232 JOSEPH P. IDDINGS 



away from NA; more sodic feldspars would be found in rocks- 

 lower in silica. 



If the anorthite molecules be replaced by orthosilicates of 

 iron, or of iron and magnesium, less silica would be required 

 than before, and the quartz limit would shift to the right. If 

 they were replaced by metasilicates of iron, magnesium or 

 calcium, more silica would be required, and the quartz limit 

 would shift to the left. If both were developed together the 

 quartz limit might remain nearly as before. If magnetite were 

 developed the effect on the quartz limit would be still more 

 marked, and it would be shifted still farther in the direction of 

 lower silica percentages. 



Second, let all of the alkali be potash. Assuming that in this- 

 case there would be two distinct kinds of feldspar present,, 

 potash-orthoclase and anorthite, the range of rocks consisting 

 wholly of these two minerals in variable proportions would lie 

 along the red line OAn, Diagram 3, and the line QOL would be 

 the range of rocks of this kind free from anorthite. All rocks 

 of this kind occurring to the right of the line OAn must contain 

 leucite ; all those to the left must contain quartz. The effect 

 of replacing anorthite molecules by ferromagnesian molecules 

 would be similar to that just stated for the albite-anorthite mag- 

 mas ; however, orthoclase not being combined with anorthite, 

 would remain as before except where changes in the silica 

 affected the orthoclase molecules by reducing them to leucite. 

 or affected the leucite molecules by raising them to orthoclase.. 

 This would occur in the neighborhood of the line OAn, 



Third, if soda and potash were both present, other things 

 being the same as postulated in the general statement of case 

 III, it is evident that the range of rocks that might consist 

 wholly of feldspars, either pure albite, pure orthoclase, or pure- 

 anorthite, or any possible mixture of these, would be within 

 the area AOAn, Diagram 3. This narrow area would be the 

 boundary between the quartz range on one side and the leucite- 

 nephelite range on the other. The effect on the albite-anorthite 

 feldspar, whose character would be that previously indicated if 



