584 CROSS, IDDINGS, PIRSSON, WASHINGTON 



Subrang. — The comparison of those bases which are united 

 to form the groups of bases recognized in forming Rangs con- 

 stitutes Subrangs. 



Grad. — The characters of the subordinate mineral group, 

 femic or salic, will be treated in a manner analogous to that 

 employed when considering the preponderant, salic or femic, 

 group, the only difference being that as the group under con- 

 sideration is subordinate fewer distinctions will be needed. 



Grads will be based on the general acidic proportions in the 

 minerals of the subordinate group, and will follow the plan of 

 the divisions for Order in the preponderant group. 



Subgrad. — These divisions will follow the lines of the sub- 

 divisions for Rang and Subrang, and will express the general 

 and special chemical characters of the bases of the minerals of 

 the subordinate mineral group. 



Sections. — The application of the above principles shows, 

 however, that in certain points more numerous subdivisions are 

 needed. This necessity is met by the formation of Sections of 

 any of the divisions above described. These Sections will be 

 based on more special characters according to circumstances. 

 No general rule can be laid down for them, but they will be 

 explained in their respective places in the subsequent descrip- 

 tion of the various divisions. 



Family and Series. — The grouping of rocks proposed in this 

 system of classification is for the purpose of bringing together 

 rocks that are alike chemically, and also mineralogically and 

 texturally. There is need, however, in the broader treatment of 

 igneous rocks, especially with reference to their genetic rela- 

 tions and to their occurrence in petrographic provinces, to group 

 them in other ways. For these purposes the terms Family and 

 Series are appropriate, and it is proposed that they be used as 

 follows : 



The term Family may be applied to a group of rocks that 

 have been developed genetically from a common magma by 

 processes of differentiation. In its broadest sense it may be 

 applied to all the rocks of a petrographic province. But it is 



