In my description of the mineral rashleighite, which was given in this magazine in 1948, I mentioned that most of the mineral from the Bunny mine, St. Austell, was in colour light blue-green, but that it was also sometimes true turquoise-blue. Likewise, at the Castle-an-Dinas wolfram mine, St. Columb Major, there occurred rarely in addition to the blue-green rashleighite a true turquoise-blue mineral which was first assumed to be turquoise and later rashleighite. It has now been found, however, that this turquoise-blue mineral is in fact true turquoise, and distinct from the green rashleighite, Specimens of this turquoise were found at the Bunny mine, associated with varlamoffite.

Geophysical Laboratory work is especially applicable to quenched rocks, but does not lend itself to the solution of problems connected with the interpretation of structural relationships between the component minerals of deep-seated rocks. The detailed examination of a rock of simple mineral composition might be expected to yield a picture of the way in which phase-boundary adjustments are made in a rock crystallizing under such conditions. Some of the Bushveld basic rocks are of such a nature, and in the present paper an attempt is made to analyse and interpret the structural relationships between the two minerals bronzite and bytownite (An78-80), of which they are composed. The material studied was collected under the guidance of Dr. A. L. Hall during the International Geological Congress in 1929, supplemented by core specimens recently made available through the courtesy of an old student of the College.

Many Jedburgh-type olivine-basalts of Carboniferous age in Scotland are characterized by a sub-ophitic texture in their groundmass and a flow structure of their lathy felspar phenoerysts. These characteristics have been frequently reported in the writings of Geikie, Flett, Bailey, Tyrrell, MacGregor, and others and are recognized as important textural features. In the main, however, they have been described as independent features and have been regarded as unrelated phenomena. Tyrrell (1909) alone has remarked on parallel felspars enclosed in plates of augite, and since his description appeared at an early date and has been available to subsequent workers, the lack of further examples indicates either that his discovery has been overlooked or that the texture is very rare.

Hydrous micas differ from normal micas in that they contain less potassium and more water. In the calculation of structural formulae from their chemical analyses, two methods have been used which rely on the following different assumptions:

1. As these minerals are often fine-grained and may contain adsorbed water, no reliance is placed on the value of H2O+ (water lost above 105° C.) as a measure of structural water. The water content is ignored and the structural formula is calculated on the basis of 20 oxygens plus 4 hydroxyls per structural unit. This method has been used by Hendricks and Ross and by Grim and Bradley.

2. The water which is recorded as H2O+ in the chemical analysis is assumed to represent hydroxyl groups in the aluminosilicate layers of the minerals. Thus, instead of having 20 oxygens and 4 hydroxyl ions per structural unit as in normal micas, formulae are derived which show less than 20 oxygens and more than 4 hydroxyls, the total oxygen plus hydroxyl remaining 24. This method has been used by Brammall, Leech, and Bannister.

Alamosite was first described by Palachc and Merwin, i who thought it might be isomorphous with wollastolfite. For that reason it has been re-examined, using X-ray methods.

A specimen from the original locality at Alamos, Sonora, Mexico (B.M. 1910,330) was made available through the courtesy of the British Museum (Natural History), and in physical appearance it tallied with the original description. The crystals were aggregates of fibres arranged in bundles which cleaved perfectly perpendicular to the fibre axes. Small tablets could be readily cut out for X-ray analysis. The refractive indices were very high and the dispersion was large. As the axial ratios found by X-ray methods agreed with those given by Palache and Merwin, the latter's chemical analysis and density determination were not repeated.

Samples of an unusual cordierite-rieh rock from near Upington, Cape Province, were collected by Dr. A. Poldervaart who kindly passed them to me for further study. This paper presents the results of the petrographic examination together with chemical analyses of the rock and of cordierite and biotite, which are two of the constituent minerals.

Garnets rich in manganese and iron, associated with bariumfelspars, have been reported by de Villiers from the manganese ores of Otjosondu, South-West Africa, in which they occur as the most abundant silicate mineral. The farm Otjosondu is 80 miles NE. of Okahandja, a railway station on the line between Windhoek and Walvis.

One specimen (referred to as no. 1), of which de Villiers published a chemical analysis, is massive and of a somewhat dark-yellowish colour. The material is traversed by veinlets of hyalophane (Cn9), about 0.5 mm. broad.

X-ray powder photographs reveal thomsonite a.s the principal product of some low-temperature hydrothermal experiments. Glasses of three compositions were used as starting materials; one of tile composition of anorthite (CaAl2Si2O8), one of a gallium-containing anorthite (Ca4[Ga3Al5]Si8O32) in which 3/8 of the Al is replaced by Ga, and a glass of the oxide composition of zoisite minus the water (4CaO. 3Al2O3. 6SiO2).

Henry How, professor of chemistry and natural history in the University of King's College, Windsor, Nova Scotia, described in 1861 the occurrence of a new calcium sodium borate, which he named cryptomorphite from the microscopic size of the crystals. The mineral was found in narrow seams of mirabilite (glauber-salt) at the junction of anhydrite and gypsum in Clinton quarry, Windsor, where he had also discovered ulexite in 1857. The cryptomorphite occurred as inclusions about the size of a small bean, between the gypsum and crystals of mirabilite.

Tobermorite, a hydrated silicate of calcium, was described by M. F. Heddle in 1880 from three Scottish localities: two near Tobermory in the island of Mull, and the third a quarry near the pier at Dunvegan in Skye. Further localities for the mineral are Sgfirr nam Boc, Loch Eynort in Skye, and Ardtornish Bay in Morven (Morvern), Argyllshire.

Because of marked similarity in chemical composition it has been suggested that tobermorite might be identical with gyrolite. In view of this and of the current interest in hydrated calcium silicates in cement chemistry the species has been re-examined.

Nacrite is the rarest member of the kaolin group of minerals. The name was first applied by Brongniart in 1807 to a mineral (talc granuleux of Haüy, 1801) the properties of which were not then adequately described. In 1832 Breithaupt recorded a mineral from an ore vein near Freiberg in Saxony, which has since been called nacrite. In the past, however, 'nacrite' has been applied quite frequently to different nacreous and micaceous minerals, not necessarily of kaolin composition, and the authenticity of the species was not definitely established until 1931 when Ross and Kerr, on the basis of an X-ray study of nacrite from the locality Brand near Freiberg in Saxony, proved the mineral to be a distinct species of the kaolin group.

In the summer of 1951 Mr. R. J. King of Leicester collected from the shatter zone of the main fault in Sheet Hedges Wood quarry, Groby, Leicestershire, specimens of slickensided syenite with quartz veins which contained a creamy white, soapy or powdery material in small cavities in the quartz and on surfaces of movement. The mineral was found from X-ray powder photographs to be the rare clay mineral nacrite. Subsequent searching by Mr. King this year has failed to produce more than a very few grams, mostly on one specimen. The mineral seems to be confined to the region of the main fault and has not been seen in any other part of the quarry.