When a quantum, satisfactory to the Secretary of the Interior, of the privately owned lands lying within the area hereinafter described shall have been acquired and transferred to the United States for park purposes, without expense to the Federal Treasury, such areas are dedicated and set apart as a national park for the benefit and enjoyment of the people, under the name of the Badlands National Park: Provided, That the State of South Dakota shall have first constructed the highways hereinafter described.