The very abundant literature dealing with the twinning of felspar phenocrysts has put on record a surprisingly large number of twin-laws. Some of these have such complex indices that they have never been accepted; others are regarded as doubtful. Yet the authors of these have well-established reputations and the accuracy of their observations cannot be doubted. The difficulty lies in the interpretation of the results. Beside numerous examples of well-defined laws, one comes across a high percentage of isolated cases, each appearing quite as plausible as these, yet difficult to assign to any definite law.

Two monoclinic pyroxenes from the Skaergaard Halvoen intrusion, Kangerdlugssuak, East Greenland, have been separated and analysed. They occur in a strongly differentiated gabbro complex which ranges from a hypersthene-olivine-gabbro at the base to a fayalite-quartz-gabbro of unique composition at the top. The differentiation has proceeded with a steady increase in iron and a corresponding decrease in magnesia, while the other major oxides remain relatively constant throughout. The clinopyroxenes in the lower horizons are in equilibrium with labradorite, normal olivine and hypersthene, and are fairly normal varieties as shown by the following optical properties:—

1. (1) … α 1·672, β 1·678, γ 1·701, 2V 42°, γ: c 39°, sp. gr. 3·34.

2. (2) … α 1·695, β 1·702, γ 1·722, 2V 43°, γ: c 41° sp. gr. 3·38.

In recent years several occurrences of clinoenstatite and of clinohypersthene have been recorded, the identification depending on the oblique extinctions measured from a prominent cleavage. This is, of course, quite inconclusive, because many sections of hypersthene show extinction-directions oblique to the trace of a prominent cleavage, and much fuller data would be necessary before most of these occurrences could be accepted as monoclinic Mg-Fe-pyroxenes.

The determination of the specific gravity of a solid which is only available in small quantity has always been a difficult problem in cases where the suspension method fails, either because the solid is too dense, or because it dissolves in or reacts with the heavy liquids available. It is hoped that the following method will be found a useful addition to the numerous methods which have been proposed from time to time to meet such cases. References to most of these methods are given at the end of the paper. Neither the new method nor any one of the older ones is universally applicable, and the best method for any particular case will depend on many circumstances.

The existence of an unidentified mineral in the riebeckite-microgranite of Mynydd Mawr has been known for almost exactly fifty years. It was first noticed by Harker and described by him in the following words: ‘The ground-mass [of the microgranite] is a finely granular admixture of quartz and orthoclase, as in an ordinary “microgranite”, but containing in addition another mineral which is often plentiful. It occurs in minute crystals of acicular or rectangular form, scattered through the ground-mass or included by the other constituents, and having usually a fluxional arrangement agreeing with that of the parallel hornblende and tourma]ine streams. The mineral is colourless, or in the larger crystals gives for vibrations parallel to the long axis a faint tint of indigo-blue. The crystals give high polarisation colours and straight, or nearly straight, extinction. They have a high refractive index, which causes them to stand out in relief when viewed by ordinary transmitted light.’

Scawtite was first described at Scawt Hill as a secondary mineral occurring in vesicles and venules in melilite of a melilite-rich hybrid of the endogenous contact-zone. Since then scawtite has been identified in a gehlenite-vesuvianite-ealcite-rock from the contact-zone of a gabbro in the Little Belt Mountains, Montana. The present note records a further occurrence of scawtite at Scawt Hill in a different patagenesis, viz. in association with and replacing spurrite of the exogenous contact-zone.