The magnetic susceptibility anisotropy of 275 specimens comprising 38 sites from the Cambrian slate belt in North Wales was measured to determine the magnetic fabric of the slates. The susceptibility ellipsoid is oblate for all sites, and the maximum/intermediate susceptibility plane always coincides with the cleavage plane of the slates which has a Caledonian strike and is nearly vertical. The maximum axes align sub-vertically and the intermediate axes sub-horizontally, trending NE-SW. The minimum susceptibility axes are normal to this foliation plane and coincide with the poles to the slaty cleavage. The orientations of the principal susceptibility axes are found to be in excellent agreement with the orientations of the principal strain directions, determined by X-ray goniometry on one of the samples from almost all of the sites. Correlation of the magnetic susceptibility anisotropy with predicted March strains (March, 1932) shows that the principal magnitudes of susceptibility can be related to those of the strain by: ( χf χ0) = ( lf lo)a (for i = 1, 2, 3. The orthogonal principal axes), where χf and χ0 are the final and initial susceptibilities along a given axis i and lf and li are final and initial axial dimensions in the same direction i of a principal strain axis. The exponent a for the North Wales slates was found to be 0.145 ± 0.005. Knowledge of such a relationship may permit rapid approximate determinations of a petrofabric in similar rocks from their magnetic fabrics. However, the exponent a will probably have to be recalibrated for each rock type. © 1979.
