CLASSIFICATION OF IGNEOUS ROCKS 593 



in this system at one time, and it is necessary to combine two of 

 the three to form a dual basis for a first subdivision. This is 

 accomplished by uniting the first two and comparing the pro- 

 portions of MgO, FeO, CaO", and the alkalies, K 2 0", Na 2 0". 

 In this case CaO", K 3 0" and Na 2 0", are the parts of these rock 

 components allotted to femic minerals. For the adjective 

 expressing the first named quality, corresponding to alkalic, we 

 propose to use the word mirlic, referring to the magnesium, 

 iron and /ime. Upon a fivefold basis of comparison of these 

 two sets of chemical constituents we form Rangs of Classes 

 IV and V, although the more alkalic Rangs here provided are 

 not yet known. They are: 



(Mg, Fe)0 + CaO" 



Rang 1 : 

 Rang 2 : 

 Rang 3: 

 Rang 4 : 

 Rang 5: 



K 2 0"+Na 2 0" 



(Mg, Fe)Q + CaO" 

 K 2 0"+Na 2 0" 



(Mg, Fe)Q + CaO" 

 K 2 0"+Na 2 0" 



(Mg, Fe)Q + CaO" 

 K 2 0"-hNa 2 0" 



(Mg, Fe)0 + CaO" 



K 2 0"+ Na 2 0" 



Sections of Rangs in Classes IV and V are necessary to recog- 

 nize the varying proportions between the ferromagnesian and 

 calcic characters of femic minerals, since the magnesium and iron 

 are so closely associated chemically and characterize certain 

 femic minerals free from calcium. For the word ferromagnesian, 

 which is used in a somewhat loose manner in petrography, we 

 propose to introduce the word, miric, to indicate strictly the 

 magnesium and iron content in femic minerals, and to be mne- 

 monic of these two metals. 



These Sections of Rangs in Classes IV and V have been 

 made on a fivefold basis as follows : 



' . (Mg, Fe)0 7 



Section 1 : - — ° ' — > - , perminc. 



CaO 1 



