1 68 ALBERT JOHANNSEN 



lower corners of the large triangle with the similar corner of the 

 small one, and indicate its intersection with the line first drawn 

 by a dot, F. The line hd and the spot F represent the rock. The 

 point F is the same as that which would have been obtained by 

 the slide rule, and gives the relative proportions of the light con- 

 stituents (quartz [mF], orthoclase [kF], and plagioclase [^'i^]), in 



Quartz 



Orth . "'" ^ ^ ^ ^ ^ ^ ^ — ^^ ^ ^ Flag . 



S m 



the rock by its distances from the sides of the triangle opposite 

 these names at the corners. The actual percentages of the minerals 

 in the rock are also represented : he or pk for orthoclase, ah or io for 

 plagioclase, hd for the dark constituents, dm for quartz. 



In a similar manner the composition of the rock at L may be 

 read directly from the diagram, giving the values: 



Quartz 23 = 27.7 



Orthoclase 45 = 54 . 2 



Plagioclase 15= 18. i 



83 100. o 

 Dark minerals 17 



100 



