778 



W. J. MEAD 



This solution assumes that the average analyses of granite, 

 basalts, shales, limestones, and sandstones used are approximately 

 correct, and that the average igneous rock may be represented by 

 some combination of granite and basalt. So far as these assump- 

 tions are not true, solution fails, but the fact that the graphical 

 method admits of selecting a definite ratio of granite and basalt 

 and that this ratio yields an average so similar to the average for 

 the igneous rock obtained by Clarke and others seems to indicate 

 that the data and assumptions are not in serious error. 



In Table II, column i is the average analysis obtained by 

 combining the average granite and the average basalt in this ratio. 

 For purposes of comparison the average analyses of igneous rocks 

 obtained by Clarke, Harker, and Washington are placed in parallel 

 columns. 



TABLE II 



6* 



SiOa. 



AL03 



FeA 

 FeO. 

 MgO 

 CaO. 

 Na.O 

 K.O. 



+ 1 



+ 



■36 

 .16 

 . 20 

 . 10 

 ■23 

 ■38 

 ■23 

 . 20 



+ 2. 2 

 — 1.03 



+ 7-5 



- 2.9 



-305 



- 8.45 



- 6.5 

 + 6.6 



* F. W. Clarke, op. cit., p. 25. 



1. Average granite and average basalt, combined in ratio of 65 to 35. 



2. Clarke's average igneous rock. 



3. Differences between 1 and 2, on basis of total rock. 



4. Differences between 1 and 2, in percentage for each constituent. 



5. Harker's average igneous rock. 



6. Washington's average igneous rock. 



The differences between column i and the three average analyses 

 of igneous rocks are small. The differences between column i 

 and Clarke's average igneous rock are expressed in terms of the 

 entire rock and in percentage of each constituent in columns 3 and 

 4 respectively. The most striking result of the comparison is the 

 marked similarity between the combined granite and basalt and 

 the average analyses. The greatest differences are for silica and 

 magnesia, the combination of granite and basalt being slightly 

 more acidic than the other averages. 



