CLASSIFICATION OF IGNEOUS ROCKS 261 



of a rock has almost always been quantitative in character, the 

 mineralogical examination is rarely anything but qualitative ; 

 and, as a natural consequence, in all classifications based on 

 mineral characters, quantitative considerations have been almost 

 entirely disregarded. As long as the minerals which form part 

 of the definition of a rock are present, they may vary in amount 

 indefinitely without in most cases affecting its nomenclature or 

 its place in classification. 



It was with the object of remedying this particular defect, 

 and at the same time replacing the prevailing discordance with 

 uniformity and simplicity, that Professor Iddings and his 

 coadjutors devoted themselves for ten years to the elaboration 

 of their quantitative classification of igneous rocks. 



They recognised at once that by far the most important 

 character of an igneous rock was its chemical composition. 



" The chemical composition of the rock," we read on p. 108 

 of the Quantitative Classification, " is its most fundamental 

 character, being a quality inherent in the magma before its 

 solidification, and is therefore of greatest importance for its 

 correlation with other rocks." 



" All rocks of like chemical composition should be classed 

 together, and degrees of similarity should be expressed by the 

 relative positions or values of the systematic divisions of the 

 classification." 



" The mineral and textural characters, being dependent largely 

 on external conditions attending rock consolidation, are to be 

 regarded as of subsidiary importance in classification, but should 

 receive due recognition in the system." 



Again, on p. no: "Structure or texture is now known to 

 depend so largely on variable conditions attending the con- 

 solidation of magmas, that it can no longer be given the 

 prominent role hitherto assigned to it. Chemical and mineral 

 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 former 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." 1 



1 Cf. Prof. Bonney's Presidential Address, Quart. Journ. Gcol. Soc, vol. xli. 

 (1885) p. 66. 



