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



appears to have been the opinion of the late Sir Eichard Griffith 

 and General Portlock. Thus, at p. 60 of his " Geological Eeport, 

 etc.," we find the latter author quoting the former — " Slieve Gallion 

 mountain is composed of granite, syenitic granite, syenitic green- 

 stone, and greenstone, and the same varieties of rock, apparently 

 graduating into each other, occupy the parishes of Lissan and 

 Kildress " ; and at p. 94, he himself states that " there are so many 

 gradations of change, from the rock which appears decidedly granitic 

 into hornblendic rocks on the one hand, and into others which are 

 little more than schists, in which the constituents have been blended 

 together and the stratification obliterated, that the mind seems forced 

 to consider the whole, even to the granite, as metamorphic." 



In other parts of the green rock basic minerals prevail, and it 

 becomes coarse diorite, diallage, or hypersthenite. One of the best 

 localities where these changes may be seen is at Termon Eock, near 

 Carrickmore. The massive crags which rise abruptly over the flat 

 through which the railway runs are diorites, traversed in every 

 direction by veins of epidote ; and at the Glebe of Athenree, one 

 mile to the east, the diorite is replaced by diallage or hypersthenite ; 

 while at the " Scalp," two miles to the north-east, a more largely 

 crystalline variety occurs. At all these places the felspar is, as we 

 might expect, labradorite, yet at Oritor, near Cookstown, there is a 

 massive rock in which hypersthene is associated with orthoclase. 



In close proximity even to the largely crystalline rocks sudden 

 variations in texture are not uncommon. These are well shown at 

 the locality just mentioned, where, in the same mass, one part is 

 compact, while the rest is crystalline often to a great degree. In 

 some cases, too, hypersthene appears in one part of the mass, while 

 the rest is compact, and traversed by veins consisting of a mixture 

 of quartz and orthoclase. Again, the compact variety often presents 

 the appearance of a dyke traversing the more crystalline portion. 

 In one case of this kind a lenticular dyke-like rock was traced over 

 a distance of thirty yards, and looked very much like basalt. In it, 

 however, white crystals of orthoclase were observed. 



These compact dykes and veins traversing the more crystalline 

 portions seem to be analogous to those of fine-grained granite rock 

 which may be seen cutting across the granite in the neighbourhood 

 of Dublin, and have been called eurites by the late Professor Jukes. 

 Such veins he regards as portions of the same magma as the granite, 

 proceeding from the parts still in fusion, and forced into cracks or 

 fissures in the partially consolidated upper portions, and the same 

 explanation may answer as well for the compact dyke-like veins just 

 described. 



Another variety of these compact and finely-crystalline rocks is 

 one full of cells, usually filled with calcite, or, though rarely, 

 chalcedony. These may be seen at Slieve Gallion, at Drumnakilly 

 near Omagh, and at several intermediate localities. Some parts, 

 from the number and size of the cells, suggesting great disengage- 

 ment of elastic fluids, present such appearances of a volcanic nature 

 that one is at first inclined to consider them as such, yet, on further 



