A REVIEW OF IGNEOUS ROCK CLASSI- 

 FICATION 



By G. W. TYRRELL, A.R.C.Sc, F.G.S. 

 Lecturer in Mineralogy and Petrology, Glasgow University 



The recent publication of the second volume of Prof. J. P. 

 Iddings' great work on Igneous Rocks (Description and Occur- 

 rence) marks an epoch in the history of petrological science. It 

 is a gallant attempt to infuse the quantitative spirit into the dry 

 bones of the older and laxer system of igneous rock classifica- 

 tion, and also to correlate the Quantitative Classification invented 

 by Iddings, in collaboration with Cross, Pirsson, and Washing- 

 ton, with the older qualitative system. In the writer's opinion, 

 it is a failure in the sense that it fails to reconcile irreconcilables, 

 but it is a great failure which will do much to turn the mental 

 outlook of petrographers from the comparatively barren qualita- 

 tive past to the hopeful and fruitful quantitative future. 



In view of the publication of this work, the present seems an 

 appropriate time to re-examine the American Quantitative Classi- 

 fication and other classifications from a point of view frankly 

 sympathetic to the quantitative idea. It may be taken that all 

 petrologists are now more or less familiar with the main lines of 

 the American Quantitative Classification, and it is therefore 

 unnecessary to describe the system in detail. Many petrologists 

 have used it as an auxiliary to older methods of classification, 

 but none, so far as I am aware, have altogether dispensed with 

 the latter. The persistence and vigour of the qualitative system 

 at the present time, although the American Quantitative Classi- 

 fication has now had ten years for its trial, requires further 

 explanation than the inherent conservatism of petrographers. 

 In the writer's opinion, the Quantitative Classification has not 

 displaced the older system because it does not provide a ready 

 means of classification for the working petrographer. The 

 chemical analysis is the unit of the system, and it is only possible 

 for the petrographer to obtain analyses of a small proportion of 

 the rocks he describes. The greater part of his work deals with 

 the actual mineral composition or mode, and the classification 



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