i58 



F. C. CALKINS 



two simple ways of dividing regularly a series of six parts: either 

 to give all parts an equal range, or to give the end parts half the 

 range of the others, which are mutually equal. The latter plan 



GROUPING OF SODA-LIME FELDSPARS BY VARIOUS AUTHORS 



Albite Oligoclase Andesine Labradorite Bytownite Anorthite 



An.% o 



1679 



I.Fouque 

 and 

 Levy 



2. Levy 1888 



and 

 Lacroix 



3. Lacroix 1897 



4 Tschermak 1865 



5. Rosenbusch 1885 



6. Dana 16321 



7. Zirkel 1894 



8. Rosenbusch 1905 



9.Johannsen 



1908 



10. Average of 4-9 



. Proposed Han 



An% 



Fig. i. — Sources of plans illustrated 



i. F. Fouque and A. Michel Levy, Miniralogie micrographique, 1879. 



2. A. Michel Levy and A. Lacroix, Les Miniraux des roches, 1888. 



3. A. Lacroix, Miniralogie de la France el de ses colonies, II (1897), 130. 



4. G. Tschermak, "Die Feldspathgruppe," Sitzungsberichte d. K. Akad. Wien, L (1865), 566. 



5. H. Rosenbusch, Mikroskopisclie Physiographic, etc., 2d ed., 1885 ("nach Tschermak"). 



6. J. D. and E. S. Dana, System of Mineralogy, 6th ed., 1892; also used by J. P. Iddings, Rock Minerals, 

 1906, and by F. W. Clarke, "Data of Geochemistry," Bull. 330, U.S. Geol. Survey, 1908. 



7. F. Zirkel, Lehrbuck der Petrographie, 2d ed., 1894. 



8. H. Rosenbusch, Mikroskopisclie Physiographic, 4th ed., 1905, Bd. I, 348. 



9. A. Johannsen, Determination of Rock-forming Minerals, 1908. 



10. Average of 4-9. 



11. Decimal grouping. 



