CHAP PEER. BY 
ROCKS—continued 
Classification of Derivative Rocks :—I. Mechanically formed Rocks, in- 
cluding Subaérial and AZolian, Sedimentary, and Glacial Rocks (Soil 
and Subsoil, Rock-rubble, Rain-wash, etc., Blown Sand and Dust, 
Laterite, Terra Rossa, Conglomerate, Grit and Sandstone, Greywacké, 
Clay, Till, etc.). II. Chemically formed Rocks—(Stalactites and 
Stalagmites, Tufa, Magnesian Limestone, Rock-salt, Gypsum, 
Siliceous Sinter, Flint, etc., Ironstones). III. Organically derived 
Rocks—(Limestone, Coal, etc., Guano, Coprolites). 
De DERIVATIVE: ROCKS 
THE rocks included under this head are of very diverse origin, 
and show every variety of composition, texture, and structure. 
Some are dominantly siliceous, calcareous, argillaceous, ferru- 
ginous, or carbonaceous ; others are mixtures of many differ- 
ent kinds of material; while a few are composed of one 
mineral substance only. As regards texture, they vary from 
smoothly compact rocks to aggregates of the coarsest kind. 
So, likewise, they exhibit much variety of structure—the large 
majority consisting of fragmental (clastic) materials, while not 
a few are crystalline or subcrystalline. All derivative rocks 
are of epzgene origin, z.e. they have been produced at or near 
the surface of the earth by the action of the various super- 
ficial agents of change—wind, rain, frost, water, etc. Hence 
many have been formed mechanically ; others, again, are due 
to chemical action; while yet others are of organic origin. 
As a rule they are characterised by a more or less pronounced 
bedded arrangement, and hence are often termed collectively 
the “Stratified Rocks.” Furthermore, as water has played 
the most important part in their formation, they are not 
infrequently spoken of as the “Aqueous Rocks.” As some, 
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