PROBLEMS OF PETROGRAPHIC CLASSIFICATION 



SUGGESTED BY THE "KODURITE SERIES" 



OF INDIA^ 



WHITMAN CROSS 

 U.S. Geological Survey 



INTRODUCTION 



Petrographic classification has often been retarded or sent on 

 the wrong course, for a time, by the repeated failure of petrog- 

 raphers, often those of deserved reputation as investigators, to 

 observe some broad principles of systematic classification. A 

 petrographic system which is to endure must surely be constructed 

 with the aid of generalizations or other factors which are widely 

 applicable to the rocks of the earth. Premature generalizations 

 have always proved harmful when introduced into system, however 

 helpful they may have been in their proper places, as aids in the 

 study of particular problems. Broad system expresses general 

 relationships of certain kinds, but many systematic propositions or 

 criticisms are made with a desire to emphasize the peculiarities of 

 the rocks of a locahty, a district, or, at most, a province. I desire 

 to offer a few observations on this last point apropos of a recent 

 discussion by L. L. Fermor, and this leads to some comments on 

 the classification of igneous rocks contained in Hatch's well-known 

 Textbook of Petrology. 



A recent publication by Fermor,^ referring to both local and 

 general problems of petrographic classification, raises a number of 

 questions of more than passing interest. Certain manganiferous 

 rocks of India called the "Kodurite Series" are believed by Fermor 

 to be "a series of differentiated igneous rocks ranging in acidity 

 from quartz-orthoclase rock, through intermediate quartz-kodurites 



' Published by permission of the Director of the U.S. Geological Survey. 



^L. Leigh Fermor, "The Systematic Position of the Kodurite Series, Especially 

 with Reference to the Quantitative Classification," Records, Geol. Survey of India, 

 XLII, Pt. 3 (1912), 208-30, Calcutta. 



791 



