Executive Order 13735: Providing an Order of Succession Within the Department of the Treasury [open pdf - 189KB]
"By the authority vested in me as President by the Constitution and the laws of the United States of America, including the Federal Vacancies Reform Act of 1998, as amended, 5 U.S.C. 3345 et seq. (the `Act'), it is hereby ordered that: Section 1. Subject to the provisions of section 3 of this Executive Order, the officers named in section 2, in the order listed, shall act as and perform the functions and duties of the office of Secretary of the Treasury (Secretary) during any period when both the Secretary and the Deputy Secretary of the Treasury have died, resigned, or are otherwise unable to perform the functions and duties of the office of Secretary."
Executive Order 13735; EO 13735
Federal Register: http://www.gpo.gov/fdsys/browse/collection.action?collectionCode=FR
Federal Register (August 17, 2016), v.81 no.159, p.54709