8 INTRODUCTION. 



site. Thus, if we examine a piece of conglomerate or pudding- 

 stone, we find it to be composed of a number of rounded 

 pebbles embedded in an enveloping paste or matrix. The 

 pebbles are worn and rounded, and thus show that they have 

 been subjected to much mechanical attrition, whilst they 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 micro- 

 scopical examination would reveal that their edges were all 

 worn and rounded. It follows from this that the mechanically- 

 formed Aqueous Rocks 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 Rocks." Every bed, 

 therefore, of any mechanically-formed rock, is an exact equiva- 

 lent of a corresponding amount of destruction of some older 

 rock. 



The Mechanically-formed Rocks may be divided into the 

 two groups of the Arenaceous or Siliceous Rocks, and the 

 Argillaceous or Aluminous Rocks. In the Arenaceous group 

 are those Aqueous Rocks which are mainly composed of 

 smaller or larger grains of flint or silica. The chief varieties 

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

 conglomerates and breccias. In the Argillaceous, group are 

 those Aqueous Rocks 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 ROCKS. In this section are com- 

 prised all those Aqueous Rocks which have been formed by 

 chemical agencies. As many of these chemical agencies, how- 

 ever, 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 import- 

 ant of the Chemically-formed Rocks are the so-called Calcare- 

 ous Rocks, comprising all those which contain a large propor- 

 tion of carbonate of line, or are wholly made up of this substance. 

 We may also shortly notice coal and gypsum. 



Chalk is merely a limestone which is soft and pulverulent, 

 with an earthy fracture. It is nearly pure carbonate of lime, 

 and is to a great extent an organically-formed rock, consisting 

 mainly of the minute calcareous shells of Foraminifcra, with 

 the calcareous shells of molluscs, sea urchins, sea-mosses, and 



