J. Nolan — Metamorphic and Intrusive Rocks of Tyrone. 159 



parts is in about equal proportion with the mica, forming hornblendic 

 or syenitic granite. This again passes into a variety devoid of 

 mica, the felspar changing to a greenish colour, and ultimately into 

 the quartzless hornblendic rock. These changes, as before men- 

 tioned, are well seen at Slieve G-allion, and also at the hills called 

 Bardahessiagh, and Craighballyharky, north of Pomeroy, where 

 instances occur, in which the granitic and hornblendic varieties are 

 so blended in the same rock-mass, that it is quite impossible to 

 distinguish them. 



Granite as an Eruptive Bock. — When the maximum of metamor- 

 phism is reached in the complete fusion of rock and conversion into 

 crystalline granite, we may expect to find that at some places such a 

 mass has been intrusive, and, accordingly, there are few areas occu- 

 pied by metamorphic granite, in which it has not been evidently 

 intrusive at some parts. Thus, in the granite of Newry, the great 

 mass of which I believe to be of metamorphic origin, parts are 

 certainly intrusive, instances of which will be found detailed in the 

 Geological Survey Memoirs descriptive of that part of the country. 

 So, in this district too, the granite has all the appearance of having 

 been intrusive at some parts. Thus, at Drumduff, near Beragh, 

 close to the margin of that rock, the sandstones are indurated, and 

 have an unusually high dip ; but by far the best evidence is to be 

 seen at Aghnagreggan Bridge, near Carrickmore. There the sand- 

 stones can be observed in actual contact with the granite, they are 

 highly disturbed and semi-vitrified, being converted into yellowish 

 quartzite, leaving little room for doubt but that the granite has 

 been intruded through them. It cannot be regarded as being due 

 to metamorphism of the sandstone, for in the immediate vicinity 

 beds were found quite unaltered. This could not be the case if this 

 part of the Old Bed Sandstone had undergone so complete a change 

 as a conversion into granite. A great extent of rock would be 

 indurated, then foliated and altered into coarse gneiss, which, 

 becoming more massive and crystalline, would ultimately pass into 

 granite. As no such gradation was anywhere observed in this 

 district, no change in the sandstones being effected, except on those 

 in contact, which, as just now remarked, have been partially 

 vitrified and altered in colour, as happens where masses of molten 

 rock have been protruded through sedimentary strata, we may 

 justly conclude that a similar action has taken place here, and that 

 some of the granite has been intrusive. If this be so, then, we have 

 proof that the granite is newer than the earlier part of the Old Red 

 Sandstone period, while the mass of the schists and other metamor- 

 phic rocks with which it is associated, and into which it appears 

 to graduate, are certainly older, being probably of Lower Silurian age, 

 coeval with those in the West of Ireland. This apparent difficulty 

 is, however, easily explained, if we suppose the granite, and perhaps 

 some of the other highly crystalline rocks, to be the result of 

 re-metamorphism. The depression, which took place during the 

 period when the great series of sandstones and conglomerates — 

 which even now have a thickness of upwards of 10,000 feet — were 



