THE COMPOSITION OF THE AVERAGE IGNEOUS ROCK 1 



ADOLPH KNOPF 

 U.S. Geological Survey, Washington, D.C. 



The composition of the "average igneous rock" has been com- 

 puted by Clarke, Harker, and Washington. Clarke's most recent 

 estimate was published in 191 5. 2 The earlier computations were 

 made by averaging the results of large numbers of analyses, and the 

 later bv averaging each constituent according to the number of 

 determinations made, and reducing the sum to 100 per cent. 

 The objection to these methods, as is well known, is that they 

 take no direct account of the quantitative distribution of the 

 rocks; each analysis or determination receives the same weight, 

 regardless of the size of the geologic body that it is held to repre- 

 sent. The force of this objection has been recognized by 

 Clarke, 3 who concludes that ''the whole land surface of the earth 

 must be taken into account before the true average can be finally 

 ascertained." 



A first approximation to this true average can be reached by 

 calculations based on data recently assembled by Daly in Igneous 

 Rocks and Their Origin. In Table IV is given the total areas 

 covered by each of the rock species named and mapped in the 

 Cordilleran and Appalachian folios of the United States Geo- 

 logical Survey. The area occupied by any rock species divided 

 by the total area of igneous rocks (16,728 square miles) gives a 

 weight-factor, and this factor multiplied by the average composi- 



1 Published with the permission of the Director of the U. S. Geological Survey. 



* Analyses of Rocks and Minerals from the Laboratory of the United Stales Geo- 

 logical Survey, U.S. Geological Survey Bulletin, No. 591, pp. 18-22, 1915. 



J The Data of Geochemistry (3d ed.), U.S. Geological Survey Bulletin, No. 616, 

 p. 26, 1916. 



620 



