Coastal configuration and submarine topography in the southern parts of the Yagami coal-field, eastern environs of Nagasaki City, controlled by the geologic structures of the Paleogene formations and the distribution of neo-volcanics. The neo-volcanics of the Makishima Island, east of the Aba Bay, may be belonging to the Nagasaki volcanic group by its petrographic features. This fact is suggesting that the Paleogene coalbearing formations may be distributing under these volcanics and. extending to the seawards. The Aba Bay originally constructed by the faults of north-south trend, cut in the Paleogene strata, and lately submerged by the upheaving of the sea level of at least 40 meters. 30 bottom sediment samples from the Aba Bay area are treated by the mechanical grain analysis. The division of the sediments which was proposed by Inman and Chamberlain (1955) are applied to this area. The coarse sediments containing organic calcareous fragments which are designated to Sediment Type V are found in the coastal area where strong influence of surf and tidal current affecting. Muddy sediments are classified to Types IIIa, III and IIIb. Fine mud found in the deeper part of the bay mouth and also in the north-western part of the bay head and the shallow embayment in the Makishima Channel. These fine-grained sediments may be yielding under the stagnant water body or lower part of circulation current in the bay. The diagrams of sand-silt-clay ratios are most useful for the division of the muddy sediments. In the longitudinal section of the Aba Bay, the amount of silt and clay increasing with distance from the bay head and water depth
