SYSTEMATIC PETROGRAPHY 483 



They merely serve to show the chemical range found within the 

 families so far as Osann's examination extends. 



From the above statement it would appear that Osann has 

 not, in reality, proposed a chemical classification in the sys- 

 tematic sense, and hence it is not desirable to enter further into 

 the analysis of this elaborate discussion of the chemical varieties 

 represented within the families of the Rosenbusch system. 



This discussion of petrographic systems proposed during the 

 nineteenth century will close with a review of three attempts to 

 bring all known rocks into orderly arrangement. Not that the 

 authors think to have formulated natural or logical systems, for 

 that is expressly disclaimed by them. Yet in presenting these 

 comprehensive arrangements of rocks, according to the light of 

 the last decade of the century, these authors define and illus- 

 trate in a most effective way the present condition of systematic 

 petrography. 



Ferdinand Zirkel, iSgj, i8g^. — The second edition of Zirkel's 

 Lehrbuch der Petrographie is the most comprehensive and com- 

 plete description of all known rocks ever published, and it there- 

 fore represents the present status of the systematic science as a 

 whole, better than any other work, and hence deserves careful 

 consideration. But the fact staring the student in the face is 

 that systematic petrography is still very largely an arrangement 

 for convenience of description, and is not, in its entirety, a logical 

 expression of relationships. Within the division of igneous 

 rocks there is at least some attempt at system, but the other 

 rocks confessedly defy logical treatment by any method as yet 

 proposed. 



The primary division of all rocks is on general geological 

 grounds into four groups : 



I. Igneous rocks — "Massige, eruptive Erstarrungsgesteine." 

 II. Crystalline schists. 



III. Sedimentary crystalline rocks (not clastic.) 



IV. Clastic rocks. 



This division is clearly based on geological considerations, 



