Mill"stone` (?), n. 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.
Millstone girt (Geol.), a hard and
coarse, gritty sandstone, dividing the Carboniferous from the
Subcarboniferous strata. See Farewell rock, under
Farewell, a., and Chart of
Geology. -- To see into, or
through, a millstone, to see into or
through a difficult matter. (Colloq.)