(1) When the table of revenue-rates for any area has been finally published, the rates specified therein shall remain in force for a period of thirty years.
     (2) Notwithstanding anything contained in sub-section (1),--
         (a) revenue-rates may be altered or revised in any year after the expiry of every ten years from the date on which the table of revenue-rates was introduced, in such manner and to such extent as may be prescribed;
         (b) when the circumstances of a local area are such that a fresh determination of the revenue-rates is in the opinion of the Administrator inexpedient, he may extend the term of settlement by such further period as he may think necessary.