102 Cote—The Rhyolites of the County of Antrim ; with a Note on Bauzite. 
At the east end of Sandy Braes, the old diggings reveal a beautifully fluidal 
and banded rhyolite, red and lithoidal, resembling the ‘ millstone-porphyry ” of 
Hungary; it is, however, of somewhat coarser structure, and by no means so 
vesicular. But its character as a true lava-flow is scarcely open to doubt. Under 
the microscope it is an unusually handsome rock (PI. IV., fig. 5), with alterna- 
tions of burnt-sienna fluidal glass, purplish-grey bands containing embryonic 
spherulites, and grey-brown lithoidal patches enclosing shattered grains of quartz 
and felspar. These crystals are often torn to pieces along their curvilinear cracks, 
and are distributed in angular shreds throughout the groundmass. The specific 
gravity of this rock is 2°48. 
In a digging on the left, as one enters the western excavations at Sandy Braes 
from the Doagh and Connor road, there is a very friable whitish rock, which 
absorbs water rapidly, and becomes muddy brown, flecked with black, after any 
shower of rain. The black specks and patches through it are evidently perlitic 
obsidian. In places this glassy material can be seen to form a more continuous 
groundwork, in which the lighter portions lie. Quartz and felspar crystals are 
porphyritically embedded in the whole. 
Examination with the lens, and with the microscope after the manner of 
Cordier, shows that the lighter portions are pumiceous rhyolite, while the obsidian 
particles include numerous small spherical vesicles, and are brown by transmitted 
light. Many fragments of both kinds show doubly refractive effects between 
crossed nicols, such as are common in pumice, which remains, on cooling, in a 
state of considerable strain. 
Here we seem to have the surface of a rhyolitic lava-flow ; but, in the quarry 
to the north, there is still stronger evidence that we are dealing with the flanks of 
a voleano. Resting on the pink-red rhyolite already mentioned (p. 95), there 
is a fragmental rock, containing volcanic material of various kinds. This became 
exposed during the summer of 1895; it would clearly be desirable to keep a 
regular watch on the walls of these shifting excavations. The chief component of 
this agglomerate is a dark grey and almost lithoidal rhyolite, in part retaining the 
aspect of a perlitic pitchstone, but in part as dull and flinty as a compact Welsh 
‘‘felsite.” Its matrix is harder than the blade of a knife, has a specific gravity of 
somewhat over 2°50, and contains the usual porphyritic crystals of quartz, 
orthoclase, and plagioclase. Some of the fragments of this rock are a decimetre 
in length. It is embedded in a ground which is also dull grey when unaltered, 
but which contains in addition abundant white fragments of pumice, often 1 cm. 
in length, and dull angular pieces of other rhyolites. Even the pumice includes 
quartz grains; and its filaments, examined microscopically, are coated over with 
minute tridymite. Quartz and felspar crystals, evidently derived from the 
rhyolites, are scattered through the compact ground of the agglomerate. In fact, 
