THE AVERAGE IGNEOUS ROCK 



W. J. MEAD 



University of Wisconsin 



The chemical composition of the average igneous or crystalline 

 rock of the earth's crust is of sufficient interest to geologists and 

 petrographers to have elicited several attempts at compiling 

 average analyses. Clarke^ summarizes the three most important 

 averages of igneous rock analyses — those by Clarke, by Harker, 

 and by Washington. These averages agree fairly well, showing 

 very small differences in the amounts of the various oxides, and 

 there is little to be said of the relative merits of the three deter- 

 minations. 



One question, however, has been repeatedly raised, i.e.: Does 

 the average of all igneous rock analyses represent with any degree 

 of certainty the actual average of the igneous rocks of the earth's 

 shell ? Obviously, if the average analyses of Clarke, Harker, and 

 Washington are to represent correctly the composition of the igneous 

 rocks of the earth's crust, it is necessary that the samples of the 

 various types of igneous rock be proportional to the amounts of 

 these several types in the earth's crust. If not, these average 

 analyses are misleading. 



In the course of certain investigations in the Metamorphic 

 Laboratory at the University of Wisconsin, the writer has attempted 

 to check Clarke's average igneous rock by comparing it with the 

 composition of the sediments, and the results obtained are thought 

 to be of sufficient interest to warrant their presentation here. 



If we are correct in the assumption that the oxides of the bases 

 and the silica of the sediments have as their ultimate source the 

 igneous rocks, then a properly weighted average of the sediments 

 (excluding water and carbon dioxide) would represent very closely 

 the average composition of the igneous rocks from which these 

 sediments were derived. Certain discrepancies are to be expected 



^ F. W. Clarke, "Data of Geochemistry," U.S. Geol. Survey, Bull. 4pi, igii. 



772 



