CoLte—The Rhyolites of the County of Antrim ; with a Note on Bauxite. 101 
from cracks and hollows, and probably represent material imported into the 
rhyolite from outside. 
The specimen from which this section was cut contains a mass of brownish and 
whitish opal and agate at its centre. Cavities occur in the dull agate, and small 
cubes have at one time developed in them. These are now represented by 
pure white epimorphs, measuring about ‘75 mm. in the side. While the opal of 
the cavity, mingled, doubtless, with chalcedony, has a specific gravity of from 
2°10 to 2°27, the material of the epimorphs is almost uniform, giving 2°26. Under 
the microscope, it is isotropic, and shows a minutely globular structure like that 
of ‘“‘beekite.” There is no doubt that it also is chalcedonic or opaline. I can find 
no trace of the original cubes, which may have consisted of rock-salt or some 
easily removable substance. 
This final deposit of chaleedony corresponds, however, in its opaque whiteness, 
to that which has spread over the adjacent surface of the opaline mass. Ina 
diffusion-column of exceptionally long range, most of this powdery substance was 
found to float at the same level as the epimorphs; but, under high powers of the 
microscope, the material proves to be of complex character. Fibrous patches of 
chalcedony are seen; and numerous minute crystals, too thin in any direction to 
affect polarised light, form the finest constituents of the powder. These crystals 
appear as rectangles, or as parallelograms with the obtuse angle measuring about 
100°; their greatest length is about ‘0125 mm., so that eight hundred would extend 
across 1 cm. ‘They are perfect in form for the most part, but have a slight 
tendency to additional growth at the edges, the rectangular sections then showing 
little prominences at their angles. On causing the rectangular bodies to roll over 
in water under the cover-glass, they show obtuse angles on other faces of the 
prism ; the prevalence of squares and rectangles makes me regard the solid form 
as monoclinic, bounded by the three pairs of pinacoids. 
After boiling in a strong solution of caustic potash, and allowing to stand for 
forty-eight hours, the chalcedonic fibres were almost all dissolved away, and an 
immense number of the microlites above described were set free from entanglement 
in them. Examination during the process of destruction of the chalcedony shows 
that the minute crystals are normally thus hidden away among the fibres. 
After boiling in strong hydrochloric acid, and standing for forty-eight hours, 
the chalcedony showed a tendency to part along its fibres, and many of the 
microlites became thus set free. They were naturally not so clean and clear as 
in the sample treated with potash. 
I am unwilling to withhold the wider observations on the rhyolites of the 
county of Antrim until the identity of these interesting little bodies can be proved. 
They are neither carbonates nor phosphates; probably they are silicates, for they 
produce a cloudiness in a bead of microcosmic salt. 
TRANS. ROY. DUB. SOC., N.S. VOL. VI,, PART III. Q 
