﻿Vol. 64.] ROCKS OF GLENDALOUGH AND GREYSTOXES. 



4S1 



In thin sections scarcely any trace of the poecilitic structure is to 

 be seen. The rock consists almost entirely of amphiboles, with a 

 little apatite, clinozoisite, sphene, magnetite, and pyrite. 



A prism of apatite was found in only one section. The sphene 

 occurs abundantly, most often within crystals of feebly-coloured 

 hornblende, in small isolated grains of more or less characteristic 

 shape. Rarely it is moulded on the magnetite, which is present in 

 small quantity. Pyrite is very plentiful, and is also confined almost 

 entirely to the colourless amphibole. 



The amphiboles exhibit less variety than in the previously- 

 described types. They are mostly pale green to colourless, and 



Pig. 3. — AnipJiibolite from GlendalougJi, consisting of green 

 hornblende often outgrown bg tremoUte. (Magnified 30 diameters.) 



occasionally form a colourless border to a pleochroic green horn- 

 blende. The brown hornblende fails almost entirely, and the deeper 

 green varieties occur only as remnants within the paler varieties. 

 The larger crystals have, as a rule, good crystalline boundaries ; 

 the smaller ones form flakes penetrating aggregates of clinozoisite. 

 Pleochroic halos round small colourless inclusions occur in the green 

 hornblende. Often the interior of the larger plates is occupied by 

 a ragged aggregate of twinned clinochlore. 



In this series, then, we see a hornblende-peridotite containing 

 augite and olivine pass by the alteration of these into a rock con- 

 sisting almost entirely of hornblende, the hornblendefels of the 



