COMPOSITION OF THE AVERAGE IGNEOUS ROCK 621 



tion of the species, which has been computed by Daly in Table II, 

 gives the percentage contribution of that species to the composition 

 of the average igneous rock. In this calculation species covering 

 less than 2 square miles were omitted, as their inclusion would not 

 affect the second decimal place of the result. The composition 

 thus calculated is that of the average exposed igneous rock; whether 

 it represents the composition of the average igneous rock of the 

 10-mile crust depends on the verity of certain petrogenic considera- 

 tions. 



The following proportional factors were used in the computa- 

 tions : 



Granite, including allied porphyries o. 23212 



Granodiorite 12195 



Rhyolite ' 12834 



Andesite 23864 



Basalt 20773 



Quartz monzonite and allied porphyry 00108 



Diorite 01802 



Gabbro 02225 



Anorthosite 0031 1 



Syenite and syenite porphyry 00389 



Monzonite 00161 



Nepheline syenite 00024 



Shonkinite . 00054 



Theralite 00036 



Peridotite 00436 



Pyroxenite 0001 1 



Diabase 01602 



Dacite 00536 



Trachyte 00036 



Latite 00030 



Phonolite 00048 



Nepheline melilite basalt 00018 



Limburgite 00012 



Total 1 • 00000 



The result of the new calculation of the composition of the 

 average igneous rock is given in column I of the accompanying 

 table; for comparison, the most recent estimate by Clarke is given 

 in column II. 



