y Co. Londonderry. 



406 Scientifc Proceedings, Royal Dublin Society. 



grain from coarse to fine. The petrographical description is 

 mainly based upon a collection made from the following locali- 

 ties : — 



One mile jST. of Raphoe, \ 



Half mile IS", of Raphoe, I Co. Donegal. 



Half mile jST. of Conyoy, J 



Half mile S. E. of Drumahoe Bridge, 

 two miles S. E. of Deny, 



One mile and a-half TV. of St. Johnstown 

 (Dooish Mountain), 



One mile and a-half S. S. TV. of New Build- 

 ings, and four miles S. S. TV. of Deny, / 



All these rocks are plagioclase-pyroxene rocks, which have under- 

 gone alteration under the influence of dynamic metamorphism. 

 Most of them were originally dolerites (Grer. Diabase) ; but it is 

 not improbable that the masses at one mile N. of Raphoe and 

 half mile N. of Convoy represent altered Grabbros. 



As a result of the metamorphism, the pyroxene has been 

 completely 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 uralitisation, the horn- 

 blende becomes split up into innumerable fibres which partake in 

 the general movement. A sort of lenticular structure is thus 

 produced, the margins of the lenticles consisting of a felted horn- 

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

 varieties, the hornblende becomes decidedly actinolitic in character, 

 forming long green or bluish-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. Derry). Liebe : has described the presence of primary horn- 

 blende in rocks of this type ; but there is no evidence of an occur- 

 rence of this nature in those under examination. A colourless, 



1 Uebersicht ueber den Schicbtenaufbau Ostthiiringens. Abhandl. zur geol. 

 Spezialkarte too Preussen u. d. Thiiring. Staaten. Bd. v. ; heft 4, p. S3. ' 



