同一岩帯の数箇所で観測された自然残留磁気の信頼性

Results of palaeomagnetic observations are discussed on the assumption that rocks were magnetized along the direction of the geomagnetic field at the time of formation. It is a matter of great importance that the natural remanent magnetization (NRM) of rocks has stably been preserved against the later change of the geomagnetic field or other physico-chemical influences. Stability of the NRM is examined either by field or laboratory tests (Graham, 1949 ; Nagata, 1961 ; Irving, 1964). However, although the NRM of a rock unit is shown to be stable, it is often unknown whether the direction of the NRM is a primary one of the rock unit or not. For example, the directions of magnetization obtained from rocks of the same time or a single rock mass must closely coincide with one another, but inconsistent examples are rarely observed in such rocks. The inconsistent example suggests that the direction of the stable magnetization of samples collected at a site in a single mass is not always representative of the primary magnetization of the rock unit. Especially, the Japanese Islands are in a zone of active tectonic movement in the Tertiary or Quaternary, and it is of a great importance to confirm that a rock mass has not been deformed by such tectonic movement, and to find out the direction of the original NRM.