CLASSIFICATION OF IGNEOUS ROCKS 63 I 



specific magmatic names can be given to it. In each of these 

 cases the critical mineral and the texture can be indicated by 

 the same qualifying terms, or more precise ones if needed. 



In this respect this system possesses a distinct advantage 

 over former ones, in which no attempt has been made to indicate 

 in the nomenclature the degree of exactness with which the rock 

 is known or is to be described. 



3. To indicate the actual mineral character of a rock when 

 its magmatic name is given it is only necessary to express either 

 the fact that it is standard or the departure from the norm by 

 mentioning those critical minerals whose presence induces 

 changes in the norm, or those varietal minerals which may be 

 present. When such exactness is desired : 



a) If the rock possesses a normative mode the actual mineral 

 composition is expressed by using the word normative before 

 the magmatic name. 



b) If the rock does not possess a normative mode and it is 

 desired to indicate the critical minerals present, it is proposed 

 that such mineral qualifiers be used in some cases without intro- 

 ducing quantitative modifications. These mineral qualifiers 

 may be used as full names attached to the magmatic name by a 

 hyphen, as is the present practice, or they may be abbreviated 

 and compounded. 



c) It is suggested that the presence of small amounts of 

 important minerals be indicated by adding the suffix -ic to the 

 mineral name. 



4. The texture is to be indicated by adjectives expressing the 

 fabric, the crystallinity, and the granularity, and may be the terms 

 in common use, those suggested above, or abbreviations of these. 



5. Either mineral or textural qualifiers may be placed next 

 to the magmatic name according to the emphasis to be given 

 them, it being generally understood that the term nearest the 

 magmatic name carries the strongest emphasis ; the magmatic 

 name coming last. The same rule is to be applied to the 

 arrangement of several mineral qualifiers, that nearest the mag- 

 matic name is to be considered the most important. 



