The area described is the city of Taira and the adjacent villages of the Middle Joban Coal Field, including Yoshima, Akai, and a few other smaller collieries.
This paper is divided into two parts for convenience. In Part I, the following subjects are discussed: (1) the changes in thickness of formations above and below the coal bearing member, (2) changes of sedimentary facies of the Shiramizu Group except the coal bearing member, (3) newly detectd formation in a borehole. In Part II, the writer interpreter and discusses the problems of the deposition and deterioration of the coal and the coal seams.
In Part I, here presented, an interpretation of the stratigraphy and tectonic geology of the area is followed by the three main subjects as below.
(1) Borehole sections reveal that the Goyasu Sandstone, the lowest member of Yunagaya Group, increases in thickness abruptly in the eastern, deeper part of the Taira District, probably with the boundary of narrower zone of NNE-SSW direction. In this deeper part, the thickened Go-yasu Member has thick vitric tuff beds in its lower part, underlain by a basal conglomerate bed which rests on the Shirasaka Shale. The vitric tuff beds are completely absent from the western part of the Taira District. Above the vitric tuff beds are coal-bearing beds having successions of remarkable cyclic sedimentation similar to those of the Iwaki Coal-bearing Member. This coal-bearing upper half of the Goyasu member is present in the western area where it is thinner than in the east. By these reasons, the depth of boring to reach Iwaki coal seams is increased. in eastern area, and the boring itself is obliged to stop frequently, jammed by the collapse of vitric tuff.
(2) The Shirasaka Shale, Asagai Sandstone, and Iwaki Sandstone, all of the Shiramizu Group, might be found in vertical succession in a borehole. However, they vary greatly in thickness, and in some places one or the other is absent, although the total thickness of the section penetrated is unchanged. These three members seem to be of simultaneous deposition only different in facies of sedimentation. Judging from an isopacous map of each member, the Iwaki Sandstone appears to be a subaqueous deltaic deposit. The deltas are believed to extend to the east or to the southeast. To the east, the direction of deepening, the Iwaki Sandstone is apparently replaced by the Asagai Sandstone which can be shown to be in the relation of interfingering with the Iwaki Sandstone in the known borehole sections.
(3) The Second Kaso, which is the lowest coal seam of the Taira District, has a thick clay bed under it, the so-called "gaerome clay". The clay bed rests directly on granite or metasediments of the basal complex, in the western half of the area, but in the eastern area it is underlain by sedimentary rocks. The latter resembles the Kasamatsu Shale of Cretaceous Age both in its lithology and in its aspects of sedimentation. From these data, the writer believes this member to be of Cretaceous age, beveloping from the north of Yotsukura, although in this area it lies completely underground with no outcrop.

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