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millstone

One of two circular stones used for grinding grain or other substance. No man shall take the nether or the upper millstone to pledge. (Deut. Xxiv. 6)

The cellular siliceous rock called buhrstone is usually employed for millstones; also, some kinds of lava, as that Niedermendig, or other firm rock with rough texture. The surface of a millstone has usually a series of radial grooves in which the powdered material collects.

(Science: geology) Millstone girt, a hard and coarse, gritty sandstone, dividing the Carboniferous from the subcarboniferous strata. See farewell rock, under Farewell, and chart of geology. To see into, or through, a millstone, to see into or through a difficult matter.