536 report — 1878. 



MONDAY, AUGUST 19, 1878. 



The following Papers were read : — 



1. On the Metamorphic and Intrusive Bodes of Tyrone. 

 By Joseph Nolan, M.B.I.A., fyc, of H.M. Geological Survey of Ireland. 



The rocks described in this paper occupy the central parts of County Tyrone, 

 extending from Omagh eastwards and north-eastwards towards Slieve Gallion. 

 They consist for the most part of an amorphous green hornhlendic rock, in the midst 

 of which is a wide lenticular tract of micaceous gneiss and schist. The author 

 shows that these two classes pass gradually into each other, and that even among 

 the amorphous hornhlendic rocks traces of schistose structure can generally he 

 observed, while local transitions into schist frequently occur. Gradations into 

 more crystalline rocks were also noted and described, those of a hornhlendic cha- 

 racter passing into a felspathic variety in which little or no hornblende occurs, 

 while quartz and orthoclase are developed, so that a coarse quartz porphyry is pro- 

 duced, passing ultimately into granite. 



It was also shown that some of the granite was intrusive during the period of 

 the Old Red Sandstone and its association with metamorphic rocks of Lower 

 Silurian age explained on the hypothesis that the intrusive granite was due to re- 

 metamorphism at the later period, so that portions of the already crystalline rocks 

 were completely fused and became irruptive. That metamorphic action in this 

 district continued up to and even after the Old Red Sandstone age seems to have 

 been the opinion of the late General Portlock, who in his ' Geological Report on 

 Londonderry with Parts of Tyrone,' &c, has described these rocks and their 

 relations to each other at considerable length. He did not seem to have considered 

 the granite to be intrusive, but merely a metamorphosed condition of what we 

 now call the Lower Carboniferous Sandstone, which was then classed with the 

 Old Red formation. 



2. On the Origin of Crystalline Rocks. By T. Sterky Hunt, F.B.S. 



3. On some Neiu Areas of Pre-Cambrian Bochs in North Wales. 

 By Henry Hicks, M.D., F.G.S. 



In addition to the areas of pre-Cambrian rocks already described by the author, 

 he now includes amongst that ancient group the following, which he has recently 

 explored, accompanied by Professor Torell, of Stockholm, and Mr. Tawney, of 

 Cambridge, and during part of the time by Professor Hughes, of Cambridge, and 

 Dr. Sterry Hunt, of Montreal. 



1. Some cupriferous schists with their associated green bands (intrusive green- 

 stone of the Survey), and including Robel Fawn, to the north of Dolgelly. 



2. The so-called ' intrusive felspathic porphyries and Greenstone breccias,' in 

 the neighbourhood of Pwllheli. 



3. The so-called intrusive syenitic (Rhos Hirwain Syenite) and felsitic masses 

 and the associated schists which together form the extremity of the promontory of 

 Caernarvonshire, and also Bardsey Island. 



4. The so-called ' intrusive felspathic porphyries ' in the neighbourhood of 

 Llanfihangel and Nevin. 



