230 JOSEPH P. ID DINGS 



amounts of alumina would behave in a manner analagous to that 

 just described for either extreme, the feldspars would be soda- 

 potash polysilicates, with quartz or with leucite or nephelite. The 

 lines indicating the range of combinations of feldspar and ferro- 

 magnesian silicates would occupy positions intermediate between 

 the corresponding lines for albite and for orthoclase. The 

 quartz-leucite-nephelite limits would be nearly the same as 

 before. 



III. Let us now postulate the case of magmas in which the 

 alkalis control an equal amount of alumina and in which lime ami 

 additional alumina occur in the proportion of one to one and co?istitute 

 orthosilicate , anorthite molecules. First, let all of the alkali be 

 soda. This case is illustrated by Diagram 2. As the relative 

 proportions of the constituents, Na 3 0= (A1 3 3 ) ', CaO~ 

 (A1 2 3 )" and SiO s vary, those rocks in which there is soda 

 and no lime will consist of albite and quartz, or of albite and 

 nephelite, and will occur along the line QAN, Diagram 2, as in 

 the previous Diagram 1. All rocks occurring below this line 

 will contain anorthite molecules in variable proportions. Pure 

 anorthite rock will occur at the point An, where the alkali- 

 silica ratio = 0, and Si0 3 =43.2. The range of rocks composed 

 wholly of albite and anorthite in varying proportions will occur 

 along the line A An. They correspond to rocks each consisting 

 of a feldspar of the albite-anorthite series. The points on this 

 line at which particular mixtures occur, such as AbgAn^ 

 Ab 3 An,, etc., are designated in the diagram. This line also 

 indicates the range of rocks consisting wholly of soda-lime- 

 feldspar. All rocks from magmas of this case, III, to the right 

 of this line must contain nephelite in addition to these feldspars. 

 All rocks to the left must contain quartz, the argument being the 

 same as for magmas of cases I and II. Assuming that the 

 feldspar present is always one corresponding to the particular 

 combination of albite and anorthite molecules in the magma, 

 we may discover the range of magmas consisting of combina- 

 tions of each of these kinds of feldspar and quartz on the one 

 hand, and of these and nephelite on the other. They are 



