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• <br />NE~~ U. S. BUREAU OF MINES CRITERIA <br />From Report RI-8507 (November, 19S0 ) <br />~o.~ <br />• u <br />m <br />c <br />U <br />O 10 <br />w <br />w <br />U <br />F- <br />[t <br />4 <br />1 <br />• <br />6 <br />~~ 100 <br />FRE~UENCY~ Nz <br />