THE FOSSILIFEROUS ROCKS. 21 



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 fhe 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 palaeon- 

 tologist, 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 structure 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 

 of lime is soluble in water holding a certain amount of car- 

 bonic acid gas in solution; and it is, therefore, found in larger 

 or smaller quantity dissolved in all natural waters, both fresh 



