THE AVERAGE IGNEOUS ROCK 



175 



stone combine to yield the same percentage of silica as the average 

 granite, and also the proportion in which shale and limestone 

 combine to the same end. These points platted on the sides of 



fc,B._ 



Fig. I.— Graphical expression of the relations of any combination of granite and 

 basalt and the ratios of sediments derived from them. 



the triangle determine the line. In a similar manner lines are 

 drawn for the various oxides' for both the average granite and the 

 " Iron is calculated as total Fe, the oxides not being considered separately. 



