f concentric rings, on fossils and fragments of
limestone in S. Devon, is known as "orbicular silica" or "beekite,"
having been named after Dr Henry Beeke, dean of Bristol, who first
directed attention to such deposits. Certain pseudomorphs of chalcedony
after datolite, from Haytor in Devonshire, have received the name of
"haytorite." Optical examination of many chalcedonic minerals by French
mineralogists has shown that they are aggregates of various fibrous
crystalline bodies differing from each other in certain optical
characters, whence they are distinguished as separate minerals under
such names as calcedonite, pseudocalcedonite, quartzine, lutecite and
lussatite. Many coloured and variegated chalcedonies are cut and
polished as ornamental stones, and are described under special headings.
Chalcedony has been in all ages the commonest of the stones used by the
gem-engraver.

See AGATE, BLOODSTONE, CARNELIAN, CHRYSOPRASE, HELIOTROPE, MOCHA
STONE, ONYX, SARD and SARDONYX. (F. W. R.*)

CHALCIDICUM, in Roman architecture, the vestibule or portico of a public
building opening on to the forum; as in the basilica of Eumactria at
Pompeii, and the basilica of Constantine at Rome, where it was placed at
one end.

CHALCIS, the chief town of the island of Euboea in Greece, situated on
the strait of the Euripus at its narrowest point. The name is preserved
from antiquity and is derived from the Greek [Greek: chalkos] (copper,
bronze), though there is no trace of any mines in the neighbourhood.
Chalcis was peopled by an Ionic stock which early developed great
industrial and colonizing activity. In the 8th and 7th centuries it
founded thirty town-ships on the peninsula of Chalcidice, and several
important cities in Sicily (q.v.). Its mineral produce, metal-work,
purple and pottery not only found markets among these settlements, but
were distributed over the Mediterranean in the ships of Corinth and
Samos. With the help of these allies Chalcis engaged the rival league of
its neighbour Eretria (q.v.) in the so-called Lelantine War, by which it
acquired the best agricultural district of Euboea and became the chief
city of the island. Early in the 6th century its prosperity was broken
by a disastrous war with the Athenians, who expelled the ruling
aristocracy and settled a cleruchy on the site. Chalcis subsequently
became a member of both the Delian Leagues. In the Hellenistic period it
gained importance as a fo