Provided always, and be it further enacted, That nothing herein contained shall extend or be construed to empower or enable the Commissioners of His Majesty's Woods, Forests, and Land Revenues, or the Surveyor General of His Majesty's Woods and Forests for the Time being, to grant in Exchange, or to make Sale of any open or accustomed Marl Pits, or any Bogs or Lands from which Peat has been usually gotten for Fuel, or any Gravel Pits now open, situate in or within any of the said Forests, or any Lands, by the Exchange or Sale whereof the Owners or Occupiers of any Estates adjoining or contiguous to the said Forests shall be deprived of their usual or accustomed Supply of Marl or Peat from the said Forests.