A REVIEW OF IGNEOUS ROCK CLASSIFICATION 69 



and III. are of the same size and shape, and are built of the 

 same material ; those in Classes IV. and V. are of entirely 

 different size and shape, and are built of different material. 

 Or three-fifths of our reticulated background is drawn to the 

 same pattern ; two-fifths to an entirely different pattern. 



The reason for changing the basis of the orders is that 

 quartz, lenads, and felspars cease to be the dominant minerals 

 in Classes IV. and V., and give place to pyroxenes, olivines, 

 etc. The change, however, not only transgresses the principle 

 that a single pair of factors should be applied consistently 

 throughout, but also the principle that classification should be 

 based throughout on factors which are dominant in the whole 

 series of igneous rocks. It can hardly be denied that the 

 salic (or felsic) minerals are by far the most abundant and 

 significant constituents when the whole field of igneous rocks 

 is considered. Taking the general mean of all analyses of 

 igneous rocks as calculated by Clarke, 1 and as recalculated 

 by Cross into the norms, 2 we find that the four salic con- 

 stituents total 79 per cent., and the femic 21 per cent. If the 

 bulk of the various igneous rocks were taken into account it 

 is possible that the ratio of salic to femic constituents would 

 be still higher. 3 The primary factors in the classification there- 

 fore should be the salic ratios, and the femic ratios should 

 only be used after the salic are exhausted. 



The convenience, simplicity, and correctness of using the 

 salic ratios right through the Quantitative Classification can be 

 shown by the consideration of some petrographic series which 

 traverse the partition between Classes III. and IV. The 

 magmatic symbols invented by C.I.P.W. are found very useful 

 in this connection. The subrang salemose, for example, is 

 represented by the symbol II. 6.3.4., indicating that a rock 

 in salemose falls into Class II. (dosalic), Order 6 (lendofelic), 

 Rang 3 (alkalicalcic), and Subrang 4 (dosodic), and thus has 

 a definite magmatic character. The persistence of some of 

 these numbers, or their sympathetic or antipathetic variation 

 one with the other, in the symbols of a related series of rocks, 

 indicates the persistence or regular variation of certain magmatic 

 characters. The consanguinity of the members of certain 



1 "Some Geochemical Statistics," Proc. Amer. Phil. Soc. xli. 1912, pp. 214-34. 



- Jour. Geol. xx. 191 2, p. 759. 



3 Mennell. Geol. Mag. 1904, p. 263 ; 1909, p. 212. 



