118 A. Holmes— Classification of Igneous Rocks 



A. Felsite porphyry ; Varese, Piedmont. 



B. Granitite; Pikes Peak, Colorado. 



C. Granitite; Crazy Mts., Montana. 



D. Obsidian ; Teneriffe, Canary Islands. 



E. Toscanite ; Bracciano, Italy. 



A, B, and C, falling in adjacent sub-rangs, bear a much closer resemblance 

 to one another than do C, D, and E, all of which fall in the same sub-rang. 



SynBOL5-oF 5(JB-RAhGS 

 4-3 2 



rc,c 



KpG 



In the above diagram the rectangular spaces represent subdivisions of the 

 C.I.P.W. Classification formed by rangs and sub-rangs. The spaces between 

 the curved lines represent the distribution of the different soda-lime felspars 

 across the C.I.P.W. Classification in Classes I-III, Orders 1-4 and part of 5. 

 In the case of other orders, the curves occupy other positions which may be 

 obtained approximately by swinging them upwards about the north-east corner 

 through an angle depending on the amount of felspathoid present in the norm. 



A.ctuilly, the classification is, as generally used, in four dimen- 

 sions, and consequently sixteen sub-rangs meet in the theoretical 

 point determined by the intersection of four super planes. Thus it 

 would be possible for sixteen closely similar analyses to fall into 

 sixteen adjacent pigeonholes, whei'eas sixteen analyses distributed 

 evenly through any one of the pigeonholes would show considerably 

 more variation. Obviously, this is not a serious fault in the 

 classification unless it can be shown that there is a natural grouping 

 of rocks. If there were, and the existing modal nomenclature is 



