﻿Vol. 64.] HOKNBLENDIC ROCKS OP GLENDALOTJGH ANB GRETSTONES. 475 



25. The HoRiTBLENDic Eocks of Glendalough and Gretstones 

 (County Wickloav). By James Allan Thomson, B.A., B.Sc, 

 P.G.S. (Read June 17th, 1908.) 



Contents. 



Page 



I. Introduction 475 



II. The Hornblendic Eocks of Glendalough 476 



III. The ' Quartz-Mica Diorite ' Series 485 



IV. The Contact-Pheuoraena of the Amphibolite 491 



V. The Hornblendic Rocks of Greystones 492 



I. Introduction. 



The two occurrences of hornblende-rocks of which this paper treats 

 have been briefly described before, that of Glendalough under the 

 name of amphibolite by Prof. SoUas,^ that of Greystones as 

 hornblende-picrite by Prof. Watts .^ Both these authors felt that 

 there were points of interest about these rocks which required fuller 

 working-out ; and when, at Prof. Sollas's suggestion, I commenced 

 the study of the amphibolite. Prof. Watts kindly offered to delay 

 the publication of an interesting communication which he had in 

 hand on the picritc, so as to allow me to give a complete description 

 of both rocks. In recording my thanks I have also to mention 

 Prof. Cole, Mr. Seymour, and Dr. Plett, to whom I am indebted for 

 facilities to examine sections in the Geological-Survey collections 

 and for many fruitful suggestions. 



Both these rocks are intrusive into Ordovician strata in the east 

 of County Wicklow. This area has been described in detail in the 

 Geological-Survey Memoir,^ its igneous rocks are well known from 

 the descriptions by Prof. Watts,"^ and its geological history has been 

 discussed by Prof. Sollas.^ The outstanding feature is, that into 

 a thic^ mass of arenaceous and argillaceous sediments and inter- 

 calate^, volcanic rocks of Cambrian and Ordovician age, there was 

 intruded in laccolitic form an immense mass of granite. The 

 intrusion of the Leinster Granite seems to be connected with the 

 Caledonian system of folding : for its outcrop runs in a north-east 

 to south-west direction for some 30 miles, from Killiney to New 

 E,oss. At a distance from the granite the Ordovician sediments 

 have been ' contorted, plicated, overthrust, and cleaved into slates ' 

 by the pressure of the folding ; but in its neighbourhood there is 



■^ ' On the Transformation of an Amphibolite into Quartz-Mica-Diorite ' Eep. 

 Brit. Assoc. 1893 (Nottingham) p. 765. 



2 Ibid. p. 767. 



a J. B. Jukes & G. V. du Noyer, Explanations to accompany Sheets 121 

 & 130, Mem. Geol. Surv. Irel. 1869. 



< A. McHenry & W. W. Watts, ' Guide to the Collections of Eocks & 

 Fossils belonging to the Geological Survey of Ireland ' Dublin, 1895. 



a ' The Geology of Dublin & its Neighbourhood ' Proc. Geol. Assoc, vol. xiii 

 (1893-95) p. 91. 



Q. J. G. S. No. 256. 2 k 



