In order to consolidate, on the Blue Ridge Parkway and the Natchez Trace Parkway, the land forming each such parkway, to adjust ownership lines, and to eliminate hazardous crossings of and accesses to these parkways, the Secretary of the Interior is authorized to acquire, by purchase or exchange, land and interests in land contiguous to the parkways. In consummating exchanges under this section, the Secretary may transfer parkway land, interests therein, and easements: Provided, That the property rights so exchanged shall be approximately equal in value.