﻿476 ME. J. A. THOMSO^' ox THE HOKXBLENDIC [NoV. I908, 



superimposed on this dynamic effect an intense contact-meta- 

 morphism. The altered rocks crop out for a distance of 2 miles 

 from the granite, although below the surface the distance of the 

 latter may be really much less. The contact-zone consists of 

 genuine mica-schists, while, near the granite, andalusite, garnet, 

 and other minerals make their appearance. 



After this uplift of the land by the early Caledonian folding, 

 there appears to have been a land-surface until Carboniferous times. 

 In the beach-deposits and conglomerates of this period are found 

 angular and rounded fragments of the Leinster Granite, ' a proof of 

 the exposure at this time of the granite from beneath its cover.' 

 Eollowing the deposition of the Carboniferous came the great earth- 

 pressures of the Armorican folding, and Prof. Sollas believes that 

 their effect may be traced in a cross-wrinkling of the Ordovician 

 schists at Glendalough. Xo other period of deposition or of 

 mountain-building has left any important mark on the area here 

 described, if we except the Glacial Epoch, to which in all proba- 

 bility is due the greater part of the profound erosion tkat granite 

 and schists have suffered and the deposition of widespread erratic 

 material. 



11. The Hoenblexdic Eocks of Glendalough. 



These form a small boss on the south side of Camaderry, a ridge 

 that separates the Yale and Lake of Glendalough from the Valley 

 of Glendassan. The Yale of Glendalough has the characteristic 

 U-shape that is associated with glacial and fluvioglacial erosion, and 

 its sides are formed of cliffs of granite and schists, with large screes 

 on the north side. The amphibohte lies about half a mile east of 

 the boundary between granite and schists, and almost touches the 

 cliffs, but no trace of it can be found on the cliff- face. That it is 

 an intrusive igneous rock is amply proved by the zone of contact- 

 alteration which it has induced in the surrounding sedimentaries. 



From the great amount of shearing which the original rock has 

 undergone in comparison with the granite, and from the fact that 

 its hardened contact-zone has withstood the foliating action of the 

 earth-pressures, there is no doubt that it is older than the granite. 

 The relations of the two rocks are analogous to those of the 

 intrusions of the Scottish Highlands, the amphibolite being com- 

 parable with the pre-foliation intrusive rocks, and the granite with 

 the newer granites. 



The interest of the hornblendic rock lies in ' the remarkable 

 change in character and composition which it undergoes on passing 

 from the eastern to the western boundary.' It passes from an 

 ' amphibolite ' to a ' quartz-mica-diorite.' Prof. Sollas attributes 

 this change to the metasomatic action of quartz-veins which traverse 

 the schists in its neighbourhood. 



A detailed microscopic examination has revealed a wonderful 

 heterogeneity in so small a boss. Five types of rock may be 

 distinguished, forming two series : — hornblende-peridotite with 

 passages to an amphibolite, a zoisite-amphibolite, an actinolite-rockj 



