The company shall not be entitled to any mines of coal, ironstone, slate, or other minerals under any land purchased by them, except only such parts thereof as shall be necessary to be dug or carried away or used in the construction of the works, unless the same shall have been expressly purchased: and all such mines, excepting as aforesaid, shall be deemed to be excepted out of the conveyance of such lands, unless they shall have been expressly named therein and conveyed thereby.