nate and oxides of iron, argillaceous material, and chert
or chalcedony. Dolomites when very pure and well crystallized may be
snowy white (e.g. some examples from the eastern Alps), but are commonly
yellow, creamy, brownish or grey from the presence of impurities. They
tend to be crystalline, though on a fine scale, and appear under the
microscope composed of small sharply angular rhombohedra, with a perfect
cleavage and very strong double refraction. They can be often recognized
by this, but are most certainly distinguished from similar limestones or
marbles by tests with weak acid. Dolomite dissolves only very slowly in
dilute hydrochloric acid in the cold, but readily when the acid is
warmed; limestones are freely attacked by the acid in either state.
Magnesian limestones, which contain both dolomite and calcite, may be
etched by exposing polished surfaces for a brief time to cold weak acid;
the calcite is removed, leaving small pits or depressions. The
distribution of the calcite may be rendered more clear by using ferric
chloride solution. This is decomposed, leaving a yellow stain of ferric
hydrate where the calcite occurred. Alternatively, a solution of
aluminium chloride will serve; this precipitates gelantinous alumina on
contact with calcite and the film can be stained with aniline dyes
(Lemberg's solution). The dolomite is not affected by these processes.

Dolomites of compact structure have a higher specific gravity than
limestones, but they very often have a cavernous or drusy character, the
walls of the hollows being lined with small crystals of dolomite with a
pearly lustre and rounded faces. They are also slightly harder, and for
these and other reasons they last better as building stones and wear
better when used for paving or road-mending. Dolomites are rarely
fossiliferous, as the process of dolomitization tends to destroy any
organic remains originally present. As compared with limestones they are
less frequently well bedded, but there are exceptions to this rule. Many
dolomites, particularly those of the north of England, show a very
remarkable concretionary structure. The beds look as if made up of
rounded balls of all sizes from a foot or two in diameter downwards.
Often they are stuck together like piles of shot or bunches of grapes.
They are composed of fibrous radiate calcite crystals, which by some
kind of concretionary action have segregated from the dolomitic material
and grouped themselves