100 CoLte—The Rhyolites of the County of Antrim; with a Note on Bauzite. 
The specific gravity of the ‘‘ Pearlstone Porphyry of Sandy Brae ” is given by 
Berger* as 2°38. He found 2°52 for the ‘ Pitchstone Porphyry ” in its ‘ blue- 
black vitreous varieties ;” nearer the surface, where it became olive green, it had 
a specific gravity of 2°50, and at the surface it was yellowish green and rather 
earthy, giving 2°40. 
I do not think that many really glassy specimens can be as high as 2°50; my 
own blue-black obsidians give 2°43, and an altered brownish glass, given me by 
Mr. A. G. Wilson, is only 2°36. 
For comparison with these figures, we may note those given by Prof. Judd +t 
for the rhyolites of Lipari, the obsidian having there a specific gravity of 2°37, 
and the most lithoidal lava rising to 2°53. 
Berger states that fragments of the perlite of Sandy Braes intumesce before the 
blowpipe ‘‘ to four or five times their first volume, fusing into a foamy and light 
glass, not unlike pumice-stone.” This, as far as I am aware, is the earliest record 
of this interesting observation, which was afterwards repeated by Beudant ¢ on 
the glassy rocks of Hungary, and which has been commented on recently by 
Professor Judd.§ 
As already mentioned, opal and chaleedony form common products of altera- 
tion in the rocks of Sandy Braes, as in Hungary, Mexico, and other rhyolitic areas. 
Vesicular cavities have been partly filled up by banded agates, and sometimes the 
ground of the rock itself has been converted into chalcedony, while retaining its 
most intimate structures. In one example of this kind, the glass has become 
purple-red and flinty, too hard to be scratched by the knife; but under the micro- 
scope it shows a multitude of primary ‘‘trichites,” arranged in a delicate and 
wavy fluidal structure. Polarised light, however, reveals long fibres of chalcedony, 
passing through the ground in spherulitic bunches, which abut against the pre- 
existing cracks of the glass and there terminate. They seem to arise from 
independent centres, and not from the porphyritic crystals, and represent a struc- 
ture superimposed during the silification of the matrix. This is one of those 
numerous cases in which the study of the microscopic section alone might lead to 
the most erroneous conclusions. 
This particular rock is of interest as containing a fair amount of warm brown 
hornblende, some erystals of which have been bleached and much altered by the 
action of the groundmass round them. ‘The red colouring-matter of the chalce- 
dony or jasper into which the ground is now converted is distributed in brilliant 
little patches, resembling hematite and affecting polarised light ; these have spread 
(Ops ctt., p. LOL: 
+ ‘Contributions to the Study of Volcanos,” Geol. Mag., 1875, p. 62, 
t ‘ Voyage en Hongrie” (1822), vol. i1., p. 362. 
§ ‘“‘ The Natural History of Lavas,” Geol. Mag., 1888, p. 6. 
