Dr. J. 8. Hyland — Epi-diorites hi Ireland. 205 



II.— On some Epi-diorites of N.W. Ireland. 1 



By J. Shearson Hyland, Ph.D. 



(Communicated by permission of the Director- General of the Geological Survey.) 



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 the 

 examination of the rocks are sufficiently interesting to deserve a 

 wider circulation. 



The rocks occur as sheets and dykes intrusive into the altered 

 sediments (quartzites, mica-schists, 2 etc.), and at St. Johnstown and 

 Raphoe they are seen to break through the stratified deposits 

 transversely to the bedding. 3 In the field, a strong foliation is often 

 to be recognized ; but this structure is at times little developed and 

 hardly perceptible. Still the absence of this macroscopic feature 

 does not preclude the possibility of reconstruction having occurred ; 

 for it has been abundantly demonstrated that molecular re-arrange- 

 ment can ensue without the development of such a structural 

 modification.* 



The specimens examined are greenish in hue and vary in grain 

 from coarse to fine. The petrographical description is mainly based 

 upon a collection made from the following localities : — 



One mile N. of Raphoe ; \ 

 Half mile N. of Raphoe ; \ Co. Donegal. 

 Half mile N. of ('onvoy; ) 



Half mile S.E. of Drumahoe Bridge, and two miles S.E. of Deny ; \ 

 One mile and a half TV. of St. Johnstown (Dooish Mountain) ; ( Co. London- 

 One mile and a half S.S.W. of New Buildings, and four miles ( derry^ 

 S.S.W. of Derry; ) 



The rocks described under the head "Epi-diorites" are plagioclase- 

 pyroxene rocks which have undergone alteration under the influence 

 of dynamic metamorphism. Most of them were originally dolerites 

 (Ger. Diabase) ; but it is not improbable that the masses at one 

 mile N. of Eaphoe and half mile N. of Convoy represent altered 

 gabbros. 



As a result of the metamorphism, the pyroxene has been com- 

 pletely altered into a greenish monoclinic hornblende, which 

 possesses the characters of " uralite." This mineral occurs in ragged 

 patches, which in most cases still preserve the ophitic structure of 

 the pyroxene which it has replaced. Where there has been move- 

 ment coincident with or subsequently to this uralitization, the horn- 



1 Read before the Royal Geological Society of Ireland, 10th February, 1S90. 



2 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, and rutile are also present. Strain-slip cleavage is well 

 developed. Iron-pyrites is to be observed macroscopically, also rutile according to 

 Giesecke. 



3 Prof. Hull in Memoir to Sheet 17, p. 7. 



4 Teall, " The Metamorphosis of Dolerite into Hornblende-schist," Q.J.G. S. 1885, 

 p. 139, and "The Metamorphosis of the Lizard Gabbros," Geol. Mag. Dec. III. 

 Vol. I. p. 487. 



