All the commoner limestones, in fact, from
the Lower Silurian period onwards, can be easily proved to be
thus _organic_ rocks, if we investigate weathered or polished
surfaces with a lens, or, still better, if we cut thin slices
of the rock and grind these down till they are transparent. When
thus examined, the rock is usually found to be composed of
innumerable entire or fragmentary fossils, cemented together
by a granular or crystalline matrix of carbonate of lime (figs.
11 and 12). When the matrix is granular, the rock is precisely
similar to chalk, except that it is harder and less earthy in
texture, whilst the fossils are only occasionally referable 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.
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