THE REPRESENTATION OF IGNEOUS ROCKS 169 



The rock at S is seen to consist of orthoclase 27 per cent, 

 plagioclase 42 per cent, quartz o per cent, and dark minerals 

 31 per cent. The relative ratios of the hght constituents are: 

 orthoclase 39.1 per cent, plagioclase 60.9 per cent, and quartz 

 o per cent. The rock at T consists of orthoclase o per cent, 

 plagioclase 66 per cent, quartz 24 per cent, dark constituents 

 10 per cent. Relative ratios of the light minerals, plagioclase 

 73.5 per cent, quartz 26.5 per cent, and orthoclase o per cent. 



The actual plotting takes much less time than the telHng. In 

 fact the small triangle is never plotted. The upper end (b) of the 

 line is located at the dividing point between orthoclase (be) and 

 plagioclase (ab) on the horizontal Hne whose length (ac) is equal to 

 the sum of the feldspars, and through this point the inclined line 

 is drawn to the percentage position of quartz (d). . One of the 

 lower corners of the small triangle is located, without drawing it, 

 and the intersection of the inclined line with the ferromagnesian 

 mineral line used to determine the locus of the rock. 



