CLASSIFICATION OF IGNEOUS ROCKS 609 



rocks of Class I and in the greater portion of those of Class II, 

 these two classes comprising over three-quarters of the known or 

 analyzed igneous rocks of the globe. The accord is also very 

 good in Class V, which includes but few rocks. Classes III and IV 

 show less constant accord, as is to be expected in view of their 

 composition. The results of our proposed method of calcula- 

 tion may therefore be regarded as satisfactory. 



Varieties. — It is evident that within the range of possible 

 variations in each of these modes for a given magma unit, there 

 may be mineralogical distinctions among the quite subordinate 

 components, and that it may be desirable to recognize these 

 differences as distinguishing features of rocks. The method of 

 accomplishing this is by considering the several subdivisions 

 upon this basis as Varieties, more specifically Modal Varieties. 



A Modal Variety of a petrographical unit of whatever degree, 

 such as Class, Order, Rang, etc., may be defined as a rock hav- 

 ing a mode with a slightly different development of the quite 

 subordinate component minerals. 



For example, a rock belonging to Class I composed of an 

 extreme amount of feldspar (5th Order) may have 10 per cent, 

 of nephelite actually crystallized, whereas the norm contains no 

 nephelite, the difference having been brought about by the 

 development of a small amount of hypersthene or of hornblende 

 instead of olivine. 



The development of a small amount of hornblende, or of 

 biotite, in a rock with a normative mode also constitutes a Modal 

 Variety. The presence of small (not notable) amounts of rare 

 or otherwise noteworthy minerals may also thus be recognized 

 as varieties. 



Indeterminable Modes. — Rocks not completely crystallized, 

 containing glass base, and those which for any reason do not 

 permit the determination of all the component minerals, must be 

 classified in the first instance in each case by comparison of the 

 observed minerals with the norm calculated from the chemical 

 analysis of the rock, and subsequently, by analogy with holo- 

 crystalline rocks of similar chemical composition whose texture 



