30 U.S.C. § 2

On and after July 1, 1916 , in the absence of the Director of the United States Bureau of Mines the assistant director of said bureau shall perform the duties of the director during the latter’s absence, and in the absence of the Director and of the Assistant Director of the United States Bureau of Mines the Secretary of the Interior may designate some officer of said bureau to perform the duties of the director during his absence.