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BOOK VII
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159
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According to Xenophon it was made into millstones by the inhabitants of that district and brought to Babylon.
Lapis molaris which melts in a fire is called μύλη by Aristotle and Varro who has described the revolving millstones of Vulsiniis. The stones that do not melt are called μυλίαν. Theophrastus and Strabo called the stones that melted μυλίαν, physicians called them μυλίτης, and those who followed the Latin nomenclature molaris. Certain Greek writers called them pyrites since
fire can be drawn from them just as from hard pyrite. Usually these
stones are quite hard. The black gem that burns the fingers when heated
by rubbing is evidently pyrites. Since this stone is unaffected
by rain, heat or cold it can be used as a building stone. Millstones
are made from it whence comes the name. According to Pliny there are
certain millstones belonging to this genus that, when polished with
flint and viewed from a distance, have the appearance of ophites. Commonly molaris is
black and sometimes spotted since it has formed from a liquid material
of this color. Actually if a reddish stone is melted to a liquid the
part that will flow becomes black when it congeals. Aristotle writes
that this material is similar to limestone and of course limestone can
be used as a building stone but as Pliny writes, this has a certain
natural unctuousness. When free of pyrite it is used for millstones. It
reduces swellings that may have come on any external part of the body.
But to return to arenaceous
rocks. Sand can be obtained from them when they are crushed finely but
this is not the only source of sand. It is also obtained from the
stones used in the manufacture of glass as I have explained in Book V,
from stony marl, from unctuous limestone, and, in fact, from almost all
stones and rocks that have a similar hardness. They make a variety of
sand that is called carbunculus from bituminous earth that has been changed into a rather soft rock.
There
are three genera of sand. The first is sedimentary sand that is dug
from pits such as those at Pelgrana, Misena, and Glogova, Lygius. The
second is fluvial that is found along rivers and streams. The third,
marine that is found on beaches and seashores. Pliny describes sand
that was obtained from the shallows of the Adriatic Sea when exposed
during the summer. Sands vary in color according to the color of the
rocks from which they are derived. That from Pelgrana is white, that
from Glogova, yellow. The sand from Mt. Peribolos, Rome, is a golden
color and for that reason they have come to call the mountain Motorius,
i.e., golden mountain. Fluvial sand is usually gray although red,
black and other colored sands do occur.
Some
sand is fine, some coarse. The fine usually comes from pits. The
fluvial and marine is usually coarse and mixed with gravel that must be
separated with a sieve. It may even contain a large amount of coarse
gravel. The sand that is to be mixed with lime must be free, not only
of fine and coarse gravel but also of all earth and congealed juices.
Earth affects the tenacity and toughness of lime and destroys its power
to hold
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