This study is an attempt to isolate and identify the primary magnetization in red cherts. Red chert samples from a middle Triassic sequence in the Inuyama area (35.4øN, 137.0øE), central Japan have a multicomponent magnetization, which is delineated by thermal demagnetization. Two of the components have high coercivity and high blocking temperatures' one is carried by hematite and the other by magnetite. The former was concluded to be the primary magnetization for two reasons' (1) it predates intraformational folding, and (2) it records geomagnetic reversals. The other component, which accounts for the dominant part of natural remanent magnetization was probably acquired at a time long after the deposition. Two paleomagnetic results were drawn from the Middle Triassic red chert sequence' (1) The mean inclination of the samples in the sequence was 1.4 ø _+ 6.8 ø' therefore the paleolatitude amounts to 0.7 ø + 3.4 ø, in contrast to the paleolatitude expected from the paleopole of Eurasian continent, which is about 70 ø, and (2) the magnetostratigraphy of this sequence indicates that two geomagnetic epochs in the Middle Triassic are considerably longer (more than several million years) than the average of those in the Cenozoic.

Palaeomagnetic studies require a theory of magnetization mechanism of sediments and a method of estimating magnetic field intensity from their remanences. This paper establishes a physical basis for the generation of the remanence in deep-sea and lake sediments experimentally.Redeposition experiments have been carried out under centrifugal force in weak magnetic fields. The centrifuging method produces post-depositional remanent magnetization (post-DRM) in the compacted sediment, and its remanence and susceptibility are compatible with those of natural sediments and reconstituted materials of other redepositional experiments. Three properties of the post-DRM have been deduced from the experiments: (1) the efficiency of acquisition of post-DRM decreases with increase in density during the compaction process, (2) the total post-DRM is equal to the sum of the partial post-DRM (addition law), and (3) time is not a substantial factor for alignment of the magnetic particles. These results lead to the conclusions that the magnetic particles do not rotate steadily but in a series of steps, and that the density change is the crucial factor giving rise to the post-DRM.A mathematical formula representing the remanence record in sediments is proposed on the basis of the experimental results and the model. The principal equation is expressed as an integral of the product of three parameters over time when sediments have been compacted; the field intensity variation, characteristic function of the sediment and the time derivative of the density change.

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