In the course of my study of the dispersion phenomena of the felspar group in the Mineralogical Laboratory of the University of Cambridge, Professor Lewis advised me to study the remarkable properties of sanidine from the Eifel, which were first noticed by Des Cloizeaux, and he very kindly placed the mineral sections in the Museum at my disposal. In order to be certain of the material used, application was made to Professor A. Lacroix of Paris, who with the courtesy and generosity characteristic of his countrymen was good enough to send some fragments of the very crystals used by Des Cloizeanx in his classical researches.

In the present paper the dispersion of the refractive indices of monoelinie felspars from four different localities is given, likewise that of the optic axial angle and of the optic axes in some of them.

The method of investigation and instruments for these studies have been already mentioned in my previous paper. As the refractive indices and dispersion of the glass hemisphere of the total-reflectometer have not so far been given, I shall in the first place state the way in which they have been determined and their values and the degree of accuracy attainable, for the dispersion of the glass hemisphere differs to au appreciable degree from that of other instruments of the same kind.

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.

During the last decade there has been no lack of controversy on the subject of rock classification, but the dispute has concerned igneous rocks almost entirely. Petrography has been threatened by an effort to reduce it to an independent science of igneous rocks, and in consequence of this there has been a tendency to neglect its wider aspects. Important as igneous rocks are, their formation is less well understood than is that of some other sub-groups of rocks ; moreover, they constitute one only of some six or seven sub-groups, and it is the business of petrography as a science to deal with these rocks as a whole. The student of petrography should be encouraged to take this wider view. He shonld look at these different sub-groups of rocks in their proper geological perspective, and approach the study of the classification of igneous rocks with a knowledge of the principles that are applicable to the classification of rocks as a whole.

The discussion concerning the classification of ore deposits has proceeded on independent lines, and in a more well-balanced manner than has that of rocks. With ore deposits, as with rocks generally, genetic principles have triumphed; but the application of these principles to rocks long preceded their application to ore deposits. Iu recent years it has become apparent that the fundamental requirements for the classification of rocks and ore deposits are essentially the same ; and in view of the identity of their interests, both subjects are treated in this paper.

The aims of this paper are : To define the basis of genetic classification ; to give a brief historical account of classification on genetic-geological principles ; to point out defects in the present system of arrangement ; and to suggest an altemmtive scheme of grouping that is in closer accord with geological and genetic principles.

Early in 1905 a small fragment of a meteoric iron from Uwet in Southern Nigeria was received at the Imperial Institute from Mr. John Parldnson, the Principal of the Mineral Survey then proceeding in Southern Nigeria. The fragment was part of a large mass which was held in great veneration by the natives of Uwet (lat. 5° 17' p N., long. 8° 15' p E.), a town on the Calabar river, about 23 miles north by west of Calabar. Later, further particulars of the meteorite were communicated to the Imperial Institute by Mr. E. D. Simpson.

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 apparatus to be described has been designed for the cutting of parallel plates and of prisms of crystallized substances in any required direction. The authors have endeavoured to design a small apparatus, simple in construction and easy to manipulate, which at the same time will be capable of yielding results of sufficient accuracy for most optical work on minerals. Several instruments have been devised with the same objects in view, notably those of E. A. Wülfing, G. Halle~ and F. Stöber. The well-known cutting and grinding goniometer of Dr. Tutton is an instrument of an entirely different class.

Virtuous Lady Mine is situated in the parish of Buckland Monachorum on the right bank of the river Tavy close to its junction with the river Walkham, at a distance of three miles south of Tavistock in Devonshire. The mine has long been famous for the charm of its situation, and in addition possesses for the mineralogist an especial interest owing to the peculiar beauty and unusual character of the mineral specimens it has yielded. The early history of the mine is lost in obscurity, as also is the origin of its curious name, which, however, local tradition ascribes to Queen Elizabeth.

Some specimens of calcite which I obtained recently from a chalkpit at Corfe Castle in Dorsetshire appear worthy of description on account of their somewhat unusual habit and of the rarity of wellcrystallized minerals in the Chalk.

The pit is situated in the Upper Chalk, not far from the great overthrust fault which traverses the Isle of Purbeck from west to east along the boundary of the Chalk and the overlying Tertiary beds. The Chalk is here steeply inclined, with a northerly dip, and is much indurated, being sufficiently hard to be worked for road-metal.

Eight years ago (in 1907) Mr. F. N. Ashcroft, who for many years has devoted much of his leisure to bringing together choice specimens of the species included in the zeolitic group of minerals, turned his attention to the specimens occurring in the cavities of the basalt of Co. Antrim. He was fortunate enough to secure the services of Mr. Robert Bell, of Belfast, ill his quest, and between them they have searched many of the quarries and other places likely to yield good specimens of these minerals. Some of the early specimens obtained through Mr. Bell, which, however, did not include any chabazite, were exhibited by Mr. Ashcroft at the anniversary meeting of the Society, on November 12, 1907.

In 1904 Professor Bowman described the occurrence of bertrandite at the Cheesewring Quarry, Linkinhorne, Cornwall; the mineral having been then recently discovered there by Mr. F. H. Butler. The following notes contain an account of the character and mode of occurrence of the same mineral at four additional localities in Cornwall.

The mineral hero described was recently collected by the officers of the Geological Survey during their work for the six-inch map of the Island of Mull. It is present as minute phenocrysts in a dark, glassy, magnetite-bearing rock/which is intruded as sheets in the Tertiary lavas nearly one mile south-south-west of Pennygael.

The small rounded crystals of augite are seldom more than ½ mm. in diameter and are traversed by irregular cracks. They contain a few negative crystals, and are occasionally darkened, probably with included magnetite. They appear to have been reabsorbed to some extent by the matrix.

In recent years a number of alkali-rich rocks have been described, containing barkevikite or 'barkevikitic hornblende.' So far as I am aware, however, no detailed examination has been made of the mineral since Brögger's work on the original material from Barkevik in Norway, with the exception of the investigation by Michel Levy and Laeroix. Hence, when a rock was found amongst the Permo-Carboniferous rocks of Ayrshire, with comparatively large crystals of barkevikite, it was decided to examine it in some detail.

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.

It is impossible to determine when this natrolite occurrence was first discovered, but it has been known at any race locally for many years, and specimens of the mineral may frequently be seen in collections, where, however, they are usually labelled as from the Giant's Causeway. This error in localization is largely accounted tbr by the fact that the guides at the Giant's Causeway regularly offer specimens of the mineral for sale to visitors, together with other so-called Giant's Causeway minerals, fossils, and flint implements.