The natural glasses recognized as tektites are characterized by their acidic composition, the silica percentage ranging from 68 to 80. Their origin is still a debatable point. Within this range glasses are known which have undoubtedly been produced by the fusion of siliceous terrestrial materials in the intense heat developed when large iron meteorites have struck the earth's surface; while others of the same origin show silica ranging up to 98 per cent., that is, an almost pure silica-glass.

From a collection of 84 chemical analyses of tektites taken from the literature, only 36 (many of them due to A. Lacroix) give data for the specific gravity and refractive index which presumably were determined on the same sample of material as that analysed. These are reproduced in table I (arranged in order of silica percentages) and plotted in figs. 1 and 2. Corresponding with the range in SiO2 from 68·00 to 80·73% there is a nearly uniform decrease in d (2·498–2·339) and n (1·526–1·4867). The specific refractivity, K = (n−1)/d, varies from 0·2050 to 0·2107 (0·2258 in the case of no. 29 with the abnormally high n 1.5380).

There is a close optical and chemical resemblance between chamosite, the chloritic mineral of the bedded ironstones, and daphnite, a low-temperature vein-chlorite common in some of the Cornish tin mines. New material has made it possible to undertake a fresh comparison of the two minerals: chemical analyses have been made by Mr. C. O. Harvey, chemist to H.M. Geological Survey, and a report on the X-ray measurements is contributed by Mr. F. A. Bannister, of the Mineral Department of the British Museum.

The new analysis of chamosite agrees with the simple formula previously assigned: X-ray examination of material from several localities has now established the distinctive crystalline nature of this fine-grained mineral, which differs structurally from ordinary chlorites such as clinochlore. Daphnite, on the other hand, has the ordinary chlorite structure, but the new analysis fully confirms Tschermak's original opinion that it cannot be represented chemically as a mixture of serpentine and amesite.

The original description of aramayoite by Spencer in 1926 contained only a qualitative discussion of the crystallography, because suitable crystals for a more rigorous treatment were not found. The pseudo-tetragonal symmetry was recognized, and cleavages were noted, but only one was defined. Twinning was later observed in polished section by Schneiderhöhn and Ramdohr, but undefined. Yardley demonstrated the triclinic symmetry of aramayoite by the Laue method, and determined the spacings of a considerable number of planes in the crystal. From these data she formulated a unit cell and correlated the X-ray data with Spencer's crystallographic results.

Several specimens of aramayoite found in our recently acquired Ahlfeld Bolivian collection contained small but excellent crystals. Measurement of these indicated that the Yardley X-ray crystallography was not consistent with an adequate morphological treatment of the form-development. The twenty-seven forms found by us lead to an obvious crystallographic unit (fig. 1), bearing a rather complex relationship to the Yardley unit. In order to test the suitability of the latter, we repeated the X-ray examination by the modern Weissenberg method and found complete accordance with our morphological treatment, as will be shown below.

The use of a magnetic separator for the separation of certain ores has long been a standard commercial practice and a laboratory scale instrument applying the same principles to the separation of mineral grains from a powdered rock has been designed by Hallimond. In this apparatus the mineral grains are spread on a rotating pan which carries them below the pole pieces of one or more electromagnets, the pole pieces being so designed that the grains pass through a strongly diverging magnetic field and so are subjected to a force proportional to their susceptibility. If this force exceeds the weight of the grains they are attracted and adhere to the pole pieces. The material thus collected is periodically discharged by interrupting the magnetic field and the grains then fall through a slot in the rotating pan into a receptacle below.

In carrying out a petrological examination of pebbles collected by Mr. F. Gossling from the Lower Greensand of Surrey and west Kent, the writer has found a low-power dissecting binocular microscope an extremeIy useful piece of apparatus. The instrument, by Watson, is fitted with two pairs of objectives on a rotating nosepiece, and with two pairs of eyepieces, so that it commands the choice of four different magnifications.

The pebbles are treated in a uniform manner. After the preliminary examination of a ‘bag’ of pebbles, those selected for study are ground flat on one side, finishing on the fine carborundum plate as if for section making. Each one is then mounted on the usual 3 by 1 inch glass slide, taking care not to over-cook the balsam. If the pebbles are treated systematically in batches of six, a dozen pebbles can be mounted in less than an hour, so that the gain of time over section-making is very considerable.