In May, 1936, a quantity of pale yellow, rounded fragments, averaging 7 mm. across, was submitted to us by Mr. Arthur Russell for examination, with the information that the material came from the Castle-an-Dinas wolfram mine, and contained bismuth and tungsten. Chemical and spectroscopic examination showed that, besides bismuth and tungsten, the mineral contained only traces of silicon, arsenic, and iron. A sample of the same material had also been given to the British Museum by Mr. E. H. Davison in 1934 and registered provisionally as bismuth tungstate. This, however, would not have provided sufficient of the yellow mineral for chemical analysis. We are also indebted to Mr. Davison for a further generous supply of yellow pellets picked from the wolfram concentrates received since tile present work was begun. The mineral has all the attributes of an alteration product probably of native bismuth. It is fine-grained and compact, with hardness 3½, and on fracture it gives a clay-like odour. Some of the pellets show traces of a micaeeous mineral and quite frequently enclose native bismuth, wolframite, and bismuthinite. Mr. Russell has picked out from tihe material he had collected a few green pellets and some showing both the yellow and the green mineral. Spectrographs of the two are identical, but an X-ray powder photograph of the green mineral is quite distinct from that of tile yellow mineral.

In the first part of this paper (33) the effects of heat on the potash-soda-felspars were considered, and a possible explanation in terms of atomic rearrangement was proposed. In this part the experiments, and deductions from them, will be applied to some questions of petrogenesis and differentiation, particularly in connexion with the granites and granite-pegmatites.

When some publicity was given recently in the press to the Gundaring and Kumerina finds, which are described below, Mr. G. E. Willard called upon me in regard to a meteorite which had fallen on Dalgaranga sheep station (27° 45′ S., 117° 5′ E.). He stated that when he was a manager of this station in 1923 he found a crater measuring 75 yards across at the top, 50 yards at the bottom, and 15 feet deep. The surface of the country was ‘hard conglomerate’ (laterite?—E. S. S.), and pieces of rock up to one cubic yard were pushed up out of the hole, as if there had been an explosion. The NW. side had been thrust upwards more than any other, indicating that the meteorite came from the SE. In and around the crater Willard found ‘a hatful’ of small ragged pieces of iron like the Henbury (Central Australia) fragments. These original specimens unfortunately had since been mislaid, but he was able to secure for me from the present manager of the station a 40-gram piece.

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.

In the investigation of a suite of leucite-lamproites from the West Kimberley area of Western Australia, carried out in the Department of Mineralogy and Petrology, Cambridge, during the period 1937–38, a number of unusual minerals were noted. Three of these were isolated and analysed and two of them proved to be new species. The present paper deals mainly with the three minerals analysed. They are—

1. (l) Magnophorite, a new amphibole allied to katophorite.

2. (2) Wadeite, a new potassium-zirconinm silicate.

3. (3) Titaniferous phlogopite.

The leueite-bearing rocks occur at a number of isolated volcanic vents, nineteen of which have been examined and mapped by Dr. A. Wade. They have in every instance broken through sedimentary déposits of Permian age. Considerable diversity is to be seen in the rock types developed, but they are all characterized by high potash, magnesia, and titania, with low alumina and soda. Analyses of some types which indicate the general chemical character of these rocks are given in table I.

The natural glasses recognized as tektites are characterized by their acidic composition, the silica percentage ranging from 68 to 80. Their origin is still a debatable point. Within this range glasses are known which have undoubtedly been produced by the fusion of siliceous terrestrial materials in the intense heat developed when large iron meteorites have struck the earth's surface; while others of the same origin show silica ranging up to 98 per cent., that is, an almost pure silica-glass.

From a collection of 84 chemical analyses of tektites taken from the literature, only 36 (many of them due to A. Lacroix) give data for the specific gravity and refractive index which presumably were determined on the same sample of material as that analysed. These are reproduced in table I (arranged in order of silica percentages) and plotted in figs. 1 and 2. Corresponding with the range in SiO2 from 68·00 to 80·73% there is a nearly uniform decrease in d (2·498–2·339) and n (1·526–1·4867). The specific refractivity, K = (n−1)/d, varies from 0·2050 to 0·2107 (0·2258 in the case of no. 29 with the abnormally high n 1.5380).

Among a series of specimens collected by Mr. H. G. Dines from the South Crofty mine, Illogan, one was found to contain bertrandite and beryl. The specimen was obtained from the back of the 290-fathom level, east of Robinson's shaft, in the no. 1 lode, which hades north and at this position is 3 feet 10 inches wide. The location of the specimen was about 9 inches from the granite footwall. In hand-specimen the rock is a normal stanniferous veinstone composed of massive quartz tinted grey-green by disseminated chlorite and tourmaline and traversed by stripes of blackish ‘peach’ and white vein quartz which run parallel to the lode walls. Microscopically it is composed essentially of quartz, chlorite, and tourmaline, with subordinate orthoclase, fluorspar, and cassiterite, accessory apatite, and local bertrandite and beryl. In its texture there is evidence of an early formation of stanniferous chlorite-tourmaline peach with quartz-fluorspar gangue, which was subsequently fractured and, later, more or less completely healed by an accession of quartz, some redistribution of the earlier minerals having taken place at the same time. The place of bertrandite and beryl in this general scheme is discussed below (p. 576).

In 1824 Henry Witham discovered a beautiful red epidote mineral in the lavas of Glen Coe, Argllshire, which was described by Brewster under the name withamite. An analysis was made by Coverdale of obviously highly impure material, but later a more complete one was carried out by Heddle on carefully hand-picked mineral. Heddle admits, however, that his analysis shows ‘by no means a satisfactory agreement with the composition of epidote’, and from his figures it would seem very likely that his analysed material was also impure. In the Glen Coe Geological Survey Memoir no further data were recorded of this mineral, Heddle's analysis and description merely being quoted.

The writer visited Glen Coe and collected material in a cutting on the new road, approximately 5½ miles west of King's House (Geological Survey, sheet 53), where it is fairly well exposed. It occurs in a fine-grained, rather altered andesitic rock, as narrow veinlets up to 120 mm. in length and 50 mm. in width ; also as infillings of vesicles. The veinlets may be straight or highly irregular; in the former case they possibly follow joint-planes, for the rock will often fracture along such a plane, producing one or two surfaces covered with the red epidote mineral. The vesicles vary greatly in size from approximately 0·3 to 70 mm., and in the larger ones calcite is a common associate.

The south-eastern margin of the Glen Tilt complex consists of a long strip of diorites with a number of small associated patches of rocks of appinitic type. Xenoliths, both of hornblendite and hornblendeschist, the latter belonging to the Perthshire series of the Dalradian, are found enclosed within the dioritic rocks, which range petrographically from diorite to quartz-mica-diorite. In all these rocks hornblende is the most important fcrromagnesian constituent always predominating over pyroxene in the basic members and frequently persisting to the exclusion of biotite in the intermediate rocks of the intrusion. The hornblendes were examined chemically because of the difficulty of estimating their composition even from a thorough optical investigation. As they play such an important role in the complex it is considered that they may give some indication of the history and mode of formation of the rocks in which they occur. Such a chemical investigation of a series of related hornblendes will also indicate the possible range of composition of common amphiboles within the diorites and related rocks of a single complex.

Direct-reading refractometers depend upon observation of the boundary between transmission and total internal reflection, when a flat surface of a specimen is placed in optical contact with the flat surface of the isotropic medium employed in the instrument. This medium is usually a heavy lead glass in the form of a hemisphere or prism. The upper limit to the range of indices which can be measured is determined by two factors: the refractive index of the medium employed in the instrument, and the refractive index of the liquid used to make optical contact between the two surfaces.

Liquids of high refractive index are essential to such instruments, and some reference to them may be conveniently made at this stage. Various pure liquids, mixtures, and melts of high refractivity have been proposed from time to time, chiefly as immersion media.

Work which has been in progress during the last few years on the low-grade regionally metamorphosed rocks of western Otago, New Zealand, has shown that members of the stilpnomelane group, hydrous iron- (or manganese-) rich silicates, are exceedingly widespread in rocks with the most varied mineral assemblages. In many cases the mineral in question is in sufficient quantity to rank as an essential constituent and may even be the most abundant one present. Hence in view of the importance of this mineral in the rocks of this region, it was considered that exact analytical data were necessary, and preliminary work in this connexion was carried out by Dr. F. J. Turner and the writer in 1934, but at that time, owing to the difficulties of separation, the pure minerals could not be obtained for analysis.

Dinas Head is a small promontory adjoining Trevose Head, some five miles west of Padstow on the north coast of Cornwall. Petrographical attention was first drawn to it by Howard Fox, who in 1894, noted the development of albite-rich rocks between greenstone and slate, and he concluded that the rock was an adinole. He also noted the peculiar spherulitic development of albite giving some rocks an igneous appearance, but, on account of the fine banding preserved in these rocks, he concluded that they were metasomatized sediments. This conclusion was subsequently supported by McMahon and Hutchings. The Memoir of the Geological Survey of the Padstow and Camelford district agrees with Fox as to the origin of the adinole, and points out the exceptional thickness of its development when compared with other greenstone-slate contacts.

The object of this paper is to demonstrate the widespread occurrence of zoned olivines in igneous rocks, to show that the zoning is continuous, that the composition varies from a magnesium-rich centre to an iron-enriched margin, and that these findings have an important bearing on the question of petrogenesis.

I propose to express the composition of olivine by stating only the percentage of fayalite, as the rock-forming olivines belong mainly to the forsterite-fayalite series. There are, however, a number of names in use for the members of this series. Assuming that the two names forsterite and fayalite are insufficient for the needs of classification, it yet seems to me that all but four of the existing names are superfluous.

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.

There is a close optical and chemical resemblance between chamosite, the chloritic mineral of the bedded ironstones, and daphnite, a low-temperature vein-chlorite common in some of the Cornish tin mines. New material has made it possible to undertake a fresh comparison of the two minerals: chemical analyses have been made by Mr. C. O. Harvey, chemist to H.M. Geological Survey, and a report on the X-ray measurements is contributed by Mr. F. A. Bannister, of the Mineral Department of the British Museum.

The new analysis of chamosite agrees with the simple formula previously assigned: X-ray examination of material from several localities has now established the distinctive crystalline nature of this fine-grained mineral, which differs structurally from ordinary chlorites such as clinochlore. Daphnite, on the other hand, has the ordinary chlorite structure, but the new analysis fully confirms Tschermak's original opinion that it cannot be represented chemically as a mixture of serpentine and amesite.

The Boxhole meteorite crater has been described previously. It is in Central Australia at latitude 22° 37′ 30´″ S., longitude 135° 11′ 59″ E. During the single afternoon that was spent examining this great crater, nearly 200 yards across, most of the time was occupied with taking measurements and photographs, and only a few shale-balls were found in the way of meteoritic material. Mr. Joe Webb, who lives nearby, was instructed to continue the search, and in due course he sent down to Adelaide three pieces of iron weighing respectively 17 lb. 9 oz., 1 lb. 8 oz., and 5 oz., shown in figs. 1 and 3, together with six more shale-balls. This material was purchased by the South Australian Museum. Two other small pieces of iron have been obtained by the author from other sources.