A QUANTITATIVE MINERALOGICAL CLASSIFICATION 

 OF IGNEOUS ROCKS— REVISED 



ALBERT JOHANNSEN 



University of Chicago 



PART I 



In response to a request published in a former paper on a quanti- 

 tative mineralogical classification of igneous rocks,^ numerous letters 

 have been received from petrographers in this country and abroad. 

 Practically no objections were raised except against the separation 

 of the feldspar molecules. With this separation the writer himself 

 was not fully satisfied, and he reverted, shortly after the paper was 

 published, to the subdivisions used by him when the classification 

 was first presented to his students some nine years ago, namely, 

 that of dividing the feldspars simply into plagioclase, on the one 

 hand, and the remaining feldspars, on the other. In the following 

 article this change is shown. A further modification is introduced 

 in Class 4, although no objection was made to it as previously given. 

 The new subdivisions are somewhat simpler than before. 



An extensive change, embracing the omission of the 72 families 

 of the monzonite series (Fig. i), was contemplated, and personal 

 letters were sent to a considerable number of petrographers asking 

 opinions. Unfortunately the answers were so far apart that it has 

 seemed best to allow the divisions to remain as they were. The 

 need of more uniformity in classification was brought out clearly 

 by the repHes to this one question, as a comparison of the granite 

 quartz-diorite series of several petrographers will show (Fig. 2). 



Essentially the system now is as follows. For a detailed descrip- 

 tion the reader is referred to the former paper. 



Classes. — On the basis of the amount of dark minerals (mafites) 



present, the igneous rocks are divided into four classes, the division 



points being 0-5-50-95-100 per cent mafites. 



' Albert Johannsen, "Suggestions for a Quantitative Mineralogical Classification 

 of Igneous Rocks," Jour. GeoL, XXV (191 7), 63-97. In that paper delete eudialyte, 

 p. 90, 1. 15, and change less to more under Class i, p. 91, 1. 26. 



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