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The method of measuring magnetic anisotropy in terms of the couple acting on a sample suspended in a magnetic field is well known. In recent years it has been used by Granar and by Stacey, among others, to determine what may be called the ‘magnetic fabric’ of rocks. In this paper two variations on Granar's magnetometer are described, designed respectively for (i) maximum sensitivity (minimum detectable susceptibility difference 10−10 emu/cm3) at fields of 20 oersteds or less and (ii) maximum convenience of operation at intermediate fields of 10 to 80 oersteds. An assessment is made of the importance of the various sources of error in measurement with the second instrument, and these experimental errors are compared with the natural scatter of principal axes of susceptibility of rock specimens.

By examining what is believed to be the physical basis of detrital remanent magnetism (DRM) we can set limits to the conditions under which it can occur. For example, detrital magnetism is not likely to be important in a sediment whose magnetic particles are less than 0.1 μ or greater than 50 μ or so in diameter. We can also set in order of importance the forces active in magnetization, and this leads to the conclusion that gravity is generally dominant at the instant of deposition. Both theories of DRM depend on this and are shown to be variants of the same basic theory. Future work should be directed toward the development of an improved form of this theory.

Opdyke's [1961] conclusion that the Triassic sediments of the Newark group of New Jersey do not show an inclination error of the kind that we have observed in recent sediments is, of course, a valid one, but his result is to be expected if, as is commonly supposed, the remanence of red sandstones is chemical in nature rather than detrital. We take his result to be strong evidence that this particular sediment did not acquire its remanence by the alignment of magnetic particles during deposition, but we should like to point out that this may not be true of all older sediments. In particular, we should expect siltstones with a nonmagnetic cement, in which the magnetic particles are not grossly different in size from the nonmagnetic, to show deviations of the remanence quite comparable with those which have been found in varves. A field study, similar to Opdyke's, of such material would be of considerable interest.