Cotu—The Rhyolites of the County of Antrim ; with a Note on Bauxite. 85 
a somewhat limited survey of the ground. So little attention, however, has been 
called to the sections on the east side of Carnearny and on Sandy Braes, that Sir 
Archibald Geikie * in 1888 omitted the glassy rocks of the area from his main 
account of the Tertiary pitchstones of the British Isles, and only referred to them 
in passing on a subsequent page of his memoir. He stated, moreover, that he 
knew of no case where acid lavas had reached the surface during the Tertiary 
era, excepting at the Sgirr of Eigg. Mr. G. H. Kinahan similarly passes over 
the glassy and fluidal rocks in his ‘‘ Manual of the Geology of Ireland,” as also 
does Mr. Teall in his comprehensive ‘ British Petrography,” published in 1888. 
The predominance, moreover, which the type-rock of Tardree Mountain has 
assumed in the minds of geologists is shown by the general reference of the 
rhyolites of the county of Antrim to massive intrusions rather than to dykes and 
lava-flows.t 
The typical rock is cut into by a series of quarries extending from south to 
north along the flank of ardree Mountain for a distance of more than a mile. 
In the acute north-western angle of the cross-roads between Carnearny and 
Tardree Cottage there are two abandoned diggings, which reveal a strongly 
reddened compact rhyolite of the central type, and also, as Mr. A, G. Wilson 
pointed out to me, pumiceous and perlitic obsidians, with large porphyritic 
crystals of felspar. 
Beyond the Cottage there is at present a roadside cutting, which shows how 
ared colour, often vivid, has been introduced into the rhyolite along vertical 
joints. Hence the prevailing colour at the cross-roads is probably also due to 
alteration. 
Immediately to the north of the roadside-cutting is an old weathered excava- 
tion, showing vertical columns 40 to 45 centimetres in diameter, which have been 
figured on p. 20 of the Survey Memoir. Then we pass to the important quarry in 
which the stone is still being actively worked for building purposes. 
Here the rock is divided by great upright parallel joints into vertical or slightly 
curving sheets, the edges of which at first give a false effect of columnar struc- 
ture. True columns occur, however, in the central part of the quarry. The same 
appearance is conspicuous on the craggy wall of the Boren of Bilin in Bohemia, 
on the domes of Sellnitz and Zlatnik, and on others of those extraordinary necks 
of phonolite which penetrate the Oligocene Brown Coal. On the vertical sides 
of the Zlatnik Mountain the joints radiate from a centre, the mass suggesting 
*« The History of Volcanic Action during the Tertiary Period in the British Isles,” Trans. Roy. 
Soc. Edin., vol. xxxv, pp. 145, 146, and 171. Also ‘‘ Anniversary Address,’ Quart. Journ. Geol. Soc. 
London, vol. xlviii. (1892), Proc. p. 168. 
+ See, for instance, H. Backstrom, “ Beitriige zur Kenntniss der islindischen Liparite,” Geol. Foren. 
i Stockholm Forhandl., Bd. xiii. (1891), p. 672. 
TRANS. ROY, DUB. SOC., N.S. VOL, VI., PART III. 0 
