Occasion arose recently when the writer required to make a number of comparative areal measurements of irregularly shaped portions of fossil corals in thin section, and it was found that the micrometric methods available were very tedious and involved considerable strain. A micro-planinmter has now been designed by means of which fairly accurate determinations of areas can be made with a minimum of eye strain. Though intended primarily for use with the petrographical microscope with the rotating stage, it can be adapted for use on microscopes of the fixed stage type.

This amphibole is an important constituent of an albite-epidote-actinolite-chlorite-calcite-schist from the summit of Coronet Peak in the Wakatipu district, western Otago. The mineral is somewhat concentrated into folia and porphyroblastie crystals up to 5·0 mm. in length, and is markedly flattened parallel to the orthopinakoid so that sections cut parallel to the schistosity exhibit numerous porphyroblasts, deeply coloured, poorly birefringent, and showing rather poor cleavage lines (fig. 1 A and B; fig. 2 A). In slices cut across the foliation numerous elongated end-sections are to be seen with development of small orthopinakoids (fig. 2 B).

The refractive indices (±0·002) are α 1·635, β 1·650, γ 1·655; γ—α 0·020. The angle γ:c = 18° and 2V 60°±1° Pleochroism follows the scheme α pale yellow, β dirty bluish-green, γ deep bluish-green, with absorption γ > β > α.

The original description of aramayoite by Spencer in 1926 contained only a qualitative discussion of the crystallography, because suitable crystals for a more rigorous treatment were not found. The pseudo-tetragonal symmetry was recognized, and cleavages were noted, but only one was defined. Twinning was later observed in polished section by Schneiderhöhn and Ramdohr, but undefined. Yardley demonstrated the triclinic symmetry of aramayoite by the Laue method, and determined the spacings of a considerable number of planes in the crystal. From these data she formulated a unit cell and correlated the X-ray data with Spencer's crystallographic results.

Several specimens of aramayoite found in our recently acquired Ahlfeld Bolivian collection contained small but excellent crystals. Measurement of these indicated that the Yardley X-ray crystallography was not consistent with an adequate morphological treatment of the form-development. The twenty-seven forms found by us lead to an obvious crystallographic unit (fig. 1), bearing a rather complex relationship to the Yardley unit. In order to test the suitability of the latter, we repeated the X-ray examination by the modern Weissenberg method and found complete accordance with our morphological treatment, as will be shown below.

In the course of work on the Barker index of determinative crystallography, a number of interesting problems have arisen owing to the selection by a few workers of unusual sets of fundamental angles for their computations. Most of these problems have been of no general application, but One appears to have a wider interest and may prove of practical value in dealing with crystals having curved or striated faces that only admit of accurate measurements along one zone.

This problem was to compute the angles and elements of a monoclinic crystal, using as fundamentals the angles from b(010) to any three faces e, f, g (of known indices) not in the zone [010], and no pair of which are co-zonal with b(fig. 1).

While crystal classes based upon symmetry furnish a clear-cut classification of crystals free from ambiguity and uncertainty, there are cogent reasons for recognizing the larger groups called systems.

It is important to recognize crystal systems, not only on account of physical properties, but also on account of the difficulty of assigning some crystals to their appropriate class. For example, a crystal consisting of a tetragonal prism and a pinakoid may belong to any one of seven classes. For the time being it is placed in the tetragonal system. When it is carefully investigated and its symmetry determined it may be assigned to the proper class.

Contrary to statements that have been made, we can have, and do have, a two-fold classification in geometrical crystallography—the thirty-two classes based upon symmetry, and the six systems based upon properties that certain classes have in common.

In the opinion of the writer the most satisfactory set of names for the crystal classes is the one based upon names of general forms. These names of Groth are comparatively simple, consistent, and lend themselves to international usage.

During the course of a survey of the mineral deposits of the area immediately south of the Murchison Range in the NE. Transvaal, a visit was paid to the Union mica mine, one of the only two producing mica mines in South Africa. Interest very soon centred round the occurrence of flattened garnet inclusions in a large number of the mica sheets inspected in the trimming hut. A description has been given by A. L. Hall of the nature of the occurrence of the muscovite, while the geology of the area surrounding the mine is shown in a general way on his map. However, no mention was made of platy garnet crystals; and the Union mine is shown on the map as the Premier mica mines, a name by which it was then known. It is situated on the farm Inyoko (or Inyoka) (24° 8' 25" S., 30° 50' 49" E.) on the western slopes of a ridge overlooking the Sandspruit valley, and lies only two miles from Mica Siding on the Selati Railway, though best approached from Gravelotte by road, a distance about 22 miles.

The systematic classification of the potash-soda-felspars is of the greatest interest and importance to the mineralogist, but problems demanding physical investigation are raised in work carried out recently by Dr. Edmondson Spencer, who has described the results of exhaustive chemical, optical, and thermal investigations of a representative collection of these minerals. Dr. Spencer has very kindly allowed us to examine by X-ray methods a large number of his specimens, so that for the first time it has been possible to make a fairly complete survey of potash-soda-felspars of known chemical composition and with accurately measured optical properties.

This paper presents the results of our general survey of potash-soda-felspars with compositions ranging from nearly pure potash-felspar to approximately 50 % soda-felspar. The natural minerals, and the same minerals after specified heat-treatment, have been examined. We do not discuss in detail the interpretation of our results in terms of the atomic structure of felspars; the subject is extremely complicated, and it is necessary to await the results of other investigations now in progress in this laboratory before attempting a complete structural interpretation.

The very abundant literature dealing with the twinning of felspar phenocrysts has put on record a surprisingly large number of twin-laws. Some of these have such complex indices that they have never been accepted; others are regarded as doubtful. Yet the authors of these have well-established reputations and the accuracy of their observations cannot be doubted. The difficulty lies in the interpretation of the results. Beside numerous examples of well-defined laws, one comes across a high percentage of isolated cases, each appearing quite as plausible as these, yet difficult to assign to any definite law.

Many attempts have been made to discover the causes of the various colours displayed by minerals. If we except fluorescence and allied phenomena, the diiteren~ theories advanced may in general be grouped under one or other of the following classes:

1. (1) Colour may be due to an intrinsic property of atoms, ions, or molecules. Examples are copper atoms, chromate ions, and ferric oxide molecules.

2. (2) Colour is sometimes attributed to structural causes. The blue tints of certain halites have been explained in this way; but a fresh suggestion is made in this communication.

3. (3) A pigmentary impurity may cause the observed colour. The colour of the ruby is believed to be caused by the presence of a red chromic oxide. The blue tint of a Yorkshire aragonite is shown later to be due to a copper salt.

4. (4) Some minerals show plays of colours due to interference and/or grating effects. Labradorite and opal are cases in point. Into this group also fall minerals like bomite which become iridescent from tarnish.

Kaalijärv is a crater-lake on the estate Kaali (järv = lake in the Estonian language) at 58°24′N., 22°40′E. Known in German as Sall, this locality is 20km. NE. of Arensburg (= Kuresaare) on the Baltic island of Oesel (= Saaremaa). On the level agricultural country a thickly wooded knoll 6–7 metres high is seen. Inside there is a crater 15½ m. in depth, the circular rim of which measures 92–110 m. across. The lake at the bottom is about 60 m. across. The rock of the district is horizontally bedded Silurian dolomite with a covering of glacial deposits. The rim of the crater consists of fragmentary materials, and the steeper inner slopes show the beds of dolomite dipping outwards as a dome at angles of 30–40°. Near by, on an area of ¾ sq. km., are five other smaller craters similar in character, and at least three other depressions that have been filled with stones collected from the tilled fields.

A number of specimens of core-like australites possess well-developed flaked equatorial zones (text-fig. 1), on which peculiar grooves and flutings are present. These markings are here referred to as ‘bubble tracks’, the term indicating their apparent mode of formation. The specimens displaying these features are round and elongate forms. The round forms vary in diameter from 34 to 41 mm., the elongate forms range in length from 30 to 75 mm., while the weights vary from 27½ to 96 grams. Australites of these sizes are of infrequent occurrence, only twenty-one having been previously recorded (9, pl. 4), (2, pl. 4), and (3, pl. 9); the illustrations of these examples show that they have comparable features, such as flaked equatorial zones and bubble tracks.

In 1850, T. H. Henry published an imperfect analysis of francolite, a supposed new mineral, previously named by H. J. Brooke. Henry stated that his material was similar to the material from Wheal Franco, Tavistock, Devon, but he does not show how he was in a position to know the composition of the material from this mine, because there is no record to indicate that the mineral from Wheal Franco had been analysed. Henry did not report CO2 for the material he analysed, but M. H. N. Story-Maskelyne and W. Flight analysed material, supposedly similar, and obtained a significant amount of CO2.

Two monoclinic pyroxenes from the Skaergaard Halvoen intrusion, Kangerdlugssuak, East Greenland, have been separated and analysed. They occur in a strongly differentiated gabbro complex which ranges from a hypersthene-olivine-gabbro at the base to a fayalite-quartz-gabbro of unique composition at the top. The differentiation has proceeded with a steady increase in iron and a corresponding decrease in magnesia, while the other major oxides remain relatively constant throughout. The clinopyroxenes in the lower horizons are in equilibrium with labradorite, normal olivine and hypersthene, and are fairly normal varieties as shown by the following optical properties:—

1. (1) … α 1·672, β 1·678, γ 1·701, 2V 42°, γ: c 39°, sp. gr. 3·34.

2. (2) … α 1·695, β 1·702, γ 1·722, 2V 43°, γ: c 41° sp. gr. 3·38.

Jamesonite is described in Dana (1892, p. 122) as a soft, brittle, steel-grey to lead-grey mineral forming acicular orthorhombic crystals, a: b = ‘0·915:1’ (in error for 0·8195:1), with perfect basal cleavage; composition, 2PbS.Sb2S3; principal occurrence, Cornwall. Jamesonite is taken as the type member of a group that includes cosalite, described in the first of these studies (1939), dufrenoysite, and other minerals of analogous composition.

The specimen, which is in two pieces cemented together, was purchased for the Museum of Practical Geology in February, 1888, from Mr. F. H. Butler, and bears the Museum's original Mineral Inventory label (M.I. 5714) on each portion. The inventory description is “Tin-stone, well crystallised, coating granite, Beam mine, Cornwall”. The exhibition label mentions quartz, and the presence of beryl had no doubt been overlooked through the general resemblance to milky quartz, which is not uncommon on some Beam mine material. Actually it was noticed during the rearrangement of the collection in 1934 that the cassiterite is accompanied by large, dull white crystals which proved to be beryl. In view of the rarity of beryl in Cornwall a brief description may be of interest.

The results of a chemical analysis of a mineral are always presented, in the first place, as percentages of elements or oxides present, and until recently the only possible further treatment of these values was to calculate atomic or molecular ratios, group them according to accepted isomorphous groups, and deduce (if possible) an integral chemical formula. The degree of approximation to integral ratios accepted as satisfactory is, of course, quite arbitrary, and a decision as to the degree of complication acceptable in a chemical fornmla has often had to rest on very inadequate evidence. Some workers prefer to adjust the final ratios to show the errors evenly distributed, a procedure which has no real advantages.

The structures of the fibrous zeolites edingtonite, thomsonite, and natrolite (with scolecite and mesolite) were determined by X-ray methods some years ago, and it was suggested shortly afterwards by M. H. Hey and F. A. Bannister in a private communication to one of us (W.H.T.) that ‘ptilolite’, another zeolite of fibrous habit, might prove to have a similar structure. An X-ray rotation photograph about the needle axis indicated a unit-cell dimension along the needle axis of approximately 7·5 Å., quite different from the axis c 6·6 Å. characteristic of the fibrous zeolites previously examined, and no further examination of ‘ptilolite’ was made at that time. We have now determined the unit cell and space-group of this crystal, and further work is in progress with a view to the complete analysis of the structure. This paper presents an account of the experimental data which we have obtained.

Cliftonite was described by Sir Lazarus Fletcher in 1887 as ‘a cubic form of graphitic carbon’. He put forward reasons for regarding it as a new allotropic modification of carbon and other reasons suggesting that it is a pseudomorph of graphite after some cubic mineral, perhaps diamond, but did not definitely favour either view. On the evidence available hitherto, both suggestions were clearly possible.

The use of a magnetic separator for the separation of certain ores has long been a standard commercial practice and a laboratory scale instrument applying the same principles to the separation of mineral grains from a powdered rock has been designed by Hallimond. In this apparatus the mineral grains are spread on a rotating pan which carries them below the pole pieces of one or more electromagnets, the pole pieces being so designed that the grains pass through a strongly diverging magnetic field and so are subjected to a force proportional to their susceptibility. If this force exceeds the weight of the grains they are attracted and adhere to the pole pieces. The material thus collected is periodically discharged by interrupting the magnetic field and the grains then fall through a slot in the rotating pan into a receptacle below.