When corundum is colored brown by impurities of iron, it is termed _corundum_, when black by greater quantities of iron, it is _emery_. These varieties are far the commonest form in which corundum occurs, and when ground to finer or coarser powder make the commercial emery. Emery is likely to be found in sands, making so-called “black sands,” where it has accumulated as a result of the weathering to bits corundum-bearing rocks. In some one of its forms, corundum is found in Massachusetts, Connecticut, New York, New Jersey, and all down the Appalachian Mountains, also in Colorado, Montana, California, etc. Bauxite Al₂O₃·2H₂O Occurs in grains, or oolitic or clay-like masses; hardness 1-3; specific gravity 2.5; color white to yellowish-white or reddish-brown. Bauxite never comes in crystals, but is usually in earthy masses, which have resulted from the decomposition of granitic or volcanic rocks, in circumstances where hot alkaline waters were present. This explanation seems to apply especially to the deposits in France, which were first the chief source of the bauxite, and may be applicable to those in Georgia and Alabama. Some of the other deposits, however, do not seem to have had any hot water available, and the deposit appears more like simple decomposition of the underlying rocks by alkaline waters. In many cases bauxite resembles limonite in being a mixture of two or more aluminum oxides with water of crystallization, such as Al₂O₃·H₂O, Al₂O₃·2H₂O and Al₂O₃·3H₂O. This is particularly true of the bauxite which resulted from the decomposition of rocks by surface water. Bauxite is the ore from which aluminum is obtained. The deposits are not large, but the United States has its share of them. It is found in Alabama, Arkansas, Georgia, Missouri, Tennessee, and California. Cryolite Na₃AlF₆ _Ice stone_ Occurs in pseudo-cubic crystals or massive; hardness 2.5; specific gravity 3; color white; luster vitreous; transparent on thin edges. Cryolite is a relatively soft mineral, colorless to white as snow; for which reason, and partly also because it comes mostly from Greenland it is called “ice stone.” It is really monoclinic but the inclination of the c axis is so slight, that, unless examined carefully, the crystals appear to be cubic. Until about 1900 great quantities of this mineral were shipped from West Greenland, and from them the metal aluminum was extracted. When bauxite was discovered, it was found to be considerably cheaper to make the aluminum from that mineral, and now cryolite is no longer sought. Aside from its occurrence in Greenland some cryolite is found in Colorado, near Pike’s Peak. The Arsenic Group