574 CROSS, IDDINGS, PIRSSON, WASHINGTON 



The intermediate group c) of aluminous ferromagnesian 

 and calcic silicates will be designated alfer or alferric group, this 

 name recalling the fact that these minerals are characterized by 

 the presence of #/umina and y^rric oxide. 



Grojip I. — The minerals of groups a) and U) have been 

 united to form the salic group (I) because of the well-recog- 

 nized relations between the development of quartz, feldspar 

 and feldspathoids in rocks and the available silica in the magmas ; 

 these minerals forming frequent series of rocks with a regular 

 range of silica. It is also done because of the association of 

 notable amounts of zircon and corundum with these minerals in 

 the more quartzose and feldspathic rocks. And further it is in 

 accord with the stronger affinities of the bases, potassium and 

 sodium, for silica and alumina, which will be discussed later. 



The kaliophilite molecule is recognized, since its presence is 

 necessary in a few magmas so low in SiOg as not to allow the 

 formation of both leucite and akermanite. Muscovite is omitted 

 from the list in order to simplify the process of calculation. It 

 may be considered as made up of orthoclase with corundum and 

 water. Analcite is also omitted for a similar reason. It may 

 be considered to be composed of albite, nephelite and water. 

 Hauynite is omitted because it may be regarded as calcic nose- 

 lite, and because it introduces needless complications into the 

 calculation. The standard SO s -bearing feldspathoid is there- 

 fore considered to be a purely sodic noselite. Cancrinite is 

 likewise omitted as being in most cases of secondary origin, as 

 well as for purposes of simplification. 



Group //.—The minerals of groups d) and e) have been 

 united to form the femic group (II) because of their freedom 

 from alumina and their association in notable amounts in rocks 

 low in silica and alumina. They are in this sense antithetical to 

 the salic minerals. 



Wollastonite (CaSiO s ) is added to the list of femic minerals 

 in order to simplify the calculation of standard minerals in 

 rocks rich in calcium, which may actually enter aluminous 

 molecules, such as garnet. 



