The mineral to be described in the following pages was found hy Mr. J. L. Popham in 1910 at Bwana Mkubwa, Northern Rhodesia, and was sent by him to Mr. A. E. V. Zealley, at that time Curator of the Rhodesia Museum, Bulawayo. Investigations conducted by one of us (A. M. M.) in Bulawayo showed that the mineral occurred as definite, though minute, orthorhombic crystals and that it consisted in the main of copper and phosphoric acid, but that it differed in many respects from the known hydrated copper phosphates such as libethenite, tngilitc, dihydrite, or pseudomalachite.

Various authors have discussed the problem how to determine the position of the optic axes of a biaxial crystal when the extinction-angles on n different faces have been observed. It has been proved that a unique solution is possible when n = 1, 2, or 4, according as the crystal is orthorhombic, monoclinic, or triclinic. It is assumed (and will be assumed in this paper) that the n faces have general positious. The result is uot true, if one or more of the faces has a specialized position. For instance, the extinction-angle ou a face of an orthorhombic crystal which is parallel to an axis of symmetry will obviously not suffice to determine the position of the optic axes.

The following notes were prepared at the request of Dr. J. E. Stead, F.R.S., the well-known metallographer. Several years ago he had obtained in a tin-antimony-arsenic alloy some remarkable crystals having the form of seganents of spherical shells (fig. 3, p. 9.72 below), and he naturally wished to learn if anything of the same kind was known amongst minerals. The notes contain no new data, but merely collect together facts already known to mineralogists. They may perhaps be of some use to metallographers in calling attention to the various habits of growth met with amongst minerals, since distortions and irregularities of crystalline growth are so frequent amongst metals.