It is well established by X-ray crystal analysis that the mineral constituent of bone and of the enamel and dentine of teeth is essentially hydroxy-apatite Ca5OH(PO4)3, and that hydroxy-apatite has a structure differing only in small details from that of the well-crystallized mineral fluor-apatite Ca5F(PO4)3. The main difference, and in fact the only fully established one, between the two structures is that the hydroxy-apatite has a slightly larger unit cell than that of the fluor-apatite. The structure of the latter has been determined by Náray-Szabó, and by Mehmel who finally agrees with Náray-Szabó. We have made what is probably a more accurate determination of the structure, using more extensive X-ray data.

At two localities on the outskirts of the township of Broken Hill—near the De Bavay shear zone, and at ‘The Piggery’—W. R. Browne reported unusual rock types rich in barium. His statement is accompanied by an analysis by H. P. White (analyst to the Mines Dept.) of one of these rocks, which contains 9·23% BaO. Subsequently S. R. Nockolds and E. G. Zies examined the felspars of the rock analysed by White with a view to ascertaining whether they are barium-containing varieties.

In view of the very high barium content of some of the occurrences now being investigated, much higher than that of the rock examined by Nockolds and Zies, a further mineralogical investigation was undertaken with the results recorded below. The rocks under examination were light-coloured gneisses containing up to 15% BaO. They are distributed as lenses and streaks in an acid gneiss. A specimen of the latter rock was analysed and found to contain only 0·09% BaO. The barium was found to occur in four felspars, one being a new variety of celsian. The minerals studied were celsian, calciocelsian, hyalophane, and barium-plagioclase. A careful search was made for other barium silicate minerals such as sanbornite, but none was found.

Calcium is one of the most widespread and abundant of metals, coming fifth on the list of elements and third among the oxides of the outer lithosphere; it is an essential constituent of a great many rock-forming minerals and is, therefore, almost always present in rocks. Barium and strontium come very much lower in the list of abundance of elements, but nevertheless barium is extremely widespread and occurs in most rocks, igneous, metamorphic, and sedimentary, while strontium is also very widespread. It has been stated that the average content in igneous rocks of both of these elements is of the order of 0·05%.

Recently Dr. M. H. Hey published in this journal a method to construct graphically a projection showing the relation of two etched surfaces M and N to the crystallographic axes of a mass of the Gibeon meteoritic shower. Upon those faces could be distinguished the traces of four octahedron planes 0l, 02, 03, 04 which are inclined at the following angles, measured clockwise from the intersection [MN] of the etched surfaces: on surface M 22°, 62°, 98°, and 135° and for the same octahedron planes on surface N 106°, 38°, 150°, and 86°, respectively.

The two faces M and N are at an angle of 84° one to the other. Hey's solution of the problem is based on a reversal of the Goldschmidt construction for drawing crystals from the gnomonic projection.