482 WHITMAN CROSS 



A. Osann, igoo, igoi. — A further attempt to utilize chemical 

 composition as a factor in the classification of igneous rocks was 

 made by A. Osann in the closing year of the century. Under 

 the title "Versuch einer chemischen classification der Eruptiv- 

 gesteine " x Osann essays to use chemical composition as a supple- 

 ment to mineral and textural characters, by establishing various 

 chemical types within rock families formed upon the Rosen- 

 busch system. The author accepts the classes of deep-seated, 

 dike and effusive rocks, in the Rosenbusch sense, and the 

 vaguely defined families established upon mineral composition. 

 Realizing that by this latter factor, as currently applied, the 

 relative abundance of the minerals is not sufficiently taken into 

 account, Osann attempts to bring out quantitative relations, 

 within the families, by establishing certain types upon the basis 

 of chemical composition. 2 



It is clear that if the quantitative element was not sufficiently 

 expressed in forming the families discussed by Osann, he fails 

 to remedy the defect. A logical subdivision of the families on 

 a chemical basis would principally serve to point out the defects 

 in this respect, and would really weaken rather than strengthen 

 the system as a whole. 



But the "types " set up by Osann are in no sense systematic 

 divisions of the families. The " type " of this author is simply 

 a chosen well-analyzed rock, differing in chemical composition 

 from other rocks within its family, according to the adopted 

 method of comparison. To a type thus established are referred 

 other rocks of nearly identical chemical characters. But the 

 types bear no definite relation to each other or to the family. 



1 Tschermak'h Mm. und petr. Miltheilungen, Bd. XIX, 1900, pp. 351-469, and Bd. 



xx, 1901, pp. 399-558. 



2 The author's standpoint may be sufficiently understood from the following state- 

 ment: "Der Hauptmissstand der mineralogisch-structurellen Classification liegt 

 darin, dass dem relativen Mengenverhaltniss der wesentlichen Gemengtheile zu wenig 

 Bedeutung zuerkannt wird, und es wird gerade die Hauptaufgabe der chemischen 

 sein, in dieser Richtung erganzend und vertiefend zu wirken." " So kann es sich 

 bei dem hier unternommenen Versuch ebenfalls nur um ein kiinstliches System han- 

 deln, welches in erster Linie dazu bestimmt ist, das mineralogisch-structurelle zu 

 erganzen." Ibid, Bd. XIX, pp. 351-352. 



