22 2 JOSEPH P. IDDINGS 



problem further, the several kinds of molecules may be pre- 

 sented in the following manner : 



Elements Kinds of molecules Kinds of minerals 



[ ( f i : 1:6 orthoclase, albite 



\ tjt xt ai I tv xt \ r\ Ain c • r\ .. i:i:4 leucite (amphibole) 



K, Na, Al, (K,Na)oO: AlOo: SiO, .. < * , i.\ y , ,., / . , 



-{ x j i : i :i nephelite, sodalite(with 



[_ excess of Na.) 



[(K,H) 2 0:Al,0 3 :SiO, :: 1:1:2 muscovite (A1„0 3 > 



KoO) 



K, Na, Al, Fe, (K,H) 2 0:(Al,Fe) 2 3 : Si0 2 : : 1 : 1 : 2 mica (biotite) 



Na, Fe, Na 2 0:Fe 2 3 : Si0 2 :: 1:1:4 acmite, riebeckite 



(_ f Na, Fe, Ca, (Na 2 ,Ca,Fe) O.Si0 2 arfvedsonite 



j Ca, Al, (CaO: Al 2 3 :Si0 2 :: 1:1:2 anorthite 



f [_ Ca, Mg, Fe, j CaO.(Mg, Fe)0.2Si0 2 pyroxene 



I Ca0.3(Mg, Fe)0.4Si6, amphibole 



•{ Mg, Fe, (Mg,Fe)O.Si0 2 pyroxene 



olivine, mica 

 |^Mg, Fe, Al, (Mg,Fe)0.(Al,Fe) 2 3 .Si0 2 pyroxene, amphibole 



The alkalis combine with an equal number of molecules of 

 alumina in the feldspathic minerals (except sodalites). These 

 are polysilicates, metasilicates and orthosilicates ; also in one 

 of the amphibole molecules ; while in muscovite part of the 

 potassium is replaced by hydrogen. The alkalis with hydrogen 

 enter into an orthosilicate, mica molecule with alumina and 

 ferric oxide in the proportion of 1 : I, the potash being generally 

 less than alumina. Soda enters into a metasilicate molecule 

 with ferric oxide, in the acmite molecule. It combines with lime 

 and ferrous oxide in the metasilicate, arfvedsonite molecule. 

 Calcium oxide enters an orthosilicate molecule with alumina in 

 proportions of 1 : 1. — anorthite. Calcium occurs with mag- 

 nesium and iron in metasilicate molecules in pyroxenes and 

 amphiboles, which differ in the relative proportions of CaO:(Mg, 

 Fe)0 ; in the first case in the ratio 1 : 1 ; in the second, 1 : 3. 

 Magnesium and ferrous iron usually occur together in variable 

 proportions, either in a metasilicate molecule with calcium in 

 monoclinic pyroxenes and amphiboles, or without calcium in 

 metasilicate molecules in orthorhombic pyroxenes, or in ortho- 

 silicate molecules in olivine and mica. They combine with alu- 

 minium and ferric iron in a subsilicate molecule in pyroxenes and 



