In a recent note in this Magazine, Dr. R. B. Elliott points out that inclined contacts of grains of considerably different index may bias the result of a micrometric analysis. That there may be some bias of this sort is hardly to be doubted, though I am inclined to feel that he greatly overestimates its importance. His demonstration, however, involves a fallacy so long and widely held as to be worth refuting.

Micaceous minerals, such as the micas, the chlorites, and most clay minerals, usually develop as thin lamellae parallel to the crystallographic basal plane (001). In sedimentary deposits these lamellae tend to lie parallel to the bedding plane and in slates parallel to the cleavage. Their degree of orientation is likely to vary with the conditions of formation of the material, and in the case of slates Bates (1947) has shown that it is closely related to their fissility. In relatively coarse-grained materials the degree of orientation may be studied directly by suitable microscopic techniques, but with fine-grained materials X-ray methods must be used, and even with coarser-grained materials the use of X-ray methods may have advantages, especially if a scanning technique, such as that described by Thewlis and Pollock (1950), is employed.

During an investigation of the mineralization of the Leadhills- Wanlockhead lead and zinc mining district, rammelsbergite (NiAs2) was identified as an associate of niceolite (NiAs), reputed to have come from no. 4 level on the west branch of the New Glencrieff vein. The identification was confirmed on further material collected from the dumps at the Glenerieff mine, Wanlockhead, Dumfriesshire, and on specimens from the Scottish Mineral Collection (nos. 1952.5.18; 20; 21).

Tricalcium disilicate, Ca3Si2O7, which was first described by Shepherd and Rankin, has been found in certain types of slag, and recently as a natural mineral at Scawt Hill, County Antrim, to which Tilley applied the name rankinite. The present work was undertaken after it was discovered that rankinite is formed when afwillite is heated to about 1000°C.

Hillebrandite was first characterized by Wright as a white, fibrous mineral with the composition 2CaO.SiO2.H2O, optical properties αNa 1·605±0.005, γNa 1·612±0.003, moderately weak birefringence, parallel extinction, optic axial angle 2E between 60 and 80° and density 2.692. X-ray powder data were first published by Vigfusson, but these have not been substantiated by later investigators (Clark and Bunn, McMurdie and Flint, and Heller and Taylor). Synthetic dicalcium silicate β-hydrate has been found to resemble hillebrandite closely.

In this paper the formation of a gelatinous hydrate phase from the natural dicalcium silicate minerals larnite and bredigite is described. Detailed study of the gel so formed indicates that the process of hydration is a solid reaction which is effeeted by diffusion. The preferred orientation of the hydrate phase suggests that the hydration is facilitated by a relationship between the structure of the anhydrous silicate and that of the hydrate phase produced.

In this investigation the variations in density, mean refractive index, and chemical composition of the natural gels have been determined and the characteristics of the adsorbed water studied by isothermal dehydration and rehydration. In discussing the mechanism of hydration it has been assumed that this obtains at constant volume. This assumption is based on the study of gel pseudomorphs after single crystals of larnite and bredigite in thin section.

The mineral gyrolite was discovered in Skye by T. Anderson in 1851. He showeditto be ahydratedcaleiumsilicateofapproximate composition 2CaO.3SiO2.3H2O. Subsequently gyrolite has been found in other localities, and has been prepared hydrothermally by Flint, McMurdie, and Wells. The ratio of lime to silica found by Anderson appears well established, but various values of the water content have been reported and more recent analyses approximate to 2CaO.3SiO2.2H2O. X-ray powder measurements were given by McMurdie and Flint, a but no crystallographic or other investigations of a detailed character have been reported.