During a study of the petrography and mineral constituents separated from various shales overlying the coal-measures of South Wales, Dr. Alfred Brammall noticed on several hand-specimens from Llandebie a white infilling to fissures or a coating on slickensided surfaces which could be readily detached from the matrix. Chemical examination showed that this coating, unlike the shales themselves, contained more sodium than potassium. The present note gives an account of subsequent chemical, X-ray, and optical work, which confirms Dr. Brammall's suggestion that the white incrustation contains a new sodium-rich mineral allied to mica. It is not, I hope, an unfitting tribute to his work in this field as well as to his interest in recent advances in mineralogy that I have named this mineral brammallite. A short account of its nature and relationship to illite, which is the chief constituent of the shale itself (see preceding paper, p. 297), will now be given.

Quartz is the commonest of minerals and reference to any dictionary will show that it is crystalline silica, but how or when this substance acquired the name quartz seems to be a still unsolved mystery.

Theophrastus, the pupil and successor of Aristotle, in his book on stones (19), mentions among other stones used for engraving seals, a stone called crystal This name, apparently, is derived from cold, and contract. It would seem from this that the ancient Greeks, as implied by Pliny, believed crystal (modern quartz) to be a variety of supercooled ice. In point of fact the formation of crystal in nature is explained by the earliest writers in a different way. Diodorus Siculus (circa 30 B.C.) suggests that crystal was formed from water hardened by heavenly fire, while Pliny the Elder (circa A.D. 60) says that crystal is ‘a substance which assumes a concrete form from excessive congelation’, and goes on to explain that crystal is only to be found ‘in places where the winter snow freezes with the greatest intensity’ (15, Book 37, 2, 9). Pliny's description of crystal, however, leaves no doubt that he applied this name to the crystalline variety of silica which is now called quartz.

The present communication deals with a calc-silicate hornfels of uncommon mineralogical and chemical composition. It may be tentatively referred to the deep-seated metamorphism of a marl, and on account of the shortage of analytical data pertaining to metamorphosed sediments of the calcareo-argillaceous type, a detailed investigation of a variety of this group seems justified.

Intercalated bands of anorthite-epidote-garnet-hornfels bordered bilaterally by pyroxene-bearing gneisses occur on the farms Zandfontein, Pella, and Klein Pella in the north-eastern part of the division of Namaqualand, South Africa (cf. geological map), On Klein Pella small exposures also extend intermittingly for approximately 1·5 miles due WNW.-ESE. of the amphibolite koppie situated on the northern boundary of the farm area. The bands exhibit a gentle pinch and swell structure with maximum widths of 15 to 41 inches, and the contact with the host rock is commonly marked by an epidotic fringe ¾ inch in thickness. In some cases the anorthite-epidote-garnet-rock is penetrated by parallel lenticles of pure quartz which may attain maximum breadths of 0·8 inch.

The fine lamellar structure (pl. IV, fig. 1 a and b) which is described here is very common in the orthopyroxenes of plutonic and regionally metamorphosed rocks. This structure can be seen under crossed nicols in sections inclined at a high angle to the optic axial plane (100), except on those sections which make a low angle with (001). When the main part of the crystal is in extinction, the fine lamellae stand out because their extinction position is different from that of the main crystal. Although this structure is common, it is not described in any of the widely used text-books, but several descriptions and references to it have been published in recent years. The purpose of this paper is to draw attention to the nature of this lamellar structure.

The olivines, which are the subject of this note, were obtained from a series of basaltic lavas and dikes on Ubekendt island, which lies off the west coast of Greenland in latitude 71 ° N., longitude 54 ° W.

The optical properties of the olivines were investigated in order to determine any range in composition; such variations might have been expected in the very thick lava series. From this aspect, however, the results are disappointing, the composition range being small. However, it is thought that the results may have a certain interest in showing the relative accuracy obtainable in the measurement of refractive indices and optic axial angles. The usual determinative methods have been slightly varied. It is the purpose of this note to describe these variations and to compare the relative speed and scope of the method used with those obtained by other methods.

Waldemar Lindgren described in 1924 a mineral present in nickel ores from southern Nevada which in colour, hardness, and stability differs from polydymite from Grüneau mine at Sehutzbach, between Betzdorf and Daaden in Rhenish Prussia, and yet matches so-called ferriferous polydymite from Vermilion mine, Sudbury, Ontario. He proposed the name violarite both for the mineral from Nevada and for the ‘polydymite’ from Ontario on account of the violet-grey colour of the mineral on polished sections. Lindgren assumed that the iron found in the first analysis of ‘polydymite’ from Ontario was due to admixture with pentlandite and thence deduced nickel disulphide as the possible composition of violarite. In the light of subsequent results it is relevant to mention that Lindgren and Davy's work was based upon the study of polished ore sections but not in polarized light.

Tricalcium disilicate, first described by Shepherd, Rankin, and Wright in their investigations on the lime–silica and lime–alumina–silica systems has since been reported as a constituent of technical slags. An account has also been given of its occurrence as a reaction product in a glass furnace.

In the ternary system CaO–Al2O3–SiO2 the compound forms with pseudowollastonite–gehlenite and larnite–gehlenite two three-phase equilibrium assemblages, associations which are in part recorded in the technical products.

G. V. Wilson recorded the disilicate together with wollastonite and melilite in reconstituted limestone fragments enclosed in glass, while S. G. Gordon has described and presented an analysis of a slag composed of pseudowollastonite and the 3:2-compound approaching in composition that of the binary eutectic in the synthetic system.