Sec. 18.5
That upon relinquishment to the United States, filed in the General Land Office within six months after the approval of this Act, of all right, title, and interest claimed and possessed prior to July 3, 1910, and continuously since by the claimant or his predecessor in interest under the preexisting placer mining law to any oil or gas bearing land upon which there has been drilled one or more oil or gas wells to discovery embraced in the Executive order of withdrawal issued September 27, 1909, and not within any naval petroleum reserve, and upon payment as royalty to the United States of an amount equal to the value at the time of production of one-eighth of all the oil or gas already produced except oil or gas used for production purposes on the claim, or unavoidably lost, from such land, the claimant, or his successor, if in possession of such land, undisputed by any other claimant prior to July 1, 1919, shall be entitled to a lease thereon from the United States for a period of twenty years, at a royalty of not less than 12½ per centum of all the oil or gas produced except oil or gas used for production purposes on the claim, or unavoidably lost: *Provided, *That not more than one-half of the area, but in no case to exceed three thousand two hundred acres, within the geologic oil or gas structure of a producing oil or gas field shall be leased to any one claimant under the provision of this section when the area of such geologic oil structure exceeds six hundred and forty acres. Any claimant or his successor, subject to this limitation, shall, however, have the right to select and receive the lease as in this section provided for that portion of his claim or claims equal to, but not in excess of, said one-half of the area of such geologic oil structure, but not more than three thousand two hundred acres.