to
the _Foraminifera_. In other cases, the matrix is more or less
crystalline, and when this crystallisation has been carried to
a great extent, the original organic nature of the rock may be
greatly or completely obscured thereby. Thus, in limestones which
have been greatly altered or "metamorphosed" by the combined
action of heat and pressure, all traces of organic remains become
annihilated, and the rock becomes completely crystalline throughout.
This, for example, is the case with the ordinary white "statuary
marble," slices of which exhibit under the microscope nothing but
an aggregate of beautifully transparent crystals of carbonate
of lime, without the smallest traces of fossils. There are also
other cases, where the limestone is not necessarily highly
crystalline, and where no metamorphic action in the strict sense
has taken place, in which, nevertheless, the microscope fails
to reveal any evidence that the rock is organic. Such cases are
somewhat obscure, and doubtless depend on different causes in
different instances; but they do not affect the important
generalisation that limestones are fundamentally the product
of the operation of living beings. This fact remains certain;
and when we consider the vast superficial extent occupied by
calcareous deposits, and the enormous collective thickness of
these, the mind cannot fail to be impressed with the immensity of
the period demanded for the formation of these by the agency of
such humble and often microscopic creatures as Corals, Sea-lilies,
Foraminifers, and Shell-fish.

Amongst the numerous varieties of limestone, a few are of such
interest as to deserve a brief notice. _Magnesian limestone_
or _dolomite_, differs from ordinary limestone in containing
a certain proportion of carbonate of magnesia along with the
carbonate of lime. The typical dolomites contain a large proportion
of carbonate of magnesia, and are highly crystalline. The ordinary
magnesian limestones (such as those of Durham in the Permian
series, and the Guelph Limestones of North America in the Silurian
series) are generally of a yellowish, buff, or brown colour,
with a crystalline or pearly aspect, effervescing with acid much
less freely than ordinary limestone, exhibiting numerous cavities
from which fossils have been dissolved out, and often assuming
the most varied and singular forms in consequence of what is
called "concretionary action." Examination with the microscope
shows that thes