304 Frof. G. A. J. Cole— The Volcano of Tardree. 



which descends northward from Scolboa Hill; jutting rocks flanking 

 the notch which runs east and west across Carnearney Hill ; two 

 small quarries in the north-west angle of the cross-roads before 

 we reach Tardree Cottage ; the well-known quarries of Tardree 

 Mountain itself; sections on the steep ascent to Sandy Braes, 

 beyond the road from Parkgate to Kells ; and a number of small 

 excavations on the high moor of Sandy Braes. 



The Tardree mass, as now denuded, forms a beautifully regular 

 dome, and presents all the characters of a volcanic core. Its division 

 by vertical joint-planes reminds one of the structure of the phono! ite 

 necks of northern Bohemia ; while its outline recalls the Grand 

 Sarcouy of Auvergne. The compactness and uniform character of 

 the rock, the abundance of porpliyritic crystals, and the absence 

 of fluidal structure, all point to its having formed a continuous mass, 

 probably in the centre of the volcano. 



But th&se characters are rapidly lost as we proceed north or south 

 from Tardree Mountain. The Tardree type of rhyolite, grey, or 

 pale pink, or yellow-brown, crops out here and there ; but the 

 larger exposures now show it to possess a distinctly laminated 

 structure. Nothing can be well made out at the cross-roads south 

 of Tardree Cottage, and the brilliant red staining of the rhyolites in 

 the quarries at this point results from irregular alteration, and not 

 from the association of lava-flows of different compositions. On the 

 rise towards Carnearney the compact type is also seen ; but in 

 the notch south of the new plantation two kinds of rhyolite are 

 visible. A small cliff faces the plantation, its upper part being 

 formed of compact but fissile rhyolite. There is no obvious flow- 

 structure to account for the parallel and gently dipping planes of 

 division in this rock; but beneath it, the plane of junction being 

 parallel to those of fissility in the upper rhyolites, lies a handsome 

 spherulitic obsidian, preserving all its character as a grey glass, 

 traversed by perlitic cracks. These rocks recall the cliff of Skleno, 

 near Selmeczbanya, with its variety of compact and glassy lavas, 

 poured out as flows one upon another. 



The humble exposure at Scolboa merely shows pink, compact, and 

 porphyritic rhyolite, somehow underlying a handsome olivine-basalt. 

 Similarly, the dip of the rhyolites on the north end of Carnearney 

 would bring them under the basalt-flows of the south end and of 

 the summit of the hill. Sir A. Geikie ' has stated that he knew 

 of no case in which acid lavas of Cainozoic times reached the surface 

 in our islands, save at the Scuir of Eigg; but his omission of the 

 rocks of Sandy Braes from his account of the Cainozoic pitchstones ^ 

 shows that the compact rhyolites of Tardree were the predominant 

 type present to his mind. The exposure on Carnearney looks 

 suspiciously like a section at the junction of two lava-flows, and 

 there is nothing surprising in its occurrence on the south flank of 

 the denuded volcano of Tardree. 



1 ' ' The History of Volcanic Action during the Tertiary Period in the Eritish 

 Isles," Trans. Eoy. Soc. Edin., vol. xxxv (1888), p. 145. 



2 Ibid., p. 146. 



