A REVIEW OF IGNEOUS ROCK CLASSIFICATION 61 



immediate to his needs must therefore be based on the mode, 

 and not on the norm (theoretical mineral composition) derivable 

 only from a chemical analysis or from determinations equivalent 

 to chemical analysis. 



This view does not necessitate the abandonment of the 

 American Quantitative Classification. The latter provides a 

 final court of appeal for cases in which the mode is indecisive or 

 indeterminate. It is, so to speak, a regularly-reticulated back- 

 ground on which we may trace the magmatic characters of the 

 rocks ; or, to change the figure, it is a more or less convenient 

 system of pigeon-holing by which we may docket our rocks 

 according to their magmatic characters. 



The advantages conferred by the American Quantitative 

 Classification and its authors on petrological science are many 

 and various. They exposed the lax, unsystematic character of 

 the older qualitative classification, and the chaotic condition 

 of its nomenclature. They forced a quantitative view-point 

 on petrographers and showed that the comparative method 

 in petrology was almost impossible under the qualitative 

 regime. Their criticisms have effected an immense improve- 

 ment in the technique of rock-analysis, and it is safe to predict 

 that in any new collection of rock-analyses the proportion of 

 " superior" to "inferior" examples will be much greater than in 

 those already published. They have insisted on the importance 

 of the chemical analysis from a petrological point of view, and 

 have raised its status from a mere ornamental but usually in- 

 accurate adjunct to petrological work to that of an essential and 

 indispensable part. The recognition of this changed status is 

 the probable cause for the great increase in the number of 

 analyses of igneous rocks now made. Furthermore, the exact, 

 detailed, and systematic methods of the authors of the American 

 Quantitative Classification have caused a great improvement in 

 descriptive work. Only of late years has it become possible 

 strictly to compare rocks from the ends of the earth simply 

 from their published descriptions ; and for this most desirable 

 consummation the more exact and detailed mineralogical and 

 textural description, and the publication of modal proportions, 

 insisted upon by the authors of the Quantitative Classification, 

 have undoubtedly been largely responsible. Finally, they have 

 given petrographers an instrument of inestimable value in the 

 conception of the norm. 



