Of the various kinds of material abraded, each calls for different grades and kinds of abrasives, and for variation in the above factors in the use of these abrasives in order to insure most efficient use. Consequently, it is almost impossible to determine arbitrarily the uses for which each of the various abrasive materials is essential. This much, at least, seems certain—that for finishing and polishing glass, particularly optical glass and plate glass, there is as yet no general agreement that satisfactory substitutes are available for the better grades of Turkish and Greek emery, although experiments in manufacture and use of suitable artificial abrasives have been successful.
GEOLOGICAL DISTRIBUTION
The known emery deposits are products of magmatic differentiation or of regional metamorphism, or of combined contact and regional metamorphism of limestone, presumably argillaceous, and of argillaceous sediments. A study of certain individual deposits, therefore, makes possible some forecast as to future supplies in some regions, particularly those in which the emery deposits are intimately related to certain beds in metamorphic sedimentary formations in close proximity to igneous rocks. The emery bodies are, however, as a rule, spotted or irregularly distributed, and reliable estimates of reserves are difficult.
Corundum, in a number of associations, is an original constituent of a great variety of igneous rocks, such as peridotites, anorthosites, syenites, nepheline syenites, and syenite pegmatites. It is also abundantly found in regionally metamorphosed rocks and in contact metamorphic zones, occurring in serpentines, mica schists, quartz schists, and crystalline limestones. A third important source of corundum is alluvial deposits. Corundum is not a characteristic or essential constituent in any of these types of rocks and is present in alluvial deposits in restricted localities only. Furthermore, its distribution and its concentration, when present, are irregular and unsystematic, and there are, therefore, no geologic guides by which future supplies can be forecast without intensive study of each individual occurrence.
GEOGRAPHIC DISTRIBUTION
The chief deposits of emery and corundum in the United States are in the eastern seaboard or Appalachian states.
The emery deposits of Chester, Massachusetts, are in a narrow band less than 500 feet wide that has been traced for nearly five miles. The Chester deposits have been worked at various times since the eighties and up to 1913.
Emery deposits near Peekskill, New York, are associated with igneous rocks in an area of 20 to 25 square miles. These deposits have been worked since 1889. Some of the material mined is a true emery, that is, an intimate mixture of corundum and magnetite, but most of it is largely a mixture of spinel and magnetite, which, while not a true emery, makes an excellent abrasive. In 1916 and 1917, the annual output of ore was approximately 15,000 tons.
In the vicinity of Whittle, Pittsylvania County, Virginia, spinel emery, somewhat like the New York emery, but containing more corundum, is abundant. The deposits in this region have already produced considerable emery and may be counted on for a large supply.
Corundum is associated with a serpentine belt extending through Lancaster, Chester, Delaware, Montgomery, and Bucks counties, Pennsylvania, and through adjoining counties in Delaware and Maryland. The deposits in this region do not seem to be of commercial importance.