CLASSIFICATION OF IGNEOUS ROCKS 567 



deserving of the same name. The consistent application of such 

 a system of grouping would therefore separate rocks which are 

 alike so far as intrinsic qualities are concerned. And this, 

 according to our conception of classification, is not classification. 

 Many of these facts are essential to the complete petrological 

 understanding and description of rocks, but are not applicable 

 to the construction of a petrological system. 



The inherent characters of igneous rocks have always been 

 prominent in the formation of petrographic systems, and are 

 plainly the features it is most natural to select. This was spe- 

 cially pointed out by the fathers of systematic petrography, von 

 Leonard and Brongniart, and has been emphasized in recent dis- 

 cussions. Of these characters chemical and mineral composi- 

 tion, structure or texture, are the most important, the others 

 being comparatively trivial or accidental. Structure or texture 

 is now known to depend so largely on variable conditions attend- 

 ing the consolidation of magmas that it can no longer be given 

 the prominent role hitherto assigned to it. Chemical and min- 

 eral composition then remain as those characters of igneous 

 rocks most available for their classification. Of these, it is to 

 be noted, that while the two are most intimately related, the for- 

 mer is more fundamental, since it pertains to a magma which 

 may consolidate as a glass or become a holocrystalline rock, and 

 in the latter case the mineral constitution varies with attendant 

 conditions. 



CHEMICO-MINERALOGICAL CLASSIFICATION. 



While the chemical composition of igneous rocks is their 

 most fundamental characteristic, it is known that there is an 

 absence of stoichiometric proportions among the chemical ele- 

 ments or components. It is further clear that there is an 

 intricate interrelationship and serial variation among these com- 

 ponents and an absence of chemical division lines, or of groups 

 or clusters of similar combinations of elements. These facts show 

 that any subdivision on a purely chemical basis must be arbi- 

 trary, unpractical and unsatisfactory. 



All holocrystalline, and many of the partially crystalline 



