A Small set of quartz crystals of somewhat peculiar habit, collected by Lieut. E. G. Spencer-Churchill near De Aar, South Africa, and given by him to the Mineral Department of the University Museum, present some features of interest; two crystals, in particular, are remarkable as exhibiting faces seldom observed on quartz. The first has a face X lying in the zone [mz] (or [mr]?), the other a well-developed face in the zone [rz].

The crystal bearing the former face, X, is depicted in fig. 1. As regards habit and other features, it is in all respects similar to the second crystal (fig. 2).

The faces r and z cannot be distinguished with certainty, but cleavage-cracks within the crystal point to the lettering shown in fig. 1. The polariscope proves the presence of both right- and left-handed material.

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.

Historical Introduction.—As given in modern text-books, the holohedral-cubic silver haloids occurring as minerals include the following species: the pure chloride, chlorargyrite, AgCl. ; the pure bromide, bromargyrite, AgBr; the intermediate embolite, Ag(Cl,Br), containing chlorine and bromine in varying proportions ; and iodobromite, containing all three halogens in a definite ratio corresponding to the formula 2AgCl.2AgBr.AgI.

The terms hornsilver (used by Gesner as far back as 1565) and cerargyrite (Beudant, 1832) are usually taken as referring only to the pure chloride. As regards their signification, however, they are equally applicable to all of the above species, and in lack of chemical analyses were certainly so applied in the past. Thus Breithaupt, after the discovery of the bromide and chloro-bromide of silver by Berthier in 1841 and 1842, remarks that many of the specimens of ‘Hornerz’ in the Werner Museum having a greenish colour probably contained bromine.

Many minerals of the rhomhie system are of markedly columnar habit, with the axis of elongation, or vertical axis, as one of the biseetrices of the angles between the optic axes. Such a crystal, in a section parallel to the vertical axis, gives straight extinction. It is natural to expect, and is perhaps generally assumed, that a section slightly oblique to the vertical axis will give nearly straight extinction. The object of the present note is to inquire how far this is true, and in what possible cases a serious error may arise.

Professor Baumhauer, in January, 1901, described five very small crystals of a new mineral, to which he gave the name seligmannite. They were associated with rathite and binnite in dolomite, and came from the Lengenbach quarries in the Binnenthal, Switzerland. In June, 1902, he described another crystal, the angles of which agreed fairly well with those of the crystals he had previously measured. In habit and twinning, these orthorhombic crystals so closely resembled bournonite that he ventured to assign to the new mineral the chemical formula Cu2S.2PbS.As2S3, but from paucity of material he was unable to make a chemical analysis.

The anharmonic ratio here considered is that of the pencil (P) formed by the four lines (lying in a plane perpendicular to the zonal axis) drawn from any point M perpendicular to four faces of the zone. The anharmonic ratio of the pencil P is evidently independent of the position of M.

Let four planes be drawn through M perpendicular to the four lines of the pencil P; these are parallel to the crystal faces and meet in a line (the line through M perpendicular to the plane of the pencil P and parallel to the zonal axis).

These four planes are cut by the plane of the pencil in a pencil having the same angles, and therefore the same anharmonic ratio, as the pencil P.

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.

The mineral kilbrickenite was analysed, described and named by Dr. James Apjohn in 1840. The specimens which he examined were sent to him by Mr. P. M. Taylor, who conducted the mining operations at the Kilbricken mine in County Clare, where the mineral was discovered. Apjohn's analysis showed the presence of lead, antimony and sulphur in proportions corresponding fairly closely with tbe formula 6PbS.Sb2S3. It was only in the preceding year that L. F. Svanberg had described a new mineral, geocronite, to which he assigned the formula 5PbS.(Sb, As)2S3. The announcement of this new mineral appears to have escaped Apjohn's notice, since geocronite is not included in the list of sulphantimonites of lead with which he concludes his paper.

The evaluation of the determinant

occurriug at the foot of page 345, Vol. XII, can be effected without first squaring it. Multiply Δ1 by

say D 1/2, which is the value of the determinant

The result is

Similarly Δ2 and Δ3 may be evaluated.

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}.

In August, 1900, several specimens of this mineral were found attached to gypsum-rock supplied to me from the quarry at Kirkby There, in Westmoreland, by Messrs. Joseph Robinson & Co., Ld., of Carlisle, who kindly inform me that the bed of this rock is about 20 feet thick, being overlaid by about 22 feet of marl, &c. There is very little anhydrite, not more than 2 or 3 inches, which, as a rule, is found in the middle of the bed of gypsum. Although I have used this rock for commercial purposes for years past in regular quantities, yet the mirabilite was never previously noticed.

A determination of the constants of elliptic polarization of light reflected from a fresh cleavage surface of antimonite enabled Professor P. Drude to calculate two of the indices of refraction of this substance as 5.17 and 4.49 respectively. Instead, however, of the symmetry-axes of the crystal, regarded as belonging to the orthorhombic system, coinciding with the principal vibration directions in the plane (010), he found that the latter were inclined to the former lines at angles which varied in different observations between 2.6° and 15.4°. These observations, which indicate that the symmetry of antimonite is of the oblique or anorthie type, stand in opposition to those of Professor O. Mügge, who found that exceedingly thin flakes of antimonite placed between crossed nicols in direct sunlight transmitted sufficient light of a deep red colour to enable him to determine the extinction as straight.

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.

In a previous paper the author pointed out the advantages of a three circle form of goniometer in the determination of the morphological characters of crystals ; the crystal is adjusted once for all, and measurements may be made in any desired zone. A description was added of such an instrument, which had resulted from the addition of a two-circle apparatus to an ordinary Fuess goniometer with a single horizontal circle. This instrument has been in almost daily use since the end of June, 1899, and the author is, therefore, in a position to appreciate the ease and celerity with which crystals, however small, may be in this manner measured. As was remarked in the paper quoted above, the adapted instrument has the disadvantage that measurements can only be made through little more than a right angle from the pole in which the zone to be measured and the zone of reference intersect, and, therefore, only half a zone can be measured without readjustment of the circles B and C.

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′.

The mode of occurrence and origin of the various forms of titanium dioxide as rock constituents have received attention at the hands of many petrologists, both on the continent and in our own country. Among the works of the former dealing with this subject, Thürach's exhaustive treatise on microscopic rock-forming rutile, anatase, and brookite, is undoubtedly the most important : in England, Mr. Maynard Hutchings has described in great detail the occurrence of rutile and anatase both as constituents of clays and slates, and as a product of contact metamorphism in the Lower Coniston Flags in the neighbourhood of the Shap granite; Mr. J. J. H. Teall has also described rutile needles and anatase crystals from clays of various ages, and anatase from the Cleveland iron-ore; Mr. Allan Dick has mentioned the occurrence of rutile in the Bagshot Sands ; and Mr. H. H. Thomas has lately described these minerals from the Budleigh Salterton Pebble-bed of Devonshire.