7Mineral rents
Where the compensation payable upon the redemption of a casualty incident to any feu is fixed under this Act, and the rental of such feu consists in whole or in part of a rental from minerals which are being or have been worked, a sum equal to four per cent. on the capital value of such minerals at the date as at which compensation is to be fixed shall be taken to be the rental of the minerals for the purpose of fixing such compensation: Provided that in ascertaining the compensation for redemption of casualties upon minerals, no minerals shall 'be brought into computation other than minerals from which casualties may be exigible in accordance with existing law.