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56 STRUCTURAL AND FIELD GEOLOGY 
however, show no trace of aqueous action, while certain 
others are not stratified or arranged in layers, we may 
designate the class by the more comprehensive term of 
Derivative Rocks. For, as we shall learn, all the rocks in 
question are composed of materials derived from the breaking- 
up and disintegration of pre-existing minerals and rocks 
by epigene agents, and from the débris of plants and animals. 
Various systems of classification have been adopted for 
the Derivative Rocks, none of which can be said to be quite 
satisfactory. Perhaps as convenient a system as any is that 
which is based on the geological origin of the various rocks. 
At all events, it has the merit of directing the student’s 
attention to the action of the various epigene agents of 
change which are so ceaselessly employed in modifying the 
crust of the globe. We shall therefore group the series 
under these three heads :—MECHANICALLY FORMED, CHEMI- 
CALLY FORMED, and ORGANICALLY DERIVED, Rocks. 
I, Mechanically formed Rocks 
The vast majority of these rocks consist of fragmental 
materials—they are, in short, aggregates of fragments of 
minerals and rocks. Some of them are due to “weathering” 
and the action of wind; others are the products of the action 
of moving water; while yet others are the result of the 
action of ice. We have thus three types of mechanically 
formed derivative rocks, namely, 1. Sudbaérial and Atohan 
Rocks, 2. Sedimentary Rocks, and 3. Glacial Rocks. 

1. SUBAERIAL AND AXOLIAN ROCKS 
Under this head are included all accumulations which are 
due to “weathering” and the action of wind. The process 
known as “ weathering” is by no means, however, exclusively 
mechanical. The subaérial disintegration of rocks is brought 
about by the operation of various agents, and it is not always 
possible to assign to each its proper share in the work 
performed, In cold regions, rocks are broken down chiefly by 
the action of frost; in many temperate countrics the chemical 
action of rain is often the most effective agent of destruction, 
or the work of demolition may be pretty equally divided 
