The protractor of which an account is given in the following pages is intended to facilitate the construction of stereographic and gnomonie projections of the sphere, especially on the part of students of crystallography. The fundamental properties of these projections, on which its design depends, have long been known, and have been the basis in the past of methods somewhat similar to those about to be described. Any novelty there may be in the device is to be sought therefore in the convenient arrangement of its parts, and in the ease with which it may be constructed and used, rather than in the principles which it embodies. We will consider first the construction of the protractor, then its application in tile case of the stereographic and gnomonic projections, and in conclusion will deal with a few points of interest in the history of these projections.

It is more than twenty-one years since Mr. Allan B. Dick communicated a description of the microscope, now generally associated with his name, to this Society. The instrument is familiar to all mineralogists, and it will suffice to say that the main feature consisted in having the polarizing and analysing prisms arranged so that they can be rotated simultaneously, instead of turning the object on the stage between them. The rotating stage is never very satisfactory, especially when it is desired to examine minute crystals in convergent light. In two pamphlets, published later, Mr. Dick extended his description of the instrument and gave some methods of using it.

Before treating of the subject before me, I must describe its history. At a soirée of the Royal Society on April 25, 1877, I showed a number of specimens illustrating a new class of optical properties, applicable to the identification of minerals. These greatly interested the late Sir G. G. Stokes, Bart., and led to a considerable amount of correspondence between us ; and he communicated a paper to the Royal Society on the mathematical part of the subject, and I sent a short one on my apparatus and observations. This was admitted to be very imperfect, since the crystals examined, though showing the general facts perfectly well, had not been cut so as to be suitable for such quantitative measurements as could be compared with Stokes's theoretical determinations.

Cornwall has long been famous for the occurrence of certain minerals, found in the neighbourhood of the great granitic intrusions, that are rare in, or absent from, the aureoles of metamorphism surrounding the granites of many other regions. It is now generally recognized that these minerals have been produced by the action of certain heated gases on the rocks they penetrate along cracks and fissures ; but the exact age of this type of alteration is not so well known. It is often held to be contemporaneous with the thermo-metamorphism produced by the granite, and rocks Containing these minerals are included in the description of the ‘contact rocks’ surrounding the granite.

This paper gives the results of an investigation at the Imperial Institute of a carnotite-bearing specimen received from South Australia.

The material represented by the specimen is described by Mr. H. Y. L. Brown, Government Geologist of South Australia, as occurring about 2 miles SSW. of Teesdale's dam and about 20 miles ESE. of Olary railway station. He remarks : ‘The ore occurs as yellow and greenish-yellow incrustations and powder on the faces, joints, and cavities of a lode formation, which consists of magnetic titaniferous iron, magnetite, &c., and quartz in association with black mica (biotite) . . . . . . The outcrops traverse metamorphic gneissic micaceous granite and granite schist, into which dykes of granite and diorite have been intruded.’

Among the chief treasures of the University Museum is a large piece of the meteoric stone which fell at or near Chandakapur in the valley of Berar, India, on June 6, 1838. The specimen has been for many years in the collection, but there seems to be no record of its acquisition or previous history. The weight was formerly 3329.5 grams, but the stone was cut in two in 1899 and a portion exchanged with the late Mr. J. R. Gregory for a number of representative specimens of meteorites. A series of five thin sections were prepared at the same time, and about five grams of fragments remained, which have been used for the analysis described below.

The new and remarkable mineral locality known as Rhodesia Broken Hill is situated in North-Western Rhodesia, at a distance of about 300 miles to tile north-east of tile Victoria Falls, and at the present terminus of the Cape to Cairo Railway. The outcrop of lead and zinc ore which occurs here was accidentally discovered in January, 1902, by Mr. T. G. Davey, while he was prospecting the country for copper, Rising out of the flat surrounding country is a series of low hills or kopjes, the largest of them being about 90 feet in height ; and these consist almost entirely of oxidized ore. The surrounding rocks are mainly crystalline limestones with some beds of sandstone, conglomerate, and phyllite.

In the present notes the writer has endeavoured to collect all the available facts concerning the occurrence of zeolites in both Cornwall and Devon. Specimens belonging to this group are rarely represented in collections of Cornish minerals, and as several new localities have recently come to light, the following observations will, he ventures to hope, be of some interest as adding to the already lengthy list of mineral species occurring in the West of England.

Among the zeolite specimens from the neighbourhood of Bombay in the Oxford University Collection are several of the well-known type, showing pale green crystals of apophyllite associated with white stilbite and white silky needles labelled as ‘poonahlite’, on a matrix of purplish-brown amygdaloid, with a layer of a bluish-green, earthy mineral lining the amygdules and underlying the zeolites.

The name poonahlite was originally given by H. J. Brooke, in 1831, to a mineral occurring on specimens which he had received from Henry Heuland. He states that the poonahlite formed slender crystals accompanying apophyllite from Poonah, and was at first taken for mesotype or needlestone, but that it had a prism-angle of 92° 20′.

The mineral carminite was first described by F. von Sandberger in 1850 in a paper which appeared in Poggendorff's Annalen, entitled ‘Carminspath, ein neues Mineral aus der Ordnung der Arseniate’. It occurred near Horhausen, Rhenish Prussia, forming clusters of fine needles of a carmine- to tile-red colour, implanted on quartz, limonite, and beudantite, and accompanied by pyrolusite, clear crystals of mimetite, and an undetermined yellow, earthy mineral. In a subsequent paper in 1858, ‘Ueber den Carminspath,’ he gave the precise locality as the Luise mine, Horhausen, Rhenish Prussia ; and a quantitative analysis, on 0.068 gram by R. Müller, showed the mineral to be an anhydrous arsenate of iron and lead : As2O5, 49.11 ; Fe2O5, 30.29 ; PbO, 24.55 = 108.95. A trace of phosphoric acid was also present.

In the year 1888 I gave before the Mineralogical Society an account of the forms and optical characters of some minute crystals of kaolinite from Anglesey. At that time I was unable to ascertain whether the mineral was optically positive or negative, and it was stated that the axial angle 2V would approximate to 90°. Subsequently, by the aid of oil-immersion lenses and condensers of high angular aperture (numerical aperture=1.25), it was found that the mineral is optically positive and that the acute bisectrix is normal to the clinopinacoid (the obtuse negative bisectrix emerging through the basal plane). The optic axial angle in air was determined with an eyepiece micrometer to be 2E = 121°, with a possible error of one or two degrees, due to the small size of the crystals and the consequent weak interference-figures.

If a drop of strong solution of potassium bichromate, placed upon a microscope-slide, be observed under the microscope as it crystallizes, and if the drop be sufficiently thin, it will generally be found that the following events take place: The first crystals make their appearance at the edges of the drop and after growing rapidly for a short period as branching fibres, they begin to grow quietly in the form of plates or flattened rods presenting the characteristic form of the crystals of this substance. These crystals continue to grow uniformly, but after a short period, at a distance from the growing crystals, and at some spot where the drop is thin, a fresh crystalline growth starts suddenly from a point, and extends with great rapidity in all directions in the form again of branching needles and fibres.

The crystallographic and other physical characters of seligmannite from the well-known quarry in the Lengenbach, Binnenthal, were first described by Professor Baumhauer in 1901. In its crystallographic characters the mineral was found to be closely allied to bournonite, which it resembled in the habit of the crystals and the repeated twinning on the prism-planes.

Although the mineral gyrolite has long been known to occur in the British Isles, namely in the basalt lavas of Skye, Mull, and the Treshnish Islands, &c., off the west coast of Scotland, and has more recently been described from the cliffs between Ardtornish Bay and the fault of Dearg Allt in Argyllshire on the mainland, no instance of its occurrence in the lavas of the north of Ireland has hitherto been recorded.

The examination, however, of a large number of specimens collected both for me and by myself during the past year and a half has shown that the mineral, though occurring in small quantity relative to the other zeolites with which it is associated, is fairly widely distributed throughout the harder lavas in the neighbourhood of Belfast.

The behaviour of a mass of molten but cooling igneous rock under moderate pressure may be compared to that of liquid cupelled silver, which at a certain temperature can no longer retain the oxygen, many times its volume, that it has occluded. Thus, during the submarine eruption off the island of Pantelleria in 1891, blocks of lava that swam for a while on the surface of the sea exploded with violence. Chilling by contact with sea-water had occasioned exactly what Lowthian Bell observed to happen on the sudden cooling of masses of slag by contact with water. There was a discharge within the set outer envelope, at something near a red heat, of previously dissolved water and gases, converting the floating ejectamenta into a species of bombs.

Previous experiments made by myself in the Oxford Mineralogical Laboratory on the crystallization of potash-alum had shown that a drop of supersaturated solution Of potash-alum, when left by itself, crystallized almost invariably in birefringent sphere-crystals, which represent a more soluble and less hydrated variety of that substance. It was further suggested (1) that the spontaneous crystallization of that particular substance always takes place as spherulites ; and (2) that this spherulitie growth is indicative of a sudden change in the solution, which is passing from the metastable state for ordinary alum (in which ordinary octahedra, when introduced, grow by themselves) to the labile state (in which the alum crystallizes as a rectangular network of needles).