1957 年 1 巻 1 号 p. 7-10
A palaeomagnetic study has been conducted of the volcanic rocks in the North Izu-Hakone volcanic region, Japan, where the complete succession of lavas has been determined by H. Kuno. By sampling 4∼7 oriented rock-specimens at each of the localities, the period from the very beginning of the Pleistocene to the Holocene has been covered, where the maximum time interval between consecutive samples may probably be not more than several tens of thousands years except that between two samples of middle to younger Pleistocene when the volcanic activity did not occur within the region concerned. Care was taken not to use the rock samples of which natural remanent magnetization may have suffered from any significant disturbances, geologically, chemically, magnetically or otherwise. Selection of proper samples was performed according to the criteria for the stability of remanent magnetization proposed by us previously (Journ. Geomag. Geoelec., VI, No.4). The major findings in this study are: 1) During the whole Quaternary age, the axis of the geomagnetic centred dipole was fluctuating around an axis of which north pole changed from φ=72°N, λ=86°E to φ=81°N, λ=32°W. 2) The direction of polarization of the centred dipole was reversed at a time in the earliest Quat rnary, namely, during the middle period of the formation of the Usami volcano.