together in this way. Other concretions from
these beds resemble bunches of corals, tufts of plants, or present
various strange imitative forms.

Dolomite, unlike calcite, is not secreted by marine animals to build up
the hard parts of their skeletons, and it is generally agreed also that
dolomite is only very rarely and under exceptional conditions deposited
directly from solution in water. On the other hand, there is much
evidence to show that limestones may absorb or be partly replaced by
magnesium carbonate, and the double salt dolomite substituted for
calcite by one of those processes which are described as "metasomatic."
Thus the Carboniferous limestones of various parts of Britain pass into
dolomites along lines of joint, fissure or fault, or occasionally along
certain bedding planes. At the same time the rock becomes crystalline,
its minute structure is altered, its fossils are effaced, and as
dolomite has a higher specific gravity than limestone, contraction
results and cavities are formed. The prevalence of crystalline,
concretionary and drusy structures in dolomite can thus be simply
explained. The process may actually be studied in many "magnesian
limestones," in which by means of the microscope we may trace the
gradual growth of dolomite crystals taking place simultaneously with the
destruction of the original features of the limestone. Recent
investigations in coral reefs show that these changes are going on at
the present day at no considerable depths and in rocks which have not
long consolidated.

All this goes to prove that the double carbonate of calcium and
magnesium is under certain conditions a more stable salt than either of
the simple carbonates, and that these conditions recur in nature with
considerable frequency. Experiments have proved that at moderately high
temperatures (100 deg. to 200 deg. C.) solutions of magnesium salts will
convert calcite into dolomite in the laboratory, and that aragonite is
even more readily affected than calcite. The analogy with dolomitization
of limestones is strong but not complete, as the latter process must
take place at ordinary temperatures and approximately under atmospheric
pressures. No completely satisfactory explanation of the change, from
the standpoint of the geologist, has as yet been advanced, though much
light has been thrown upon the problem. Many limestones are rich in
aragonite, but this in course of time tends to recrystallize as calcite.
Magn