Other so-called sands and sandstones,
though equally mechanical in their origin, are truly calcareous in
their nature, and are more or less entirely composed of carbonate
of lime. Of this kind are the shell-sand so common on our coasts,
and the coral-sand which is so largely formed in the neighbourhood
of coral-reefs. In these cases the rock is composed of fragments
of the skeletons of shellfish, and numerous other marine animals,
together, in many instances, with the remains of certain sea-weeds
(_Corallines_, _Nullipores_, &c,) which are endowed with the
power of secreting carbonate of lime from the sea-water. Lastly,
in certain rocks still finer in their texture than sandstones,
such as the various mud-rocks and shales, we can still recognise
a mechanical source and origin. If slices of any of these rocks
sufficiently thin to be transparent are examined under the
microscope, it will be found that they are composed of minute
grains of different sizes, which are all more or less worn and
rounded, and which clearly show, therefore, that they have been
subjected to mechanical attrition.
All the above-mentioned rocks, then, are _mechanically-formed_
rocks; and they are often spoken of as "Derivative Rocks," in
consequence of the fact that their particles can be shown to
have been mechanically _derived_ from other pre-existent rocks.
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