It shall be lawful for the authorities of any State having troops engaged in the Battle of Fort Donelson to enter upon the lands and approaches of the Fort Donelson National Battlefield for the purpose of ascertaining and marking the lines of battle of troops engaged therein: , That before any such lines are permanently designated, the position of the lines and the proposed methods of marking them by monuments, tablets, or otherwise shall be submitted to the Secretary of the Interior and shall first receive the written approval of the Secretary.Provided