274 



E. Warth — Composition of Igneous Rocks. 



The percentage of silica varies from 80 to 40, and the usual 

 limits between acid, intermediate, and basic rocks are indicated by 

 the vertical lines on the diagrams. The diagrams show the per- 

 centage of the different bases which corresponds with the percentage 

 of silica indicated on the straight diagonal boundary-lines of the 

 silica which rise from the points marked 80 per cent, on the left of 



Vertical sca^e. Percentaoe 



■iS- O 20 -W "^ Si 



Diagram-Fig. 2. 



each diagram to the points marked 40 per cent, on the right of each 

 diagram. As it would not be possible to give so many details on 

 the diagram, small quantities of other acids have been reckoned 

 with the silica and treated as such. These acids are titanic acid, 

 phosphoric, carbonic, and in rare cases also sulphuric acid. In the 

 case of the bases I also included the manganous oxide, and a few 

 other rarer metallic oxides with the ferrous oxide. The total number 

 of specimens which were calculated amounted to 428. About sixty- 

 six of Eoth's analyses were omitted, partly because they referred 

 to strongly weathered specimens, and partly to exclude serpentines 

 and some other more purely magnesian silicates. The specimens as 

 given by Eoth are from generally distributed localities, and the 

 following are the compositions of extremes and of the total average 

 in numerical figures : — 



Most acid. . . 

 Most basic 

 je ... 



