About 70 per cent of the domestic mica comes from North Carolina and 25 per cent from New Hampshire. The deposits are small and irregular, and mining operations are small and scattered. These conditions are largely responsible for the heterogeneous nature of the American product. It is hardly possible for any one mine to standardize and classify its product, although progress was made in this direction during the war by the organization of associations of mica producers. This lack of standardization and classification is a serious handicap in competition with the standard grades and sizes which are available in any desired amounts from foreign sources. For ground mica, the domestic production exceeds in tonnage the total world production of sheet mica, and is adequate for all demands. GEOLOGIC FEATURES Mica is a common rock mineral, but is available for commerce only in igneous dikes of a pegmatite nature, where the crystallization is so coarse that the mica crystals are exceptionally large. Muscovite mica occurs principally in the granitic pegmatite dikes. The phlogopite mica of Canada occurs in pyroxenite dikes. The distribution of mica within the dikes is very erratic, making predictions as to reserves hazardous. The associated minerals, mainly quartz and feldspar, are ordinarily present in amounts greater than the mica. Also, individual deposits are likely to be small. For these reasons mining operations cannot be organized on a large scale, but are ordinarily hand-to-mouth operations near the surface. A large amount of hand labor is involved, and the Indian deposits are favored by the cheapness of native labor. The output of a district is from many small mines rather than from any single large one. Pegmatites which have been subjected to dynamic metamorphism are often not available as a source of mica, because of the distortion of the mica sheets. The mining of a mica is facilitated by weathering, which softens the associated feldspar, making it an easier task to take out the mica blocks. On the other hand, iron staining by surface solutions during weathering may make the mica unfit for electrical and certain other uses. Scrap or ground mica is obtained as a by-product of sheet mica and from deposits where the crystals are not so well developed. Black mica (biotite) and chlorite minerals, which are soft and flexible but not elastic and are found extensively developed in certain schists, have been used to a limited extent for the same purposes. MONAZITE (THORIUM AND CERIUM ORES) ECONOMIC FEATURES