The Secretary of the Interior is authorized and empowered, under such rules and regulations as he may prescribe, to establish rental rates for the lease of reserved lands of the United States situate within the exterior boundaries of Boulder City, Nevada, and, without prior advertising, to enter into leases therefor at not less than rates so established and for periods not exceeding fifty-three years from the date of such leases: , That all revenues which may accrue to the United States under the provisions of such leases shall be deposited in the Treasury and credited to the Colorado River Dam fund established by .section 617a of this titleProvided