Hyland — On some Epi-diorites of N. W. Ireland. 407 



tremolite-looking hornblende is sometimes apparent, but is not a 

 constant accessory. 1 



Intergrown with, and embedded 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-yellow, 



j3 and 7 = brownish-yellow to dark-brown. 



Epidote and zoisite 2 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 

 possibly account for the statement made over fifty years ago by 

 Gr. Rose, that " uralite " was only to be found in those greenstones 

 in which felspar was absent or little apparent. 3 Still, notwith- 

 standing 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. 4 



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

 and the consequent formation of new products. This granulation 

 is referred to a " crystallising process going on under the influ- 

 ence and control of powerful mechanical stresses." 5 The original 



1 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.) 



2 For crystallographic details see Memoir, Sheet 17, pp. 35 and 36. The occur- 

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

 Portlock (Report of the Geology of the Co. Londonderry, &c. : Dublin, 1843, p. 209) ; 

 also to Giesecke (I. c. p. 208). 



3 Pogg. Ann. d. Ph., 1833, 1 St., p. 103. " Immer 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 Mineralien scheint die Bildung des Uralite 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 Jahrbuch f. Min., 

 1832, p 237). 



4 Reise nach d. Ural, Bd. ii. 575. 



5 Judd, On the Processes by which a Plagioclase Felspar is converted into a Scapo- 

 lite, Min. Mag. viii., No. 39, p. 186 (13.) 



SCIEN. PROC. R.D.S. — VOL. VI., PART VIII. 2 I 



