CHEMICOMINERAL RELATIONSHIPS IN ROCKS 23 I 



indicated by the lines QA, Qb, . . QAn and NA, Nb, . . NAn, 

 which also indicate the limits for magmas of this kind. None 

 would be possible to the right of the line NAn, unless containing 

 other elements than those already mentioned for this case, III. 



Since within the boundaries QNA/i, rocks may exist com- 

 posed wholly of quartz and the feldspathic minerals mentioned, 

 the presence of any ferromagnesian compound necessitates the 

 diminution of some of the first-named constituents, and since 

 the silica percentages and alkali-silica ratios are the coordinates 

 by which comparisons are made, we may assume that the 

 presence of ferromagnesian constituents affects the lime and its 

 equivalent alumina. Let them be affected equally. The ortho- 

 silicate anorthite molecules may then be replaced in some 

 uniform manner by non-aluminous ferromagnesian orthosilicates 

 or metasilicates. If the anorthite molecules be replaced by 

 orthosilicate of iron, Fe 2 Si0 4 , the varying results are indicated 

 in Diagram 2. If wholly replaced by Fe 2 Si0 4 , the line AFa 

 would represent the range of the resulting rocks composed of 

 varying proportions of albite and Fe 2 Si0 4 . And the area 

 AFaA?i would be the range of rocks composed of varying pro- 

 portions of albite and Fe 3 Si0 4 and anorthite. The lines 

 bFa, cFa, etc., indicate the ranges of rocks composed of Fe 2 Si0 4 

 and AbgAnj, Fe 2 Si0 4 and Ab 2 An 1 , etc. That is, ranges in 

 which the character of the lime-soda feldspar is constant, but 

 the proportions of Fe 2 Si0 4 vary. The line Ay shows the range 

 of rocks composed of albite, with varying amounts of anorthite 

 and Fe 3 Si0 4 in the proportion of 4 to 3. At the points 

 b' , c' , d' , etc., where this line intersects the lines bFa, etc., the 

 rocks would consist of Fe 2 Si0 4 and the feldspars Ab 6 An x , 

 Ab 3 An 1( etc. From this it is evident that as the molecules of 

 anorthite are replaced by ferromagnesian molecules, the feld- 

 spars in a rock of any given silica percentage and alkali-silica 

 ratio become relatively higher in albite molecules, that is, the 

 lines Ob, Oc, etc., would shift farther from QA; more sodic 

 feldspars would occur in rocks with lower alkali-silica ratios. 

 There would be a similar movement of the lines Nb, Nc, etc., 



