June 1, 1898.] 



KNOWLEDGE 



123 



Mourne, some sixty miles north across the sea, to admit 

 that the northern group has at least a supremacy of form. 

 This is apparent, also, in the details of the landscape, 

 as one may come across them in the higher passes of the 

 mountains. We have referred to the bold peak-like tors of 

 Slieve Bernagh, and to the frequent vertical rock-walls ; but 

 the most impressive scene of all is, perhaps, the group of 

 granite pinnacles weathered out on the south side of Slieve 

 Commedagh. We may come upon these suddenly as the 

 mist lifts from the great dome of Donard, leaving the deep 

 valleys filled with cloud below us ; and close against us is, 

 as it were, a fantastic temple, the columns rising on each 

 side of a little gorge. The vertical joints have here had a 

 dominant eiiect, while the horizontal ones cut up the 

 pinnacles with a fictitious air of masonry. The neigh- 

 bouring cliflfa also display the level tabular joints, so 

 characteristic of granite, in a remarkable degree, and the 

 whole hill-side suggests an acropolis given over to decay. 

 The same air of titanic masonry is seen in the analogous 

 granite mass of Goatfell in the Isle of Arran. 



This brings us to the petrological characters of the granite 

 of the Mournes. While, as in the quarries that scar the 

 hills near Annalong, the rock is often coarsely crystalline, 

 the general mass is of finer texture, with a ground in which 

 the quartz and the alkali-felspar may be intergrown with 

 one another. The ferromagnesian constituent is a dark 

 mica. Throughout the whole region a drusy structure is 

 very common — that is, cavities occur, varying from a 

 microscopic size up to four or five inches across, in which 

 minerals have developed out freely, with all their proper 

 forms (Fig. 2). The orthoclase felspar here appears in dull 

 white or yellowish white crystals, as clean and neat as the 

 wooden models that are placed before students of miner- 

 alogy. The quartz in these cavities is usually a smoky 

 variety, forming prisms capped by pyramids, in complete 

 contrast to its ordinary mode of occurrence in igneous 

 rocks. The mica forms the most exquisite little hexagonal 

 tables, standing up on edge ; and, in addition, blue-green 

 beryl and colourless topaz are not uncommon, and have been 

 much sought for by collectors. One must conceive such a 

 rock as having been saturated with liquids under pressure, 

 each knot, if we may so say. of the liquid acting as a 

 hydi'othermal laboratory — at first delaying crystallisation, 

 but finally allowing of free growth, and of the production 

 of the most delicate prismatic forms. Few pleasures can 

 be greater to the mineralogist than the breaking up of 

 these granite blocks in the high passes of the Mournes, and 

 the sight of the perfect little crystal-groups, lying there 

 fresh as when made, and never before bared to human 

 eye. 



The granite of Arran, above referred to, is closely 

 similar to that of the Mourne lMountain3,f and we meet 

 allied, but less drusy, masses in the heart of Mull and 

 Skye. The latter rocks are among the more recent pro- 

 ducts of the great period of volcanic activity in the 

 Hebrides, which opened in Lower Eocene times. \ Hence 

 the peculiar fine-grained granites of Mull and Skye are, 

 at the earliest, of Eocene age. 



South of Carlingford Lough there is another granite 

 mass, which is intrusive in the dark gabbro of the Carl- 



* See the fine illustratiou in Sir A. Geikie's ■' Ancient Volcanoes of 

 Great Britain," Tol. II., p. 419. 



t See Judtl, '' Secondary Rocks of Scotland," Quarterly Journal 

 Geological Socief;/, Tol. XXX (1874), p. 275 ; and Teall, " British 

 Petrography," pp'. 328 and 330. 



X See J. Starkie Gardner, " Lower Eocene Plant-Beds of Ulster," 

 (^arferli/ Jourant Geological Socittii, Tol. XLI. (1885), p. 82, and 

 "Leaf-Beas of Ardtun." ibid., Tol. XLIIL, p. 292. 



ingford promontory. The relations of these rocks have 

 been admirably described by Prof. Sollas ; and there is 

 no doubt as to the correlation of the granite with that of 

 the Mourne Mountains. The gabbro is represented on 

 the Mourne coast by a multitude of dykes of basaltic 

 andesite and basalt, which form a marvellous picture of 

 the fracturing to which the Silurian rocks were subjected. 

 These dark ribs of igneous rock have altered the Silurian 

 shales and sandstones, which appear as a fringe along the 

 coast ; but they are cut off abruptly by the granite of the 

 adjacent hills. The flakes of Silurian strata that remain 

 here and there on the surface of the mountains are 

 similarly seamed by dykes ; but the granite cuts off all of 

 them, and is clearly later than this first eruptive series. 

 A few basic dykes, however, which may be well seen as 

 grey-green bands in the granite north of Slieve Bernagh, 

 cut through the granite, and represent a return of basaltic 

 conditions. Hence we have three igneous series, two being 

 basic, with a highly siliceous one between them. 



This is precisely the order of events in the Eocene 

 volcanic centres 

 of Mull and Skye ; 

 and, even in 

 microscopic de- 

 tails, the rocks of 

 the one area may 

 be paralleled by 

 those of the other. 

 Moreover, in the 

 county of Antrim, 

 the outpouring of 

 the sheets of the 

 "Lower Basalts" 

 was followed by 

 local eruptions of 

 rhyolite, a highly 

 siliceous lava, 

 agreeing in com- 

 position with the 

 granite of the 

 Mournes. t This 

 series was in turn 

 buried by the 

 " Upper Basalts." 

 All this volcanic 

 material in An- 

 trim seems to be 

 of Eocene age ; 

 and the sequence of events practically clinches the argument 

 that the Mourne granite belongs also to the Eocene period. 



Here, then, we have a granite, one of those rocks formerly 

 supposed to be of very ancient origin, brought near the 

 surface as a fluid mass as recently as Cainozoic times, and 

 probably not exposed, even in its upper layers, until shortly 

 before the glacial epoch. The geological history of the 

 Mournes, of Carlingford Mountain, and of the high volcano 

 of Slieve Gullion in Armagh, is seemingly, then, a very 

 modern matter compared with that of the adjacent Newry 

 granite and the old weather-beaten core of Leinster.J 



Possibly the little dome of Ailsa Craig, which has 

 suffered so heavily from denudation that its pebbles lie 



* " The Tolcanic District of Carlingford and Slieve Gullion," 

 Trans. S. Irish Acad., Tol. XXX. (1894), p. 477. 



t See A. McHenry, "Age of the Trachytic Kocks of Antrim," 

 Geol. Mag., 1895, p. 264; also G. Cole," " Rhvolites of Coontv 

 Antrim," Sci. Trans. S. Dublin. Soc, Tol. TI. (189d), pp. 84 and 

 104. 



t See Knowledqb, Vol. XXI. (1898), p. 76. 



Fig. 2. — Speciuieu of Mourne Granite, 

 showing crystals developed in a drusy cavity. 

 The pointer, marked T, inilicates a crystal 

 of topaz. 



