5. Natural Remanent Magnetization of Sedimentary Rock, Part II.

The intensity and direction of natural remanent magnetization of Narita bed (a horizontal layer of Quaternary deposit widely distributed in the Kwanto District) were examined. In practice, the horizontal layer of Narita bed 470cm in thickness from its upper boundary was divided into many horizontal layers 2cm thick, from every one of which a specimen of 1.5cm cube was cut off. The intensity and direction of natural remanent magnetization of all samples were measured by means of the apparatus described in the preceding paper, The results of measurement are snown in Fig.1, where the direction of magnetization is expressed by both of the inclination I and the declination D, the latter being referred to the present geomagnetic meridan. As will be seen in Fig.1, the direction of remanent magnetization changes quasi-periodically with the depth ; this result will be more cleary seen in Fig.2, where the smoothed curves of D and I are given. Next, the physical meaning of the remanent magnetization of sedimentary rocks was examined from various points of view. Among a few probable processes of causation of the remanent magnetization, only a possible one is the followings: - Many particles of magnetite contained in the igneous rocks, which had obtained the thermo-remanent magnetization during their cooling after their ejection or intrusion, were made free owing to weathering or other causes, depositing with the other non-magnetic minerals on the bottom of the sea or rivers. In the process of their deposition, those magnetized particles should be more or less affected by thegeomagnetic field. Receiving the above-mentioned assumption, we examined the reliability of the established relation between the direction of the remanent magnetization and that of the geomagnetic field. (i) The ratio (α) of the intensity of natural remanent magnetization(J_n) to that of thermo-remanent magnetization obtained du ing cooling in vaccum and in 0.45 Oe magnetic field (J_ was measured, the result being, α=J_n/J_=0,10～0.23, while the average value of α, is a^^-=0.14 (ii) The volume-percentage and number of magnetites contained in the specimens were measured, the result showing that the intensity of natural remanent magnetization J_n is nearly proportional with the volume percentage, as shown in Fig.3 ; and that the number of magnetite particles contained in the 1.5cm cube specimen is larger than 7000. (iii) The probability P of that the superposed intensity of N particles of magnetized element (j_i) becomes larger than α×Σ^^N|j_i| as the result of their random distribution was calculated. Putting N^^>7,000 and α>0.10, we get P. This result will show that the natural remanent magnetization of Narita bed is not the resultant behaiviour of random accumulation of magnetized particles but that of their regular arrangement, though there should be some disturbances. Seeing that the effect of mutual magnetic interaction between those magnetized particles can be practically neglected, we can say that a possible external force to arrange them is the geomagnetic field alone, this conelusion leading the fairly reliable presumption that the change in D and I with the depth, given in Figs.1 and 2, approximately shows the change in declination and inclination of geomagnetic field with time during the period of deposition of Narita bed