Professor Arthur Holmes has described in the preceding paper a suite of rocks from Mafuru, Uganda. During his study he observed in thin section a mineral which in optical properties is indistinguishable from nepheline. He gives chemical evidence, however, for the presumption that a potassium aluminosilicate is present in mafurite which would suggest either that the nepheline-like mineral is kaliophilite or a polymorphous form of KAlSiO4. Kaliophilite is not isomorphous with nepheline, but has similar optical properties, and the two minerals could be easily confused in thin section. They may, however, be rapidly distinguished by X-ray methods. Even minute grains insufficient for chemical analysis can be determined with certainty; so it seemed probable that an X-ray study of the nepheline-like mineral from Uganda would give additional information and decide whether or no a separation and chemical analysis of the mineral should be attempted. Professor Holmes accordingly invited me to examine his specimens and rock-sections.

The study and identification of the gem minerals present difficulties which are not to be resolved by the ordinary methods of mineralogy. The commercial value of the material forbids recourse to chemical reagents and the blowpipe, and when faceted and mounted, the criteria of crystal form and specific gravity are also inapplicable. Optical phenomena then constitute the sole means of discrimination, and the refractometer, spectroscope, polariscope, and dichroscope become the only arbiters between gemstones of similar appearance but different species. The function of the polariscope is to narrow the field of inquiry by differentiating between isotropic and anisotropic gemstones; the verdict of the spectroscope is unequivocal in gemstones characterized by absorption spectra, which, unfortunately, form a minority ; dichroism, when present, can be regarded as supplementary evidence only; the refractometer, an instrument of wider application than the spectroscope, and capable of yielding information which is usually equally unambiguous, thus assumes a role of primary importance.

Haddenrig (Haddon Rig) quarry is situated three miles east of Kelso in Roxburghshire, near the road from Kelso to Mindrum in Northumberland. The rock quarried is marked on the Geological Survey Map (Scotland, Sheet 26) as a basalt intrusive into the Lower Carboniferous lavas (Kelso traps). The absence of exposed contacts excludes any direct field evidence of its intrusive character, but the appearance of the fresh basalt from the quarry suggests that it belongs to a sill or a plug, as it is in many ways similar to other intrusive basalts of the Kelso district. The evidence of the orientation of the felspar laths in the basalt is not entirely conclusive. With the exception of the northern part of the quarry where the orientation is vertical, elsewhere it is more or less horizontal. This suggests a sill. The vertical orientation of laths in the northern part of the quarry may be due either to the presence of a feeding channel or to eddies in the sill.

During the investigation of a migmatite complex near Strathy, north Sutherland, cummingtonite and gedrite were found to be common constituents of certain schists and granulites. Calcium-poor amphiboles have not hitherto been recorded from rocks of the Moine Series, and it is the purpose of this paper to give the results of the optical and chemical study of three examples of this group. The methods of separation and purification employed are described in some detail as it is considered that many of the previous analyses of these minerals are vitiated by faulty procedure. To avoid errors in separating two amphibole species, the investigated samples were taken from rocks containing a single amphibole phase. The paper concludes with a discussion of the correlation of optical properties and chemical composition of cummingtonites and gedrites.

During a petrological examination of samples of bauxite from Boolarra, in south Gippsland, Victoria, it was noted that some specimens of the bauxite, which is largely derived from Tertiary olivine-basalt, contained numerous grains of yellow-brown to amber-yellow leucoxene. The leucoxene is clearly pseudo-morphous after ilmenite, residual particles of ilmenite being enclosed in many of the leucoxene grains. Most of the leucoxene grains are opaque, but occasional grains are translucent to transparent, though isotropic. Some of them show parallel markings suggestive of cleavage, but probably a residual structure from the replaced ilmenite. In view of the highly aluminous nature of the enclosing rock, there seemed some possibility that this mineral might be the little-known aluminium titanate, xanthitane. It was thought, therefore, that if a pure sample of the mineral could be prepared, a chemical analysis would establish its identity.

The problem of determining the orientation of Widmanstetter figures has recently arisen in a paper by Dr. L. J. Spencer on the Gibeon shower of meteoritic irons in South-West Africa: the method of attacking it to be described here will be applied to the crystals illustrated in plate II of his paper.

By producing the faces of a regular octahedron in all possible directions a set of six congruent prisms is formed, as shown in fig. 1. The Widmanstetter figures are the traces of these prisms in a plane of section. The section across any one of the prisms by a plane normal to its axis is a rhomb whose angles are those of a face of the rhombic-dodecahedron, that is to say, 70° 31′ and 109° 28′.

In the mineralogical literature, stibnite is usually stated to change by oxidation into kermesite and finally into valentinite or senarmontire. Another alteration product is described as antimony ochre, a term loosely applied to several minerals, viz. cervantite, stibiconite, and volgerite (Dana, System of mineralogy, 6th edit., 1892, p. 203).

In my metallurgical experience with stibnite ores, I have never met with kermesite or valentinite. Whenever I have encountered altered stibnite in ores from widely separated localities, it showed a yellowish to buff coating which I took to be cervantite. After spending two years in central China where I examined hundreds of tons of stibnite ores from Hunan province, I had to purchase a specimen of kermesite from a mineral dealer in order to become acquainted with the mineral. The locality of the specimen was given as Bräunsdorf, Saxony.

During a study of the sources of the dusts in South Wales coal mines, carried out on behalf of the Industrial Pulmonary Disease Committee of the Medical Research Council, a mineral of the mica group was found as the main constituent of the shales overlying the coal in several mines. Of six shales investigated four were associated with anthracite, one with semi-bituminous, and one with bituminous coal. Between the coal seam itself and the shale which forms its roof there usually occurs a more coaly shale referred to locally as ‘clod’. Five of the samples investigated were isolated from ‘clods’ and one from a roof.

In the investigation of the system CaO–MgO–SiO2 by Ferguson and Merwin the orthositicate 3CaO.MgO.2SiO2 was not observed, and the existence of this compound was first proved in 1921 by Larsen and Foshag who detected it in a contact-altered limestone at Crestmore, California, and gave to it the name merwinite. Since then this compound has been found in contact-altered limestones at Scawt Hill, Antrim, near Neihart in the Little Belt Mts. of Montana, and at Velardeña, Mexico. The history of the recognition of merwinite as an artificial mineral is less easy to trace, but it seems that research workers on refractory bricks were the first to recognize its existence. Thus U.S.A. Patents applied for in the years 1934 and 1935 and British Patents applied for on 11.10.1935 and 19.5.1936 by Non-Metallic Minerals Inc. specify merwinite and periclase as the essential constituents of a refractory brick produced by firing mixtures of dolomite and serpentine, dolomite and sand, or impure magnesite and dolomite at temperatures varying between 2700° and 2900° F.

Some years ago, when working through large parcels of mixed faceted gem-stones in a search for blue spinels of high refractive index, several unusual specimens were encountered, two of which were later identified as kornerupine by Dr. G. F. Claringbull, using X-ray methods. The other stones in these parcels were all quite typical products of the Ceylon gem gravels, consisting mainly of zircons, garnets, and spinels. Since kornerupine is so rare a mineral, and had not previously been reported from Ceylon, it was considered worth while to attempt to place the new occurrence beyond doubt by searching through rough gem gravel imported direct from Ceylon. This was made possible by the kindness of Mr. Hans Van Starrex, who sent a consignment weighing several pounds from the Matale district (Anderson and Payne, 1940).

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 Chapel, or Charlestown Main, Limestone horizon lies in the Lower Limestone Group of the Carboniferous Limestone Series. At Chapel quarry the limestone is underlain by a thick sill of quartz-dolerite. The contact, however, is not exposed but probably lies only a few feet below the quarry floor. Other intrusions are seen in the vicinity: a very thin dike of decomposed basalt, which connects with a small ‘float’ of white trap, cuts the limestone about 30 yards north-eastwards of the point where the quarry crosses the Chapel road, and a small sill of olivine-dolerite or teschenite occurs at a horizon about 100 feet above the top of the limestone. In the quarry the limestone is about 60 feet thick. The contact-alteration described in this paper is most evident near the base of the limestone face and its extension laterally in the quarry has no relation to the outcrop of the thin dike. The alteration is therefore considered to be essentially connected with the underlying, unexposed, sill of quartz-dolerite, and, as will be shown ih the following pages, has been effected by the action of hot solutions rather than by thermal reconstruction.

Large specimens of native gold showing crystalline form are not at all common in Southern Rhodesia, but the recently discovered gold with adherent silver from the Old West mine, Penhalonga, would be outstanding wherever it had been found.

About the middle of December 1940, Mr. D. V. Burnett, Director and Consulting Engineer to the Rezende Mines Limited, the owners of the property, lent two specimens to the Geological Survey for examination. The following week Mr. B. Lightfoot, Director of the Southern Rhodesia Geological Survey, visited the mine and examined the cavity where the specimens had been found. He made notes and collected specimens of the associated minerals, and borrowed a few other specimens which had been retained at the mine.

During the X-ray study of a new phosphate mineral, not yet published, the usual comparison was made with cell-side measuremerits of related minerals. One of these is ‘eggonite’, a rare hydrous aluminium phosphate said to occur on silver ores at Felsőbánya, Hungary. Rotation photographs of ‘eggonite’ are different from those of the new mineral, but yielded dimensions close to those of sterrettite, a new mineral from Fairfield, Utah, described last year by Larsen and Montgomery. These authors generously presented a specimen of sterrettite to the British Museum, and rotation photographs taken in the Mineral Department not only give cell-dimensions close to those published but also identical with those of ‘eggonite’. A brief account of this identification is now given not only to justify abandoning the mineral name ‘eggonite’, but also to place on record a European occurrence of a rare mineral so far described from only one American locality.

Hintze (1930) has pointed out the considerable variation in analyses of hydrozincite and the various formulae assigned to this mineral. Further data concerning this comparativeIy rare mineral are therefore desirable. Hydrozincite has been found at Narlarla, Western Australia, and this material is described below.

Narlarla is situated in the Napier Range, West Kimberley district (long. 124° 43′ E., lat. 17° 16′ S.), 75 miles due east from Derby, on the south side of the Barker River gorge (fig. 1). Several small silver-lead-zinc ore-deposits occur in the hills at Narlarla which are made up entirely of Devonian limestone. The occurrences have been described by Woodward (1907, p. 11), who, in his report, gives the results of assays of several specimens of the ore. These occurrences are Unique as they are the only known post pre-Cambrian primary ore-deposits in Western Australia. (Secondary gold deposits occur in the Donnybrook sandstone which is considered to be probably of Permian age (Forman, 1936).) The unique character of the Narlarla deposits has been noted by Simpson (1939, p. 218), who, however, offers no explanation as to their genesis. The absence of any post-Devonian igneous intrusives, other than the leucite-lamproite plugs (Wade and Prider, 1940) which are intrusive into the Permian rocks of the Fitzroy River area to the south of Narlarla, seems to indicate that these deposits are genetically related to the lamproite magma. The nearest known occurrence of the leucite-lamproites is at Mr. North, some 15 miles to the south (fig. 1), but there may be closer occurrences as the area has not been very closely examined, and the widespread distribution of the lamproite magma has been demonstrated (Wade and Prider, 1940).

The deposits of sulphur at Krisuvik, Iceland, have long been known. They were described geologically in detail by Vincent who also discussed their probable origin, but gave no chemical analyses other than rough determinations of the percentages of sulphur and matrix in various samples. From the flame spectrum Vincent thought that thallium might be present.

Apart from popular descriptions by travellers and casual references in guide-books, little attention appears to have been given in literature to these deposits in more recent years. Sporadic attempts at working the sulphur economically were made by the Danish Government during the eighteenth century and later by a Danish merchant round 1832. These were abandoned, however, owing to cost of transport. There is little likelihood of their renewal in the immediate future unless the deposits are found to contain some other element(s) of sufficient market value.

The hornfels discussed here was collected from Sparcraigs, near Whitecairns, eight miles to the north of Aberdeen, very close to the eastern contact of the basic igneous intrusion of Belhelvie. The Highland Schists of the district are very poorly exposed, but at this locality there are a few small isolated outcrops of high-grade hornfels, nearly all silica-poor types, with garnet, spinel, hypersthene, cordierite, plagioclase, and biotite as their chief mineral constituents. The actual contact is obscured, but the igneous rock, an olivine-gabbro, is exposed at about twenty yards from the hornfels.