66 ALBERT JOHANNSEN 



sufficient data in the literature as to the modes of rocks, the publica- 

 tion was delayed. In the present paper the writer presents the 

 system in a tentative form, hoping to receive from other petrog- 

 raphers expressions of opinion and suggestions for modifications. 

 Later he hopes to show the relationships, both mineralogical and 

 chemical, existing between the rocks falling into the various groups. 



The system here proposed is strictly mineralogical, quantitative, 

 and modal, and is directly applicable to all plutonites and to prac- 

 tically all extrusives. The writer's objections to the percentage 

 values set by various other authors will be given below. Appar- 

 ently the dividing lines have previously been arbitrarily selected, 

 and no attempts have been made to gather published data with 

 respect to the modes of rocks. There is, in fact, a surprising lack 

 of such data, the writer having been able to find published reports 

 of less than 600 quantitatively determined rocks. 



If the reader has ever attempted to find, from the average 

 report, the relationship existing between a newly described rock and 

 the older types, he will in many cases have found it impossible. 

 This is clearly shown by the fact that Rosenbusch himself, by the 

 misinterpretation of descriptions, has misplaced rocks, grouping 

 them with totally unrelated types. If the reader will turn at 

 random to almost any petrographic report, 1 and will read a descrip- 

 tion and then attempt to picture to himself the rock described, 

 in most cases he will find that owing to lack of quantitative data 

 no idea as to its appearance can be obtained. 



A name should convey an idea as to the character and appear- 

 ance of a rock, and it should not be necessary, as it now unfortu- 

 nately is, for one to read the description of a rock to know what a 

 writer means. So far as the name itself is concerned, it is of slight 

 importance, provided the texture is described and accurate quantita- 

 tive details of the average rock are given. But without quantitative 

 details serious errors may arise. Thus in a recent petrographic 

 report a rock was said to contain orthoclase, andesine, quartz, 



1 The writer is guilty of having written, indefinite descriptions himself. As an 

 exception to the general rule of poorly written and indefinite reports, he likes to refer 

 his students to Dr. H. S. Washington's "Roman Comagmatic Region," Carnegie 

 Publication No. 57. Here there is never the least doubt as to the mineralogical 

 composition and appearance of a rock. 



