214 PROCEEDINGS OF THE AMERICAN ACADEMY. 



averages were reduced to 100.00 per cent and entered in Table II. 

 Each average analysis was then recalculated to 100-00 per cent after 

 H 2 (and C0 2 ) had been subtracted. The results are also given in 

 Table II., in which plutonics and corresponding effusives are grouped 

 together. Magniatic relationships are often less obscured if these 

 volatile oxides, which may be wholly or in part of exotic nature, are 

 excluded. Finally, in order to facilitate -reference to the tables, an 

 index to the different rock-types was prepared and may be found 

 below Table II- 



It will be observed that certain rock-types have been omitted from 

 the tables. The large class of " aschistic " dike-rocks is not represented 

 because of their chemical similarity to the corresponding plutonic 

 species. Other named varieties are omitted since their analyses are 

 too few to give useful averages. In a few cases the mineralogical and 

 chemical variations within each variety are so great that it has not 

 seemed advisable to regard their averages as worthy of entry. Many 

 other subordinate varieties of rock, though given special names, are 

 chemically almost identical with the more important types entered in 

 the tables and therefore have been excluded. 



Sources of Information. 



The immediate sources of the analytical statements used in the 

 computations are as follows : — 



1. Beitrage zur chemischen Petrographie, zweiter Teil, by A. Osann. 



Stuttgart, 1905. 



2. Chemical Analyses of Igneous Rocks published from 1884 to 1900, 



by H. S. Washington. Prof. Paper, No. 14, U. S. Geological 

 Survey, 1903. 



3. Elemente der Gesteinslehre, 2nd edition, by H. Rosenbusch. 



Stuttgart, 1901. 



4. Lehrbuch der Petrographie, 2nd edition, by F. Zirkel. Leipzig, 



1893. 



5. Studien uber die Granite von Schweden, by P. J. Holmquist. Bull. 



Geol. Institution, University of Upsala, Vol. 7, 1906, p. 76. 



6. Some Lava Flows of the Western Slope of the Sierra Nevada, Cal- 



ifornia, by F. L. Ransome. Amer. Jour. Science, Vol. 5, 1898, 

 p. 355. 



7. Matenaux pour la Min^ralogie de Madagascar. Nouv. Archives 



du Museum, (4), Vol. 5, Paris, 1903. 



8. Geology of the Yellowstone National Park, by A. Hague and 



others. Petrography by J. P. Iddings. Monograph No. 32, 

 Part 2, U. S. Geological Survey, 1899. 



