene Tertiary volcanics of basalt-rhyolite series of the so-called Green Tuff formations in the south east part. The Tertiary plutonics are generally distributed at the transitional zone of the above two units.
2. This area is consisted of two geological units of the basement complex and the Tertiary plutonics. The former consists of many kinds of crystalline schist of probably Paleozoic formation and potassh-feldspar porphyritic granites of the Cretaceous igneous activity, and the latter consists of quartz gabbro, diorite porphyrite, and quartz diorite. Between these two units above mentioned, ultramafic rock and diabase and leucocrate are distributed, evidently suffered from contact effect of the quartz gabbro and seems to have intruded into the basement.
3. The quartz gabbro is remarkable of their peculiar mineral assemblages composed of pyroxene, hornblende, plagioclase, biotite, quartz, albite, and potassh-feldspar, all of which can be often observed even in one thin section. It is evident that these constituent minerals have at least two stages of mineralization, i. e., pyroxene and plagioclase in the earlier stage and hornblende, biotite, albite, quartz, and potassh-feldspar in the later stage which may well be called metasomatic stage.

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Usu, an active volcano situated in southwestern Hokkaido, consists of a somma, central lava domes, parasitic lava and crypto-domes.
In early Holocene, on the southern wall of the Toya caldera, Usu volcano was constructed as a stratocone by repeated eruptions of lava flows and scoriae of basalts and andesites. After a quiescence, its summit was broken by violent explosions which led to the formation of a small caldera.
Then the composition of magma changed to dacitic, and consequently the type of activity was converted into formation of lava domes and explosive eruptions of pumice or volcanic ash. Such activities have taken place intermittently also in historic times. Showa-Shinzan is the latest lava dome which protruded on the eastern foot of this volcano in 1943-1945.
In the present paper, petrology of the somma lavas is mainly concerned. Compositions of the somma lavas varied gradually from augite-hypersthene-olivine basalt to pigeonite-hypersthene andesite, in accordance with the sequence of eruption. Simultaneously the composition of olivine and Ca-poor pyroxene, contained in the later somma lavas, became richer in Fe, and finally pigeonite crystallized out as phenocryst.
Mineralogical and chemical studies on the somma lavas show that the somma lavas were derived from tholeiitic magma by fractional crystal-lization.
Dome lavas are all very similar, but have quite different characters from those of the somma lavas both in mineralogy and chemistry. Another interesting problem about Usu volcano, is what kind of process took place during the crystallization of magma at the stage of sudden change of character as above mentioned.
The writer describes some petrographical observations on the dome lavas, and he considers that contamination of granitic rocks by mafic magma is most suitable to explain these observations and sudden change in the composition of magma.

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The massive gypsum deposits of the Yonaihata and the Iwasawa mines occure in the sedimentary rocks of middle Miocene age. At the Yonaihata mine, the mudstone of the wall rocks are partly altered into Mg-chlorite zones by hydrothermal solutions. In this paper, the dolomite from the both mines are examined by chemical analysis, and the relation between dolomitization and Mg-chloritization has been stressed. And then the characteristics of hydrothermal dolomie and typical sedimentary dolomite are ascertained from the results of differential thermal and spectrographic analyses.

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