Mesolite was first recognized as a separate species by J. N. Fuchs (1816), being one of the three species into which he subdivided R. J. Haüy's species mesotype (1801). Its individuality has been repeatedly doubted, many workers regarding the mesotype group as an isomorphous series with natrolite and scolecite as the endmembers, and mesolite merely an intermediate isomorphous mixture. The frequent occurrence of intergrowths of mesolite with natrolite or scolecite is largely responsible for this confusion, these intergrowths frequently passing for single minerals, and being adduced as evidence of the occurrence of all intermediate compositions in the supposed isomorphous series. Faroelites, and mixtures containing faroelite, have also been analysed as mesolite from time to time.

In the memoir dealing with the geology of the Broken Hill district, New South Wales, mention is made 1 of a remarkable rock type found in a few places in the vicinity of Broken Hill, which looks not unlike an aplite in the field. A brief description of its appearance in thin section is given. It consists mainly of what appears to be very basic plagioclase. A fair amount of quartz, some epidote and bluishgreen hornblende were noticed, whilst a notable amount of sphene is present.

Since the early work of Dmde and others on the reflection of light by opaque crystalline media, little progress has been made in the direct determination of the indices of refraction and absorption for such substances. Interest in the optical properties of the opaque minerals, however, has been greatly stimulated by the rapid development in the last few years of the methods of ore-microscopy first brought into prominence by Campbell in 1906. Full theoretical discussions have been given recently by Berek, Koenigsberger, and others, but the only quantitative measure feasible in routine microscopy at present is that of the reflectivity.

A statistical study of clay and bauxitic minerals, which consisted of plotting all available analyses of these minerals on two triangular diagrams (figs. 1 and 7) and constructing frequency curves for the cardinal ratios, led to a classification of these minerals on a purely chemical basis. The suggestion is also made that among substances claimed to be minerals we can select only a few as belonging to definite mineral species, and that the others are probably mixtures of these selected minerals, or their hydration products, or colloidal hydrates of variable composition. The distinct crystalline mineral species thus singled out are as follows :

Besides these minerals, other substances, such as termierite, anauxite, allophane, &c., are suggested as probable species, without, however, any clear statistical evidence. The bentonitic minerals, not belonging strictly to the pure hydrous aluminium silicates, are placed in a separate group.

In conclusion, I should like to thank Dr. G. Hickling and Mr. G. S. Mockler for reading the MS. of this paper and for their valuable suggestions.