250 Prof. F. Lcewiwson-Lesxing — Origin of the Igneous Rocks. 



On Table I are given: (I) The average from basalt (after Daly l ) 

 and granite (after Holinquist 2 ) ; (II) Washington's 3 and (III) Clarke's 

 average for igneous rocks ; (IV) the average between II and III. 

 We can easily see that this last average agrees in its essential features 

 with the composition of an earth magma, presumed to consist of 

 equal parts of a granitic and a gabbroidal magma. 



The same result is reached by taking the new data of Daly, 4 as 

 can be seen from the figures on Table II. Washington's average and 

 the mean from granite and gabbro are in the essential lines identical. 



TABLE II. 



Our first conclusion can be formulated as follows : The average 

 composition of the crust, calculated by the method hitherto in use, 

 corresponds to the composition the crust would have if it consisted 

 of equal parts of granite and gabbro. What does this hypothetical 

 fundamental magma represent ? Examining the answers to this 

 question given by different authors, we perceive that the current 

 opinions are erroneous. Thus Daly, with whom I agree on many 

 points, is wrong when he defines this magma as a dioritic one, because 

 in calculating the average composition of diorite he has taken 

 analyses, not of diorites only, but also of quartz-diorites (granodiorites) 

 representing a different magmatic type from that of true diorites. 

 In reality the average III on Table II corresponds almost exactly to 

 the syenitic magma, as can be seen by the following calculation : — 



1 B. Daly, " Origin of the Augite-andesites " : Jour. Geol., 1908, p. 409. 

 3 Holmquist, " Studien iiber die Granite von Schweden " : Bull. Geol. Inst. 

 Upsala, vii, 1906. 



3 H. S. Washington, " Chemical Analyses of Igneous Bocks published from 

 1884 to 1900 " : U.S. Geol. Surv., Prof. Pap. No. 14, 1903. 



4 B. Daly, "Average Chemical Composition of Igneous Bock-types": 

 Proc. Amer. Acad. Arts and Sciences, xlv, No. 7, p. 209, 1910. 



