The Secretary of the Interior is authorized, in his discretion, at any time within ten years from October 6, 1949, to accept from the State of Washington on behalf of the United States title to any lands in the State of Washington which he deems chiefly valuable for wildlife refuge purposes, and which are equivalent in value to the lands of the United States within the Skagit National Wildlife Refuge, and in exchange therefor to convey by deed on behalf of the United States to the State of Washington the said lands of the United States in the Skagit National Wildlife Refuge.