632 F. F. GROUT GRAPHIC STUDY OF IGNEOUS ROCK SERIES 



sixty years or more, but which were coming into disfavor as the idea 

 of differentiation came to be accepted as explaining the variety in igneous 

 rocks. 31 Since then Daly 32 and Bowen 33 have each presented strong argu- 

 ments for considering all primary magmas as basaltic. Meanwhile, others 

 have been influenced, probably by the calculation of the "average igneous 

 rock/ 734 to consider magma of intermediate composition as the most 

 abundant, if not the only primary magma. 35 Several petrographers still 

 write as if they considered the primary magma in some regions very dif- 

 ferent from that in others. The diagTams furnish data that are, to say 

 the least, very suggestive. 



EVIDENCES FROM THE DIAGRAMS 



The simple fact of the occurrence of a differentiated series does not 

 necessitate starting with basaltic magma nor any of the "normal" series 

 of derivatives of basalt toward granite. Using the best estimates that 

 could be found or made from the published data, the writer has tabu- 

 lated the compositions of the magmas as intruded into the chambers 

 where differentiation has been strongly active and the results well ex- 

 posed. The magmas show a wide range — from olivine gabbro as at Pres- 

 ton to granophyr diabase as at Sudbury ; from quartz syenite as at Assynt 

 to nephelite syenite as at Ice Eiver, with branches to teschenite as at 

 Lugar, Scotland, and to essexite as at Saint Hilaire. Many show no 

 phase approaching basalt, but all are sharply differentiated. 



The course of differentiation of several bodies of magma that are not 

 basaltic as intruded is not in a direction to indicate that the process be- 

 gan with basalt. To be sure, some series grade from intermediate rocks 

 to granite, like those at Pyramid Peak and the Slesse batholith, and some 

 grade from alkaline basalts to yet more alkaline types, like the Square 

 Butte series, all assuming a trend on the diagram as if the primary 

 magma had been basaltic. Others are so different in trend as to sug- 

 gest no relation to basalt. The Castle Mountain stock, and the series 

 from Belknap Mountain, and that from Almunge give curves that do 

 not pass near basalt, and have a trend that is certainly not to be ex- 

 pected, if the primary magma was average basalt. 



31 J. P. Iddings : Igneous rocks, vol. 1, gives a review of the development of these 

 theories. 



32 R. A. Daly : Igneous rocks and their origin, pp. 264-5 and p. 458. 



33 X. L. Bowen : Op. cit., pp. 66-73. 



34 The latest calculation, referring to others, is by H. S. Washington. Chemistry of 

 the earth's crust. Jour. Franklin Inst., vol. 190, 1920, p. 773. 



35 Monzonite magma is taken as a logical start, by Pirsson and Cross. See Am. Jour. 

 Sci., vol. 31, p. 430 (1911). Diorite is considered most reasonable by Campbell. See the 

 October, 1920, meeting of the Institution of Mining and Metallurgy 



