The President of the United States is authorized, upon the joint recommendation of the Secretaries of the Interior and of Agriculture, to add to the Lassen Volcanic National Park, in the State of California, by Executive proclamation, any or all of the lands within sections 3 and 4, township 29 north, range 6 east; and sections 29, 30, 31, 32, 33, 34, 35, and 36, township 30 north, range 6 east, Mount Diablo meridian, not now included within the boundaries of the park: Provided, That no privately owned lands shall be added to the park prior to the vesting in the United States of title thereto.