A REVIEW OF IGNEOUS ROCK CLASSIFICATION 81 



consideration of a number of rocks in which both norm and 

 mode are available. Hence, in the absence of the mode, the 

 norm may be used for modal classification with little possibility 

 of error in rocks belonging to persalane and dosalane. 



Whilst the modes of many rocks are normative, they are pro- 

 bably more frequently somewhat abnormative ; and this has given 

 rise to some curious anomalies in Iddings* classification. For 

 instance, the anorthite appearing in the norm of a " monzonite " 

 may in part be taken up in the mafic minerals, and there- 

 fore fail to appear in the form of lime-soda felspar in the mode. 

 Rocks of this type, therefore, called monzonites in Iddings' 

 classification on the strength of the ratios of alkalic to lime-soda 

 felspars calculated from the norm, may actually be devoid of 

 modal lime-soda felspar. But the presence of the latter in 

 amount roughly equal to the orthoclase, is the essential part of 

 the original definition of monzonite. Many similar anomalies, 

 caused in the same way, could be cited ; but until as many 

 modes have been accumulated as there are norms, it is probable 

 that this difficulty will not be completely obviated. 



It may be remarked that the range of rocks covered by 

 Iddings' divisions, excluding that based on the mafic minerals, 

 might equally well be treated as divisible into nine equal 

 compartments, based on ratios between quartz or lenads to 

 felspars analogous to those that form the nine orders in 

 Classes I., II., and III. of the American Quantitative Classification. 

 The diagram makes this clear. 



A quantitative mineralogical classification of igneous rocks, 

 based on essentially the same principles as that of Iddings, 

 has recently been devised by F. C. Lincoln. 1 The chief difference 



1 F. C. Lincoln, " The Quantitative Mineralogical Classification of Gradational 

 Rocks," Economic Geology \ viii. 191 3, pp. 551-64. 

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