Whilst some atention has in the past been devoted to the question of the properties of natural glasses, very little has been done to indicate the interrelations of the properties determined. In the case of tile natural volcanic glasses, this is unfortullately only too evident from a study of the standard text-books. A determination of such properties as density, refractivity, ttud chemical composition, and the interrelations of these for particular glasses, is rarely to be found in the literature. The chief contribution to this subject is due to Stark, who has examined the refringence of a large number of volcanic glasses. Unfortunately, there is no record of the density of these rocks, and the interrelation of these remained unknown.

It is now well recognized that quartz undergoes a sudden change near 575° C., and Wright and Larsen (1) have shown that the characteristics of quartz formed above or below this point are so marked that they can be utilized to determine the temperature of formation of rocks containing quartz. Quartz formed above 575° so-called β-quartz, is of distinctly hexagonal development, and occurs generally as regular hexagonal bipyramids with or without a subordinate prism-face. These external characteristics are of course preserved on cooling, and the bipyramids are converted into apseudomorphous aggregate of α-quartz.

It has been proved by the work ot Messrs. A. Hutchinson and A. M. Macgregor, recently published in this Magazine (1921, vol. 19, pp. 225-232) under the title 'Oil cornetite from Bwana Mkubwa, Northern Rhodesia', that the only difference—if any—between the Rhodesian mineral and the original cornetite from Katanga consists in the presence of cobalt as determined by Professor G. Cesàro in the crystals from the Étoile du Congo mine. It may be of interest to give here the results of my researches on the chemical composition of the cornetite from the Étoile du Congo mine in Katanga.