THE QUANTITATIVE CLASSIFICATION 

 ON IGNEOUS ROCKS 



By JOHN W. EVANS, D.Sc, F.G.S. 

 Imperial Institute, London 



In the course of my work on the crystalline rocks of the cataracts 

 of the Rio Madeira and the adjoining rivers, I thought it would 

 be of interest to ascertain the position of such of the rocks as 

 had been analysed, in the quantitative classification of igneous 

 rocks which has recently been brought forward 1 by four of the 

 leading petrologists of the United States, and almost universally 

 accepted in that country. 



As I have given some attention to the details of this classifi- 

 cation, and as it is still comparatively little known, it may be 

 useful to give a brief account of its main features, to show how 

 it is applied in concrete cases, and to state — for what they may be 

 worth — the conclusions to which I have come as to its claims for 

 adoption beyond the limits of the land that gave it birth. 



It is impossible not to sympathise with the ideal which the 

 authors of this bold attempt at reform set before them. Every 

 petrologist deplores the confusion that reigns in the classification 

 and nomenclature of igneous rocks. It is true that the un- 

 precedented advances that have been made in the last thirty 

 years in our knowledge of their composition and structure, 

 origin and relations, have not been without result. The old 

 point of view that attached an illogical importance to geological 

 age and stratigraphical relations has been to a large extent 

 discarded ; but although the recognition of the importance of 

 chemical and mineral composition and of structure as the only 

 legitimate bases of classification marks an important step in 

 advance, yet there has been — and still is — such lack of agree- 



1 Quantitative Classification of Igneous Rocks, by Whitman Cross, Joseph 

 P. Iddings, Louis V. Pirrson, and Henry S. Washington (Chicago, 1903 ; also 

 published in vol. x. of The Journal of Geology, 1902, p. 555) ; Chemical 

 Analyses of Igneous Rocks published from 1884 to 1900, by Henry S. Washington 

 (Washington, 1903) ; Chemical Composition of Igneous Rocks expressed by means 

 of Diagrams, by Joseph P. Iddings (Washington, 1903). 



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