A REVIEW OF IGNEOUS ROCK CLASSIFICATION 63 



However manipulated, the chemical analysis is the unit dealt 

 with in the American Quantitative Classification. The analysis 

 of an igneous rock is first calculated into a set of standard 

 minerals (the norm), under fixed rules which, it is generally 

 admitted, follow and succinctly express most of the laws of 

 mineral formation in igneous magmas as we know them. 

 Certain important rock-forming minerals (the alferric — augite, 

 hornblende, biotite, muscovite, etc.) are not utilised in the norm 

 because of their complex chemical composition, although they 

 may actually be present in the rock. They are split up and 

 their components distributed to the normative minerals. The 

 norm is therefore a possible mode of crystallisation of all 

 magmas under certain conditions. Thereafter the classification 

 proceeds by taking factors from the norm two at a time, and 

 applying them consistently throughout (with one exception 

 explained later). It follows that the American Quantitative 

 Classification is a classification of chemical analyses or magmas, 

 not of the actual rocks. It is in effect a normative classification, 

 as contrasted with the modal classification which is forced on 

 the working petrographer by the sheer impossibility of obtaining 

 chemical analyses of all the rocks he wishes to describe. 1 



The norm is first divided into salic (quartz, felspars, fels- 

 pathoids) and femic (pyroxenes, olivine, ores, etc.) groups, 

 whose relative proportions furnish the first line of subdivision 

 into Classes. Five Classes, bounded strictly by arithmetical 

 ratios between the salic and femic groups, are thus formed, and 

 express quantitatively Brogger's division of igneous rocks into 

 leucocratic and melanocratic phases ; not, as stated by Cross, 

 the old subdivision into acid, subacid, sub-basic, basic, and 

 ultrabasic. The first three Classes are then each divided into 

 nine Orders on the basis of the ratios of normative quartz or 

 lenads (felspathoids) to the felspars present. This really ex- 

 presses the variations of rocks in respect to the ratio between 

 alkalis and silica. Assuming the ferromagnesian minerals and 

 anorthite to have their necessary quota of silica, the presence 

 of quartz or felspathoid in a rock depends on whether the 

 remaining silica does or does not exceed that necessary to the 

 formation of alkali-felspar. The latter may be independent, or 



1 The utility of the norm is unquestionable. As an instrument for comparing 

 rocks, especially those it is impossible to compare modally, it is of great value, as 

 well as for other purposes. 



