伊豆半島西側の大陸斜面から採集された軟体動物化石と伊豆半島の地質構造

抄録

1) Pieces of basement rock containing numbers of molluscan fossils have been collected by dredging along the continental slope on the west coast of the Izu Peninsula. The stations with depths sounded 1,200-1,500 m were occupied at the mouth of the Suruga Bay. The fossil molluscs belonged to species of the shallow-water type. 2) The submarine stratum containing the fossils (Table 1) is comparable with Shirahama formation distributed on the land of the Peninsula. Its geological age is believed to be somestage between the latest Miocene and the early Pliocene. 3) The fact that such a stratum as bearing characteristics of the shallow sea lies beneath the continental slope should be attributed to rise of the sea level since the late Tertiary. 4) The evidences suggest that flat plains known lying at depths of 2,000m at various localities of the ocean must have been formed around the beach-line that existed in the late Miocene. 5) The region extending from the Izu Penisula to the Izu Islands has geological structure similar to that of the Japan Sea side of the southwestern region of this country where green tuff of the Neogene is distributed over metamorphic rocks and granite. Mines which produce gold and silver are located in that region. 6) It is assumed that the Izu-Mariana arc was not formed in a single geological age and that ancient platforms may be included in the arc.