356 
DEMESNE OF PETROLOGY 
understanding of its life history and the genetic meaning of the 
physical unit to which it belongs may be acquired. 
The proper mission of Petrology is, then, to reveal that life his¬ 
tory of rocks. Applied Petrology must have for its chief aim 
the discovery of the meaning of particular rocks in terms of some 
special requirement. Any method, any criterion, or any nomen¬ 
clature that serves this end- is a part of the legitimate equipment 
of the earnest petrologist. Yet such investigator must be before 
all a broadly grounded geologist, or he cannot expect to register 
in this class. 
