form of stalactites
calcite is of extremely common occurrence. Each stalactite usually consists
of an aggregate of radially arranged crystalline individuals, though
sometimes it may consist of a single individual with crystal faces
developed at the free end. Onyx-marbles or Oriental alabaster (see
ALABASTER) and other stalagmitic deposits also consist of calcite, and so
do the allied deposits of travertine, calc-sinter or calc-tufa.

The modes of occurrence of calcite are very varied. It is a common gangue
mineral in metalliferous deposits, and in the form of crystals is often
associated with ores of lead, iron, copper and silver. It is a common
product of alteration in igneous rocks, and frequently occurs as
well-developed crystals in association with zeolites lining the
amygdaloidal cavities of basaltic and other rocks. Veins and cavities in
limestones are usually lined with crystals of calcite. The wide
distribution, under various conditions, of crystallized calcite is readily
explained by the solubility of calcium carbonate in water containing carbon
dioxide, and the ease with which the material is again deposited in the
crystallized state when the carbon dioxide is liberated by evaporation. On
this also depends the formation of stalactites and calc-sinter.

Localities at which beautifully crystallized specimens of calcite are found
are extremely numerous. For beauty of crystals and variety of forms the
haematite mines of the Cleator Moor district in west Cumberland and the
Furness district in north Lancashire are unsurpassed. The lead mines of
Alston in Cumberland and of Derbyshire, and the silver mines of Andreasberg
in the Harz and Guanajuato in Mexico have yielded many fine specimens. From
the zinc mines of Joplin in Missouri enormous crystals of golden-yellow and
amethystine colours have been recently obtained. At all the localities here
mentioned the crystals occur with metalliferous ores. In Iceland the mode
of occurrence is quite distinct, the mineral being here found in a cavity
in basalt.

The quarry, which since the 19th century has supplied the famous
Iceland-spar, is in a cavity in basalt, the cavity itself measuring 12 by 5
yds. in area and about 10 ft. in height. It is situated quite close to the
farm Helgustadir, about an hour's ride from the trading station of
Eskifjordur on Reydar Fjordur, on the east coast of Iceland. This cavity
when first found was filled with pure crystallized masses and enor