628 E. E. GROUT GRAPHIC STUDY OF IGNEOUS ROCK SERIES 



tioned this feature years ago, 18 and similar ideas were stated by Cross 19 

 and by Pirsson. 20 



5. There is a rongh suggestion of divergence, or radiation, in the lines 

 of figure 3, from the area of low alkalies and low silica, to all the rest 

 of the areas, but there are several pronounced exceptions, and it can not 

 be assumed that the approximate center of the radial lines was "pri- 

 mary magma." In fact, after a general glance at figure 3, two or more 

 centers of radiation are almost as clear as one. Probably none is espe- 

 cially significant. 



6. The alkaline rocks are approached by series from either gabbro or 

 granite. Numerous good examples can be selected from the diagrams 

 and need not be mentioned specifically. 



Two or three regions furnish a series ranging from nephelite syenite 

 toward both gabbro and granite. No series, however, furnishes data for 

 a connected series from gabbro to nephelite rocks by way of granite. 



Normal Sequences 



Igneous rock series have long been known to be of several sorts, and 

 Eosenbusch has described three "magmatic series" 21 — the granito-diori- 

 tic, gabbro-periodotitic, and foyaitic-theralitic. Cross has criticised the 

 series as no more "natural series" than the series connecting one of them 

 to the other. Pirsson shows alkalic and sub-alkalic series with monzo- 

 nite between and suggests that crossing or connecting series are possible 

 in various directions. 22 Bowen gives several series and selects one as a 

 "normal line of descent," 23 though he presents this normal series with 

 some hesitation because of its rigidity, and offers some alternatives. 

 While these several schemes are based on considerable field and labora- 

 tory data, it is planned here to show the relation of theoretical series to 

 the actual series now known. 



Many of the actual series plotted fall along the line from the general 

 gabbro area to the general granite area. This is clearly a "normal" 



18 J. P. Iddings : Absarokite-shoshonite-banakite series. Jour. Geology, vol. 3, pp. 955- 

 959 ; see also vol. 6, p. 107. 



10 W. Cross : The natural classification of igneous rocks. Quart. Jour. Geol. Soc, vol. 

 66, 1910, p. 481. 



20 L. V. Tirsson : Petrography of Tripyramid Mountain. Am. Jour. Sci., vol. 31, 1911, 

 p. 430. 



21 H. Rosenbusch : Mikroskopische Physiographic Massige Gesteine, 4th ed., vol. 2, 

 pt. 1, 1908, p. 13. 



22 L. V. Pirsson : Petrography of Tripyramid Mountain. Am. Jour. Sci., vol. 31, 1911, 

 p. 430. 



23 N. L. Bowen : Evolution of igneous rocks. Jour. Geol. Supplement, 1915, p. 75. 



