J. Nolan — Metamorphic and Intrusive Hocks of Tyrone. 157 



examination, there is no evidence of extrusion, the rock changing by 

 the gradual disappearance of the cavities into the ordinary horn- 

 blendic close-grained variety. 



The passages from these varieties into schistose rocks are also very 

 remarkable. They not only occur in the vicinity of the schist series, 

 but everywhere throughout the mass, no considerable tract of which, 

 indeed, is without some trace of foliation. Thus, at Termon Eock, 

 which is mostly composed of coarse diorite, foliation was observed 

 in tbe more finely crystalline portions, associated with a variety, 

 which, from its close texture, has much the look of an indurated 

 or highly altered slate, full of strings and ramifying veins of 

 serpentine ; and to the north-east of Omagh, west of the road to 

 Mountfield, similar schistoid rocks, also abounding with serpentine, 

 may be seen. 



A very interesting series is met with at Beaghbeg, some six miles 

 north of Pomeroy. Here, within a small area, are diallage and 

 quartz -porphyry, having between them finely crystalline and com- 

 pact rocks. Some of these latter have minute scattered prisms of 

 hornblende in a felspathic-looking base, and change into partially 

 foliated rocks of porcelainic appearance, with numerous strings and 

 veins of quartz. Close to this, on the north-west, are chloritic aud 

 talcose schists, and schist breccias, containing pieces of red iron 

 jasper and green flinty-looking rocks with quartz grains like altered 

 felstones, or perhaps very quartzose grits. These latter were again 

 noticed at Creggan, four miles to the south-west, but among the 

 finely-crystalline hornblendic series. They have all the appearance 

 of metamorphosed conglomerates, and still further confirm the 

 evidence as to the original sedimentary character of these rocks. 



Thirdly. — Quartz, Porphyry, Granite, and Syenite. — It has been 

 before remarked, that the compact varieties of the green metamorphic 

 rock frequently lose all trace of hornblende, and become felspathic. 

 The base in these cases is usually of a light green or bluish, changing 

 to a pink or reddish grey colour. This occurs very gradually — first 

 there appear a few spots of a pink or reddish shade, these then 

 extend and form the prevailing tint of the base, and some quartz 

 appears. Crystals of flesh-coloured orthoclase and a light olive- 

 green felspar that seems to be oligoclase, are now observed, the 

 blebs of quartz increasing in size and quantity, with an admixture 

 of dark green mica of resinous lustre occurring in hexagonal plates. 

 We thus get a quartz-porphyry — a rock having all the constituents 

 of granite, differing only in the disposition of the silica, which, as 

 just remarked, is in individual crystals, and does not form the base. 

 It is easy to see that a rock of this kind can readily pass into true 

 granite, requiring but such an increase in temperature as to keep 

 the silica in solution while the other minerals have crystallized out, 

 and, accordingly, we find that in many parts of this district the 

 quartz-porphyry does pass into granite. A very remarkable example 

 may be seen at Mullanamore Bridge, north of Carrickmore. The 

 porphyry was observed in the stream a little above the bridge, close 

 to it is a rock which it is impossible to refer with certainty to either 



