That all mineral lands shall be exempted from the grants made by this act. But if sections sixteen and thirty-six, or any subdivision or portion of any smallest subdivision thereof in any township shall be found by the department of the interior to be mineral lands, said states are hereby authorized and empowered to select, in legal subdivisions, an equal quantity of other unappropriated lands in said states, in lieu thereof, for the use and benefit of the common schools of said states.