CLASSIFICATION OF IGNEOUS ROCKS 275 



small, the salic lime is diminished, and the ratio of salic alkali 

 to salic lime is correspondingly increased, and vice versa. 



The rangs of the first three classes being divided into 

 sub-rangs on the basis of the ratio between the salic potash 

 and salic soda, the variations in chemical composition remain 

 in almost all other respects as great as in the rangs, the chief 

 exception being the sub-rangs of the orders in whose determina- 

 tion the lenads play a part. Here the variation caused by the 

 different silica percentages of leucite and nepheline practically 

 disappears when the ratio of the salic potash and soda becomes 

 fairly constant. 1 



Throughout the classification, both in the large and small 

 divisions, the extent of the silica variation is very striking, 

 and though it may not be relatively greater than that of the 

 other oxides, yet it has on the whole a greater influence on 

 the character and mineral composition of the rock. 



It is sufficiently apparent that the lines of division of the 

 Quantitative Classification do not stand in any logical relation 

 to the chemical composition, and that it does not satisfy the 

 condition laid down by the authors that " 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 principle of employing larger groups than the oxides 

 as units of classification undoubtedly finds considerable support 

 from the phenomena attending the differentiation and consolida- 

 tion of igneous rocks. 2 A scientific classification should, as 

 far as possible, express the origin and genetic relations of 

 the divisions and subdivisions to which it gives rise. Igneous 

 rocks, being formed by the consolidation of magmas, should 

 be classified with reference to the laws in accordance with 

 which igneous magmas are differentiated, either while still 

 in the liquid state or in the course of consolidation. 3 



These laws are still imperfectly known, but there is no 



1 It is for this reason, I presume, that the sub-rangs are most frequently used 

 in nomenclature. 



2 Iddings, The Chemical Composition of Igneous Rocks, p. 69. 



3 While it is probable that the majority of igneous magmas are the result of 

 the differentiation of the original molten substance of the earth, there are some 

 rocks which appear to be formed from magmas of an unusual type, due to the 

 absorption of solid material by a liquid magma. These would have to be specially 

 dealt with in any scientific classification. 



