The funnel or basin shaped rhyolitic complex occupies almost the whole area of the Ashio mining district, and its surface distribution makes an imperfect ellipse-like outline, reaching about 3.3km in its minor axis and about 4.4 km in major axis, and the volcanic activities of these volcanic complex are supposed to have taken place in the late Tertiary period.
The rhyolitic complex is composed of rhyolite massive flow and rhyolitic pyroclastics, such as volcanic breccia, tuff, tuff breccia, lapilli tuff and also welded tuff. They are accumulated alternately, forming nearly horizontal beds. The lower part of the complex consists mainly of rhyolitic lava, intercalating some rhyolitic volcanic breccia and tuff. In the upeer parts are found alternations of tuff breccia, lappili tuff and welded tuff, the last one being most conspicuous. Thus, the volcanic complex of the Ashio mining district seems to form a stratovolcano.
The basal breccia, named by the writer, occurs along the contact surface between the rhyolitic complex and the Palaeozoic sediments or sometimes the late-Mesozoic granodiorite of basement complex, and is composed mainly of angular or subangular fragments of basal rocks, such as chert, clay slate, sandstone, siliceous slate, schalstein and granodiorite. Judging from the mode of occurence, it may be concluded that the basal breccia is not of explosion, but of talus origin, formed on the steep slope of the wall of the basin.
The funnel or basin shaped hollow, filled up with rhyolitic rocks, may represent a caldera, formed by the depression preceding the volcanic explosion. The central vents of this rhyolitic volcano are supposed to be situated near the pyrophyllitized and brecciated zone of Tenguzawa, and the original crater, now indistinct by erosion, might be located also at the same place.
Fissure veins are formed abundantly in rhyolite flow and welded tuff, but scantly in tuff and tuff breccia. Ore shoots of veins are mostly localized in favorable beds such as rhyolite or rhyolitic welded tuff, which lies nearly horizontally, and then the shoots may have a flat elongation. The localization of ore shoots may be attributed mainly to the deflection of each vein according to the different physical characters of each bed. Bonanzas of so-called "Kajika type" are formed sometimes in tuff and tuff breccia, only when the fissure veins swarm closely, though it is not the case where an isolated vein is present.

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The ore deposits of Kato mine, Fukuoka Prefecture, are probably mesothermal fissure-filling veins in the Mesozoic porphyrite. Minerals in the ores consist of quartz, sphalerite, chalcopyrite, and galena with minor amounts of hematite, pyrite, and klaprothite. The most characteristic feature of the ores is their high selenium content as well as that of gold and silver. Microscopic studies show that the silver content depends on silver-selenide, aguilarite Ag₄SeS, precipitated in place of argentite from the selenium-rich residual solution. Microscopically, the mineral has low hardness, probably Talmage's hardness A, light brownish grey colour, and weak anisotropism showing mimetic twinning. It always occurs as minute interstitial grains among sulphides.
Though native gold is mostly found penetrating sulphides as veinlets along minute fissures and other open spaces, some of them associating with galena and aguilarite occur as rosary and irregular aggregates of rounded granules, 1 to 10μ in diameter.

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Jacobsite, with chemical composition MnFe₂O₄, belonging to magnetite series of spinel group, has been found in the manganese deposits from the Kiuragi Mine, about 15 kilometers west of Saga-City, Kyushu, Japan. This mineral is fine-grained and in tephroite ores associated with rhodonite, spessartine, rhodochrosite and a small amount of penwithite. It shows the following characters;
Colour is magnetite-like iron black with metallic luster. Streak, blackish brown. Strongly magnetic. Hardness, 6± In thin section this mineral is opaque, usually associated with galaxite which shows close similarity in crystal structure. In the polished section it is greyish white with olive tint. Standard etch reactions are negative except that of HCl(conc.)+SnCL₂.
Its X-ray powder pattern is in very close agreement with those of jacobsite from Jacobsberg and artificial MnFe₂O₄. Analysed specimens contain a small amount of galaxite (5.46% wt.). Analytical results are as follows; SiO₂ tr., Fe₂O₃ 56.19, MnO 29.34, MgO 0.10, CaO 0.13, TiO₂ 3.37, AL₂O₃ 11.62, BaO none, total 100.75. After deducing MnAL₂O₄ for galaxite as impurities, the chemical formula can be written as (Mn, Fe^II, Ca, Mg)_0.990 (Fe^III, Al, Ti)_1.950O_4.000. A precise determination of the lattice dimension by the extrapolation-method gave 8.452±0.004A.

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The Hitachi ore deposit is generally known as a cupriferous pyrite bedded deposit, and consists of several layers of ore zones, the principal ores being the Hitachi main and the Irishiken ore zones. The former has a trend from NE to SW, while the latter lies paralell, about 1, 000 meters to the North-West of the former. These two ore zones used to be considered separate ones running parallel one with the other.
Since 1953, the writer has come to believe in the existence of a folded structure, after restudying past data. As the result of our recent geological surveys, these two ore zones can now be interpreted as forming a synclinal structure having its apex near the Sasame ore body on the eastern extremity of the main ore body where the folding is prominent.
The distribution of the country rocks and ore bodies are controlled by this synclinal structure, and the trend of the lineations of the rocks are not parallel on the two sides along the axial plane of this folding.
Although follow-up studies are being made, it appears that this structure has an important bearing upon the genesis of the Hitachi ore bodies.

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