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.