CLASSIFICATION OF IGNEOUS ROCKS 625 



tion and the actual, there is obviously no need of mentioning 

 details of the actual mineral composition, since this is fully 

 expressed by the norm. To indicate this mode it is only neces- 

 sary to use the word ?wrmative, before the magmatic name and 

 connected with a hyphen. 



As has been already remarked, though all the various possi- 

 ble combinations of the feldspar molecules are regarded as 

 standard, yet it will often be found needful to indicate the 

 special feldspars actually crystallized. In this case, since the 

 albite most often crystallizes with the anorthite molecule, form- 

 ing a soda-lime plagioclase, and since the character of this will 

 condition that of the alkali-feldspar, it will in general only be 

 necessary to name the soda-lime plagioclase which is present. 

 If, however, it is desirable to indicate the presence of microline 

 or anorthoclase, this can also be done. 



To express the extent to which the mode of a rock differs 

 from the norm it is necessary to state the kinds of minerals which 

 are different as well as the amount to which they have been 

 developed. 



It is proposed to use the names of the minerals in question 

 as adjective qualifiers of the magmatic name, as has been the 

 custom with such names as mica-diorite, and to express their 

 quantitative relations by means of prefixes or suffixes, or by the 

 order of their arrangement when several mineral names are 

 employed. 



Owing to the interdependence of the minerals in any rock on 

 each other and on the chemical composition of the magma, it is 

 evident that, given the norm of a rock, which is involved in its 

 magma name, it is only necessary to state the presence and 

 amount of certain minerals developed, which are not in accord 

 with the norm, in order to determine the modifications of the 

 standard minerals consequent upon the development of these 

 particular ones. 



And since such modifications take place in all degrees, from 

 the slightest possible change to the greatest possible, it is neces- 

 sary to recognize in a systematic manner differences of degree. 



