﻿Vol. 64.] BOCKS OP GLENDALOUGH AND GEEYSTONES. 



489 



In the types poor in quartz, the felspars form large plates, with 

 coarse albite-twinning and very well-developed pericline-twinmng« 

 A zonary structure is rare. Where quartz is abundant a clear 

 mosaic of smaller grains occurs, but often large plates of felspar 



Pig. 8. — The ' mixed roclc^ Glendalough : corroded crystals of hiotite 

 (penetrated by acicular spliene)^ actinolite, and apatite lie in a 

 matrix of felspars. (Marpnjied SO diameters.) 



are bordered with a graphic intergrowth. The regularity of this 

 is disturbed by the crushing that the rock has undergone. All 

 the felspars have refractive indices higher than quartz, and the 

 extinction-angles show that the chief species is labradorite. 



I made the following percentage analysis of one of the most acid 

 types ; — 



SiOa 59-78 



TiO, j 



PA \ 16-43 



aiaJ 



FeA 0-92 



FeO 6-84 



MgO 2-94 



CaO 915 



K,0 236 



Na.,0 1-44 



H.,0+ 0-93 



Hfp- 0-12 



Total 100-91 



