The Secretary of the Interior, in order to encourage and promote development of production of furs in the Territory of Alaska, is authorized to lease to corporations organized under the laws of the United States, or of any state or Territory thereof, citizens of the United States, or associations of such citizens, public lands of the United States in the Territory of Alaska suitable for fur farming, in areas not exceeding six hundred and forty acres, and for periods not exceeding ten years, upon such terms and conditions as he may by general regulations prescribe: Provided, That where leases are given hereunder for islands or lands within the same, such lease may, in the discretion of the Secretary of the Interior, be for an area not to exceed thirty square miles: Provided further, That nothing herein contained shall prevent the prospecting, locating, development, entering, leasing, or patenting of the mineral resources of any lands so leased under laws applicable thereto: And provided further, That this section shall not be held nor construed to apply to the Pribilof Islands, declared a special reservation by section 646 1