Vol. 67.] 



SK0MER VOLCANIC SERIES. 



209 



VII. The Chemical and Miner alogicai. Composition of the 

 Lavas. 



Chemically and mineralogically, the rocks of the Skomcr Series 

 show some marked peculiarities. Primarily they form a group 

 rich in soda and poor in lime, a fact which shows itself in the 

 prevalence of soda-rich plagioclase felspars. 



The abundance of soda-rich felspars and the unusual character 

 of many of the rocks might reasonably, in view of recent research, 



Kir. 13. 



20 



15 



10 



-Diagram illustrating the percentage chemical composition 

 of the Skomer igneous rods. 



& 



/ 



m^o— 



— YqP 



MsO- 



40 



45 



50 



60 



05 



70 



80 



suggest that some of the rocks were originally more basic, but 

 had undergone a process of albitization similar to that described 

 by Messrs. Bailey & Grabham. 1 In the Skomer Series, however, if 

 albitization has taken place, it has been complete and has left no trace 

 of intermediate stages as an indication of the change. Again, if 

 the rocks are albitized, albitization has been restricted to certain 



1 Geol. Mag. dec. 5, vol. vi (1909) p. 250. 



