CLASSIFICATION OF IGNEOUS ROCKS 



60 1 



portions of miric and calcic constituents. They need only be 

 used in Rangs I, 2, and 3. 



Section 1 

 Section 2 

 Section 3 



(Mg, Fe)0 ^ 5 

 CaO" > 3 ' 



(Mg, Fe)0 S3 

 CaO" "35 



(Mg, Fe)Q 3 

 CaO" ^ 5 ' 



preminc. 



calcimiric. 



precalcic. 



Subsections of Sections I and 2 are based on the proportions 

 of magnesic and ferrous components as in the case of Subrangs 

 in Classes IV and V. They are : 



MgO^ 5 



Subsection 1 

 Subsection 2 



Subsection 3 : 



FeO 

 MgO 5 3 

 FeO ^ 3 5 

 MgO 3 



FeO ^ 5 ' 



premagnesic. 

 magnesiferrous. 



preferrous. 



It is understood, of course, that all the divisions just 

 described are not needed, as has been explained in regard to 

 those based on the preponderant group of minerals. Thus a 

 calcic Subgrad is not needed in a Grad with dominant lenads. 

 In such cases it is simply inactive. 



THE ROLE OF ACTUAL MINERAL DEVELOPMENT AND TEXTURE IN 

 ROCK CLASSIFICATION. 



The methods employed in forming Classes, Orders, Rangs, 

 Grads, and their subdivisions are distinctly chemico-minera- 

 logical and chemical, the minerals considered being those 

 denned as standard. These divisions are applicable to magmas 

 regardless of their mode of solidification or crystallization. The 

 classification so far is purely magmatic. 



The chief object attained up to this point by the system 

 proposed is the uniting of all igneous rocks having like chemi- 

 cal compositions into divisions or groups which conform to our 

 present conceptions of chemical petrographical unity. And it 

 has been our endeavor at the same time to prepare such a 



