Notwithstanding the provisions of any Act of Congress to the contrary, any person who on and after prospects for, mines, or removes by strip or open pit mining methods, any minerals from any land included in a stock raising or other homestead entry or patent, and who had been liable under such an existing Act only for damages caused thereby to the crops or improvements of the entryman or patentee, shall also be liable for any damage that may be caused to the value of the land for grazing by such prospecting for, mining, or removal of minerals. Nothing in this section shall be considered to impair any vested right in existence on .June 21, 19491949-06-21June 21, 19491949-06-21