rmer must
have been of far greater magnitude and frequency than those of recent
times.

See Lacepede, "Eloge historique de Dolomieu," in _Memoires de la
classe des sciences de l'Institut_ (1806); Thomson, in _Annals of
Philosophy_, vol. xii. p. 161 (1808).

DOLOMITE, a mineral species consisting of calcium and magnesium
carbonate, CaMg(CO3)2, and occurring as rhombohedral crystals or large
rock-masses. Analyses of most well-crystallized specimens correspond
closely with the above formula, the two carbonates being present in
equal molecular proportions (CaCO3, 54.35; MgCO3, 45.65%). Normal
dolomite is thus not an isomorphous mixture of calcium and magnesium
carbonates, but a double salt; and any variations in composition are to
be explained by the isomorphous mixing of this double salt with
carbonates of calcium, iron, magnesium, manganese, and rarely of zinc
and cobalt.

[Illustration: FIG. 1.]

In crystalline form dolomite is very similar to calcite, belonging to
the same group of rhombohedral carbonates; the primitive rhombohedron,
_r_ (100), parallel to the faces of which there are perfect cleavages,
has interfacial angles of 73 deg. 45', the angle of the cleavage
rhombohedron of calcite being 74 deg. 55'. A specially characteristic
feature is that this rhombohedron is frequently the only form present on
the crystals (in calcite it is rare except in combination with other
forms); the faces are also usually curved (fig. 1), sometimes to an
extraordinary degree giving rise to saddle-shaped crystals (fig. 2).
Crystals with plane faces are usually twinned, there being an
interpenetration of two rhombohedra with the vertical axes parallel. The
secondary twin-lamination, parallel to the obtuse rhombohedron _e_
(110), so common in calcite, does not exist in dolomite. In the degree
of symmetry possessed by the crystals there is, however, an important
difference between calcite and dolomite; the former has the full number
of planes and axes of symmetry of a rhombohedral crystal, whilst the
latter is hemihedral with parallel faces, having only an axis of triad
symmetry and a centre of symmetry. This lower degree of symmetry, which
is the same as that of dioptase and phenacite, is occasionally shown by
the presence of an obliquely placed rhombohedron, and also by the want
of symmetry in the etching and elasticity figures on the faces of the

primitive rhombohedron.

[Illustration: FIG. 2.]

Dolomite is bot