The terms so fixed shall be operative for 20 years from the date of the grant. Upon the expiration of such 20-year period and each succeeding period of 20 years the commission or other duly constituted authority, may prescribe for each ensuing period of 20 years a different royalty rate or rates to be paid as a condition of the continued exercise of the grant.

(Added by Stats. 1941, Ch. 548.)