CHEMICOMINERAL RELATIONSHIPS IN ROCKS 229 



sodium-aluminium orthosilicate. If we conduct the discussion 

 in the same manner as in the previous case we should proceed 

 from the point 0, the position for a pure potash-orthoclase rock. 

 At this point the alkali silica ratio — . 166 and SiO s =64.7. Rocks 

 consisting of pure orthoclase and leucite will occur along the line 

 OL; those composed of pure orthoclase and quartz along the 

 line OQ. Rocks having an alkali-silica ratio of .166 and less 

 than 64.7 per cent, of silica must contain some other constituents 

 than K 2 = A1 3 3 and Si0 2 . The same is true for all rocks 

 lying below the line QOL. The range of rocks that might 

 consist of pure orthoclase and orthosilicate of iron is shown by 

 the red line OFa, close to the corresponding line for soda 

 magmas. All rocks occurring above this would necessarily 

 contain leucite, all below it would contain quartz. The lines 

 001 and OFo represent the range for rocks consisting respectively 

 •of orthoclase and olivine, and of orthoclase and orthosilicate of 

 magnesium. They also mark the limit of the quartz-bearing 

 rocks of this kind on one side, and of leucite rocks on the other. 

 The lines for rocks composed of orthoclase and metasilicates 

 of iron, magnesium and calcium are OF, OC, OM. They are 

 somewhat further from the corresponding ones for soda magmas 

 than in the case of the orthosilicates. Comparing their positions 

 with those of the corresponding lines for soda magmas, it is 

 seen that the quartz limit is lower in the scale of silica percent- 

 ages for potash magmas than for soda magmas. That is, other 

 things being equal, quartz will occur in less siliceous potash 

 rocks than in the corresponding soda rocks, the most marked 

 differences being in rocks highest in alkalis. 



The development of magnetite would have the same effect 

 in shifting the quartz-leucite limits toward the right, that is, 

 toward rocks with lower silica percentages as in the case of 

 soda magmas. The line LFa is the rano;e for magmas com- 

 posed of leucite and orthosilicate of iron and seems to be the 

 lowest limit for potash magmas, as stated on page 102 of this 

 volume. 



Magmas in which both soda and potash occur with equal 



