In a previous paper two well-known crystalline compounds, sodiumpotassium tartrate, XaKC4H4O6. 4H2O, and sodium-ammonium tartrate, Na(NH4)C4H4O6.4H2O, were described, and it was therein shown how, in the crystals obtained by mixing these two isomorphous substances in any proportion, the dispersions of the ojjtic axes in the freshly prepared crystals were incompatible with orthorhombic symmetry. The crystals have been frequently measured and found to be orthorhombic bisphenoidal in external symmetry and this has been further confirmed by the author's measurements. A. full description of all the various dispersions to be found in the orthorhombic and monoclinic systems may be obtained from any text-book of crystallography, but in the following pages two of these are specially important.

In the first part of this paper it was shown that the acid section of the mica group could be divided into two main series, the potash micas and the lithia micas. The molecular ratio of potash to silica was 1 : 6 and the micas were therefore represented as salts of an acid characterized by this value. The present section contains the result of a similar investigation of the basic micas, especially biotite and phlogopite; here again it will be seen that the grouping K2O. 6SiO2 is present throughout. For comparison of the molecular composition, the analyses of the basic micas have been recalculated in molecular ratios, (Si,Ti)O2 being made equal to 600 according to the method already used for the acid micas.

The system of intrusive sheets in the Lower Carboniferous formations of the north of England, familiar to geologists throughout the world as the Great Whin Sill, has a voluminous early literature which it is unnecessary to summarize here. It will be sufficient to refer to the classical papers by Topley and Lebour on the field relationships (1) and by Teall on the detailed petrography (4), each of which contains an ample bibliography of the earlier literature. The most recent general account is by Prof. E. J. Garwood (9), who contributed a careful discussion of the evidence bearing on the period of intrusion.

Teall's great paper closely followed on one devoted to some of the dikes of the north of England (3), and in it he drew attention to the remarkable petrographical Similarity between the rocks of the Heft and High Green dikes and that of the Whin Sill (4, p. 656).

Considerable interest attaches to the estimation of the volume occupied by the several atoms in a crystalline compound. For the complete molecule a volume can at once be calculated from the density and the chemical composition, but the further subdivision of this space into domains belonging to the constituent atoms or groups can only be made by methods that are limited and frequently very indirect. In general, the volume is not additive, and. it seems clear that the same atom may occupy very different volumes in its different compounds; indeed, it is only within the limits of a truly isomorphous series that the volume can be treated as an additive property. But even in this case it is not possible directly to calculate volumes for the constituents of the molecule if the chemical formulae are of the same type throughout; when, however, the formulae differ in type a solution may be possible.

The place-names of mineral-localities in central Europe have presented many difficulties in connexion with the topographical indexes to 'Mineralogical Abstracts', and also in the much more extensive cardindex of localities represented by specimens in the Mineral Collection of the British Museum which has been in progress for many years. Further, I have been asked from several quarters to prepare a short check-list of equivalent names of those localities most frequently mentioned in mineralogical literature. When, in August 1927, Prof. F. Slavík was good enough to send me a copy of the manuscript list of Czechoslovakian localities and minerals which he had prepared in French for Prof. A. Lacroix for use in the Paris Museum, I at once recognized that with his co-operation a much more useful list could he prepared. This he readily assented to, and he has since extended his original list to include other countries.

As a first-aid in the identification of non-metallic minerals I have for many years made use of specific gravity, which is a character that can be easily and rapidly determined hy the use of heavy liquids. A small, clean fragment of the mineral to be tested is dropped into one or other of a series of tubes containing methylene iodide with various small known Crystals as indicators. Adding benzene from a dropping-bottle, the density of the liquid is quickly brought to that of the mineral. This preliminary determination of the specific gravity gives a useful clue as to the nature of the mineral—what it may or may not be ; and the same fragment can then be used for confirmatory optical and micro-chemical tests.

The occurrence of chloritic minerals as infillings of vesicular cavities in volcanic rocks is very common in rocks of all geological ages. Numerous chlorites have been described and analysed, but their classification and nomenclature are complicated by the fact that, as in the case with other hydrous alumino-silicates such as zeolites and clay minerals, the chloiites present a great variety of chemical compounds, usually of indefinite composition.

The twofold division of metamorphic rock-series into (a) Isophysical rock-series and (b) Isochemical rock-series, as proposed by P. Niggli in his paper, 'Die Gesteinsassoziationen und ihre Entstehung', introduces into metamorphic petrology two very useful terms, which will be of service in the metamorphic researches that await the attention of the petrologist.

Among such investigated rock-series one may call to mind the studies on the chloritoid-schists of the St. Gotthard massif," the glaucophaneschist suite of the Bagne valley (Wallis), the calc-silicate series of the Trondhjem region, the contact rocks of the Kristiania and Comrie areas. Of these the last are particularly illustrative of an isophysical rock-series, wherein a chemically wide group of assemblages conforming to the composition of shale-limestone sediments is represented in the free-silica and quartzless hornfelses.

In their paper on 'The structure of α and β quartz', Sir William Bragg and R. E. Gibbs touch on the question of twinning of quartz, showing how X-ray determination of structure can help to explain the different types of twinning. They only mention, however, twinning with parallel axes and twinning on . It would be interesting to see this extended to the examination of all known laws of twinning of α-and β-quartz. There is, however, still much doubt about the laws of the twinning of quartz. Many varieties of twins have been described, but a number of these are much open to doubt, while some are decidedly nonexistent. G. Friedel has within recent years published valuable papers on the laws of twinning in general, and has made more special application of his theories to the laws of twinning of quartz. It would be interesting to see how far all cases of well-established quartz twins occurring ill nature confirm his results.

Since the original experiments of M. von Laue on the diffraction of X-rays by crystals, other and more suitable methods of research have been devised. The Bragg ionization-chamber method and the Debye-Scherrer powder method are now quite well known. The newest technique is that of the so-called 'rotating crystal', which is based essentially on some experiments of M. de Broglie. This method was first employed and developed in the Kaiser Wilhehm Institut für Faserstoffchemie in Berlin. It has recently been extended by the work of J. D. Bernal in the Royal Institution in London.

There are two methods of making this type of photograph. In both a small crystal is rotated in a beam of X-rays and the reflected beams recorded photographically, in one case on a flat plate, and in the other on a cylindrical film the axis of which coincides with the axis about which the crystal is rotated.

Labradorite of exceptional purity, occurring as phenocrysts in two basaltic dikes just below the lighthouse at St. John's Point, Ardglass, County Down, Northern Ireland, was described by Professor A. Hutchinson and myself in 1912. The refractive indices for sodium-light were determined on an Abbé-Pulfrich total-reflectometer. Carefully selected material was analysed by Mr. G. G. Knighton and by Mr. M. C. Burkitt, and the specific gravity of the material analysed was determined.

This paper presents the results of an investigation of the granite composing the eastern part of Bodmin Moor, Cornwall, as mapped on Sheet no. 337 of the Geological Survey. The work was undertaken on the suggestion of Dr. J.W. Evans, and comprised the field- and laboratory-work necessary for a detailed study of the petrology and tectonics of the area.

The boundary of the granite, as mapped by the Geological Survey, has been retained, and also the distribution of the two main granite types ('coarse' and 'fine') shown on the Survey map. The coarse granite, however, has been sub-divided into two varieties, and it has been found practicable to map the distribution of each (fig. 1).

The killas inclusions in this granite have not hitherto been studied, and these have received due attention. Similarly, the minor intrusions described include some that have not been previously recorded.

It is unfortunate that so many published analyses of mica are not accompanied by the physical data for the material used. Nevertheless, enough information is available to permit a general comparison of the density relations for the group. This is most conveniently made in terms of the specific volume, for, in most isomorphous series, the volume is an additive property eveu when the molecular volumes of the endminerals differ appreciably. Such a relationship will be shown to hold good approximately for the micas; the volume can be calculated from constants assigned to the constituent oxides, the difference from the observed value being rarely more than 2 per cent.

The untimely death of Sir John B. Harrison on February 8, 1926, just as he was returning home after over forty-six years' service in the West Indian colonies; unfortunately prevented the publication by him of a complete account of the interesting new mineral which, as a result of much patient chemical work, he had discovered in British Guiana. As he had corresponded with me on this matter since April, 1924, and further, as this is the same mineral that I had myself erroneously described as 'allopalladium', I feel it my duty to place on record the full facts so far as can now be ascertained.

I had suggested that the new mineral might be named after Sir John Harrison, but this he objected to ; and, in consultation with the Governor of British Guiana, he proposed the name potarite (letter of April 8, 1925), but he did not himself record this name in print.