86 Cote—-The Rhyolites of the County of Antrim ; with a Note on Bauxite. 
a book partly opened and set up on end. In this way the edges always face 
outwards, and are seen one after another as the observer walks round the volcanic 
neck. It is interesting to find the structure of these trachytic cores of Bohemia 
repeated thus boldly at Tardree ; the rock in both cases was probably of similar 
viscid character, weathering out at length as a coherent dome, bounded by a wall 
of cliffs. At Tardree Mountain the northern climate and the greater antiquity of 
the mass have now combined to soften down its flanks and to cover them with 
grass-grown taluses. (PI. III., fig. 1). 
Oblique cross-joints may also be seen, breaking up the vertical sheets. In the 
second large quarry, south-west of the imn on the road from Doagh, similar features 
oceur, with some attempt at the formation of columns 60 to 70 centimetres in 
diameter. 
The rhyolite of Tardree Mountain, when freshly quarried, is full of water and 
of a deep grey colour, turning to white on drying. Hence specimens of the 
curb-stones and other blocks, when broken in the quarry after being shaped, may 
appear white on the surface and still damp and grey within. 
The type-rock itself, the pale grey mass of the well known quarries at Tardree, 
has been frequently examined and described. Berger* styled it a ‘“ Clay- 
porphyry,” the “porphyry of Sandy Brae,” containing embedded in it concre- 
tions of smoky quartz, earthy and glassy crystals of felspar, and olivine. The 
‘Colivine” may perhaps refer to epidote. He determined the specific gravity 
of the rock as 2°43; Mr. Hardmant subsequently gave 2:433; and I have obtained 
2-46. It will be seen later that these figures are probably too low, owing to the 
abundance of minute drusy cavities. 
In calculating the proportions of the mineral constituents of the rock, Hard- 
man regarded it as consisting of quartz and orthoclase only. Von Lasaulxt 
recorded his discovery of tridymite in the rock, and stated, in his popular work,§ 
that the fine dissemination of this mineral through the groundmass accounted for 
the high percentage of silica found by Hardman. At the same time he described 
the groundmass as rich in glass, thus agreeing with Prof. Hull, || who had 
) 
previously regarded it as “amorphous.” But von Lasaulx, for some reason, in 
his formal paper,{] states that, ‘contrary to Prof. Hull’s opinion,” the ground- 
mass shows no glass, but is a very compact mixture of quartz, tridymite, and 
felspar, the last-named mineral not affording proof of its monoclinic or triclinic 
* Op. cit., Trans. Geol. Soc. London, ser. i., vol. ii1., p. 189. 
} Op. cit., Journ. R. Geol. Soc. Ireland, vol. ii., p. 27. 
{ ‘On the discovery of Tridymite in the Trachyte Porphyry of Co. Antrim,” Journ. R. Geol. Soc. 
Treland, vol. iv. (1877), p. 227. 
§ ‘Aus Irland,” pp. 167 and 168. 
|| Memoir to Sheets 21, 28, 29, Geol. Survey of Ireland (1876), p. 18. 
q Tschermak’s Mittheilungen, Bd. i. (1878), pp. 416 and 418. 
