In 1911 an interesting find of barium-felspars was made in the Benallt manganese mine near Rhiw in the Lleyn peninsula of Carnarvonshire. A brief preliminary note on the occurrence was given by Mr. Arthur Russell in the same year. At the same time he very generously handed over to me for crystallographic and optical description a selection of the material he had collected, while he himself made a start on the chemical analysis of the material. Chemical analyses have now, at last, been made by Miss Hilda Bennett in the Mineral Department of the British Museum, and it therefore seems time to collect together the large volume of notes on the work done in 1911.

The study and identification of the gem minerals present difficulties which are not to be resolved by the ordinary methods of mineralogy. The commercial value of the material forbids recourse to chemical reagents and the blowpipe, and when faceted and mounted, the criteria of crystal form and specific gravity are also inapplicable. Optical phenomena then constitute the sole means of discrimination, and the refractometer, spectroscope, polariscope, and dichroscope become the only arbiters between gemstones of similar appearance but different species. The function of the polariscope is to narrow the field of inquiry by differentiating between isotropic and anisotropic gemstones; the verdict of the spectroscope is unequivocal in gemstones characterized by absorption spectra, which, unfortunately, form a minority ; dichroism, when present, can be regarded as supplementary evidence only; the refractometer, an instrument of wider application than the spectroscope, and capable of yielding information which is usually equally unambiguous, thus assumes a role of primary importance.

During the investigation of a migmatite complex near Strathy, north Sutherland, cummingtonite and gedrite were found to be common constituents of certain schists and granulites. Calcium-poor amphiboles have not hitherto been recorded from rocks of the Moine Series, and it is the purpose of this paper to give the results of the optical and chemical study of three examples of this group. The methods of separation and purification employed are described in some detail as it is considered that many of the previous analyses of these minerals are vitiated by faulty procedure. To avoid errors in separating two amphibole species, the investigated samples were taken from rocks containing a single amphibole phase. The paper concludes with a discussion of the correlation of optical properties and chemical composition of cummingtonites and gedrites.

The hornfels discussed here was collected from Sparcraigs, near Whitecairns, eight miles to the north of Aberdeen, very close to the eastern contact of the basic igneous intrusion of Belhelvie. The Highland Schists of the district are very poorly exposed, but at this locality there are a few small isolated outcrops of high-grade hornfels, nearly all silica-poor types, with garnet, spinel, hypersthene, cordierite, plagioclase, and biotite as their chief mineral constituents. The actual contact is obscured, but the igneous rock, an olivine-gabbro, is exposed at about twenty yards from the hornfels.