Southwestern Hokkaido, a northern part of the Green Tuff region, is divided into two different geologic provinces by the Kuromatsunai Lowland. One is the east province which is characterized geologically by NW-SE trending structure, whereas the other is the west province with N-S trend. Since the Shimamaki district is located in the northeastern corner of the latter province, the study of this district should contribute to the understanding of geological developement of southwestern Hokkido. The Neogene formations developed in Shimamaki district are divided into the Obiragawa, Orikawa and Honme Formations in ascending order. The Middle Miocene Obiragawa Formation consists of green tuff with a basal conglomerate, which is overlain conformably by the Upper Miocene Orikawa Formation composed of hard shale and hyaloclastites. The Pliocene Honme Formation of hyaloclastites covers the lower two formations with a partial unconformity, which has been dated as 4.7 Ma by fission track method. The Quaternary Tokotangawa Formation of sandstone rests unconformably upon the Neogene formations. The Obiragawa Formation unconformably covers the Mesozoic rocks partially with a distinct relationship of high-angle abutting. At the base of the formation it has a poorly sorted talus-like conglomerate. These evidences suggest that the basin of this formation has been formed by the same mode of the collapse movement as in the Green Tuff region of Honshu Island in Miocene time. At Late Miocene time, a rapid subsidence and violent volcanisms occurred in this district, followed by the separation of the west and the east province by the NW-SE trending "Hikagenofuchi Fault Zone" which a close association with Kuromatsunai Lowland. A rapid upheaval with violent volcanisms at Early Pliocene time in this district is ascribed to the Island Arc Disturbance which has been proposed by FUJITA (1978).

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The Mesozoic system, newly named Tomarigawa Group, is distributed in Shimamaki area, Southwest Hokkaido. It is divided into following two formations, namely the Kanayamagawa and the Garozawagawa Formations in ascending order. The Kanayamagawa Formation is divided into three members, that is the Lower, Middle and Upper Members. The Lower Member is composed mainly of shale and shaly flysh (alternations of sandstone and shale), associated with chert, limestone and basic tuff. The Middle Member is characterized by the dominance of chert. The Upper Member consists of flysh, associated with chert, pebbly shale, conglomerate and thick arkosic sandstone. This formation yeilds some Upper Jurassic radiolarians and Upper Triassic conodonts from cherts. The total thickness exceeds 1100 meters. The Garozawagawa Formation, conformably overlying the Kanayamagawa Formation, is divided into the Upper and Lower Members. The Lower Member is composed mostly of limestone, basic tuff and chert, associated with alternations of chert and shale. The Upper Member consists of greenstones and chert, associated with limestone, shale and arkosic sandstone. Greenstones are made up largely of reworked hyaloclastite (hyaloclastic or epiclastic volcanic breccia, sandstone and siltstone), but include small amounts of pillow lava and massive lava. Some fossils such as calcareous algae, hexacorals and Upper Triassic conodonts have been obtained from limestones of this formation. However, these limestones are largely conglomeratic, or blocks of various size, which are included in greenstones. Such feature described above suggests that limestones of this formation include partly allocthonous ones. Based on radiolarian fossils obtained from chert and shale, the age of this formation seems to range from Upper Jurassic to early Cretaceous. The total thickness exceed 1400 meters. The sediment of this group seems to have been transported from following two provinances, namely, granitic hinterland and shallow seamount composed of greenstones and limestone. This group was deposited well away from granitic hinterland, but in the vicinity of shallow seamount. The Tomarigawa Group, which includes allocthonous seamount materials, may be comparable to the submarine terrace sediments on the continental slope or trench sediments, but further detailed study is necessary before we come to our final conclusion.

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The area investigated around Mt. Nikoro-yama is made up of the Mesozoic sedimentary and volcanic rocks which are divided into the Yubetsu Group, Nikoro Group, and Saroma Group. The Nikoro Group is composed mainly of greenstones such as basaltic and trachytic pillow lavas,, hyaloclastites, dolerite and trachyte dykes, ultramafic cumulates, and a small amount of chert and limestone. Volcaniclastic conglomerate, sandstone, siltstone, and shale are associated with the greenstones and are found in the Lower Part of the Nikoro Group. The volcanic piles of the Nikoro Group are considered as remnants of Jurassic seamounts from the following reasons; (1) explosive volcanism producing a large amount of hyaloclastite and strongly vesiculated pillow lavas, (2) alkali basalt magmatism from alkali olivine basalt to trachyte, (3) presence of volcaniclastic sedimentary rocks, and (4) lack of terrigenous sediments. Boulder and cobble conglomerate observed at the base of the Saroma Group irregularly covers greenstones of the Nikoro Group. No evidence of submarine basaltic volcanism is recognized in the Saroma Group composed entirely of clastic sedimentary rocks. The rapid change of lithofacies from the Nikoro to Saroma Groups as well as the distribution and occurrence of the basal conglomerate might indicate the presence of unconformity The basal conglomerate contains large numbers of well-rounded boulders and cobbles; 58〜78 modal % pyroxene andesite, 0〜10% hornblende andesite, 0〜2% rhyolite, 0〜4% monzonite, 0〜16% porphyritic monzonite, 0〜2% monzogabbro, 0〜2% slate and sandstone, 2〜4% trachyte, 5〜18% basalt and hyaloclastite. The abundance of calc-alkaline andesites and alkaline plutonics suggests that the materials have been derived mainly from a continental margin or an island arc. In this connection, it is noticed that the similar rock types are known in the Cretaceous formations of the Nemuro Group. Accordingly, the boulders and cobbles are considered to be originated in the Paleo-Kurile Arc in late Cretaceous time. At the time forming the unconformity, seamounts of the Nikoro Group might have been emplaced to the western margin of the Paleo-Kurile Arc, where a large amount of terrigenous elastics were supplied.

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Six fossil teeth of the great white shark, Carcharodon carcharias, from five localities of the Boso Peninsula and Shimosa Upland are described. Results are summarized as follows. 1) The horizons which occured the fossil shark teeth are the Kurotaki and Ichijiku Formations of the Kazusa Group, and the Jizodo and Narita Formations of the Shimosa Group. The geological age ranges from the Pliocene to the Upper Pleistocene. 2) Comparing with the teeth of the Recent Carcharodon carcharias, these fossil teeth are identified as two of upper left anterior teeth, one of upper left intermediate tooth, two of lower left lateral teeth and one of lower right lateral tooth. 3) Judging from the size of these fossil teeth, the ancient great white sharks are estimated about 4.5 meter to 5.7 meter in total length based on the study of RANDALL (1973). 4) The origin and evolution of the great white shark will be clear by the following discoveries and the morphological and biostratigraphical studies on many fossil teeth from the Pliocene to Pleistocene strata in the Japanese Islands and other countries in future.

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Two radiolarian assemblages, namely, Follicucullus monacanthus and Fo. scholasticus assemblages are discriminated within the Permian Maizuru Group in the Maizuru Belt, Southwest Japan. The species of Follicucullus ORMISTON and BABCOCK, representing these assemblages, are described and the significance of this discovery in the Maizuru Belt is briefly discussed. The Follicucullus scholasticus Assemblage Zone is probably correlated with some part of fusulinid-zone, Lepidolina kumaensis. The age of the Maizuru Group is regarded to be younger than that of the Shimomidani Formation which is distributed in the area adjacent to the north of the study area.

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