﻿494 ME. J. A. THOMSON OlS THE HOENBLENDIC [NoV. I908, 



fibrous outgrowths of tremolite. The green hornblende is partly- 

 replaced by tremolite and ores. There is a little sphene. The 

 poecilitic structure has entirely disappeared. Talc is practically 

 absent, but there is a great development of chlorite, in which small 

 elliptical grains of quartz are abundant. There is no well-marked 

 parallel structure, but the fibres of tremolite are often bent, and the 

 chlorite winds among the amphiboles like the ground-mass of an 

 augen-gneiss. Undulose extinctions are common in most of the 

 minerals. 



The f elspathic rocks found within both dykes are composed chiefly 

 of a pale-green hornblende, an acid plagioclase, and a greenish- 

 j'ellow epidote. The hornblende is often in the form of elongated 

 idiomorphic crystals, showing in cross-sections the 6-pinacoid (010) 

 besides the prisms. Twinning parallel to (010) is common. The 

 frequent occurrence of tremolite-patches within the hornblende 

 seems, however, to relate it to that of the amphibolite. The force 

 exerted by growing crystals, which has formed the subject of 

 experimental investigations by Messrs. G. F. Becker & A. L. Day,^ 

 is often exemplified in these rocks by the manner in which calcite 

 and chlorite have forced apart the hornblende aloug its cleavages. 

 Granular epidote is very abundant, and occasionally the mineral is 

 developed in radiating aggregates. Apatite is more abundant than 

 in the peridotite, and forms fine needles resembling sillimanite in 

 habit. 



All these minerals lie in a mosaic of felspar and quartz. The 

 felspar is twinned on the albite-law, and seems to be mostly 

 oligoclase. Quartz is seldom abundant, and forms local graphic 

 intergrowths with the felspar. 



The relation of this felspathic rock to the peridotite cannot be 

 gleaned from the evidence so clearly as at Glendalough. That it 

 is also an intrusive acid vein which has partly absorbed the 

 peridotite is possible, and is suggested by the tremolite-patches 

 within the green hornblende. The rocks differ from the mixed 

 rocks of Glendalough in the absence of mica, garnet, zoisite, and 

 sphene, in the abundance of epidote, and in the more acid nature 

 of the felspar. 



Peridotites occur as common difi'erentiation-products with much 

 more acid rocks in Scotland. It is possible that the intrusive vein 

 was thus related to the Greystones peridotite. 



Discussioir. 



The Peesidext commented on the importance of the results ob- 

 tained by the Author, which seemed to afi'ord conclusive proof of 

 the transformation of a highly-basic rock by the introduction of 

 granitic material. Observations in the field had left some doubt in 

 his own mind as to the precise nature of the intrusive veins in this 



^ ' The Linear Force of Growing Crystals ' Proc. Wash. Acad. Sci. vol. vii 

 (1905) p. 283. 



