The State Treasurer shall invest all sinking fund monies in direct obligations of the United States of America or in
certificates of deposits from banks in the State of Oklahoma acceptable as depositories by the State Treasurer when such certificates of deposits are secured by acceptable collateral and yield as much or more than direct obligations of the United States of America, to mature in time to meet the principal and interest payments on the 1961 Building Bonds, which earnings shall be deposited in the sinking fund. Laws 1961, p. 466, § 10.