SUGGESTIONS FOR A QUANTITATIVE MINERALOGICAL 

 CLASSIFICATION OF IGNEOUS ROCKS 



ALBERT JOHANNSEN 



University of Chicago 



It is with considerable hesitation that the writer introduces a 

 new classification of igneous rocks. He knows that he who adds 

 a single term to an already overburdened vocabulary is looked upon 

 with disfavor, while he who brings in many has hearty objurgations 

 heaped upon him; yet he hopes, as others who have gone this way 

 before him have hoped, by fixing definite boundary lines beyond 

 which the different families cannot pass, to eliminate the multiplica- 

 tion of names for rocks which differ in no essential particulars from 

 previously described types. 



It is being recognized, more and more, that there is need for 

 three classifications of igneous rocks. Of these, one must be for 

 field use 1 and megascopic. Another must be chemical, after the 

 manner of the systems of Osann 2 and C.I.P.W. 3 The third must 

 be mineralogical. The old classifications of Rosenbusch and Zirkel 

 are more or less mineralogical, it is true, and are not to be discarded 

 lightly, but they fail especially in their lack of the quantitative 

 element. Furthermore, they are neither purely mineralogical, 

 purely chemical, nor purely geological. For example, certain 

 dike-rocks are classified by Rosenbusch, on the basis of their field 

 associations with nephelite-syenites, essexites, etc., as rocks of the 

 alkali series, and to them he gives specific names, yet they are miner- 

 alogically and chemically identical with normal rocks of the alkali- 

 lime series. He depends in part, therefore, on field associations 



1 Field classifications are given by Cross, Iddings, Pirsson, and Washington, 

 Quantitative Classification of Igneous Rocks (Chicago, 1903), p. 180; L. V. Pirsson, 

 Rocks and Rock Minerals (New York, 1908), p. 202; Albert Johannsen, " Petrographic 

 Terms for Field Use," Jour. Geol., XIX (1911), 317-22. A revised form of the latter 

 will appear shortly. 



2 A. Osann, Tschermak's Mitteilungen, XIX, XX, XXI, XXII (1899-1903). 



3 Cross, Iddings, Pirsson, and Washington, op. cit. 



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