THE AVERAGE IGNEOUS ROCK 



in 



lines have been placed shows that there is marked uniformity in 

 the ratio of granite to basalt, as expressed by the positions of inter- 

 section on these several lines. The average of the ratios expressed 

 by the points of intersection on each line has been indicated by a 

 large dot near the line, and the positions of these dots on the scale 



Basalt IQ0% 



CraniTe.««, Basalt- 35% 



Fig. 2. — Proportional triangle used as a scale on which to measure the granite- 

 basalt ratios expressed by the position of intersections on the full black lines in Fig. i. 



of the proportional triangle show a distinct concordance. By 

 averaging the ratios expressed by the dots on the several lines, 

 the average ratio of granite to basalt is found to be 65 parts granite 

 to 35 parts basalt, i.e., this is the proportion of granite to basalt 

 which may be most closely approximated by some combination of 

 derived sediments represented by their average analyses here 

 employed. 



