Volga Occurrence

F1 < BROWN, C. E., 1967, FLUORITE IN CRYSTAL-LINED VUGS IN THE MAQUOKETA SHALE AT VOLGA, CLAYTON COUNTY, IOWA: AMERICAN MINERALOGIST, V. 52, P. 1735-1750.F2 < WORL, R. G., VAN ALSTINE, R. E., AND HEYL, A. V., 1974, FLUORITE IN THE UNITED STATES, EXCLUSIVE OF HAWAII: U.S. GEOLOGICAL SURVEY MAP MR-60, SCALE L:3,168,000.F3 < HEYL, A. V., AND WEST, W. S., 1982, OUTLYING MINERAL OCCURRENCES RELATED TO THE UPPER MISSISSIPPI VALLEY MINERAL DISTRICT, WISCONSIN, IOWA, ILLINOIS, AND MINNESOTA: ECONOMIC GEOLOGY, V. 77, P. 1803-1817.F4 < BROWN, C.E., 1966, PHOSPHATE DEPOSITS IN THE BASAL BEDS OF THE MAQUOQUETA SHALE NEAR DUBUQUE, IOWA, IN GEOLOGICAL SURVEY RESEARCH 1966: U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 550-B, P. B152-B158.F5 < BROWN, C. E., 1974, PHOSPHATIC ZONE IN THE LOWER PART OF THE MAQUOKETA SHALE IN NORTHEAST IOWA: U.S. GEOLOGICAL SURVEY JOURNAL OF RESEARCH, V. 2, NO. 2, P. 219-232.