Cotr—The Rhyolites of the County of Antrim ; with a Note on Bauxite. 93 
This pyroxenic character of the rock is borne out by the porphyritic crystals. 
The order of prominence in these is plagioclase, orthoclase, quartz, soda-pyroxene, 
magnetite, and zircon. The plagioclase shows both albite and pericline twinning, 
and is bored through and through by glass intrusions; it is thus often reduced to 
a mere shell, two walls of the prism, with the angle between them, sometimes 
alone remaining. It often includes the pyroxene, as the pyroxene does magnetite, 
he three minerals forming ‘‘ glomeroporphyritic” groups. The simply twinned 
felspar behaves like the plagioclase with regard to the pyroxene, and the two 
felspars evidently arose in the same original rock. The quartz resembles ‘that of 
the other rhyolites, and is traversed by curvilinear cracks. The pyroxene 
developed very little earlier than the plagioclase, so as to form at times inter- 
growths that are almost micropegmatitic. The spreading and branching groups of 
pyroxene, however, are in these cases granular in structure, and are not optically 
continuous. Face-pleochroism is not perceptible in thin sections of the pyroxene, 
and the axis-colours differ but little from one another, changing from a pale 
yellow-green to a slightly bluer green. We may safely refer this mineral to the 
soda-augites poor in iron. Detached prisms and granules of it, including 
magnetite, occur freely. 
Magnetite is also fairly abundant, and in one instance includes zircon; the 
latter mineral is by no means rare, and even its uniaxial character can be deter- 
mined. A few granules of epidote are seen, collected in one hollow in the section. 
This pitchstone is, then, a glassy pyroxene-rhyolite on the verge of the rhyo- 
litic andesites. In a previous consolidation, as shown by the porphyritic groups, 
it was in the form of a pyroxene-plagioclase-granite ; whereas the rock at present 
hidden beneath Tardree Mountain is probably a more ordinary mica-granite. 
Since the Carnearny pitchstone underlies the normal Tardree-type of Rhyolite, it 
may be that the earlier eruptions were more andesitic than the later ones. This 
would imply, also, a gradual change, leading from any previous basaltic conditions 
in the area up to the remarkable obsidians of Sandy Braes. However, it is rash 
to found so wide a generalisation on this interesting but solitary section on 
Carnearny. 
The pitchstone of Carnearny has a specific gravity, in the mass, of 2°44. 
The glassy and microlitic groundmass, determined in a diffusion column of 
methylene iodide, gives 2:42 to 2-44. A fragment of one of the spherulites is as 
high as 2°61, but their average specific gravity is 2°544; a large one, detached and 
determined on Walker’s balance, gives 2:56. The determination of the specific 
gravity of the felspars is vitiated by the frequency of more basic inclusions; but 
the glass-inclusions in others may serve to balance these. The results, yielding 
an average of 2°65, and pointing to oligoclase, are thus probably somewhere near 
the mark. 
TRANS. ROY. DUB. SOC.,N.S. VOL, VI., PART III, P 
