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



Assuming that the great flows from fissure eruptions give us our best 

 approximation to primary magma, Daly finds probably 95 per cent are 

 basalts/ 58 though the total areas of andesites in the earth's exposures are 

 greater than those of basalts. The actual data 39 for such basalts were 

 plotted and enclosed in the dotted outline in figure 1. 



They are seen to range more widely than average basalt differs from 

 average igneous rock. Numerous other basalts range yet more widely, 

 but are smaller in bulk and less certain to be "primary" magma. The 

 range is probably greater than one would expect after reading of the 

 "uniformity" of primary magma. 



Furthermore, taking these great flows as the best illustrative material, 

 it is noteworthy that the average is not olivinitic, but has normative 

 quartz. Most petrographers, assigning normal basalt the mineral compo- 

 sition of an olivine gabbro, think of its chemical composition as also near 

 that of olivine gabbro. On the contrary, these great basalt eruptives are 

 more siliceous than even an olivine-free gabbro, and range both above and 

 below average gabbro in amount of alkalies. 



It may be seen that, if primary basalt varies as these samples indicate, 

 some are very close to diorite. Those petrographers who have decided 

 that dioritic magma was primary in certain regions may be no more in 

 error than those who would claim average basalt for all regions. As 

 Pirsson stated it, 40 "whether the magma shell of the molten earth (sup- 

 posing there ever was such a thing) was ever homogeneous or not . . . 

 is purely an academic question . . . the magmas underlying the 

 crust are different now in different regions." The variation in alkalies 

 is marked and the excess or deficiency of silica may reach 10 per cent. 



Summary 



The process of differentiation is not to be outlined in one sequence, nor 

 even in two. There are many diverse series. The nature of the differ- 



Op. cit., p. 120. 



Analyses obtained as follows : 



Deccan, rough analysis. Geol. Mag., vol. 2, 1905, p. 21. 



Washington State. U. S. Geol. Survey Folio 106, p. 6. 



Lake Superior Archean ; average by Zapffe. Econ. Geol.. vol. 2, p. 149. 



Idaho, Columbia lava. U. S. Geol. Survey Folio 106, p. 8. 



Iceland. U. S. Geol. Survey Prof. Paper 99, p. 649. 



Lake Superior Keweenawan. Average of the best given in Jour. Geol., vol. 18, pp. 



644-650. 

 New Jersey. Average of those in New Jersey Geol. Survey Ann. Kept. 1907, p. 159. 

 Idaho, Snake River. TJ. S. Geol. Survey Bull. 199, p. 87. 



Purcell lavas, B. C. Rept. of Chief Astronomer, 1910, Appendix No. 6, p. 209. 

 British Isles. Average between dolerite described in Geol. Mag., vol. 2, 1905, p. 21, 



and the flows in Skye, Mem. of Geol. Survey of United Kingdom, 1904. 

 Am. Jour. Sci., vol. 20, 1905, p. 47. 



