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The Orito Formation, which was first defined by T. UTASHIRO in 1950, is distributed on Sado Island, Niigata Prefecture. K. TSUDA (1956, 1962) reported that the Orito Formation is subdivided into upper and lower members, and that the lower member which consists of dacite, conglomerate and siltstone is correlated to the Daijima stage, and the upper member to the Nishikurosawa stage. On the other hand, T. SASAKI (1972) reported that the dacite of the lower member is covered by conglomerates and siltstones with unconformity and he redefined that the Orito Formation is restricted to conglomerate and siltstone facies of TSUDA'S lower member. He thought that the Orito Formation is correlated to the Nishikurosawa stage. There are two different opinions about the stratigraphy of the Orito Formation. The detailed stratigraphy and lithology of the Orito Formation in Nishimikawa district are hereby described, and also the extensive marine transression of the Miocene epoch is discussed in this paper. Our results are summarized as follows ; 1) The Orito Formation abuts on the pyroclastics, dacite, andesite and propylite, and it is covered uniformably' by the Tsurushi Formation. 2) The Orito Formation is mainly composed of siltstone, sandstone, calcareous sandstone and conglomerate without pyroclastic rocks. The Orito Formation is lithologically subdivided into silty rock facies and conglomeratic facies. 3) In our mapped area, the silty rock facies of the Orito Formation, which yields Vicarya, Vicaryella and other brackish fauna, is distributed in the eastern part. The conglomeratic facies, which consists of enormous conglomerates, a few sandstone and calcareous sandstone beds, occupies the western part. This calcareous sandstone contains Miogypsina and Operculina.. 4) The silty rock facies changes laterally into the conglomeratic facies interchangeably. 5) The conglomeratic facies is about 100 m in thickness. The gravels of the facies are 5 to 15cm in diameter and rounded to subrounded in shape. The main components of the gravels are igneous rocks, dacite, andesite and propylite of the pre-Orito Formations. They were eroded out from the adjacent area, and reflect the lithofacies of the adjacent lower formation area. The Orito Formation depositted on the eroded and subsiding seabed with great relief and sharp slope. 6) The Orito Formation is wholly correlated to the Nishikurosawa stage in Miocene epoch, because it yields Miogypsina and Operculina and so on. . Therefore the first Miocene marine transgression in Nishimikawa district arose during the Nishikurosawa age.

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The Ryoke metamorphic terrain is placed in the southern front of acid volcano-plutonism during the Late Mesozoic age in Japan. The Median Tectonic Line (MTL) is the southern boundary of the present Ryoke terrain (=the boundary between the Ryoke and the Sambagawa metamorphic terrains), but, judging from pattern of zonal arrangement of metamorphic facies, it was he initially developed within the southern marginal part of the initial Ryoke terrain. Namely, the part of the Ryoke terrain developed initially in the southern side of MTL was lost through Late Mesozoicrecent tectonic movements along MTL. This paper deals with the initial form and initial movement picture of MTL. The Ryoke metamorphic terrain shows a syntaxis like arrangement on both sides of Fossa Magna. The structural trends in the Ryoke terrain, except for those around Fossa Magna, which are shown by gneissosity and linear structures of granitic rocks and metamorphics, are at low to moderate angles to the general trend of the Ryoke terrain and MTL, forming en echelon structures. The main structure around Fossa Magna shows horizontal axial trend subparallel to them, but it is superposed by new mineral lineation of dip-direction. These structural trends continue into a narrow shear belt (=southern marginal shear belt) developed along MTL. Around Fossa Magna the structure of later phase continues into the shear belt. The granitic rocks and metamorphics in the shear belt are generally mylonitic in texture. However, their constituent minerals such as brown biotite, hornblende, K-feldspar and plagioclase are stably recrystallized, though considerably decreased in size. This shows that the shear belt was formed under the high-temperature condition. On the basis of observed c-axis fabrics of quartz (pseudo-two-girdle type), the gneissosity (mylonite band) in the shear belt is regarded as corresponding to the plane including the longest and the intermediate principal axes of strain, and the lineation on it to the longest principal axis of strain. The longest and the shortest principal axes of strain (X and Z) in the shear belt are generally approximately horizontal, and the angle between X and dexceases MTL gradually towards MTL, except for in the shear belt around Fossa Magna, where X is oriented in dipdirection. The orientation patterns of the principal axes of strain in the shear belt are comparable with those in the half sides of the shear belts described by RAMSAY et al. (1970) and HARA et al. (1973). MTL is placed along the part of the highest shear strain, and it shows that the southern half side of the initial southern marginal shear belt was lost by tectonic movements of later phase along MTL. The sense of shear strain in the shear belt is left-lateral 011 western side of Fossa Magna, and its magnitude appears to increase with increase of distance from Fossa Magna, showing lateral displacement of ca. 30km near Deai (Central part of Kinki Province). In the shear belt around Fossa Magna, lateral displacement does not occur but dip-direction displacement. On eastern sidd of Fossa Magna the Ryoke granitic and metamorphic rocks are so sporadically developed that it is difficult to examine their structures. It has been concluded in this paper that the geological structures (en echelon structures, southern marginal shear belt etc.) of the Ryoke metamorphic terrain was produced through the process of formation of syntaxis-like arrangement of the terrain under non-uniform compression from the Pacific Ocean side, and that the stress concectration along the southern margin of the Ryoke terrain (=the formation of southern marginal shear belt) occurred when the syntaxis-like arrangment of the terrain was produced to some extent and the shear movement of dip-direction began around Fossa Magna.

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From 4 to 13 August '77, the pre-Congress excursion (40 A) of the 25th International Geological Congress was made in the route from Perth to Kalgoorlie-Norseman across the major Archaean Yilgarn Block. The excursion, at first, examined Archaean granulites, charnockites, gneisses and post-kinematic granites in the Southwestern Province in the Block. However, the greater parts of the progamm were spent for the study in the typical-greenstone belt sequences consisting of, in the ascending order, ultramafic and mafic volcanic members, felsic volcanic and pyroclastic complex, and clastic metasediments including polymict conglomerates in the Eastern Goldfields Province. Above all our greatest matter of interest was the famous "spinifex" textured ultramafic lava, komatiite, which may have been considered to be the supracrustal equivalent of the primodial mantle rock. The specimens clearly showing the quench texture were collected from the Serpentine Bay of the Hannans Lake, 15km south of Kalgoorlie, and were thinsectioned and photographed (Plate II). The chemical affinity of komatiites to the assumed primodial mantle rocks including 'pyrolite' model by RINGWOOD (1966) as well as GORAI's models (1973) calculated from carbonaceous chondrites is apparent as illustrated in Fig. 4. Besides a granitoid pebble from the Kurrawang conglomerate belonging to the uppermost sequence of the Greenstone belt was examined microscopically (Fig. 5). It is a striking fact that the pebble is much alike to the granitoid pebbles commonly found in the Permian Usuginu conglomerates (KANO, 1973) in Japan especially in its trondhjemitic composition. Lastly we are very much grateful to Drs. R. A. BINNS and I. R. WILLIAMS, the leaders, for their kind arrangement and guidance through the excursion.

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