During the last few years I have applied myself to the study of some minerals occurring in the sedimentary rocks of a definite geological horizon. The success of my investigations was greatly favoured by the fact that the mineralogical material was collected during the course of the systematic geological exploration of the phosphate deposits of Russia, which during the past eight years has been under my immediate supervision.

The explorations just mentioned were begun in the north-eastern part of European Russia, and several occurrences of barite were found in the first year during the field study of the phosphate deposits in the government of Kostroma.

In Professor G. Cesàro's note 'Sur une propriété numérique de l'ensemble des axes de symétrie situés dans los plans de symétrie d'un polyèdre' is given the formula ((1) on p. 174)

This species, a hydrous zinc phosphate, Zn3P2O8.4H2O, dimorphous with hopeite, was first described by Spencer in 1907. While the material then available was suitable for the physical and chemical description of the new mineral, it did not permit a full crystallographic investigation. Spencer noted that the crystals resembled in a general way those of hemimorphite, and made sufficient measurements to indicate the triclinic character of the mineral. In his sketch of a crystal he indicates that the mineral is twinned, and that this twinning is best seen on the perfect cleavage which is parallel to the brachypinacoid.

The Pilbara goldfield lies in the north-west division of Western Australia within the tropics, and has been described in considerable detail by A. Gibb Maitland in Bulletin 40 of the Geological Survey of Western Australia. Though the chief mineral product at present is gold, the district abounds in other economic minerals, notably ores of copper and tin, and has already become famous as being by far the most important producer of tantalum in the world, the chief source of supply of this metal being the manganotantalite of Wodgina.

The geometrical representation of the faces and edges of a crystal is obtained (i) by drawing lines through a fixed point O perpendicular to each crystal-face meeting a fixed sphere with centre O in a point (face-pole) representing that face, (ii) by drawing lines through O parallel to each crystal-edge meeting the sphere in a point (edge-pole) representing that edge. This representation is inconvenient as being in three dimensions, and therefore it is customary to map the sphere on a plane. This is usually done by means of the stereographic or gnomonic projection.

Certain optical properties of crystals, and more particularly the refractive index, may be determined either in the directions-image, often referred to as the 'image in convergent light', or in the ordinary object-image in which the object itself is seen. In the former case, in which the index of refraction is 'usually determined by means of the critical angle of total-reflection, every point in the image corresponds to a single direction of propagation of the wave-front through the crystal-structure and to the two corresponding directions of vibration. One of these can, however, be eliminated by the insertion of a nicol in an approximate position, and thus all ambiguity in the determination of the refractive index is removed.

The specimens of the mineral examined were obtained from a limestone quarry near the town of Renfrew. The tourmaline occurs near the contact of grey granite-gneiss and crystalline limestone of the Grenville series. The gneiss is intrusive into the limestone, and contact effects are observable both in the intrusive and in the intruded rock. The gneiss consists of quartz, plagioclase, micrccline, and a green pyroxene. The limestone is composed (almost entirely) of calcite, but contains some graphite. The secondary minerals occurring in both limestone and gneiss are, besides the tourmaline, pyrrhotite, titanite, and muscovite.