Dans les questions relatives aux biréfringences on emploie des formules approximatives, et je ne trouve relatde nulle part la formule dormant exactement la biréfringenee d'une face en fonction des angles que sa normale fair avec les axes d'éasticitd optique. Le but de cette note est d'indiquer cette formule, d'ailleurs très simple, et surtout de montrer clue la formule approximative habituelle se déduit de la formule exacte en y supposant l'indice moyen infini.

Les axes de symétrie d'un polyèdre ne sont pas toujours situés tous dans ses plans de symétrie ; ainsi, tandis que le cube a tous ses axes

situés dans ses plans de symétrie, le rhomboédre

a son ʌ3 situé dans des plans de symétrie, tandis que les 3L2 ne sont pas situés darts ces plans.

The Cambridge Museum recently acquired a number of albite specimens from Alp Rischuna, near Yals Platz in Switzerland. Many of the crystals had good faces, so that measurement of the angles seemed likely to give trustworthy crystal-elements.

The refractive indices of this mineral for different wave-lengths were determined by the Abbé-Pulfrich total-reflectometer. The dispersion of the glass hemisphere of the instrument was determined by the aid of a highly polished glass prism, the dispersion of the latter being found by the method of minimum deviation. In order to obtain light of definite wave-length Hilger's wave-length spectrometer was used with a Nernst lamp or an electric arc as the source of illumination.

Up to the present time only one of the several ehildrenite occurrences known has been examined both chemically and crystallographieally. This is the well-known occurrence at the George and Charlotte mine near Tavistock, from which nearly all the specimens seen in collections were obtained. The crystals often attain a dimension of 1 cm., so that material for analysis can be easily obtained free from gangue. Crystals from this locality were analysed by Rammelsberg, Church, and Penfield, and it is on the results obtained by the last-named that the comparatively simple formula for this mineral is based.

In 1906 the present writer gave a brief account of the interesting alluvial diamond field which is situated on the borders of the Somabula Forest in Southern Rhodesia. The area is not of very great economic importance, but it has nevertheless yielded to date about 10,500 carats of diamonds (valued at over £45,000). These are extracted from thick beds of gravel which, as described in the paper referred to, generally consist of well-rounded pebbles in a matrix of sandy clay.

The colour of the well-known iron sulphide, pyrrhotite or pyrrhotine, is given in most treatises on mineralogy as 'bronze-yellow'. Dana calls it ' between bronze-yellow and copper-red ', and also notes that it is 'subject to speedy tarnish'. The accuracy of the last statement will not be doubted by any one who has had much acquaintance with mining operations in districts where pyrrhotite is a common lode mineral, but it does not appear to have been put on record in any scientific publication that the characteristic colour, not very accurately designated 'bronze-yellow', does not seem to be the original one of the unaltered mineral.

There exists in the Lake District a peculiar type of metamorphism of which no analogue has, as far as the writer is aware, been described elsewhere, with the exception, perhaps, of South America. It is indicated chiefly by the presence of garnets and 'streaks ', the former being found indifferently in every variety of igneous rock of the district, the latter being mainly confined to the rhyolites or quartzkeratophyres. The most similar known to the writer is at Oyacachi in Ecuador, whence veto Rath has described flow-brecciated soda-rhyolites which sometimes contain red garnets. He also refers to rocks which may be 'streaky'.

Six years ago, in a paper read before this Society, Dr. Herbert H. Thomas drew attention to the relative abundance of andahsite in the Plioeene deposits of Cornwall, East Anglia, and Kent, and also in various glacial and recent sands. When that paper was written, the only recorded occurrence of unaltered detrital andalusite in sedimentary rocks of greater age than the Pliocene was that in the Middle Chalk of Beer in Devonshire.

The name autunite was originally g!ven to the well-known mineral from Autun, but it would seem that no crystals of this material have been found that would afford measurements oi1 the goniometer. As the result of optical examination Des Cloizeaux (2) concluded that the mineral was biaxial, and he observed further that the angle decreased when the specimen was heated (6); the extinction was apparently straight, and he therefore referred the trysts,Is to the rhombic system ; with the contact goniometer he found mm'=90°48'.