SYSTEMATIC PETROGRAPHY 33 
ation of the store of new information concerning rocks before 
its true bearing upon classification could be correctly appreciated, 
and partly to prevailing differences of opinion as to the relations 
of petrography to the broader science of rocks — petrology.* 
At the present time students in many parts of the world, 
dissatisfied with existing petrographic systems, are turning their 
attention to classification, and are attempting to apply the newly 
acquired knowledge, with or without the aid of theoretical con- 
siderations, to the construction of improved systematic arrange- 
ments of rocks. In view of this situation it has seemed 
appropriate to present at this time a review of the development 
of systematic petrography. An understanding of the steps 
taken in bringing the science to its present condition must cer- 
tainly be useful to those who would assist in causing important 
advances in the future. 
In this review no attempt will be made to give a complete 
historical sketch of petrography, but rather to study in a some- 
what critical way the course of development through which the 
science has passed, to analyze the more important contributions 
to its advancement, and especially to examine the principles 
nominally applied in formulating schemes of classification and 
to test the methods of application as to their logical directness 
and consistency. From some cause the existing systems are 
commonly regarded as unsatisfactory and inadequate, and this 
condition indicates either that the principles of classification 
have not been wisely chosen or that they have been incorrectly 
applied. 
That the standpoint of the reviewer may be clear, it may be 
«The terms “ petrology” and “ petrography’’ have been so widely used in various 
senses that the writer wishes to urge an agreement among students of rocks to apply 
each in future in accordance with the scope implied in its etymology. On this basis 
the broad science or treatise of rocks is manifestly pefro/ogy, and the descriptive, 
systematic science, leading to the nomenciature of these objects, should be petrography. 
This usage corresponds to the definitions given in the “ Lexique Pétrographique ”’ 
prepared by F. Loewinson-Lessing, with the co-operation of many other petrog- 
raphers, published in the Compte Rendu of the Eighth International Congress of 
Geologists (Paris). 
