20 PRINCIPLES OF PALZ ONTOLOGY. 
ing carbonate of lime from the sea-water. Lastly, in cer- 
tain 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 derzved 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 Avenaceous (Lat. arena, 
sand) or S7/iceous 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, after- 
wards 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 amount of clay or 
hydrated silicate of alumina in their composition. Under this 
head come clays, shales, marls, marl-slate, clay-slates, and 
most flags and flagstones. 
B. CHEMICALLY-FORMED Rocks.—In this section are com- 
prised all those Aqueous or Sedimentary Rocks which have 
been formed by chemical agencies. As many of these chemi- 
cal agencies, however, are exerted through the medium of 
living beings, whether animals or plants, we get into this 
section a number of what may be called “ orvganically-formed 
rocks.” These are of the greatest possible importance to the 
palzontologist, as being to a greater or less extent composed 
of the actual remains of animals or vegetables, and it will 
therefore be necessary to consider their character and struc- 
ture in some detail. 
By far the most important of the chemically-formed rocks 
are the so-called Calcareous Rocks (Lat. calx, lime), com- 
prising all those which contain a large proportion of carbonate 
of lime, or are wholly composed of this substance. Carbonate 
