90 Cotr—The Rhyolites of the County of Antrim ; with a Note on Bauaite. 
margins. These are probably what von Lasaulx* described as an alteration- 
product, resembling hyalite. Their mode of occurrence certainly suggests a 
deposit of iron-stained opal; but the material gives fairly vivid colours, even of 
the second order, between crossed nicols, although it is of very trifling thickness. 
The hexagons of the tridymite from the browner cavities of the rock are thus seen 
to be ringed about with a coloured and brighter margin. ‘This doubly refracting 
substance is that which has developed as a crust on the surface of the drusy 
cavities after the formation of the tridymite. In the hand-specimen, the crust is 
seen in many cases to consist of epidote, and its occasional brown colour may, 
after all, arise from associated limonite. The marginal zone is absent in the 
tridymite extracted from the colourless druses. Von Lasaulx has pointed out that 
minute crystals of quartz may also be obtained from these cavities. 
Here and there far smaller druses appear in microscopic sections (Pl. IV., 
fig. 1). They may be some *3 mm. in diameter, with platy crystals, of weak 
double-refraction, projecting from their walls. These crystals are also referable 
to tridymite, and sometimes this mineral has completely filled the cavity, or the 
end of it traversed by the section. Precisely similar microscopic druses occur in 
the compact and banded rhyolite at Hlinik in N.W. Hungary, the ‘‘ millstone- 
porphyry ” of older writers. 
In one of the Tardree druses, a delicate needle, with oblique extinction, is the 
only crystal present, and this I hold to be a felspar. Other crystals in these 
drusy cavities are probably minute sanidines of the Templepatrick type; and the 
occurrence of the cavities themselves may form an interesting link between the 
central mass of Tardree and the drusy granites and eurites, the so-called 
‘“‘ oranophyres,” of the Mourne Mountains and the Inner Hebrides. Professor 
Hullt has opposed any such comparison; but Mr. M‘Henry ¢ has distinctly 
supported it, and has poimted to the neck of rhyolite (‘‘trachyte porphyry”) 
between Dromore and Moira, Co. Down, as occupying an important position 
midway between Tardree and the Mournes. 
In these petrographic notes, the occurrence of black manganese stainings and 
of minute plates of heematite should be recorded. The latter are found abundantly 
as black glancing specks in the groundmass; and they occasionally form clusters 
in the cavities, which are then stained delicately pink. If they are crushed with 
a knife-point, their strong red streak becomes apparent.§ 
In the specimens in the Royal College of Science for Ireland, as well as in 
* Op. cit., Tscherm. Mittheil., Bd. i., p. 412. Also Journ. R, Geol. Soc. Ireland, vol. v., p. 31. 
} ‘‘ Phys. Geol. and Geogr. of Ireland,’ 2nd ed., p. 97. 
t Op. crt., Geol. Mag., 1895, p. 264. 
§ Mr. Watts (op. cct., Quart. Journ. Geol. Soc. London, vol. 1., p. 873) has observed specular iron 
in cracks in the obsidian of Sandy Braes. 
