REPORT OF THE DIRECTOR, 1922 10g 
antiquity of its materials and the reliability of the physical record 
even down to the minutest detail and the refinement of methods 
out of which and by means of which their histories are read. Any 
methods and any criteria and any nomenclature that serve these 
ends are the legitimate equipment and instruments and media 
of the petrologist; but he himself is first of all a geologist, helping 
in his own way to unravel the history of the earth, or else he is not 
in this class. 
The petrology of the past has been developed and advanced in 
considerable part by men who were, first of all, mineralogists; and 
it is not at all surprising, therefore, that something of the point of 
view of the mineralogist should have dominated in this intermediate 
field. Under this leadership, the mineralogy of rocks, comparison 
of rocks by means of mineral differences, and classifications empha- 
sizing mineral proportions were perfectly natural growths. Geologic 
principles thus at first did not make much of an appeal. Other 
interests of petrology have been brought forward more recently and 
almost wholly by geologists — applied and economic geologists — 
rather than by professional petrographers. It has been forced by 
an attempt to use the methods of petrography as a geologic aid. 
These attempts are not new, but to regard them as belonging to the 
true field of petrology is new and no systematic presentation of this 
kind of petrology has ever been made. 
Methods and Criteria 
Every refinement of method that can promise any new insight 
into the obscure features of rocks must be resorted to, for rock 
interpretation requires all the skill of discrimination that can be 
brought to bear. The difference, however, between the new and the 
older use of them lies in the purpose. The new effort is not only to 
detect differences, but to see meaning in them in terms of life history 
or in terms of any other claim that a given, special problem may 
present. 
Much attention is given to structure, quite as much as to com- 
position. Structural features are much more likely to have genetic 
and historical significance than are mineral proportion § or 
composition. | 
As an equipment one must have a sound appreciation of the work- 
ing of geologic processes, the complex interplay of chemical change 
under varying physical conditions, and the influence of the forces 
and agents that make and modify rocks. One must assemble the 
