600 CROSS, ID DINGS, PIRSSON, WASHINGTON 



and V, but are on a threefold basis because of the smaller 

 amount of femic minerals, as follows : 



(Mg, Fe)0 + CaO" 5 



Subgrad 1 = K a O'+Na a O' > 3 ■ P remirllc - 

 qk a (Mg, Fe)0 + CaO" ^5^3 ur . r 



Subgrad 2 : K a O' + Na a O' < 3 > 5 ' alkahmirhc - 

 n ,_ J (Mg, Fe)0 + CaO" 3 „ ,. 



Sllb§rad 3: K a O' + Na a O' < 5 ' P realkahc - 



In Class III Subgrads are formed on a basis of the chemical 

 characters of the femic minerals by analogy with those of Class 

 II, but because of the greater proportions of these minerals the 

 division is on a fivefold basis, yielding : 

 (Mg, Fe)0 + CaO" 



Subgrad 1 : 

 Subgrad 2 : 

 Subgrad 3 : 

 Subgrad 4 : 



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)0 + CaO" 



K 2 0" + Na a O" 

 (Mg, Fe)0 + CaO" 

 Subgrad5: K 2 0" + Na a O" 



In Class IV the minerals forming Grads are salic, and since 

 they are subordinate to the femic it is advisable to make only 

 three divisions instead of five, as follows : 



Q , K / + Na 8 / 



Subgrad 1 : — - — ,, ,, . — 



CaO 



Subgrad 2 : 



Subgrad 3 



K 2 Q' + Na 2 Q' 



CaO' 

 K 2 0' + Na a O' 



CaO' 



Sectio?is of Subgrads in Class III are established because the 

 amount of femic minerals is equal or nearly equal to that of the 

 salic, and it is desirable to make further chemical distinctions as 

 in the case of Sections of Rangs in Classes IV and V. The 

 divisions are the same in both cases, and are based on the pro- 



