DEMESNE OF PETROLOGY 
355 
Nor can there be such a thing- as a science of Petrology wholly 
apart from the great science of Geology. Whatever of vital sig¬ 
nificance attaches to a rock is gathered to it from the consideration 
of the processes, and the agents, and the forces, and the materials 
of its geological setting and from its own history. Virtually all 
of its own contribution, its meaning to be read, returns to Geology 
again to enrich the knowledge of that great field, the reserves of 
geological history. 
In so far as nice discrimination and true sense of proportion 
serve in detecting evidence of former conditions, or processes, or 
changes, or sources, they play directly to the new aim. Of course 
it is patent that such methods of investigation and such fine dis¬ 
tinctions made with this intimate dependence upon close and ac¬ 
curate observation have immense disciplinary and training value 
quite apart from any particular aim of the science itself. For 
its highest motif discrimination for discrimination's sake alone is 
not enough. Quality, and meaning, and interpretation, and life 
history are higher objectives. 
The successive steps in the determination of the origin and the 
succeeding modifications which an ore-body, or a vein matter, or 
a residuary product undergo are as properly a petrographic theme 
as is the like history of an igneous rock or of a sedimental terrane. 
Whether a rock undergoes much alteration since the time of its 
original formation, or whether much of its present make-up is 
added to it since its initial appearance, or whether it records some 
peculiar reversal in the course of its development along which it 
first started, may be infinitely more important geologically than 
its name, or its mineral composition, or an elaborate description 
of it. 
Work for many years over a rather wide range of materials 
and problems convinces me that the real aim of petrologic study 
is practicability, and that the true sphere of the science of Petrol¬ 
ogy is in some way always involved in unraveling the mode of 
origin and course of the life history of the rocks. It is, then, a 
grievous mistake to be content with too narrow a prospect. For 
this kind of petrology a rock needs revised definition, a character 
sufficiently representative of some structural unit to justify dis¬ 
tinct consideration in all of its petrologic, structural, economic and 
geographic phases, besides correlative treatment, so that a working 
