At the suggestion of Professor H. H. Read the optic axial angle and the refractive index y were measured in four hypersthenes from Aberdeenshire norites. As the values obtained suggested that the minerals were iron-rich, it was decided to have chemical analyses made. These and the optical data are set out in table I below. At the same time the literature has been searched for analyses and optical data of enstatites and hypersthenes, in order to supplement those used by A. N. Winchell in his diagram of variation of the properties of the series. The references given by Winchell have been examined, and it is found that some of the data is not reliable.

Helvine and genthelvite are recorded for the first time in Britain; of the former, three occurrences in Cornwall and one in Devon, and of the latter, one occurrence in Cornwall, are described. British danalite has hitherto been represented by two old specimens with the vague locality ‘Redruth’, Cornwall; occurrences at four Cornish localities are described, and a further Cornish locality is provided by two old specimens previously labelled as garnet. Partial chemical analyses for the helvine, genthelvite, and danalite specimens are presented; helvine and genthelvite occur in pyrometasomatic-hydrothermal deposits in metamorphosed calcareous sediments and, in one instance, in greenstone; danalite in hydrothermal deposits or lodes in metamorphosed greenstone; the parageneses of the three minerals are discussed.

The occurrence of dickite in the gold-bearing rocks of the Witwatersrand system was first reported by Mr. H. C. M. Whiteside. In addition, among the many mineral samples given to me by students of the Witwatersrand Technical College, Johannesburg, were several dickite specimens from mines all along the Witwatersrand area. Dickite may therefore be regarded as fairly widespread in the rocks of at least the Main-Bird series, although not in large quantity.

The only occurrence of which a detailed record is available is that from the Daggafontein mine. Little is known of the other samples except from the immediate mineral associations. It is, however, clear that all the occurrences are of hydrothermal origin.

The minerals to be described in this paper were collected, in conjunction with W. E. Fraser, who was carrying out a detailed study of the borolanite, from the road-metal quarry, opened in 1938, at Allt a' Mhuillin, Loch Borolan, Assynt, Scotland. They occur in pegmatitic patches and veins in borolanite.

This mineral is one of the chief constituents of the pegmatites. It forms large cleavable masses, associated with orthoclase, melanite, and a little dark mica. Calcite, sphene, zircon, and orthite are present as accessories. It often fills interstices between crystals of orthoclase. A preliminary examination has indicated that it is a common constituent of the spots in some types of borolanite, and further work will have to be done to determine its importance in this connexion. This is the first definite record of the occurrence of cancrinite in Britain.

The crystal used in the first part of this investigation, an untwinned specimen from Ceylon in the British Museum collection, was measured goniometrically and described in detail by Dr. G. F. Herbert Smith, who gave as its crystallographic constants :

Density (p) 5.82 (grs./c.c.). Monoelinic prismatic,

Owing to the comparatively largc size of the crystal and its high absorptive power it was unsuited either to the 'rotating crystal' or Laue photographic methods of X-ray examination, Reflections from the faces were observed on the Bragg ionization spectrometer and spacings measured with an accuracy of about 2 per cent.

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.

This new ilmenite-like mineral, containing lead and manganese, was found in rounded fragments and rough crystals in the diamond bearing sands of Diamantina, Minas Geraes, Brazil (at Dattas, Rio Cipó, &c.).

The albitisation of pelitic metasediments to form adinoles adjacent to minor mafic intrusions has in the past been attributed to Na-bearing metasomatizing fluids emanating from the intrusions themselves. The chemistry and mineralogy of adinoles associated with dykes forming a high-intensity swarm in the Tallaght area, County Dublin, Ireland, confirms a metasomatic origin for the adinoles described, with Na and to a lesser extent Ca introduced into the country rocks at the expense of K. We suggest, however, that the source of the metasomatizing fluids was external to the dykes — possibly involving a hydrothermal system driven by an underlying magma reservoir that was parental to the dykes and also contributed volatiles to the hydrothermal system by degassing. Heat flow from the dykes was instrumental in promoting the reaction between muscovite in the country rock pelites and the Na(Ca)-bearing fluids to form albite, temperatures of ∼350°C in the country rocks within a metre of the contact being indicated. The mechanism of adinolization proposed is shown to be compatible with recent experimental work on the hydrothermal alteration of greywackes and basalts and also with the likely temperature gradients adjacent to dykes.

Pinnoite, first described by Staute in 1884, is straw-yellow, granular to faintly fibrous. It occurs in the upper kainite layers at Stassfurt, and is found associated with earthy boracite from which it is freed by washing with water. The analysis corresponds to the formula MgO.B2O3.3H2O, and Staute assumes the mineral to be an alteration product of boracite, 6MgO.MgCl2.8B2O3. Groth classes pinnoite together with CaO.B2O3 as a hydrated metaborate. This assumption does not seem justified, as will be pointed out later.

The two exceedingly interesting specimens which form the subject of the present notes came under my observation in the following manner. In 1905 there was sold by public auction in London an old and very fine collection of minerals, mostly Cornish, which had been formed by John Hawkins, F.R.S. (b. 1758, d. 1841), of Trewithen, Probus, Cornwall; a gentleman well known in his day both as a scientist and traveller. The major part of the collection unfortunately left this country, having been purchased by the dealer F. Krantz of Bonn. A few lots were, however, bought by myself, and one of these contained the first specimen to be here described.

In the Geological Survey memoir a dike is recorded cutting the Lewisian gneiss near Achmelvich, north-west of Lochinver, Sutherland. At the time the mapping was carried out no thin section was prepared of the rock, and the dike was presumed to be a continuation of the Canisp porphyry dike which is exposed in the river Inver near Lochinver and farther south-east.

During an investigation of the post-Cambrian sills and dikes of Assynt and the adjoining districts of north-west Scotland, the writer examined the Achmelvich dike in some detail and recognized its affinities with the nepheline-syenites. It is the purpose of the present note to describe the field occurrence and petrography of the dike.

The specimen of cordierite-gneiss under notice was collected by Mr. T. N. Muthuswami (Assistant Professor of Geology, Presidency College, Madras) in a quarry at Ganguvarpatti in the Madura district. On examining tliin sections of the rock, he found that many of the cordierite crystals were optically positive, and, thinking that the mineral might be worth further investigation, he sent a specimen of the rock to me. The investigation was carried out ill the Geological Department of the Imperial College of Science and Technology, and my thanks are due to Professor W. W. Watts and Dr. J. W. Evans for their kindness in affording me facilities for this work and also to Dr. H. H. Thomas, of H.M. Geological Survey, for his kindness in placing Fedorov's universal stage at my disposal for the measurement of the optic axial angle.

The rare mineral stichtite, a complex hydroxy-carbonate of magnesium, chromium, and iron, was defined in 1910 by W. F. Petterd on material from the Adelaide silver-lead mine, Dundas, Tasmania. Petterd's description, together with detailed optical and chemical studies by A. S. Wesley, L. K. Ward, L. Hezner, and A. Himmelbauer, were collected by W. H. Twelvetrees and issued by the Tasmanian Department of Mines in 1914. A new analysis of the Tasmanian material is included by W. F. Foshag in his investigation of the hydrotalcite group.

The mineral was next recorded by E. Poitevin and R. P. D. Graham from the Megantic mine, Black Lake area, Quebec; no chemical data of this occurrence are available.

It is the author's good fortune to collaborate with Dr. J. W. Evans in describing the rock specimens which he collected on the expedition to Caupoliean in Bolivia during 1901-2. These specimens contain a very large proportion of shales, slates, and phyllites, and in their investigation certain petrographic methods devised for a detailed study of the slates of North Wales, Devon, and Skiddaw were followed. The present paper is intended primarily to describe these methods as applied particularly r the Bolivian rocks and to a Skiddaw slate facies which, for purposes of comparison and contrast, is of special interest. It is not suggested that between groups of rocks from places so widely separated as Bolivia and Skiddaw close correlation is possible; the products of the reconstitution processes which the two groups record differ considerably in detail; but as providing data for the study of the mechanism of these processes the two groups can be conveniently examined together.

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.

When investigating the constitution of antunite or lime-uranite, I secured an extremely fine specimen (of what was then supposed to be this mineral) from the mine at Falkenstein in the Saxon Voigtlaud. I was puzzled by the low percentage of water and other anomalous results which my analysis of the specimen in question gave. While at work upon the solution of this difficulty, Weisbach's note upon uranocircite or baryta-uranite appeared, Winkler's analysis of the Falkenstein mineral clearly proves that we have in it a pure baryta-uranite—a new species of the uran-mica group. I did not, however, abandon my own researches into the nature of this mineral, for the mode in which the water was held by this mineral remained unknown. If isomorphous with autunite, it might be expected to contain 10 aq. in the molecule, 2 of which would probably be retained at 100° C.

Thin-section analysis is essentially an areal measurement, the measurement area usually being the upper surface of the section. If transmitted light is used for the measurement, the apparent areas of opaque grains will in general be somewhat larger than their true areas on the measurement surface. For strictly spherical opaque particles in a transparent matrix the expected excess of apparent over true area is shown to be (πr2k)/(2r + k) where r is the spherical radius and κ is the thickness of the thin section. A table shows the relation between true and apparent area as a function of r/k.

In April 1909 a paper on artificial reproduction of meteoric structure in alloys of iron aud carbon was read by the author before the Russian Mineralogical Society. In that paper special reference was made to the work of Sorby, who, to cite his own words, for ‘more than twenty years … commenced to carefully study the microscopical structure of iron and steel in order, if possible, to throw light on the origin of meteoric iron’. A brief survey of the work of Stodart and Faraday, of Osmond, Rinne, and Berwerth was also given and, subsequently, the structure of a carbon-steel containing 0.55 % carbon with perfectly developed Widmanstätten figures was described.