Uraninite and Thorianite
WASSERSTEIN1 suggested that the generation by radioactive decay of Pb4+ of smaller ionic radii within the crystal lattice of uraninite caused a shrinkage in the unit cell proportionate to the geological age and the shrinkage was of the order of 0.0025–0.0041 Å. per hundred million years. The anomaly in the relation of cell dimensions of Bisundni and Gaya uraninites with the age, reported by Holmes2, was explained by Wasserstein3 by classifying uraninites into two different types—α-type for Bisundni uraninite and γ-type for Gaya uraninite, the latter being apparently anomalous and requiring heat treatment under oxidizing conditions in order to reduce the cell size to correspond to that of an α-type analogue. Wasserstein did not, however, state the temperature and period of heating and the conditions under which the cell size of γ-uraninite was reduced to correspond to age values consistent with those of the α-type.
The Age of Uraninite and Monazite from the Post-Delhi Pegmatites of Rajputana
Abstract Uraninite and monazite collected by Dr. H. Crookshank from pegmatites occurring respectively at Bisundni and Soniana (Rajputana) have been analysed for U, Th, and Pb by Mr. A. A. Srnales, and the isotopic constitution of the lead separated from each mineral has been determined by Dr. W. T. Leland and Prof. A. O. Nier. The three ages calculated from the uraninite ratios AcD/RaG, RaG/U and AcD/U are 740, 733, and 733 m.y. The monazite is considerably altered but the corresponding ratios indicate that its age lies between 700 and 865 m.y., which is consistent with the more reliable estimate determined from the uraninite. It is shown from tectonic evidence that the Delhi erogenic belt is younger than the Satpura belt (previously dated at about 900 m.y.) and that both are younger than the Aravalli and Dharwar belts.
The Pre-Cambrian and Associated Rocks of the District of Mozambique
I. General Description of the Area. Beyond the coastal and volcanic belts of Mozambique—already described in a previous contribution1—the country assumes the form of a gently undulating plateau that gradually rises towards the west. As one proceeds inland its surface becomes increasingly diversified by inselberge and clusters of abrupt hills until, west of Ribawe, where the plateau reaches an elevation of nearly 2000 feet, the scenery becomes more typically of a highland character and the stretches of unbroken plateau less extensive. Throughout the whole area traversed by the staff of the Memba Minerals Ltd. (after the Cretaceous and Tertiary formations had been left behind) no rocks were found that were not of igneous or metamorphic origin, or that, like laterite,1 could not be traced immediately back to an igneous or metamorphic parent-rock or to solutions percolating through such rocks. (See index-map, fig. 1, p. 32.) The dominant rock of the country, persistent to a degree that often becomes monotonous, is a grey biotite-gneiss. Interfoliated with the gneiss are occasional lenticular masses of hornblende-gneiss and amphibolite, and with these, smaller bands and lenticles of crystalline limestone are sometimes intimately associated. Elsewhere the gneisses become coarse and garnetiferous, and in some places eclogites and basic granulites occur. Schists were found very sparingly, and the known exposures are limited to three areas: the north-east of the territory behind the broadened coastal plain in the neighbourhood of the mouth of the Lurio, the district around Memba, and the central coastal region between the Mitikiti