In 1937 G. Nagelschmidt described a new silicophosphate mineral obtained from basic slag. It resembled the earlier described silicocarnotite (A. Carnot and A. Richard, 1883; D. P. Riley and E. R. Segnit, 1949) in its solubility in citric acid. Nagelschmidt gave optical, chemical, and X-ray data which indicated that the material had a composition of approximately 7CaO.P2O5.2SiO2, and agreed closely with a phase reported by G. Trömel (1943) in his investigation of the system CaO-P2O5-SiO2. The material was further considered by M. A. Bredig (1941), who published cell dimensions worked out from powder photographs, assuming that nagelschmidtite was a solid solution of tricalcium phosphate in a predicted high-temperature hexagonal form of dicalcium silicate. R. L. Barrett and W. J. McCaughey (1942) gave further data, chiefly connected with phase relationships in the binary system Ca2SiO4-Ca3P2O8. Finally, S. O. Agrell (1946) published further data on the slag mineral, giving the chemical analyses and optical properties of two samples, together with a discussion of the phase relationships of the material in slags, and hence in the binary system mentioned above. The name 'nagelschmidtite' is due to Barrett and McCaughey.

For the cutting of chips from rocks preparatory to thin-slicing, it has been standard practice for many years to use notched soft iron disks, manually armed with diamond powder. The latest machines employing cutting-disks of this type at the Geological Museum were built at the Government Training Centre, Ascot Works, Letchworth, to the design of the late Dr. H. H. Thomas and of Mr. W. Ward of the Centre. These are powered with 0.25 horse-power variable speed direct current motors, giving a maximum disk speed of 280 revolutions per minute. By means of rheostats, control of speed as sensitive as that obtainable with a treadle type of machine (see A. V. Weatherhead) is achieved. The normal speed for cutting is 160 to 180 r.p.m. In cutting a hard rock such as chert, it may be necessary to recharge the disk with diamond several times during the operation. The rock-specimen is held in a steel clamp with hardwood liners, mounted on a swinging arm. The rock is held in contact with the disk by the tension of a spring, or, during delicate work, by the pressure of the hand.

A rapid and accurate method of determining the density of a mineral has long been sought for by mineralogists and gemmologists. The hydrostatic method gives a high degreeof accuracy, but has the drawback that each estimation occupies about a quarter of an hour so that if some ten to twenty specimens have to be investigated the time taken will be considerable. The use of heavy fluids is rapid, but for finer distinctions a large number of fluids is required and they tend, either from evaporation or chemical change, to become variable, while the Westphai balance is only reliable for fairly large portions of the mineral. Several attempts therefore have been made to develop a balance which will give a direct reading. Most of these such as the Kratschner balance have been based upon Leonardo da Vinci's principle of the bent lever, but generally have not been sufficiently delicate or accurate for small specimens.

In the course of the investigation of the crystal structure of sodiumthyroxine- dl it appeared to be necessary, in order to find the approximate orientation of the molecule, to determine the orientation of the indicatrix and the values of the principal refractive indices.

The crystals are flat and very thin (about 0-03 mm. thickness), and when put on a glass slide they all lie on their (100) face. This suggested the use of the universal stage, but the high birefringence of the crystals prevented it from giving good results. Moreover, as the crystals belong to the triclinic system and have no regular contour, it would have been difficult to refer the indicatrix to the unit cell. The interference figures seen when the crystals lie on a glass slide were mis-oriented, though sufficient to determine the sign of the indicatrix, and no clear interference figures could be seen when the crystals were far from horizontal.

To overcome these difficulties the method which follows was developed, and proved to be very useful for all uniaxial crystals and highly birefringent biaxial crystals, whatever their habit.

During a universal stage investigation of the pegmatitic felspars in the Central Australian charnockites, several alkali-felspars were found which, according to the stereograrms published by Emmons (3, pl. 12), should have been near ' anorthoclase' in composition. This identification, based purely on optical properties, was questioned in the first instance because of the absence of other evidence for a soda-rich suite of minerals in these apparently potassic pegmatites. Two felspars which suggested a high soda-content were chemically analysed. Neither proved to be notably high in soda.

Although there have been many publications on the fluorescence MX. of minerals in ultra-violet light, very little systematic work has yet been done in utilizing this property for the elucidation of petrological problems, The response of various minerals would seem to be entirely capricious; but the extensive work done on phosphors shows that this depends on the presence of traces of impurities. The spectacular display given by certain minerals has made a strong appeal to amateur collectors.

In 1943 the present writer made use of a fluorescent variety of zircon to assist in the subdivision of the Archaezoic gneisses and igneous rocks of the Mnsgrave Ranges in Central Australia; and after further fieldwork preliminary observations were published in 1947.

The Ballachulish slates, exposed to the north and south of Loch Leven in Argyllshire, contain, in most places, cubes of pyrite up to half an inch in diameter. During a visit to the area in the spring of 1949 the writer's attention was attracted by the dark colour of the 'pyrite' cubes in the North Ballaehulish slate quarry a little more than a mile east of Onich. On examination these proved to consist of a mass of haphazardly orientated crystals of pyrrhotine with irregular outlines. Slates collected from the main working quarry on the south shore of Loch Leven, on the other hand, contain cubes which are single crystals of unaltered pyrite.

In the past decade, differential thermal analysis has developed into a very useful mineralogical technique. The theoretical background for this method has been treated by Speil, Berkelhamer, Pask, and Da vies (1945) and has been modified by Kerr and Kulp (1948, 1949). The application of the method to the clay minerals has been carried out with considerable success by a number of workers in Europe and America. In particular, Grim and co-workers (1942, 1947, 1948) have produced a number of significant papers dealing with the thermal analysis of clays. The method has been extended to other mineral groups such as the carbonates (Faust, 1949; Beck, 1946; Kerr and Kulp, 1947; Cuthbert and Rowland, 1947; Kulp, Kent, and Kerr, 1950), phosphates (Manly, 1950), sulphates (Kulp and Adler, 1950), quartz (Faust, 1948), and the hydrous iron oxides (Kulp and Trites, 1950).

In 1938 large crystals of almandine garnet were collected by the writer from a cutting excavated during road-widening operations, on the main Huntly—Keith road, near Cairnie church, about 4½ miles from Huntly. In view of their exceptional size and excellent crystal form, it is felt that the occurrence is worth recording. Garnet crystals of comparable size have been recorded by Heddle from several localities. The Cairnie crystals, considering their size and excellent form, must rank with the finest recorded from Scotland.

The rocks at this cutting are coarsely banded garnet-biotite-sillimanite-gneisses with abundant quartz, oligoclase, orthoclase, and biotite, subordinate muscovite, and accessory magnetite, apatite, and zircon. Sillimanite is sporadic in occurrence, forming elongated colourless prisms up to 2 mm. across. The biotite-rich bands generally contain only small garnets, if any, but garnet is very abundant in bands rich in quartz and felspars, and often forms a large part of the rock.