There are but few commercial deposits of the unusual ores which present more interesting geologic as well as economic features than do the deposits of natural zirconium oxide in Brazil. The Caldas region (visited in 1915 by the writer) in which these zirconia deposits occur, is situated partly in the State of Minas Geraes and partly in the State of Sao Paulo, approximately 130 miles north of the city of Sao Paulo. It is a mountainous plateau, the main elevation of which is about 3,600 feet. The surface is undulating, presenting differences in level of from 300 to 600 feet. The whole area is bounded on all sides by ridges rising abruptly from 600 to 1,200 feet above the general level and forming a roughly elliptical enclosure with a major axis of approximately 20 miles in length and a minor axis of 15 miles. This peculiar arrangement of the higher ridges is very significant when coupled with the fact that the predominant rock of the plateau is a phonolite and the presence of highly mineralized thermal water of considerable medicinal value.[100] No thorough geological survey has been made of this area with a view to determining the origin of the zirconia. The character of the ore, however, and the formation, seems to point to pneumatolitic agencies. A careful study of the relationship of the large masses of coarsely crystalline nephelite-syenite in this area, with pronounced segregations of eudialyte, might throw some light upon this subject. [100] DERBY, O. A., “Nepheline-rock in Brazil:” _Quart. Jour. Geol. Soc._, August, 1887. Zirconia ore can be roughly divided into two classes: First, alluvial pebbles ranging in size from one-half inch to three inches in diameter, generally carrying about 90 per cent. to 93 per cent. zirconium oxide. These pebbles, known as favas and having a specific gravity ranging from 4.8 to 5.2, are found along small stream beds and on the talus slopes of low ridges. Second, zirconia ore proper, or zirkite (a trade name), which ranges in shade from a light gray to a blue black, the lighter colored material carrying a higher percentage of zirconium silicate, as evidenced by analyses, which in some cases show a minimum of 73 per cent. zirconium oxide. The blue-black ore generally carries from 80 per cent. to 85 per cent. zirconium oxide. By careful sorting, however, a uniform grade carrying about 80 per cent. is produced.