CLASSIFICATION OF THE PLUTONIC ROCKS 255 



portion of orthoclase as compared to plagioclase in the members 

 of the Monzonite Series. It will be observed that the alumina 

 curve falls slightly at the acid end of the series. 



The Potash Division of the Alkali Series 



Table of Analyses of Rock-types 



SiO, 

 A1A 

 Fe,0 3 

 Feb 

 MgO 

 CaO 

 Na 2 

 K 2 

 H,0 

 TiO, 



Other constituents 

 (P 2 5> MnO, etc.) 



I. 



Potash-granite. 



70-17 



15*07 

 •88 

 179 

 I'll 

 1-13 

 2 '69 



573 

 •88 



•41 

 \ 75 



II. 



Potash-syenite 

 (Plauenite). 



60 57 

 15-85 



8-23 



2-59 



4*44 

 2-13 



6'02 



I '06 

 o'53 



III. 



Shonkinite. 



47-85 



I3'24 

 ) 2-74 

 I 2-65 



5-68 



14-36 

 372 

 5-25 



274 



2-42 



ioo'6i 



101-42 



100-65 



I. Potash-granite, Lamorna, Cornwall. W. Pollard, Geology of Land's End, 

 Mem. Geol. Surv., 1907, p. 59. 

 II. Potash-syenite (Plauenite of Brogger). Brogger's average of three analyses 

 (namely, Plauen near Dresden, Biella in Piedmont, and Reichenstein 

 in Lower Silesia), loc. cit. p. 31. 

 III. Shonkinite, Poohbah Lake, Rainy River District, Ontario. F. L. Ransome. 

 Quoted by Rosenbusch, Elemente der Gesteinsle/ire, 1898, p. 176. 



Of the three rocks represented by these analyses, the 

 shonkinite belong to the Basic and the potash-granite to the Acid 

 Group, while the syenite falls within the limits of the Intermediate 

 Group. Referring to'the diagram (fig. 4), the Potash Series is 

 in general characterised by the fact that the potash is present in 

 nearly equal molecular proportion to the soda. The sum of the 

 two bases remains practically constant throughout the series. 

 Lime is very high in the shonkinites (in the example chosen 

 14-4 per cent), but falls rapidly as the syenites are approached, 

 and this base is scarcely represented in the muscovite-granites 

 which form the extreme acid end of the series. There is a similar 

 rapid diminution in the proportions of magnesia and iron, 

 although these bases are not present in the shonkinites in such 



