H. Warth — Composition of Igneous Rocks. 



273 



tlie lower felsitic rock spoken of as lava is really of a fragmentary 

 character; secondly, the existence of dykes and intrusive masses of 

 felstone ; and thirdly, the highly metamorphic rocks of Cwm Llan 

 and Llyd Nadroedd. These three things are all new. Fourthly, the 

 occurrence of diorite in the Snowdon area is a discovery. 



X. — Diagram of Composition of Igneous Rocks. 

 By H. Warth, Esq. 



RECENT studies about the average chemical composition of larger 

 numbers of igneous rocks in the aggregate have shown that 

 figures obtained from any one hundred or more samples are 

 very similar, in fact practically equal. (See A. Harker, " On the 

 Average Composition of British Igneous Rocks " : Geol. Mag., 

 No. V, May, 1899.) This will be found also the case when com- 

 paring the following average which I calculated from the analysis 

 of igneous rocks compiled by Roth in his " Petrographie der 

 plutonischen Gesteine." 



Now if it is of interest to discuss these average results, notwith- 

 standing the wide discrepancies of many individual compositions, 

 it may also be worth while to find out how on the average the 

 compositions vary with the relative acidity of the rocks. For this 

 purpose I have arranged the bulk of Roth's samples in the order 



TGO 



Diagram-Fig. 1. 



of acidity, and after calculating average compositions of eighteen 

 groups of closely approaching acidity, I constructed from the figures 

 the subjoined diagrams. The first diagram (Fig. 1) shows the actual 

 results, the second (Fig. 2) shows them rounded off". 



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