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.