PAPERS OF THE PETROGRAPHIC SECTION 533 



The paper has since appeared in fall in the report of the New York 

 State Paleontologist for 1902. 



Remarks were made by Bailey Willis, H. M. Ami, and the author. 



Section of Petrography 



During Tuesday afternoon, while the papers described above were 

 being read in the general session, a number of petrographic papers were 

 read in an adjacent room, Professor B. K. Emerson in the chair. 



The first paper presented was one from the program of Section E, 

 American Association for the Advancement of Science, presented by 

 J. P. Iddings and H. S. Washington, entitled : 



QUANTITATIVE C HEMICO-MINERALOGICAL CLASSIFICATION OF IGNEOUS ROCKS ' 

 BY WHITMAN CROSS, J. P. IDDINGS, L. V. PIRSSON, AND H. S. WASHINGTON 



The matter of the paper is published in a book by the same authors, 

 printed by the University of Chicago Press, entitled " Quantitative Classi- 

 fication of Igneous Rocks." 



The second and third papers were read and discussed together. 



CHEMICAL COMPOSITION OF IGNEOUS ROCKS EXPRESSED BY MEANS OF 



DIAGRAMS 



BY J. P. IDDINGS 



[Abstract] 



The diagrams expressed the molecular proportions of the chief chemical compo- 

 nents of igneous rocks ; the range of their variation ; the gradations of igneous 

 rocks chemically between extremes ; the grouping of them according to the system 

 of quantitative chemico-mineralogical classification recently proposed by Cross, 

 Iddings, Pirsson, and Washington. The paper closed with a correlation of igneous 

 rocks classified on the quantitative basis with the same rocks classified on the 

 qualitative basis. 



QUANTITATIVE DISTRIBUTION OF ROCK MAGMAS 

 BY HENRY S. WASHINGTON 



[Abstracf] 



The relative abundance of rock magmas belonging to the various divisions of the 

 quantitative, chemico-mineralogical system of classification recently proposed, was 

 discussed, with remarks on the special distribution of some of them. The advan- 

 tages of the new system for the treatment of such problems were pointed out and 

 compared as to certain respects with the old. A large collection of analyses of 

 igneous rocks, on which the discussion was based, was briefly described. 



