206 Dr. J. 8. Hyiancl — Epi-diorites in Ireland. 



blende becomes split up into innumerable fibres whicb partake in 

 the general movement. A sort of lenticular structure is thus pro- 

 duced, the margins of the lenticles consisting of a felted bornblendic 

 mass in a state of fine division. In the much altered varieties, the 

 hornblende becomes decidedly actinolitic in character, forming long- 

 green or bluisb-green prisms. As the actinolitic nature becomes the 

 more evident the more decided the foliation, it is not surprising 

 that planes of movement are coated with amianthus-like actinolite 

 (one mile S.S.W. of New Buildings, Co. Deny). Liebe l has 

 described the presence of primary hornblende in rocks of this type ; 

 but there is no evidence of an occurrence of this nature in those 

 under examination. A colourless, tremolite-looking hornblende is 

 sometimes apparent, but is not a constant accessory. 2 



Intergrown with and imbedded in the hornblende there are to be 

 observed numerous patches and flakes of biotite. This mineral is 

 singularly devoid of inclusions, and appears to stand in a genetical 

 relation to the hornblende. The pleochroism is : — 



a = pale straw colour. 

 |8 and 7 = brownish-yellow to dark-brown. 



Epidote and zoisite 3 are very common, whilst secondary quartz is 

 not infrequent. There is also a little calcite present. The quantity 

 of felspar varies more or less, but is always subordinate to that 

 of the hornblende. It can eventually become very small, as the 

 hornblende appears to displace the felspathic constituent. This 

 almost entire disappearance of the felspar may account for the state- 

 ment made over fifty years ago by G. Eose, that " uralite " was only 

 to be found in those greenstones in which felspar was absent or little 

 apparent. 4 Still, notwithstanding this opinion, put forth shortly after 

 his discovery of the mineral, mention is especially made in his work 

 on the Urals of the constant association of uralite and oligoclase. 6 



The metamorphism of the felspar leads to its "granulation," and 

 the consequent formation of new products. This "granulation" is 

 referred to a "crystallizing process going on under the influence and 



1 Uebersicht ueber den Scbicbtenaufbau Ostthiiringens, Abhandl. zur geol. 

 Spezialkarte von Preussen u. d. Tkiiring. Staaten, Band V. Heft 4, p. 83. 



2 Tremolite occurs in the district comprised in Sheet 17, viz. near Curley Hill, 

 Co. Tyrone, but in limestone. (C. L. Giesecke, Minerals of the Eoyal Dublin 

 Society, to which is added an Irish Mineralogy, Dublin, 1832, p. 227.) 



3 For crystallograpbic details, see Memoir, Sh. 17, pp. 35 and 36. The occurrence 

 of zoisite in this district, at Holly Hill, near Strabane, Co. Tyrone, was known to 

 Portlock (Report of the Geology of the Co. Londonderry, etc., Dublin, 1843, 

 p. 209) ; also to Giesecke (loc. cit. p. 208). 



4 Pogg. Ann. d. Ph. 1833, 1 St. p. 103. "Immer aber haben sie (Uralite) sich 

 nach den gemachten Erfahrungen nur in den Griinsteinen gefunden in welchen 

 Albit oder Feldspath nicht vorkommen oder wenigstens nicht deutlich ausgeschieden 

 vorkommen; mit der Bildung dieser M ineralien scheint die Bildung des Uralits 

 aufzuhoren und statt dessen Hornblende an seine Stelle zu treten." The term 

 ' ' uralite ' ' is used in a strict sense as applying to a mineral possessing the outer 

 form of augite and the cleavage planes of hornblende (V. Pogg. Ann. 1831, xxii. 

 p. 321 ; and Jahrb. f. Min. 1832, p. 237). 



5 "Eeise nach d. Ural," Band II. 575. 



