C . W. Bunn and H. Emmett have observed the spreading, of multi-molecular layers on growing crystals of artificial compounds, and have suggested that deposition takes place on the high-index surfaces at the edges of the layers. Their work led the author to examine crystals of several mineral species for evidence of the mechanism of growth, and this account is the result of a preliminary survey of the growth structures found.

So many growth structures could be seen with the naked eye or with a hand-lens that it was decided to use low magnifications in order that comparatively large areas of the crystal faces could be examined and photographed at one time. A metallurgical microscope with a prism reflector and 1-inch objective was used for the observations and most of the photography. This work is qualitative rather than quantitative, but it is intended to carry out a quantitative examination of the same specimens later.

This investigation was concerned with two aspects of the colour-zoning in tourmaline: firstly, the origin of the green and pink colours ; secondly, the reason for the apparent incompatibility of these two forms. Much of the previous work on the colour of tourmaline has involved the use of emission spectra. This method provides information as to which elements are present; it does not, of course, indicate which of the twenty or so elements identified is responsible for the colour, although it may be possible to infer this. Absorption spectra, on the other hand, are directly concerned with colour. They have been employed quite extensively in Germany, and more recently in Russia, but the earlier data and instruments were often inadequate. The last decade has seen marked improvements in the technique of absorption spectroscopy, particularly in the development of photo-electric methods for the quantitative recording of spectra, and the method is likely to prove one of increasing value.

The ejected blocks found in the 'peperino' of the Alban Hills have long attracted the attention of mineralogists on account of their variety in content of well-crystallized minerals. It may be recalled that this Roman region provides the earliest and finest examples of the white octahedral haiiyne, at first referred to a separate species (berzeline), and that this mineral with leueite, yellow garnet, wollastonite, green clino-pyroxene, and melilite form characteristic assemblages in the tufts from Albano, Frascati, and other places.

Shortly after the discovery of the mineral kalsilite, the pure potash analogue to nepheline, as a natural mineral occurring in some volcanic ]avas of SW. Uganda by Bannister and Hey, Professor Arthur Holmes suggested that the ncpheline-looking ruineral occurring in a very potash-rich lava at San Venanzo, Umbria, Italy, also might be kalsilitc. His suggestion was based on the high potash : soda ratio of the bulk rock and on the optical properties of the mineral. No X-ray evidence or chemical analysis on the supposed kalsilite has been presented so far.

Examination in reflected polarized light is now widely used in metallography as well as for mineral sections. The recent introduction of graded diamond abrasives has overcome most of the difficulties in polishing that have previously hindered the widespread use of this important method.

The present instrument (fig. 1) has been designed to give the best available control over the conditions of illumination and extinction, and to provide for the complete analysis of the reflected elliptically-polarized beam. A robust stand of normal grade suitable for general research purposes and works control is provided with a stage rack, reflector unit, and the usual polarizing equipment. It may be adapted for transmitted light by the addition of the detachable substage shown on the right of the figure.

In a previous paper (Brindley, 1951) chamosites from a number of localities were examined, and their structure was shown to be of kaolin-type. Furthermore, it was shown that many of the variations in X-ray powder diagrams of chamosites can be accounted for by the existence of two kaolin-type structures, one hexagonal and one monoclinic, with simply related ceil dimensions.

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.

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.

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.