[ 405 j 



XLIV. 



ON SOME EPI-DIOEITES OF NORTH-WEST IRELAND. By J. 

 SHEARSON HYLAND, Ph. D., M.A. (Communicated by per- 

 mission of the Director- General of the Geological Survey). 



[Read February 10, 1890.] 



In an appendix to the explanatory memoir on sheet 17 of the 

 Map of the Geological Survey of Ireland (Dublin, 1889), I have 

 furnished a description of the petrographical characters of the epi- 

 diorites of the district. Some of the facts elicited by an examina- 

 tion of these rocks are sufficiently interesting to deserve a wider 

 circulation. 



The rocks occur as sheets and dykes, intrusive into the altered 

 sediments (quartzites, mica-schists, &C 1 ), and at St. Johnstown 

 and Raphoe they are seen to break through the stratified deposits 

 transversely to the bedding. 2 In the field a strong foliation is 

 often to be recognized ; but this structure is at times little deve- 

 loped and hardly perceptible. Still, the absence of this macro- 

 scopic feature does not preclude the possibility of reconstruction 

 having occurred ; for it has been abundantly demonstrated that 

 molecular re-arrangement can ensue without the development of 

 such a structural modification. 3 



The specimens examined are greenish in hue, and vary in 



1 The dark, bluish-grey mica-schist is seen under the microscope to consist of a 

 plexus of light green, uniaxial mica and minute grains of quartz : calcite, hematite, 

 tourmaline, rutile, are also present. Strain-slip-cleavage is well developed. Iron- 

 pyrites is to be observed macroscopically ; also rutile, according to Gieseeke. 



2 Prof. E. Hull, in Memoir to Sheet 17, p. 7. 



3 Teall, The Metamorphosis of Dolerite into Hornblende- Schist, Q. J. G. S. Lon., 

 1885, p. 139; and The Metamorphosis of the Lizard Gabbros, Geol. Mag. iii. vol. i. 

 p. 487. 



