446 Joseph Nolan — Volcanic District of Slieve Gullion. 



gneissose bands merging into harder and more solid granite," and in 

 the hill east of this mountain, where the granite is similar, though 

 in places nearly losing its mica, there is an unmistakable passage 

 from micaceous schists into foliated granite" (ibid. p. 14). 



The age of formation of this, called the Newry granite, cannot be 

 determined, but there is strong reason for believing that it took 

 place prior to the Upper Silurian period, as suggested by Prof, Hull.^ 

 The basal conglomerates of that system contain pebbles and boulders 

 of granite, as well as of schist and gneiss, and as granite fragments 

 are not met with, in Ireland at least, in formations older than the 

 Lower Silurian, we may fairly assume that it was formed with the 

 schist and gneiss, the eruptive and metamorphic granites being but 

 varied results of the same plutonic action. 



Micaceous Dolerite. — Eruptive through the granite is a coarsely 

 crystalline rock of massive character, forming most of the northern 

 and western parts of the mountain. It has the usual characteristic ap- 

 pearances of dolerite, having a rude prismatic structure and weather- 

 ing in globular exfoliating masses. It consists of augite and plagio- 

 clase felspar, of which there may be more than one variety, those 

 most largely developed being labradorite. In addition, it has a 

 bronze to black -coloured mica, sometimes in considerable quantity, 

 a large proportion of magnetite, olivine, and a green structureless 

 mineral that is probably epidote. 



Elvanite or quartz porphyry. — Associated with this dolerite, and 

 forming the remaining part of the mountain, is a granitoid rock 

 consisting chiefly of orthoclase felspar with free silica, seldom de- 

 veloped into well-formed crystals, a little mica, and, usually, some 

 hornblende. This rock is seen in many places to penetrate the 

 micaceous dolerite, and is therefore probably older, though as the 

 dolerite also, though rarely, sends veins into the elvanite, it is 

 most likely that both were protruded about the same time. To 

 the north-west, this elvanite composes the remarkable ridge of hills 

 that partly inclose the mountain on the north, west, and south. 

 At half the extent of its course, at Cashel Lough, it changes gradually 

 from a granitoidal rock to one having a finer grained or compact 

 base, and containing double pyramidal crystals of quartz with felspar, 

 thus becoming a highly silicated or quartzose felstone porphyry. 



Volcanic Agglomerate. — Simultaneously with this change in the 

 elvanite, suggesting conditions of less intense heat and pressure, a 

 fragmental rock of most remarkable character makes its appearance. 

 It is here entirely composed of granite pieces, and might indeed be 

 taken for a disintegrating portion of that rock, but that, on exami- 

 nation, the base is found to present a mechanical and not a crystalline 

 structure. This conclusion is confirmed on following the ridge 

 farther south, where, in the neighbourhood of Forkill, the associated 

 fragmental rock occupies a large area, and though still mainly 

 composed of granite, yet contains several fragments of other rocks, 

 all of which, however, are of local origin. Eastward of Forkill, 



1 See Physical Geology and Geography of Ireland, p. 141, note. 



