IV. Summary: 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: Pyrophyllite H₂O.Al₂O₃.4SiO₂ Kaolinite and its isomers 2H₂O.Al₂O₃.2SiO₂ Diaspore H₂O.Al₂O₃ Gibbsite and its isomers 3H₂O.Al₂O₃

Besides these minerals, other substances, such as termierite, anauxite, allophane, etc., 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.