J. Nolan — Metamorphic and Intrusive Rocks of Tyrone. 155 



more north-eastwards to Fir Mountain, having a width of from 

 three to four miles at the greatest, and terminating at both ends 

 among the green metamorphic rocks presently to be described, in 

 which, indeed, they may be considered as forming a huge lenticular 

 mass. The prevailing character is a coarse massive gneiss passing 

 into schist, generally micaceous, and often containing some horn- 

 blende. Close to the junction with the green metamorphic rocks 

 just alluded to, the foliation is less strongly marked, while the 

 texture is more crystalline, and much hornblende appears, as if 

 passing gradually into that rock. Eemarkable examples of this may 

 be seen at Fir Mountain, and at Lissan, in the stream forming the 

 county boundary. In the former locality, we find at the base of 

 the hill, micaceous schists and well-foliated gneiss, succeeded by a 

 coarser quality, till at the summit we have massive rocks, in which 

 all traces of foliation seem nearly obliterated, approaching so closely 

 to the hornblendic type that no exact line of division is at all 

 possible. The stream section, north of Lissan demesne, exposes 

 similar rocks, which are more quartzose, and change into granite and 

 syenite. From these observations it would appear that the schistose 

 and amorphous green rocks are but parts of one system, the varieties 

 being due either to differences in the chemical composition of the 

 original rocks, aided, probably, by conditions of greater intensity of 

 heat and pressure, or perhaps, that the more crystalline parts were 

 the result of re-metamorphism. That some such action has taken 

 place over that part of the district at least, I shall endeavour to 

 show subsequently. 



Secondly. — Green metamorphic rocks, generally hornblendic or 

 pyroxenic. — These, as has been remarked, occupy the country around 

 the gneiss series just described. The most prevailing kind consists 

 of an aggregate of plagioclase felspar, which is often labradorite, 

 with hornblende, or pyroxene, or both, developing in some places 

 into largely crystalline hypersthenite, in others becoming more 

 finely crystalline, passing into a compact greenish or bluish variety. 

 Free quartz, too, often occurs, sometimes in grains scarcely perceptible, 

 but in other places developing into great crystalline blebs, converting 

 the rock into a coarse syenite. 1 Examples of this may be observed 

 in the hills north of Pomeroy, where, at the base, the hornblendic 

 rock with a little quartz is seen, which mineral increases in the 

 space of a few yards to great crystals, that weather out in warty 

 excrescences, giving to the rock, as Portlock remarks, " a peculiar 

 mechanical aspect." 



In other localities the compact varieties become highly felspathic, 

 and pink felspar (orthoclase) appears, with quartz, until a corase 

 elvanite or quartz porphyry is produced, which usually contains 

 mica and passes into granite, or syenitic granite by the addition of 

 hornblende. 



That the green metamorphic rocks thus pass gradually into granite, 



1 I use the term syenite in the same sense as it is used in Jukes's Student's 

 Manual of Geology, viz. " A crystalline granular aggregate of felspar, hornblende, 

 and quartz." 



