The anisotropy of magnetic susceptibility of some basic igneous and metamorphic rocks has been found to be due to the preferred orientation of the long axes of grains of magnetite. The degree of anisotropy is in a few samples as great as 40 per cent but usually is less than 10 per cent. The variation in anisotropy is believed to be due to variation in the degree of preferred orientation, and in susceptibility due to grain size and chemical composition. Half of the susceptibility ellipsoids determined were found to approach an oblate shape while only a quarter of the remainder approached prolate shapes. The magnetic ellipsoids of a number of Tertiary lava flows, banded gabbros, dikes, cone sheets, a ring dike, and Precambrian basic gneiss have been determined. The principal susceptibility directions of most of the lavas were widely dispersed but in some flows were tightly grouped—the minima round the pole of the flow plane, the maxima normal to the flow direction in the flow plane, and the intermediates in the flow direction. Similar groupings were found with the banded gabbros but not with the other intrusives. Basic gneisses show a very marked concentration of maxima in the direction of the visible lineations and of minima normal to the observed foliation. The applicability of anisotropy measurements in structural petrology is discussed.

SuriiniaryThe iron-titanium oxides of 45 samples from a single tertiary lava flow from Skye, Scotland, have been subjected to detailed examination and quantitative measurement.A basically simple assemblage of titanomagnetite (93-100 per cent) and discrete ilmenite (0-7 per cent) has, in most samples experienced one or both of two types of alteration, titanomagnetite high-temperature oxidation and ' titanomagnetite granulation ', which we define in this paper.Titanomagnetite high-temperature oxidation, describing the process of ilmenite exsolution and further oxidation, varies rapidly but largely systematically within the flow. Parallel alteration of discrete ilmenite and olivine also occurs.Titanomagnetite granulation, describing the alteration of titanomagnetite to impure rutile granules, only affects samples with limited development of high-temperature oxidation and varies throughout the lava in a rather unsystematic fashion. While high-temperature oxidation is almost certainly wholly of deuteric origin titanomagnetite granulation is shown usually to require burial to greater than 900 m for its formation, and thus may not take place until several million years after the extrusion of a lava flow. The mechanisms by which these two types of alteration take place, and their implications for palaeomagnetic interpretation, are discussed.

The remanent magnetization of the basic igneous rocks of Skye, the hypersthene gabbro of Ardnamurchan, and the layered gabbro of Rhum has been examined. All the rocks possessing stable magnetization were found to be reversely magnetized. This is believed to be due to the Earth's magnetic field being reversed during the half million years or so during which the rocks were formed.Reasons are given for supposing that the dips of the Skye rocks are formational. The ancient pole position during part of the Lower Miocene is estimated to be 70O.8 (k 3O.8) N. Lat., and 159O.6 ( L-5O.1) E. Long., a position which is significantly different from that of the present geographic pole. It is concluded that there has been relative movement between North-west Europe and the pole since the Lower Miocene.The directions of magnetization are used in an attempt to correlate some of the lavas and dykes of Skye. It is also tentatively suggested that the rocks of Skye, Rhum and Ardnamurchan are of approximately the same age.

A selection of samples representing extremes of magnetic and opaque petrological properties represented in a single lava has been used to investigate the possibilities of obtaining the intensity of the ambient field (FAN,-) at the time of extrusion of the flow.Two methods have been used to obtain values of FAN,-; the first using a version of Thellier's method of comparison of partial natural remanences (PNRMs) and partial thermoremanences for several temperature intervals and the second using Van Zijl's method of comparison of progressive a.c. demagnetization of the NRM and of a laboratory TRM.Initial assessment of the results was based on the similarity of form of natural and laboratory weak field remanence demagnetization curves together with the internal consistency of apparent F A N C values. Using these criteria, results from a number of samples give acceptable values for FANC. While mean values for these samples from the two methods are not significantly different, the a.c. method gives the highest internal consistency; the RMS error of the mean value being + 3 per cent against 8 per cent for the PNRM-PTRM method. The overall mean value for F A N C was 0.38 Oe corresponding to a VDM (virtual dipole moment) of 7-2 x c.g.s. units. Clear relationships were found to exist between the strong field magnetic and opaque petrological properties on the one hand, and their suitability for FANC measurement on the other. Suitable samples were characterized by reversible strong field heating curves with single high Curie points, high titanomagnetite deuteric oxidation (magnetite oxidation numbers of 3.3 to 4.4, the highest value found in the flow) and the absence of titanomagnetite granulation. Unsuitable samples were characterized by double Curie points, magnetite oxidation numbers of less than 3.3 and, in four of five samples, titanomagnetite granulation.

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