Berg, Richard B. 1994. Geology of microcrystalline silica (Tripoli) deposits, southern most Illinois. Illinois:Illinois State Geological Survey;
Berg, Richard B. (1994) Geology of microcrystalline silica (Tripoli) deposits, southern most Illinois (รายงานผลงานวิจัย). Illinois State Geological Survey:Illinois.
Berg, Richard B. Geology of microcrystalline silica (Tripoli) deposits, southern most Illinois. Illinois:Illinois State Geological Survey, 1994.
Berg, Richard B. (1994) Geology of microcrystalline silica (Tripoli) deposits, southern most Illinois (รายงานผลงานวิจัย). Illinois State Geological Survey:Illinois.