The interesting results which have followed the investigation by one of us of the isomorphous series, of double sulphates and selenates of the general formuh, R′2M′′[(S, Se)O4]2.6H2O have led us to extend the investigation to the corresponding double chromates in which M is represented by magnesium.

The main portion of this memoir as far as it relates to wiltshireite was in the Editor's hands in January, 1911 ; but before it was sent by him to press I learnt that Mr. R. H. Solly had, at the Anniversary Meeting of the Mineralogical Society in 1903, described an unnamed crystal, which, from the statement made to the Society on March 21, 1911, and published in the ‘Mineralogical Magazine’ (1911, vol. xvi, p. 121), is undoubtedly the same substance as that named by me in August, 1910, wiltshireite. In the statement of March, 1911, Mr. Solly pointed out the striking relation of the crystals of wiltshireite to those of rathite ; and suggested that the latter is ohlique and not orthorhombic, as described in this Magazine (1901, vol. xiii, pp. 77-85).

Crystals of Inosite, C6H6(OH)6.2H2O , were first measured by Cloёtta, and somewhat later by Zepharovieh, and by Lewis; they are monoclinic, and the elements adopted are those of Zepharovieh, viz., a : b : c = 1.0872 : 1 : 1.5602 ; β = 111° 39′. The observed forms on the crystals were {010}, {110}, {100}, {001}, , , {210}, and {410}, and it is seen that the form-development is of a somewhat anomalous character, the indices of the last three forms being remarkable for their complexity.

Hitherto the mineral prehnite has been recorded from one locality only in Cornwall, namely, in the cliffs between Wheal Cock and the Crowns Rock, Botallack, St. Just. It occurs there as pale-green, globular aggregates composed of indistinct, curved crystals, and forms veins traversing a narrow strip of greeustone which has intruded itself through the granite mass; associated minerals are stilbite and axinite.

In the summer of 1910 the writer collected several crystallized specimens of this mineral at Pare Bean Cove, Mullion, the locality from which Dr. J.S. Flett and Mr. W. F. P. McLintock obtained the fine crystallized specimens of datolite, recently described by the latter. The prehnite occurs here in two distinct and somewhat unusual types of crystals—tabular and prismatic.

There occurs frequently in coal a white carbonate which, when it is referred to at all by scientific writers on coal, is usually described as calcium carbonate. It occurs as a rule in the form of thin layers infilling the joint-cracks which lie normal to the bedding-planes, though at times it may be seen infilling other divisional planes which may cut across the bedding obliquely.

The mineral dealt with in this paper was sent for examination the Imperial Institute by Mr. J. B. Scrivener, Government Geologist to the Federated Malay States. It occurs on the river Sebantun, about half a mile above Salak North village, Kuala Kangsar district, Perak. The ground on which it was obtained was held on a tin-mining lease, but had to be abandoned owing to the presence of the unknown mineral, which rendered mining unprofitable.

The relationship between the optic axes and the vibration-directions in a section of a biaxial crystal was enunciated by Biot in 1819, and may be stated as follows : The vibration-directions in any section of a biaxial crystal are given by the bisectors of the angles between the optic axes as projected on the plane of section.

Dr. Fletcher and Mr. Harker have elaborated the idea, but there appears to be room for a simpler presentment, more adapted to the needs of students, which does not involve the use of the optical indicatrix or the spherical projection.

The accompanying diagram (Plate VIII) has been prepared in order to facilitate the determination of indices of refraction with the Kohlrausch total-reflectometer, by affording a means of rapidly werking out the results by a graphical method.

The Kohlrausch instrument enables measurements to be made of the critical angle θ, at which total reflection begins, when monochromatic light is reflected at the plane surface of a solid of refractive index μ immersed in a liquid of higher refractive index μ′.

The beautiful crystals of colemanite from the Death Valley, Inyo County, and from the Calico district, San Bernardino County, California, were described almost simulfmneously in the autumn of 1884 by several different observers, The most complete examination we owe to Jackson, who identified forty-forms and determined the axial ratios which are usually accepted for the mineral, namely, a : b : c = 0.774843 : 1 : 0.540998 : β = 110° 9′ 15′′. Very similar values for these constants were adopted by Hiortdahl and by vom Rath.