The mode of occurrence and origin of the various forms of titanium dioxide as rock constituents have received attention at the hands of many petrologists, both on the continent and in our own country. Among the works of the former dealing with this subject, Thürach's exhaustive treatise on microscopic rock-forming rutile, anatase, and brookite, is undoubtedly the most important : in England, Mr. Maynard Hutchings has described in great detail the occurrence of rutile and anatase both as constituents of clays and slates, and as a product of contact metamorphism in the Lower Coniston Flags in the neighbourhood of the Shap granite; Mr. J. J. H. Teall has also described rutile needles and anatase crystals from clays of various ages, and anatase from the Cleveland iron-ore; Mr. Allan Dick has mentioned the occurrence of rutile in the Bagshot Sands ; and Mr. H. H. Thomas has lately described these minerals from the Budleigh Salterton Pebble-bed of Devonshire.