CLA SSIFICA TION OF IGNEO US RO CKS 643 



too small, and because of the incomplete crystallization of all 

 more or less glassy rocks. 



The variability in the development and chemical composi- 

 tion of the alferric mifierals justifies us in treating them as com- 

 binations by readjustment of salic and femic molecules. Their 

 chemico-mineralogical relations and the method of calculating 

 their proportions will be stated later on. 



The method of calculation adopted is based upon a number of 

 commonly observed chemico-mineralogical relations that obtain 

 in the rock-making minerals, which may be stated as follows : 



Chemical relations among salic minerals. — 1. The con- 

 stant relation between the molecules of Al 2 O s and K 2 and 

 Na 2 in orthoclase and albite, leucite and nephelite (A1 3 3 : 

 K 3 b + Na 2 : : 1 : 1). 



2. The somewhat similar relation between these constituents 

 in the sodalites, where the soda is slightly in excess. The ratio 

 in sodalite is Al 2 O s + - 3 C1 2 : Na 2 -f- |Na 3 : : 1:1; in 

 noselite it is A1 2 3 + ^SO s : Na 2 + |Na 2 : : 1 : 1. 



3. The constant relation between A1 2 3 and CaO in the 

 anorthite molecule ( A1 2 3 : CaO :: 1:1). 



4. The development of corundum under favorable conditions 

 in rocks with excess of A1 2 3 over K 2 0, Na 2 0, and CaO. 



5. The relation between the development of alkali-feldspar 

 (polysilicates) and of feldspathoids (meta- and ortho-silicates) 

 and the available silica in the magma, so that free silica 

 (quartz) does not crystallize together with leucite and nephelite. 



6. The stronger affinity of A1 2 3 for K 2 and Na 2 than 

 for CaO, A1 2 3 forming alkali-feldspars and feldspathoids 

 (lenads) in preference to anorthite. 



7. The constant ratio between ZrO a and Si0 2 in zircon, 

 ZrO s : Si0 3 : : 1 : 1. 



Chemical relations among femic minerals. — 1. The con- 

 stant relation between Fe 3 O s and Na 2 in the acmite molecule 

 (Fe 3 3 : Na 3 : : 1 : 1). 



2. The general fact that this molecule is developed in mag- 

 mas when K 2 and Na 3 are in excess of A1 2 3 . 



