626 CROSS, IDDINGS, PIRSSON, WASHINGTON 



Those already suggested in the chapter on classification with 

 respect to actual mineral classification are two: (i) modifica- 

 tions of the norm, which are so slight as not to interfere with 

 the classification of the rock directly from the mode; and (2) 

 modifications of the norm so considerable that the rock cannot 

 be classified directly from the mode without readjustment of the 

 molecules in accordance with the method of calculation for 

 obtaining the norm. 



In other words, we may distinguish (1) variations among the 

 actual minerals which fall within the limits possible in rocks with 

 normative modes, and (2) those variations among the minerals 

 which cause the rock to possess an abnormative mode. 



1. Minerals of the first kind may be termed varietal minerals. 

 They may be defined as minerals whose presence serves to char- 

 acterize and distinguish different rocks of one magma unit, but 

 whose amount is so small that they do not affect the character 

 of the mode as normative or abnormative. Varietal minerals 

 include: standard minerals whose presence is not indicated by 

 the general expression of the norm, that is, the magmatic name, 

 though an exact statement of the norm would recognize their 

 presence ; or they may include any of the non-standard minerals. 

 For example : small amounts of quartz or feldspathoid in rocks 

 of Order 5, in Classes I, II, III ; or small amounts of horn- 

 blende and biotite, etc. 



It is proposed to express in the nomenclature the presence 

 of varietal minerals in the rock by adding to the name of the 

 mineral the suffix -ic. When it is necessary to use several varie- 

 tal qualifiers at one time the mineral names are to be connected 

 by hyphens and the suffix applied to the last, as hornblende- 

 biotitic alsbachose. 



2. Minerals of the second kind, whose presence and amounts 

 are such as to produce abnormative modes are called critical 

 minerals, as already mentioned. They are for the most part 

 alferric, but may be salic or femic, according to the mode of the 

 rock. 



It is proposed to express the presence of a critical mineral 



