In 1934 a contribution from America (Parmelee and Badger, p. 2) reported that a certain glaze, containing rutile, showed rapid reversible darkening in daylight. It was stated that C. H. Zwermann, Jr., ascribed this photosensitivity to titanium. In the present author's experience the phenomenon is often exhibited, in varying degrees, by commercial glazes based on rutile or on the purified titanium dioxide available in industry, as well as by glazes made up in the laboratory from chemical titanium dioxide of high purity. Lee, also in America, observed reversible darkening in sunlight in commercial titanium dioxide itself, and the existence of this effect was confirmed by Williamson (1937) for some brands of commercial titanium dioxide available in Britain. These brands were affected by daylight which had passed through glass, and some tentative suggestions were made regarding the mechanism of the reversible darkening. Later it was demonstrated (Williamson, 1939) that reversible darkening in daylight could be produced in a sample of titanium dioxide of considerable purity, which was itself not discoloured by light, by contamination with iron according to a special method.

Thin sections of clays are not easily prepared by the routine petrographic methods. The chief difficulties are due to the extremely fine texture of the clays and to the soft and plastic nature of these materials. These difficulties are only too well known, and, although in some cases they have been partially overcome by impregnation in vacuo, it is safe to assume that really little work has been accomplished in the microscopic examination of clays on account of the difficulty of producing satisfactory thin sections. The writer has attempted to extend the scope of the microscopic investigation of clay rocLs by means of the technique outlined below.

The method has been devised in response to an appeal by a friend, who, working in a laboratory at a well-known pottery works and thus connected with the clay industry, desired a method that would make possible the examination of thin sections of clays under high powers with polarized light.

In the course of work on the Barker index of determinative crystallography, a number of interesting problems have arisen owing to the selection by a few workers of unusual sets of fundamental angles for their computations. Most of these problems have been of no general application, but One appears to have a wider interest and may prove of practical value in dealing with crystals having curved or striated faces that only admit of accurate measurements along one zone.

This problem was to compute the angles and elements of a monoclinic crystal, using as fundamentals the angles from b(010) to any three faces e, f, g (of known indices) not in the zone [010], and no pair of which are co-zonal with b(fig. 1).

At Brockhill in Worcestershire, some 11 miles west of Droitwieh, in the valley of the river Teme, a small dike of igneous rock is intruded into the red marls and sandstones of the Downtonian Series.

The dike was figured and described by Phillips in 1848. On the Old Series one-inch Sheet 55 N.E. it was indicated as having a very limited outcrop on the eastern side of the river Teme; but a report of an excursion to Brockhill in 1874 records the presence of igneous rock on the opposite bank. The intrusion was examined during the resurvey on the six-inch scale of the New Series one-inch map, Sheet 182 (Droitwich), and is of interest both on account of its petrographic character and its remoteness from other igneous rocks. It has recently been traced by magnetic methods for a distance of over three-quarters of a mile.

A very interesting kornerupine-bearing rock was sent by Mr. W. H. Cross during 1938 to the Geological Survey for identification. The material, of which about 800 grams were available for study, was stated to have been found in the Port Shepstone district. The occurrence has, however, not been visited by the writer.

The hand-specimens examined were all somewhat weathered. Macroscopically these consisted of an intergrowth of grey, columnar crystals of kornerupine, up to 5 centimetres in length, and adhering iron-stained quartz. Shiny ‘books’ of biotite occur both in the kornerupine and the quartz, while small patches and specks of dark brown tourmaline and bluish grandidierite were observed enclosed in the kornerupine.

Nickel-Speiss (Nickelspeise), an impure arsenide of nickel obtained as a by-product in the manufacture of smalt, has long been known as a source of distinct crystals representing a definite chemical compound. Such crystals were already analysed by Döbereiner (1823), who concluded that the material was an artificial form of niccolite, NiAS. Later analyses showed that the composition of the crystallized furnace product approaches Ni3As2, a compound unknown in nature at that time.

A substance corresponding to the crystallized nickel-speiss, but thought to be of natural origin, was first described by Breithaupt (1841), who named the material Plakodin (πλăκώδης, broad) in allusion to the tabular habit of the crystals. These were said to have occurred in the Jungfer mine at Müsen in Westphalia, associated with chalybite and gersdorffite and to have been salved by a smalt-works official from a shipment of ores.