﻿Vol. 64.] EOCKS OF GLENDALOIJGH AND GEEYSTONES. 493 



The two dykes that I examined lie practically in the bedding-planes 

 of the Ordovician rocks, which strike E. 30° N. to W. 30° S. and dip 

 about 70° northwards. The dykes are each about 20 feet thick, 

 and are separated by 30 feet of sedimentary deposits. They have 

 suffered considerably from crushing, are phacoidal on the margins, ' 

 and show asbestos on the joint-planes. The sea has been able to 

 erode the centres of the dykes, so that they cause small indenta- 

 tions of the coast. 



The appearance of the rocks is very different from that of the 

 Glendalough series, although they seem to have had a similar 

 history. The lustre-mottling is not exhibited on so large a scale, 

 and the colour is much greener. There is a similar impregnation 

 with sulphides. In both dykes there is present a reddish rock, 

 with felspar and a bladed type of hornblende. Veins of this rock 

 may be found traversing the normal peridotite. 



Microscopic examination shows that a transformation from peri- 

 dotite to amphibolite has gone on, as at Glendalough, but with a 

 greater development of talc. 



The sections of least-altered peridotite (Survey-slide, No. 658) 

 show pseudomorphs of idiom orphic olivine enclosed in poeeilitic 

 plates of brownish-green and green hornblende. The olivine seems 

 to have predominated over the hornblende. No traces of augite 

 are seen, although a few colourless patches within the hornblende 

 may represent it. Mica is very rarely present within the horn- 

 blende, and apatite occurs only in small quantity. 



The pseudomorphs after olivine preserve in many cases the 

 characteristic form of the original crystals, and are traversed by 

 strings of magnetite and pyrite along the irregular cracks of the 

 olivine. Set perpendicularly to these cracks is the usual zone of 

 serpentine. Complete serpentinization is not, however, the rule. 

 The interior is usually filled with tremolite, which gives so many 

 sections with straight extinction and parallel cleavages that it was 

 at first mistaken for iddingsite. It has sometimes a regular dis- 

 position with regard to the olivine. Besides the pseudomorphs 

 retaining the olivine-shapes, there are irregular areas of the section 

 which consist of a finely-granular serpentinous aggregate crowded 

 with small, idioblastic, green hornblende- crystals, and occasionally 

 penetrated by fibres of chlorite and tremolite. Talc in fine fiakes 

 and calcite in irregular grains lie scattered through the aggregates. 



The next stage might be termed a talc-amphibolite. The 

 former presence of olivine can only be detected by the occurrence of 

 colourless patches of trem.olite within the hornblende, and by the 

 abundance of talc. Serpentine is rare, but chlorite becomes more 

 abundant. The hornblende is frequently outgrown by tremolite. 

 Sphene occurs sparingly, and zoisite has been detected in only one 

 slide. 



In the sheared rock from the walls of the dykes, the large plates 

 of hornblende are broken up to small ragged pieces, often with wavy 



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