20 PRINCIPLES OF PALEONTOLOGY. 



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 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, 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 " organically -formed 

 rocks" These are of the greatest possible importance to the 

 palaeontologist, 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 



