At many places in ‘The Peak District’ of Derbyshire, the igneous rocks, locally called ‘toadstones’, have, to varying extents, suffered an alteration with, as a result, the production of clays—locally called ‘toadstone-clay’. These clays attracted the attention of the present author some years ago, and since that time, whilst conducting extensive ‘field’ investigations in the course of a survey of the minerals of Derbyshire, observations on them have been ahnost continuously made and the deductions drawn therefrom carefully considered.

As it is not always easy to obtain for the use of students suitable specimens of crystals which exhibit inclined or horizontal dispersion of the mean lines to a marked extent, a ready method of artificially producing optic pictures which show the appearances characteristic of these dispersions may be found of service by teachers.

It is ohvious that, if the optic picture of an ordinary biaxial crystal is viewed or projected through a prism whose refracting edge is either parallel or perpendicular to the axial plane of the crystal, the dispersion produced by the prism will displace the components of the optic picture due to the various colours of the spectrum through distances increasing from red to violet, and will thus reproduce the phenomena of horizontal and inclined dispersion respectively.

A Committee on Nomenclature and Classification of Minerals was appointed at the first annual meeting of the Mineralogical Society of America in December 1920, and two of its reports have been recently issued in the ‘American Mineralogist’. Such a committee for standardizing nomenclature is clearly needed, but it must take a far wider view. If the recommendations can be, or are likely to be, adopted only in their country of origin then the result will be still further confusion in the nomenclature of the science. It is especially important in this connexion to bear in mind that science is world-wide, and its language should, as far as possible, be adapted to meet international needs. The tendency to adapt names to particular languages should rather be discouraged.

The occurrence of melanterite, pisanite, and chalcanthite as secondary minerals resulting from the oxidation of pyrites masses containing copper is well known. It is not so well known that other metallic sulphates are from time to time found, and a brief description of the mode of occurrence of a few of these, in addition to some details of the composition of pisanite and chalcanflSte, may be of interest.

Pisanite.—This mineral is of common occurrence at all the pyrites mines of Huelva and elsewhere, ibrming stalactites and stalagmites in those parts of the mines that are above drainage level though subject to more or less percolation of water. It is rarely found as well-formed crystals, save occasionally when a pool ef strong sulphate water has been temporarily dammed hack in some way, and then supersaturated through evaporation.

We are unable to observe crystalline structure directly, and can only infer it by observation of the physical and chemical properties possessed by a crystal. It is quite reasonable to suppose that every vectorial property possessed by a crystal is dependent upon its leptonic structure. The X-ray method, which has produced such a revolutionary change in our Knowledge of crystal-structure, is not sufficient in some cases for the determination of leptonic structure. The X-ray spectral analysis gives only the spacings between the atomic planes of a crystal. The application of the method of intensities, which was so successful in the case of simple cubic crystals, may not be so in the case of crystals of a Inore complex chemical composition and of a lower symmetry. It is thus desirable to combine several methods and so approach the problem from different directions.

It has long been recognized that if specific gravity determinations are to be accurate to a unit or two of the third decimal place corrections must be applied for the error due to the circumstances (i) that the weighings are made in air instead of in a vacuum, and (ii) that the temperature at which the determinations are made usually differs considerably from the standard temperature of 4° C.

Recently A. Johnsen has given a graphical solution by means of the stereographic projection of the problem of determining the constants of a homogeneous shear of a crystal, knowing the initial and final indices of two faces or two edges.

The same problem may be solved more easily by means of the gnomonic projection.

Native arsenic has hitherto ranked as a distinctly doubtful British species. The following notes contain, however, an account of its occurrence at two localities in Cornwall. John Garbyin his ‘Catalogue of Minerals found in Cornwall’ states that native arsenic had been found in Dolcoath mine, Camborne, associated with ores of cobalt. Greg and Lettsom repeat this statement with a doubtful qualification. J. H. Collins also mentions Cooks Kitchen mine, Illogan, as another doubtful locality. The occurrence as an associate of smaltite and native bismuth, minerals that have on several occasions been found in Dolcoath, is by no means unlikely, although, as far as I have been able to ascertain, no specimens of native arsenic from any of these reported localities have been preserved.

The name chlorophaeite was given by John Macceulloch in 1819 to a mineral exhibiting striking colour change on exposure which he found at Craig nan Stàrdean (=Sgùrr Mòr) in the island of Rum. Sixty years later Professor Heddle visited the type locality and redescribed the mineral, giving for the first time chemical analyses. He also noted an occurrence of the mineral in Canna. In 1908 A. Harker showed that the rock which contained the chlorophaeite belongs to the mugearites.

Records of chlorophaeite arc not always trustworthy since the term has been used somewhat loosely. Professor Heddle (loc. cit., p. 88) confirmed the occurrence at the Giant's Causeway, Co. Antrim, first recorded by Greg and Lettsom in their Manual of Mineralogy.

The widespread use of methods based oil total-reflection for finding the refractive indices of crystals either directly, by the Abbe-Czapski crystal reflectometer or by the Kohlrausch method, or indirectly by means of liquid mixture of known refractive intex which require to be verified and standardized from time to time, renders desirable a rapid graphical method of ascertaining with some accuracy the index of the substance under test, direct from the instrumental data without calculation.

An attempt in this direction was made by the author some years ago. The equation to be solved is n' = n sin θ, where n' is the index to be found, n that of the glass hemisphere, prism, or highly refracting liquid employed in the total-reflectometer, and θ the critical angle of total-reflection observed.