There are two predominant causes for the anisotropy of magnetic susceptibility of rocks and ferrimagnetic minerals: the shape of the magnetic material and crystalline anisotropy. In general, these factors may operate simultaneously, but in rock magnetism their effects are frequently separable. In the case of shape‐effect anisotropy, one can predict the degree of anisotropy by an ellipsoidal approximation, provided that the magnitude of the bulk susceptibility and the dimensions of the body are known. The anisotropy is chiefly due to the shape effect in the cubic titanomagnetites and in the rocks containing them, whereas crystalline anisotropy dominates in crystals of lower symmetry (pyrrhotites, hematites, ilmenites). In magnetically anisotropic rocks and minerals, the direction of thermoremanent magnetism is found to deviate sometimes markedly, and always systematically, from that of the external magnetic field. Several examples and some theoretical considerations are given.

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A description is given of the bottom topography, seismic refraction profiles, magnetic and gravity observations in the Red Sea. The deep trough along the centre is associated with positive Bouguer gravity anomalies, large magnetic anomalies and seismic velocities of 7.1 km/s. It appears that this represents a fissure in the continental crust, partly filled with basic, igneous material. A structural map, based on all the geophysical evidence, has been prepared and it is suggested that the complex Red Sea rift was formed as a result of crustal tension. Finally, a discussion is given of the Red Sea as part of the world rift system. H.M.S. Dalrymple on a cruise to the Persian Gulf and obtained total intensity magnetic field records along more than 3 ooo km of track in the Red Sea. The track was planned to supplement the data obtained aboard Vema the previous year and is also shown in Figure I. The data obtained by Dalrymple are being published separately.

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