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~` l ..: <br />REFEREiJCES CITED <br />(1) Allen, A. S., and Paone, J., Surface Subsidence Control, U. S. <br />Bureau of t•lines Report. <br />(2) Obert, Duvall, and i•terrill, Design of Underground Openings in <br />Competent Rock, U. S. Dureau of clines, Bulletin 587, 1960. <br />(3) tJilson, A. H., Research into the Determination of Pillar Size: <br />Part [ an Hypothesis Concerning Pillar Stability, Mining <br />Engineering, June 1972, Val. 131, IJo. 141. <br />(4) Dunrud, C. R., Same Engineering Geologic Factors Controlling Coal <br />t4ine Subsidence in Utah and Colorado, U. S. G. S. Prof. Paper 969, <br />1976. <br /> <br /> <br />(19) <br />