﻿Vol. 64.] EOCKS OF GLENDALOUGH AND GEEYSTONES. 485 



The zoisite-amphibolite differs from, the rocks already described, 

 only by the greater proportion of the zoisite-minerals. Of these 

 there are two. Besides separating off a colourless monoclinic zoisite 

 from orthorhombic zoisite on the one hand, and from coloured or 

 strongly birefringent epidote on the other, Dr. Weinschenk distin- 

 guishes two varieties of orthorhombic zoisite, according to the position 

 of the plane of the optic axes. Zoisite a gives anomalous blue inter- 

 ference-colours, zoisite /3 normal pale-grey of the first order. The 

 latter variety occurs somewhat plentifully in some sections with 

 the clinozoisite. It has a superficial resemblance to apatite, from 

 which it may be easily distinguished by the absence of cross-fracture, 

 as well as by its biaxial character. Clear untwinned felspar, 

 probably albite, is associated with it. These minerals are not 

 restricted to aggregates lying between the hornblende-crystals, but 

 also occur scattered throughout the poecilitic plates. 



The hornblende is of the same general character as in the other 

 rocks. It is mostly of the bleached type, full of separated sphene, 

 and with only occasional remnants of green or brown. The idio- 

 blastic structure is not so well developed. Mica is more abundant 

 than in any of the previous types, and is of, the same pale colour. 

 It forms jagged plates within the hornblende, and very often is 

 elongated across the length of the latter. 



The abundance of zoisite seems to point to a former content of 

 felspar in this rock. The scarcity of albite and the absence of 

 muscovite, which generally accompany zoisite in saussurite, further 

 show that the felspar must have been very basic. The rock may 

 have been a picrite. 



III. The ' QuABTz-MiCA-DioEiTE ' Seeies. 



This series of more acid rocks occurs in the south-western corner of 

 the boss, as big boulders similar in size and shape to those of the 

 other rocks. At one place, a vein was found penetrating the 

 amphibolite. In appearance the rocks are much whiter, due to the 

 presence of quartz and felspars. In the ' bladed type,' the horn- 

 blende is elongated into long greenish prisms, on the cleavage-faces 

 of which a profusion of scales of a pale-brown mica may be seen.^ 

 A few boulders of a whiter colour, due to a larger proportion of 

 quartz and felspar, contain hornblende with a more actinolitic habit 

 and greener colour, and in addition numerous dodecahedra of a 

 pale-red garnet. The specific gravities are variable, with a mean 

 of 2-95. The geological relationships of these differences could not 

 be made out. 



These rocks are abnormal, both in mineral composition and in 

 structure, and are conceived to be mixed rocks formed by the 



^ Prof. G-. A. J. Cole ('On the Growth of Crystals in the Contact-Zone of 

 Oranite & Amphibolite' Proc. Roj. Irish Acad. vol. xxv, 1905, sect. B, 

 p. 117) agrees with Prof. Sollas in attributing this rock to the action of acid 

 veins on the amphibolite, and points out the stimulus given to recrystallization 

 by the conditions of the interaction. 



