rock, its component grams
being equally the result of mechanical attrition and having equally
been transported from a distance; and the same is true of the
ordinary sand of the sea-shore, which is nothing more than an
unconsolidated sandstone. 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.
It follows from this that every bed of any mechanically-formed
rock is the measure and equivalent of a corresponding amount of
destruction of some older rock. It is not necessary to enter
here into a minute account of the subdivisions of these rocks, but
it may be mentioned that they may be divided into two principal
groups, according to their chemical composition. In the one group
we have the so-called _Arenaceous_ (Lat. _arena_, sand) or
_Siliceous_ Rocks, which are essentially composed of larger or
smaller grains of flint or silica. In this group are comprised
ordinary sand, the varieties of sandstone and grit, and most
conglomerates and breccias. We shall, however, afterwards see
that some siliceous rocks are of organic origin. In the second
group are the so-called _Argillaceous_ (Lat. _argilla_, clay)
Rocks, which contain a larger or smaller amo