In the description of an improved polarizing microscope, recently published by the present authors, provision was made for the use of polarizing film in place of calcite prisms, and the consequent changes in the design of the substage and tube were discussed. The model then described was, however, of smaller size than is required for some patterns of Fedorov stage now in use, and a larger stand wan under consideration. For technical reasons this could not be produced at the same time as the smaller instrument, and it was therefore necessary to postpone the description which forms the subject of the present supplementary paper. The changes involved are mainly dimensional, the optical and manipulative arrangements being in principle the same as in the smaller model, but the general standard of equipment in details such as stage racks, ball-bearing stage, and interchangeable bodies is on a scale corresponding with that in the largest existing types of stand.

The separation and analysis of the olivine phenocrysts from the Spiegel river and Klaasvoogds melilite-basalts was undertaken in order to ascertain whether the olivines contain significant amounts of Ca2SiO4 in their constitution in view of the fact that the closely allied rock-types, alnöite (Bowen, 1922) and the basalt from Shannon Tier, Tasmania (Tilley, 1928), are known to contain monticellite, and that Taljaard (1936) has tentatively identified monticellite in one of the melilite-basalts from the Sutherland district, Cape Province.

The Spiegel river melilite-basalt from the Riversdale district of the Cape Province was first described by Corstorphine (1898). Petrographic descriptions of the rock have been given by Rogers and Schwarz (1898), Rogers and du Toit (1903), Rogers (1905), Wagner (1914), du Toit (1926), Taljaard (1936), and Holmes (1936).

The mineral tilleyite was first discovered by Larsen and Dunham in thermally metamorphosed limestones from the contact zone at Crestmore, California. It is associated there with merwinite, wollastonite, spurrite, and gehlenite, and the rock is traversed by streaks of idocrase with grossular. The amount of tilleyite in these rocks was apparently small, but in certain thermally metamorphosed limestones from Carlingford it is abundant and some specimens are practically pure tilleyite-rocks, it has been possible, therefore, to make a more detailed optical and chemical examination of this mineral. The tilleyite occurs as subidioblastic crystals when abundant, or as rounded grains in calcite-rich rocks, or as irregular plates.

In previous editions of Hatch's ‘Petrology of the igneous rocks’ including the last two, for the revision of which the writer was largely responsible, historical precedent was followed and the main lines of classification were based on variation in the silica percentage. In preparing for the new post-war edition considerable thought has been given to this matter. The weakness of the scheme has become increasingly apparent to the writer, and it is now obvious that no juggling with the selected ranges of silica percentage chosen to delimit the ‘acid’, ‘intermediate’, and ‘basic’ categories can result in a workable, logical scheme. Silica percentage as a basis of rock classification must be abandoned for the reasons discussed below.

The andesine-dolerite of Squilver Hill, situated nearly 3 miles due north of Bishops Castle, has been described in another place. This somewhat coarse-textured rock, with a distinctive greenish-blue colour, contains up to 20 per cent. of a fresh monoclinic pyroxene as crystals of irregular shape, many of which are about a millimetre in length. The rest of the rock consists of saussuritized plagioclase, some interstitial chlorite and prehnite, and accessory apatite, sphene, magnetite, and ilmenite; the unaltered nature of the pyroxene is noteworthy.

The ionic radius of zinc (0·83 Å.) is identical with that of divalent iron (0·83 Å.) and nearly the same as that of magnesium (0·78 Å.). One would therefore expect zinc to be mineralogically closely related to iron and magnesium. It is, however, well known that this is not the case. An examination of a list of the zinc minerals which have so far been described shows that only in a few cases are the corresponding isomorphous ferromagnesian species known. As a rule zinc minerals and iron-magnesium minerals with corresponding chemical formulae are entirely different in their crystal-structure.

This stage-attachment simplifies microscopic comparison of refractive indices of small transparent grains by the immersion-liquid methods of O. Maschke, Schroeder van der Kolk, and other workers. It was designed in 1938 for use in mineralogy and petrology, but has applications in several branches of pure and applied chemistry, patticularly for determining the relative purities of numerous raw materials and refined products of the chemical industry and in checking the constituents of glasses, slags, refractories, and cements. It can be used with advantage, also, in the optical study of soils.

While studying some of the early works on minerals I came across an obvious difficulty: several minerals, amongst which lapis-lazuli is the most startling, are described in Latin as flavus, yellow or flaxen-coloured, not blue. This error has been copied into a book in English. The solution is less obvious than the difficulty, and it may therefore be worth while to put it on record for other workers on similar lines.

A noticeable feature of many of the soils of north-east Scotland is the presence in them of glistening yellow flakes of decomposed biotite. The light-reflecting property of this material catches the eye even though the flakes themselves are often of very small size.

An analogous process to the breakdown of biotite as reported below occurs in the soil-clays of the area, and the present study was undertaken primarily in order to throw light on the mineralogical nature of the clay fractions of the soils, since the large amounts of anmrphous material associated with the soil-clay minerals renders less satisfactory the direct approach. The mineralogy of the soil-clays will be dealt with in a separate communication.

In recent years the clinopyroxenes of the system clinoenstatite–diopside–hedenbergite–clinoferrosilite have received a great deal of attention. As research in this field advanced, the need for a balanced and generally acceptable subdivision of the system has made itself increasingly felt. Hess (1941) proposed a classification for these clinopyroxenes, which was subsequently extended by Benson (1944).

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.

It has been shown recently by Brindley and Robinson that anatase could be recognized by X-ray diffraction technique in fireclays from Scotland and Yorkshire which contained from one to two per cent. of titanium dioxide. These authors remark that the anatase appeared to be of a fineness comparable to the kaolin mineral which was the main constituent, and although kaolinites from various sources throughout the world were examined, in no case was either anatase or rutile detected.

The object of this note is to draw attention to the presence of anatase in the sedimentary kaolins of Georgia and perhaps South Carolina, U.S.A. According to Smith 115 kaolin samples from Georgia had on an average 1·13 % of TiO2 by chemical analysis. The values ranged from 0·5 to 2·2 % of TiO2. This is in marked contrast to most primary kaolins; those of Cornwall and Devon, for instance, have as a rule below 0·2 % of TiO2.

Measurements with the petrological microscope may be divided into those made with the orthoscopic and those with the conoscopic arrangements. It was early realized that if the micrometer scale or compensator was inserted in the focal plane of the ocular, the same apparatus would serve for measurements of both kinds, provided that a Bertrand lens was available. The method was described by A. B. Dick as early as 1888 in a form of microscope which has remained popular with many workers up to the present time. Additional slots at the level of the focal plane of the ocular were cut through the mount and the upper part of the microscope tube. Most European manufacturers, however, continued to provide only the usual slots in the lower part of the tube.

Pyrophanite was recorded by one of us in a recent paper as one of the minerals occurring at the Benallt manganese mine in the vein from which material for the chemical analyses of the new manganese-rich chlorite, pennantite, was separated. As this was only the third record of pyrophanite in the world it seems necessary to give, very briefly, the evidence for its identification.

During the course of recent work on minyulite (KAl2(PO4)2 (OH,F).4H2O) X-ray, optical, and density data were obtained for several hydrous K-Al phosphate minerals. These included one specimen of a soft white compact mineral labelled by P. Pallary as minervite from Sebdou, Orau, Algeria, which was presented in 1940 to the Trustees of the British Museum by Major M. Connolly. Minervite was originally described and analysed in 1895 by A. Carnot from Minerva grotto, Fauzan, dép. Hérault, France. In the same year Carnot also described the same species from Tour-Combes grotto, Misserghin, Oran. Sebdou is some 80 miles SSW. of Oran and geological maps show there a patch of Pliocene on late Jurassic. No doubts were entertained that the specimen under discussion belonged to the same species as minervite from Misserghin, since the geological formations of both localities are closely related.

Interest in the orthopyroxene series is evidenced by the great number of publications on that subject which appeared in the literature. Correlations of the chemical composition and optical properties were attempted by Winchell (1923, 1933), Walls (1935), Henry (1935), and Hess and Phillips (1940). The nomenclature of the series was discussed by Winchell (1923), Alling (1936), and Hess and Phillips (1940). The obscure phenomena exhibited by certain orthopyroxenes in polarized light, attributed by some to twinning, by others to exsolution, was noted by a great many petrologists, of whom are mentioned here Lewis (1908), Holmes and Harwood (1928), Guimarães (1933), Scholtz (1936), Hess and Phillips (1938, 1940), Kuschke (1939), Wager and Deer (1939), Nel (1940), Walker (1940, 1943), Walker and Poldervaart (1940, 1941), Edwards (1942), Henry (1942), and Poldervaart (1944, 1946).

During soil survey in the Deeside district of Aberdeenshire, a light greenish-yellow clay-like material was observed penetrating and coating the altered rock along a vein in a granite outcrop freshly exposed in road-widening operations one mile north-east of Ballater. The material occurs in a crush-band in the rock, and the alteration is apparently due to the action of water, but since the band was still highly altered at the base of the exposure (about 25 feet from the surface) it is not certain from the field relationships whether the water was of hydrothermal origin or whether it percolated from the surface.