Upon satisfactory proof of full compliance with the provisions of the laws under which the location, selection, entry, or purchase is made, the locator, selector, entryman, or purchaser shall be entitled to a patent to the land located, selected, entered, or purchased, which patent shall contain a reservation to the United States of the deposits on account of which the lands so patented were withdrawn or classified or reported as valuable, together with the right to prospect for, mine, and remove the same, such deposits to be subject to disposal by the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 sections 121 to 123 of this title 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 [30 U.S.C. 181 et seq.], 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 by the Secretary of the Interior of a bond or undertaking to be filed with him as security for the payment of all damages to 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 reenter 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. Nothing herein contained shall be held to deny or abridge the right to present and have prompt consideration of applications to locate, select, enter, or purchase, under the land laws of the United States, lands which have been withdrawn or classified as phosphate, nitrate, potash, oil, gas, or asphaltic mineral lands, with a view of disproving such classification and securing patent without reservation, nor shall persons who have located, selected, entered, or purchased lands subsequently withdrawn, or classified as valuable for said mineral deposits, be debarred from the privilege of showing, at any time before final entry, purchase, or approval of selection or location, that the lands entered, selected, or located are in fact nonmineral in character.