CHIEF DIVISIONS OF THE AQUEOUS ROCKS. 13 



have been mechanically transported for a greater or less 

 distance from the rock of which they originally formed part. 

 In the case of an ordinary sandstone, the component grains 

 of sand are equally the result of mechanical attrition, and 

 have been equally transported from a distance. In the case 

 of still finer rocks, such as shale, the particles have been so 

 much water -worn that their source cannot be recognised, 

 though a microscopical examination would reveal that their 

 edges were all worn and rounded. It follows from this that 

 the mechanically-formed Aqueous Eocks are such as can be* 

 proved to have been derived from the abrasion of other pre- 

 existent rock : hence they are often spoken of as " Derivative 

 Eocks." Every bed, therefore, of any mechanically-formed 

 rock, is an exact equivalent of a corresponding amount of 

 destruction of some older rock. 



The mechanically-formed Eocks may be divided into the 

 two groups of the Arenaceous or Siliceous Eocks, and the 

 Argillaceous or Aluminous Eocks. In the Arenaceous group 

 are those Aqueous Eocks which are mainly composed of 

 smaller or larger grains of flint or silica. The chief varie- 

 ties are the various kinds of sand and sandstone, grits, and 

 most conglomerates and breccias. In the Argillaceous group 

 are those Aqueous Eocks which contain a certain amount of 

 clay or hydrated silicate of alumina. Under this head come 

 clays, shales, marls, clay-slate, and most flags or flag-stones. 



B. CHEMICALLY-FORMED EOCKS. In this section are com- 

 prised all those Aqueous Eocks which have been formed by 

 chemical agencies. As many of these chemical 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. The 

 most important of the Chemically-formed Eocks are the so- 

 called Calcareous Eocks, comprising all those which contain 

 a large proportion of carbonate of lime, or are wholly made 

 up of this substance ; but there are other rocks, of different 

 composition, which are formed by chemical or organic agency, 

 and which may be briefly noticed. 



As to the origin of the so-called Calcareous Eocks (Lat. 

 calx, lime), carbonate of lime is soluble in water holding 



