CHIEF DIVISIONS OF THE AQUEOUS ROCKS. 1 3 



general sense to signify " any group of rocks which have some char- 

 acter in common, whether of origin, age, or composition " (Lyell) ; 

 so that we may speak of stratified and unstratified formations, aqueous 

 or igneous formations, fresh-water or marine formations, and so on. 



Chief Divisions of the Aqueous Rocks. 



The Aqueous Rocks may be divided into two great sections, the 

 Mechanically-formed and the Chemically-formed, including under 

 the last head all rocks which owe their origin to vital action, as well 

 as those produced by ordinary chemical agencies. It must not be 

 forgotten, however, that such a division, though convenient in prac- 

 tice, is largely artificial. Thus many organically-formed rocks are to 

 a large extent the product of mechanical action, since, though con- 

 sisting of the skeletons of organisms, their component materials have 

 been mechanically broken down and transported by water. More- 

 over, no sharp line of demarcation can be drawn between the above 

 two groups of rocks, innumerable transitions existing between rocks 

 which are purely mechanical in origin and those which are the direct 

 result of vital action. 



A. Mechanically-formed Rocks. — These are all those Aqueous 

 Rocks of which we can obtain proofs that their particles have been 

 mechanically transported to their present 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 microscopical 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 the equivalent 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 



