CLASSIFICATION OF IGNEOUS ROCKS DO 5 



the same as those calculated from the rock analysis. Variations 

 may occur in the kind of subordinate minerals for reasons which 

 will be explained presently. 



On the other hand, magmas containing nearly equal amounts 

 of both salic and femic molecules or dominant amounts of the 

 latter may develop alferric (aluminous ferromagnesian) minerals 

 to such an extent as to prevent the crystallization of any appre- 

 ciable amount of the standard minerals. A well-known example 

 of this is the hornblendite of Brandberg, in Gran, Norway, 1 a 

 rock of Class III, which crystallized as almost pure hornblende. 



A similar magma, however, may crystallize into minerals 

 corresponding closely to the norm, as in the case of the nephelite- 

 basanite of Colfax county, New Mexico. 



It follows from this that in some divisions of the system the 

 actual mineral composition of holocrystalline rocks will accord 

 very closely with their norms, while in others it may or may not. 

 And in these cases there may be various degrees of correspond- 

 ence between the mode and norm. 



In general a number of degrees of correspondence between 

 the actual and the standard mineral composition of igneous rocks 

 may be recognized. They may be expressed as follows: 



1. Complete or almost complete accord between the 

 mode and norm. — In the first case the two correspond exactly 

 both qualitatively and quantitatively. By almost complete accord 

 is meant one in which all the principal minerals recognized in 

 establishing the norm are actually present in the rock in approxi- 

 mately the same proportions as in the norm, or else standard 

 minerals not calculated in the norm or non-standard minerals 

 are present in such small amounts as not to affect the posi- 

 tion of the rock in the system. In these cases of complete or 

 almost complete accord the rock may be said to have a normative 

 mode. 



A nonnative mode may be defined as one in which the actual 

 mineral compositio?i of the rock is so nearly the same as the 

 standard mi?ieral composition calculated from the analysis that 

 either may be used to classify the rock correctly. 



1 Loc. cit. 



