Provided always, That nothing in this Act contained shall extend the Mineral Laws, Usages, and Customs herein-before mentioned, and by this Act defined and amended, over any Lands or Hereditaments within any of the Manors or Liberties herein-before mentioned which are not now subject to Mining Customs, nor be held to subject or make liable any Lands or Hereditaments to the said Customs which are now exempt therefrom, nor to give Authority to search for Mines and Veins of Lead Ore in or upon any Lands or Hereditaments over which the Mineral Customs of the saidKing’s Field, or of the said Manors or Liberties, or some of them respectively, have not heretofore extended.