﻿Yol. 66.'] CLASSIFICATION OF IGNEOUS ROCKS. 471 



from the purely geological side, 1 but there should ultimately be but 

 one petrographic system, based on thorough knowledge, comparable 

 with that of zoology or botany. - 



In recent years there has been much discussion as to the proper 

 basis for the systematic classification of igneous rocks. In this 

 discussion the adjectives natural, arbitrary, and artificial 

 are often applied to current or proposed bases of classification. 

 In common, I presume, with all other petrographers, I should be 

 glad if a petrographic scheme of classification were possible to 

 which the term natural might be appropriately applied. But I 

 do not believe that such a system is possible, and, further, it seems 

 to me that the systems devised for igneous rocks during the last 

 century are unnatural, and without good reasons for most of 

 their artificial or arbitrary features. Certain American colleagues, 

 who entertain similar beliefs, have united with me in the formula- 

 lation of the ' Quantitative System for the Classification of Igneous 

 Rocks,' 3 in the construction of which several factors prominent in 

 earlier systems have been ignored. Some of these discarded factors 

 are corner-stones in the systems advocated by believers in a 

 possible natural system. In current criticism of the Quantitative 

 System it is commonly called arbitrary or artificial, with the 

 implication or direct assertion that a much more nearly natural 

 system is in existence or may reasonably be hoped for. With a 

 desire to contribute to the general discussion of this important 

 question, as well as to explain once more my personal point of 

 view, which led me to join in proposing the Quantitative System, 

 I wish to present some considerations as to the availability of 

 the natural relations of igneous rocks as factors in their systematic 

 classification. 



It may be well, however, before taking up this subject, to call 

 attention to some general considerations which are no doubt in the 

 minds of many petrographers but should be regarded by all, in 

 discussions of classification. First may be mentioned the import- 

 ance of definite names for rocks. Our use of a name should imply 

 a certain meaning in our own consciousness, and the use is certainly 

 without desired effect if the same significance is not conveyed to 

 others. This necessitates a definition, which, in the case of a rock 



1 W. Cross, ' The Geologica] versus the Petrographical Classification of 

 Igneous Eocks ' Journ. Geol. Chicago, vol. vi (1898) p. 79. 



2 The obscure character of many igneous rocks on the one hand, and the 

 need for a nomenclature based on external characters, where chemical and 

 microscopical stud}' is impossible, on the other hand, seems to render a field- 

 nomenclature advisable. Such a scheme has been proposed by Cross, Iddings, 

 Pirsson, & Washington, and elaborated by L. V. Pirsson in a textbook 

 intended for the field geologist and mining engineer ; but such a classification 

 is clearly not for the petrographer as a specialist. ' Eocks & Eock Minerals' 

 1908, pp. 414. 



3 W. Cress, J, P. Iddings, L. V. Pirsson, & H. S. Washington, ' Quantitative 

 Classification of Igneous Eocks' Journ. Geol. Chicago, vol. x (1902) pp. 555- 

 690, and in book form, Univ. of Chicago Press, 1903. 



