15 ), and millstones, salt (from a marsh at the east end of the harbour), and gypsum are still exported.
Rockland county quarries considerable trap rock, used mostly for road-making and concrete, and Ulster county has for more than a century produced most of the domestic millstones used in the United States.
The surface soil is clay in which are embedded fragments of siliceous sandstone, used for millstones and constructional purposes; the subsoil is limestone.
The sandstones are used for building, and for millstones and grindstones.
The mineral products of the district also include lignite, copper, manganese, vitriol, lime, gypsum, volcanic stones (used for millstones) and slates.
Millstones and pumice were also exported, but for the former the more gritty lava of Rocca Monfina was later on preferred.
Granite and other hard stones, having but a limited use (for millstones and the like), have the best chance of survival.
The large circular millstones of Roman age worked by horse-power are usually made from slices of granite columns.
The slurry, which is wet enough to flow, is ground between millstones so as to complete the process of comminution begun in the wash mill.
The grinding of the hard rock-like masses of clinker is effected between millstones, or in modern plants in ball-mills, tube-mills and edge-runners.
Minerals produced in small quantities include gypsum, millstones, salt and sandstone, and among those found but not produced (in 1902) in commercial quantities may be mentioned allanite, alum, arsenic, bismuth, carbonite, felspar, kaolin, marble, plumbago, quartz, serpentine and tin.