A group of silicic volcanic rocks hitherto called "quartz porphyries" with subordinate intercalation of clastic sediments in Central Japan was named Nohi rhyolite in 1961 by the first two writers and their co-workers. Now it is revealed that it is mostly consisted of a large number of pyroclastic flow deposits, densely welded and wholly devitrified. These welded tuffs range from rhyolite to rhyodacite in chemical composition. They include the abundant crystal fragments (30〜50 volume percent of the rocks in the most parts), in the compact cryptocrystalline to holocrystalline matrix, such as quartz, plagioclase, orthoclase, biotite, hornblende, rarely ortho- and clinopyroxenes and, very rarely, Fe-rich olivine. Commonly, they have essential lenticluar fragments which are nearly equal to their enclosing rocks in chemical and mineralogical composition, and, also have lithic fragments, in a more or less degree of abundance, mainly derived from the underlying Paleozoic rocks. Emplacement of the rhyolite is inferred to have been in Late Cretaceous period from geolgical evidences and, more definitely, in a certain period between 100 m.y. and 80m.y. ago according to the isotopic radiometry of the granites both prior and posterior to it. The Nohi rhyolite, accopanied with hypabyssal intrusives of co-magmatic origin, constitutes an enormous mass whose total area of distribution attain to about 5,000km2, mean thickness is 2,000 m or more and, consequently, original volume is about 10,000km3 or more. It is extending with NW-SE trend mainly in the non-metamorphic Upper Paleozoic terrain (so-called Mino belt) and, partly, in the northern-neighboring metamorphic terrain (Hida metamorphic belt), and evidently truncate such the basement structure as a whole. The rhyolite seems to have effused out of the fissures of NW-SE trend which took place in the sheared zones of the basement and deposited in a large graben with the same trend of elongation. In fact, vent-breccias found in the western margin of the rhyolite mass, paleo-talus deposits covered by some welded tuffs on the fault scarp in the above area and frequent alternation of welded tuffs and nonsorted very coarse-grained sediments express eloquently the repeating of the eruption and the volcano-tectonic depression, at least in its western marginal part of the mass. On the basis of detailed stratigraphic survey, the volcanic activities resulted in the deposition of pyroclastic flow can divided into five stages, between which existed some time intervals indicated by deposition of the lacustrine sediments (Atera formation, Shirakawa formation and others). Each stage is consisted of two or more welded tuff sheets each of which is generally 200 m or more in thickness, nearly homogeneous, in the most cases, both vertically and horizontally in their lithological, petrographical and petrochemical features and fairly different to others in the same features. The first stage is represented by the highly silicic and relatively crystal-poor welded tuffs, in general, and the second stage, on the contrary, by rather mafic (rhyodacite) and crystal-rich welded tuffs. Soon after the second stage, granodiorite porphyries, chemically and mineralogically similar to the welded tuffs of the second stage, were intruded into the welded tuffs of both the first and the second stage as several stocks now occupying the central and southern parts of the Nohi rhyolite mass. The third and the fourth stages are represented by the welded tuffs which have intermediate composition between the first and the second ones, and their areal distribution area was rather shifted to the east compared to the preceding stages. Products of the last stage volcanism are only poorly developed in the western marginal part of the mass, and mainly consisted of the explosion breccia deposits with several thin layers of rhyolite welded tuffs. Soon after the last stage volcanism,

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Geology of some Cenozoic silicic pyroclastic flow fields-Valles caldera, New Mexico; rhyolite plateau of North Island, New Zealand and that of Chillean Andes ; Basin and Range province, western United States-is reviewed. These large-scale pyroclastic flow fields, except the first one which is a single caldera, had been formed in a fairly long time duration by intermittently repeated eruptions of pyroclastic flows. One such pyroclastic flow field contains calderas or resurgent cauldrons each of which is formed by one-cycle eruption of pyroclastic flows. Between cycles of pyroclastic flow eruption occurred regional tumescence and tensional fracturing, erosion and sedimentation. Those pictures seem to conform to the geology of the Nohi rhyolite so far known.

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The Mesozoic large batholiths in the North American Cordillera have been considered to be the product of the so-called Nevada orogeny in the late Jurassic age. On the other hand, our studies on the late Mesozoic acid igneous activities in the Inner Belt of Southwest Japan made clear that the activities are exceptional from the concept of geosyncline and orogenic belt and that they are plutonism intimately connected with acid volcanism in the geanticlinal regions. The late Mesozoic acid igneous activities of this nature has been known also in the southeastern margin of the Asiatic Continent, occupying the area of 2,000 km wide and 4,000 km long, including the Inner Belt ot South-western Japan as a part. In this paper, the writer attempted to reconstruct the Mesozoic geohistory immediately with the activities of granite batholiths in North AmericanCordillera, located in the same geological situationof the Circum-Pacific region from the above viewpoint. A belt of Cordillera occupied by large batholiths of granite (the so-called Nevada orogenic belt) is situated almost only within the Paleozoic eugeosynclinal and miogeosynclinal regions. In the Mesozoic age, the belt was in geanticlinal circumstance and here subaqueous or subaerial acid volcanisms were violent, especially in the Jurassic age. Granitic activities preceded side by side with these volcanisms. On the other hand, the eugeosynclinal belt of the Mesozoic age was situated in the Western Jurassic belt of the Klamath Moutains, the western half of the Western metamorphic belt of the Sierra Nevada and Coast Ranges. This belt underwent a regional metamorphism of the glaucophanitic type. There is granite in the eastern margin of the belt. But in the the Western Juras, -sic belt of the Klamath Mountains the intrusive relation to the Mesozoic sediments is questionable, and only in the restricted area of the Sierra Nevada granite is in intrusive relation to the Mesozoic sediments. American geologists have generally distinguished the eastern margin of the belt from Coast Ranges. They have considered the former to be pre-Tithonian and the latter to be post-Kimmeridgian. The gap represents the so-called Nevada orogeny. The writer can not accept the idea from the following facts. Firstly, the activities of granite were not restricted in this age and ranged from the Triassic(?) to the Cretaceous age. Moreover, the activities were mainly restricted in the Mesozoic unsubsiding regions, where were in geosyncline during the Paleozoic age. Secondly, it was suggested recently that the Galice Formation is continuous to the Franciscan Group through the South Fork Mountain schist, at least in some stratigaraphic horizon. Lastly, the metamorphism in the Western Jurassic belt of the Klamath Mountains is not a type taking a granite plutonism. The writer regards the Mesozoic acid igneous activities in Cordillera as a substancially same type of those in the Inner Belt of Southwest Japan. The term "Nevada orogeny" of American geologists should be used as a synonym with our Hiroshima disturbance or Yenshan movement.

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Isotopic age determinations in Japan have been carried out systematically since 1960, and recently it has been tried to summarize these results to clarify the transition of acid igneous activities in the Japanese Islands. In this paper these summaries are reviewed and some problems are proposed to confirm the ages of late Mesozoic to Paleogene igneous rocks. K-Ar ages of biotite imply either the time when the temperature of the rocks had been lowered enough to retain radiogenic argon in course of upheaval following to emplacement and solidification, or the time when the rocks beeing cooled from the temperature which was brought with secondary thermal effect. K-Ar ages of biotite summarized here show a pattern of distribution resembling to that given by KAWANO and UEDA (1967), and the writers pointed out following problems from the pattern. 1) In the Northeast Japan, a part of granitic rocks shown as early Cretaceous age should be considered to have emplaced in middle Jurassic age. These rocks seem to be rejuvinated by the effect of younger granitic rocks. Such examples are found in the Kitakami intrusive belt, in the Abukma intrusive belt and in the Ryoke belt respectively. 2) None of acid and intermediate volcanic rocks in the inner side of Southwest Japan has been measured on the ages, despite that these rocks are considered to be important constituents of late Mesozoic to Paleogene acid igneous activities. 3) The pattern of zonal arrangement of age groups were reported from the east coast of Australia and from the Sierra Nevada batholiths of western America. In these areas igneous activities are related to some structural movements. Detailed age determinations are now necessary to get reasonable explanation of the pattern in the Japanese Islands.

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(I) Analogy of the pattern of tectogenesis (1) The most important ground for the writer's assertion that the Green Tuff movement began with collapse lies in the character of the basal conglomerate. From the fact that a similar conglomerate is found in the sedimentary basins of the Late Mesozoic to Early Paleogene period, the writer presumed that the tectogenesis of this period must have begun also with collapse. Later, his presumption was corroborated by MURAKAMI and others. (2) The Green Tuff movement occurred cutting the pre-Neogene tectogenic belt. In this respect, the Green Tuff movement is identical with the Late Mesozoic〜Paleogene tectogenesis. (II) Tectogenic correspondence between Inner Zone and Outer Zone The tectonic movement of the Outer Zone during the Late Mesozoic〜Early Paleogene period is characterized by subsidence, as exemplified by the Shimanto geosyncline and other basins. The Green Tuff movement took place mostly in the Inner Zone, whereas in the Outer Zone sedimentary basins were developed, accompanied by almost no igneous activity. When such a tectogenic correspondence is considered for the later period, the Inner Zone of the Late Neogene〜Quaternary period may be defined as a volcanic belt represented by the active volcanoes distributed on land, and the Outer Zone as a subsidence belt represented by the submarine trenches observed today. The above-mentioned correspondence is applicable only when the tectogenesis of the near-ocean side of the Inner Zone is dealt with. In actuality, however, the tectonic movement of the Inner Zone covers a vast area of the continent more than 1,000 km in width, as shown in Figs. 1, 2. This fact makes it difficult to explain those tectonic movements by a single plane of deep-seated earthquakes as advocated in the theory of the so-called plate tectonics. (III) The progress of the igneous activities in the three tectonic movements of the Inner Zone since Late Mesozoic shows a tendency of gradual basification. BELOUSSOV'S view that the igneous activities on the earth have been gradually basified since Mesozoic seems to be based on some areas centering on the rift systems. The areas discussed in the present paper are island arcs and adjacent areas, so that it is possible that the basification in these areas progressed more slowly than in the rift areas, and was delayed to the latter half of the Cenozoic area.

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