﻿Vol. 64.] EOCKS OF GLENDALOUGH AND GEEYSTONES. 



477 



and the ^ quartz-mica-diorite ' series. All these rocks weather to 

 immense rounded boulders which can seldom be in place. A few 

 distant though strong joint-planes cross the more massive rock, but 

 yield no evidence of shearing on their faces. Exact mapping is 

 frustrated by the gradual passages between the different rocks, and 

 by the jumbling of the boulders. The actinolite-rock is restricted 

 to the boundary, and this is exposed only on the north-east and east 

 sides. The ' diorite ' is confined to the south-western corner of the 

 boss, and the zoisite-amphibolite is found most abundantly between 

 it and the amphibolite on the north. Each of these types will now 

 be described in detail. 



(1) The Hornblende-Peridotite (fig. 1). 



In hand-specimens, this rock is seen to consist almost entirely of 

 large crystals of a greenish hornblende, on the cleavage-surfaces of 

 which small, rounded, dull patches are apparent — the typical lustre- 

 mottling of hornblende-peridotites. The crystals of hornblende 



Fig. 1. — Hornblende-peridotite from Glenddlough, showing poecilitic 

 structure in Jiornhlende (shaded) witli tremolite-rims (unshaded). 



[0 = Olivine ; A = Augite. The augite is surrounded by a resorption-margin 

 of magnetite, and the oHvine is lai'gely converted into chlorite (colourless) 

 spotted with magnetite. The magnification is 30 diameters.] 



attain an inch and a half in their greatest diameter. Occasionally 

 small scattered scales of mica appear on the cleavage-planes of 

 the hornblende. The specific gravity is 3*04. 



2k2 



