* * * such deposits to be subject to disposal by the United States only as shall be hereafter expressly directed by law: Provided, however, That all mineral deposits heretofore or hereafter reserved to the United States under this Act which are subject, at the time of application for patent to valid and subsisting rights acquired by discovery and location under the mining laws of the United States made prior to the date of the Mineral Leasing Act of February 25, 1920 (41 Stat. 437), shall hereafter be subject to disposal to the holders of those valid and subsisting rights by patent under the mining laws of the United States in force at the time of such disposal. Any person qualified to acquire the reserved deposits may enter upon said lands with a view of prospecting for the same upon the approval of the Secretary of the Interior of a bond or undertaking to be filed with him as security for the payment of all damages of the crops and improvements on such lands by reason of such prospecting, the measure of any such damage to be fixed by agreement of parties or by a court of competent jurisdiction. Any person who has acquired from the United States the title to or the right to mine and remove the reserved deposits, should the United States dispose of the mineral deposits in lands, may re-enter and occupy so much of the surface thereof as may be required for all purposes reasonably incident to the mining and removal of the minerals therefrom; and mine and remove such minerals, upon payment of damages caused thereby to the owner of the land, or upon giving a good and sufficient bond or undertaking therefor in an action instituted in any competent court to ascertain and fix said damages: