eral
obtainable in large pieces of perfect transparency. Owing to the strong
double refraction and the consequent wide separation of the two polarized
rays of light traversing the crystal, an object viewed through a cleavage
rhombohedron of Iceland-spar is seen double, hence the name
doubly-refracting spar. Iceland-spar is extensively used in the
construction of Nicol's prisms for polariscopes, polarizing microscopes and
saccharimeters, and of dichroscopes for testing the pleochroism of
gem-stones.

Chemically, calcite has the same composition as the orthorhombic aragonite
(_q.v._), these minerals being dimorphous forms of calcium carbonate.
Well-crystallized material, such as Iceland-spar, usually consists of
perfectly pure calcium carbonate, but at other times the calcium may be
isomorphously replaced by small amounts of magnesium, barium, strontium,
manganese, zinc or lead. When the elements named are present in large
amount we have the varieties dolomitic calcite, baricalcite,
strontianocalcite, ferrocalcite, manganocalcite, zincocalcite and
plumbocalcite, respectively.

Mechanically enclosed impurities are also frequently present, and it is to
these that the colour is often due. A remarkable case of enclosed
impurities is presented by the so-called Fontainbleau limestone, which
consists of crystals of calcite of an acute rhombohedral form (fig. 3)
enclosing 50 to 60% of quartz-sand. Similar crystals, but with the form of
an acute hexagonal pyramid, and enclosing 64% of sand, have recently been
found in large quantity over a wide area in South Dakota, Nebraska and
Wyoming. The case of hislopite, which encloses up to 20% of "green earth,"
has been noted above.

In addition to the varieties of calcite noted above, some others, depending
on the state of aggregation of the material, are distinguished. A finely
fibrous form is known as satin-spar (_q.v._), a name also applied to
fibrous gypsum: the most typical example of this is the snow-white
material, often with a rosy tinge and a pronounced silky lustre, which
occurs in veins in the Carboniferous shales of Alston Moor in Cumberland.
Finely scaly varieties with a pearly lustre are known as argentine and
aphrite (German _Schaumspath_); soft, earthy and dull white varieties as
agaric mineral, rock-milk, rock-meal, &c.--these form a transition to
marls, chalk, &c. Of the granular and compact forms numerous varieties are
distinguished (see LIMESTONE and MARBLE). In the