The acquisition by the Cambridge Mineralogical Museum of a series of well-crystallized specimens of chalybite, raised within recent years from the mines in the neighbourhood of Camborne in Cornwall, afforded a favourable opportunity for an investigation of the chemical composition and optical characters of this substance. This seemed the more desirable since no analysis of the chalybite crystals from the Camborne district appears to have been published, and our knowledge of the optical characters of the substance is confined to some observations of Ortloff on material containing large quantities of manganese carbonate from Wolfsberg in the Harz.

The molecular volumes of the members of an isomorphous group of minerals in general show an approach to equality. In other words, minerals having an analogous chemical composition show not only closely related crystalline forms, but have also approximately equal molecular volumes. Many years ago J. D. Dana endeavoured to show that a simple relationship as regards molecular volume also subsisted in the case of various minerals which were closely related crystallographically, although unlike chemically. He concluded that in these cases the similarity in form depended solely on the relationship in molecular volume, and had nothing whatever to do with the chemical composition.

Mode of Occurrence.—This new mineral was discovered at Gangapitiya, near Ambakotte and about 12 miles east of Kandy, Ceylon. In this locality quarries have been dug for the well-known Ceylon moonstone, which occurs in large porphyritic crystals in an acid granulite.

Bands of this granulite, which is composed mainly of quartz and felspar in finely granulitic and occasionally graphic structure, alternate with bands of limestone up to 18 inches wide. Between limestone and granulite occur contact-zones consisting, next to the limestone, almost entirely of a coiourless diopside, but near the granulite of a mixture of diopside with other minerals, viz. blue spinel, a little apatite, occasionally scapolite or plagioclase, and the blue mineral which is the subject of the present note.

The following notes on the petrology of British East Africa are the result of an examination of rock-specimens collected by Professor J. W. Gregory on his well-known expedition from Mombasa to Mt. Kenya and Lake Baringo in 1892-3, and of rock-collections from the Uganda Protectorate which have been recently presented to the British Museum by Sir Harry Johnston.

The collections include examples of the Archaean gneisses, schists, and granites which constitute the prevailing basement rocks of Central Africa; of ferruginotls schists, coarse sandstones, and quartzites belonging to the Palaeozoic Karagwe series ; and of an interesting series of Tertiary volcanic rocks comprising phonolites, phonolltic traehytes, riebeckite-rhyolites, kenytes, and basalts from the volcanoes of the Great Rift Valley, as well as of nephelinites and basaltic rocks containing melilite and perofskite from Mr. Elgon.

Some interest attaches to crystals of krennerite, since this mineral has apparently the same composition as calaverite, the crystalline development of which presents such peculiar features. After completing the study of crystals of the latter substance, the author naturally turned his attention to krennerite to ascertain whether any similar peculiarities were noticeable, but nothing abnormal could be detected.

Good crystals of krennerite are rare. The Mineral Collection of the British Museum has however a specimen from Nagyág, on which are dotted numerous small crystals possessing a large number of brilliant faces. This specimen was examined by Prof. H. A. Miers rather more than twelve years ago, and the measurements of one good crystal are recorded in this Magazine.

Since the occurrence of tellurides of gold in Western Australia was first recognized by Mr. Arthur G. Holroyd in May, 1896, these minerals have proved to be of the greatest importance as a source of gold; and in fact the telluride mines at Kalgoorlie in the East Coolgardie gold-field now yield as much gold as all the remaining gold-fields of Western Australia taken together.

Although the occurrence of traces of telluride of gold has been reported from one or two other localities in the neighbouring portions of Western Australia, it is only in a quite small area around Kalgoorlie that the tellurides are mined. Here they occur as lodes, or rather as large lenticular masses and as impregnations, in schistose rocks.

Crystals of mispickel are occasionally found in long brilliant tin-white prisms of simple habit in the cavities of the dolomite at Lengenbach, near Binn ; but they are more frequent as isolated crystals entirely imbedded in a rather compact mass of the dolomite. The crystals are elongated in the direction of the brachy-axis, and are terminated by two minute faces which give good images and prove to be the faces usually adopted as the prism {110}. The long faces are much striated parallel to their mutual edges of intersection ; and the images were consequently so elongated in this zone that only roughly approximate measurements of the angles could be obtained. The brachydomes satisfactorily established were s {012} and l {011}.

Whilst determining some crystals of proustite in the Mineralogical Museum at Cambridge, and which came from Bolivia or some other part of South America, my attention was attracted by faces of trigonal pyramids, which, as shown in figs. 1 and 2, are the most conspicuous on the crystals. On measurement, these faces were found to correspond very nearly to those of the rhombohedra {73̄3̄} = 10 R (fig. 1) and {94̄4̄} = 13R (fig. 2). The figures represent with fair accuracy the relatively large development of the faces of these new forms. Another form, {11.7̄.7̄} = –6 R, represented by a single face on the lower end of one crystal, is also new for proustite.

Mallard in 1879, noted that crystals of the metallurgical products ‘Spiegeleisen’ and ‘ferro-manganese’ are of two kinds. Both show a prism-zone of six faces, but when the percentage of manganese reaches a certain amount there is an abrupt change in the habit of the crystals and in the angles between their faces. Crystals containing from 11 up to 52 or 55 per cent. of manganese are lamellar, and were found to have a prism-angle (mm′′′ = 110 : 11̄) of 6° 27′, while crystals containing a larger amount of manganese are acicular and have a pseudo-hexagonal prism-zone, the angular measurements of which varied between 58° 42′ and 61° 20′.