A sedimentological study was made of the Flysch-type alternations of Otadai formation, Kazusa group deposited in the central part of the Boso peninsula in upper Pliocene epoch (Fig. 2). The formation consists of rhythmic alternation of sandstone and mudstone and the relative amounts of the two rocks vary in places. Each layer is correlated for more than 30 km in extent, as it has their own characteristics in thickness, texture, composition and colour and is arranged in similar manner at the neighbouring sections (Fig. 3, 4). Several key beds of tuff are the most important in the correlation because of their distinct features. The shape, textural distribution and grain size variation in the layers has been definitely shown by the method stated above. The thicker each layer of sandstone is, the more spacial extent it acquires in general. The layer over 10 cm in thickness at the thickest part reaches more than 30 km in extent. It is asymmetrical in shape owing to the more rapid decrease toward west (Fig. 6). On the other hand, the thickness of mudstone layers increases gradually toward west within the studied area but seems to decrease very rapidly westward (Fig. 5). It is concluded that the thickness variation of sandstone and mudstone assembly is determined by sandstone, that is, the layers of sandstone are very sensitive to the subsidence of the basin. Of course, the subsidence is the neccessary condition for the formation of layers. A layer consists of lamina which are units of mass movement of grains, as will be seen from the Photo. 1. A relatively thick sandstone layer is divided into three intervals based on the nature of lamina, namely, massive graded, parallel-laminated and cross-laminated intervals from the base respectively. But a thin sandstone layer is devoid of massive graded interval and/or parallel-laminated intervals. The arrangement of these lamina is closely related to the thickness variation of a layer (Fig. 6). The grain size distribution and consituents in a layer are also related to the textural arrangement as well as the shape (Fig. 9). The boundaries of textures are nearly parallel to the isometrical lines of median diameter of grain size and sorting coefficient. Shell fragments are concentrated at the bottom of the graded interval, while pumice and plant fragments are often seen in the parallel and cross-laminated intervals. The grain size variation in the mudstone layer seems to be more monotonous and the mean size and sand grain content gradually decrease toward west. As will be known from the fact stated above, sandstone layers are very different from mudstone layers in many respects. And it is observed that the sandstone layer is formed by different way from the mudstone. The inference is substanciated by the difference of faunal assemblages found in both layers. The sandstone has the shell fragments and worn-out foraminifers which are found in the upper neritic zone in the recent environment, while molluscan shells and foraminifers contained in the mudstone are similar to the fauna living in the bottom over 400 m in depth in the Pacific off the Boso peninsula. This fact indicates that sand deposited temporarily under the bottom of shallow sea is transported into the bathyal environment where mud is usually deposited. The direction of current transporting sand grains should be from west to east as is assumed from the sole markings developed under the bottom of sandstone layers and cross laminations (Fig. 10). The nature of flow is inferred from the result of laboratory experiments and observations of alluvial channels. It is controlled by many variables such as depth, slope, size and shape of grains, viscosity and density of sediment-water mixture, etc. So the concept of flow regime (SIMONS & RICHARDSON, 1961) is very useful as it allows grouping of the combined effects of those factors. The classification of flow regime is based on form of

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The Adoyama formation is composed mainly of red, grey, dark grey cherts including sandstone layers. Chert is finely banded in general, and is often intercalated with thin limestone lenses. Just under the Adoyama formation, the Nabeyama formation is conformably underlying, which yields the Permian fusulinids, such as Parafusulina, Pseudofusulina, Schwagerina and Neoschwagerina. Then, the Adoyama formation is undoubtedly the Permian in age. The author collected the samples from only one locality at the base of the Adoyama formation near Kuzu-Machi, Tochigi Pref., Central Japan. Conodonts were obtained by dissolving chert in 5-8% hydrofluoric acid. The author found about 1500 individuals, among which 19 genera and 42 species are described in this paper. Moreover new genus Metapoygnathus is proposed. Characteristic species in this fauna are Gondolella carinata, G. constricta, G. navicula, Gladigondolclla abneptis, G. tethydis and so on. These species have been commonly recognized as the Triassic ones. Polygnathus cf. communis is reported from the Carboniferous, and P. cf mungoensis from the Cretaceous. Both are also included in this fauna. In addition to these, porifera, radiolaria, foraminifera and fish remains are found in the residues of chert.

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The surveyed area is situated in the north-eastern foot of Yatsugatake Volcano on the upper stream of the Chikuma river. The loam in this district is devided by uncomformity into four; the upper and the lower parts of the older loam, the middle loam, and the younger loam. The lower part consists mainly of the eolian sediment of the Oishigawa scoria, the older ejecta of Yatsugatake Volcano. On the contrary, the lake deposit of the scoria is called the Oishigawa formation. Moreover, the scoria falls on the Kanto district and the lower part of the Tama loam is composed of it. The white pumice of the lower part covers widely from this district to Hanno city, 45 km north-west to Tokyo and it is, therefore, a useful key bed. The upper part of the older loam consists not only of the ejecta of Yatsugatake Volcano, but also of the volcanic ejecta from Central Japan, west to Yatsugatake Volcano. These materials make also the upper part of the Tama loam in north-western Kanto. Especially, the three pumice beds (B1 B2, B3) of the upper part, though the ejecta of volcanoes in Central Japan, are widely deposited to northwest Kanto and are therefore useful as key beds. After these volcanic activities, Yatsugatake Volcano was erupted pyroclastic flows several times and on all such occassion to Chikuma river was dommed up, forming lake and marsh. By this study, the older loam in the foot of Yatsugatake Volcano was correlated to the Tama loam in the Kanto district, which has been regarded as type area of the Quaternary tephrochronology in Japan, and consequently the tephrochronology and geological history of the early Pleistocene in the foot of Yatsugatake Volcano was clarified.

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