When title to such lands as may be determined by the Secretary of the Interior as necessary for recreational park purposes within the boundaries to be determined by him within the area of approximately one million five hundred thousand acres, in the counties of Brewster and Presidio, in the State of Texas, known as the "Big Bend" area, shall have been vested in the United States, such lands shall be established, dedicated, and set apart as a public park for the benefit and enjoyment of the people and shall be known as the "Big Bend National Park": Provided, That the United States shall not purchase by appropriation of public moneys any land within the aforesaid area, but such lands shall be secured by the United States only by public and private donations.