In quartz-seheelite veins traversing rocks from the metamorphic complex in the Central Rhodopian mountains, a mineral was found closely resembling bismuthinite. Its composition is Bi 66·62 %, Pb15·50 %, Cu 0·85 %, and S 16·71 %, corresponding to the formula PbBi4S7. It is orthorhombic, long prismatic to acicular with an axial ratio a:b:c = 0·9004:1:0·3249. Observed forms: {110}, {100}, {010}, and {011}. Sp. gr. 6·92. Hardness about 2 1/2. Displays a perfect cleavage along {100}. It is distinctly anisotropic. Strongest X-ray intensities: 3·50 Å. (10), 3·08 (8), 1·939 (8), 2·78 (6), 1·733 (6), 2·50 (5). The name is after the eminent Bulgarian mineralogist and petrographer G. Bonchev.

The absorption spectra of talc, saponite, and hectorite between 4000 and 400 cm. −1 are closely related, although the bands of the smectites are more diffuse as a result of isomorphous substitutions in the tale structure. Using oriented specimens, vibrations in which the change of dipole moment is perpendicular to the sheets of the minerals are identified, and the results compared with theoretical predictions. Three bands arising from the stretching vibrations of interlayer water molecules in the smectites are distinguished, one of which corresponds to a very weak hydrogen bond. Spectral changes arising from vigorous grinding are discussed.

The main minerals (garnet, pyroxene, hornblende, and scapolite) in certain Dahomeyan (Archaean) garnet-hornblende-pyroxene-scapolite gneisses from Ghana have been separated and analysed. Both the garnet and the pyroxene show marked eclogitic affinities, and the rocks are considered to occupy a transitional position between the amphibolite and eclogite facies. The scapolite is a sulphatic variety containing more than 4 % SO3 and is believed to be a product of autometasomatism. A revised scheme for the distribution of mineral facies within the temperature-pressure field of metamorphism is proposed.

This paper describes a development for the universal stage of the extinction curve method for determining the orientation of the optical indicatrix in biaxial crystals. The extinction curve may be defined as the locus of the poles of the extinction directions when the crystal is rotated between crossed polarizers about an arbitrary but fixed axis perpendicular to the microscope axis.

When two or more extinction curves, or parts of them, are plotted on a stereographic projection, their intersections give the directions of the axes of the indicatrix, of which at least two are directly accessible. The main application of this new orthoscopic method is to provide a means of refining the orientation of the indicatrix obtained by this or any other method. The accuracy that can be obtained for minerals of low birefringence such as the felspars is considerably greater than with the conventional orthoscopic method.

A complete account is given of the details of the plotting procedure.

When one optic axis of a biaxial crystal has been located accurately by the usual orthoseopic or conoscopic methods, the second may be located by determining the extinction positions at normal incidence and applying the Biot-Fresnel construction. When use is made of the extinction directions associated with two or more wave normals inclined to the section, this method gives more reliable values of the optic axial angle than can be obtained by the doubling of the angle V over the pole of a bisectrix. If the optic axes have been located with sufficient accuracy by this procedure, then this also determines the orientation of the indicatrix.