Cote—The Rhyolites of the County of Antrim ; with a Note on Bauxite. 108 
when they and the small rock-particles fall out, on the weathered surface of the 
rock, they leave behind them a mere meshwork, representing the coherent paste 
of fine voleanic material by which they have been bound together. 
Hence I believe that this remarkable rock* gives us a true tuff to add to the 
products of the T'ardree volcano. 
The form of rhyolite most common in this tuff is not at present exposed 
anywhere as a flow, and was probably blown out from some previously consolidated 
layer. A flinty pebble of it, found in the loose surface-materials on the north 
side of Sandy Braes, not far from the agglomerate, gives a specific gravity 
of 2°57, and is practically a very compact eurite. Under the microscope, its 
density is explained by the microgranular condition of the ground, speckled 
with an enormous number of very small grains of magnetite, and by the abundance 
of a blackened and altered biotite, which has suffered from the attack of the 
molten goundmass, after the manner of the amphiboles in many andesites. This 
rock is linked to the Selmeczbénya type of rhyolite by the prominence of dark 
mica, ferromagnesian constituents being decidedly rare in the other lavas of 
Tardree. Minute but well-formed orthoclase crystals are numerous in the ground- 
mass, which in this respect recalls the rock of Templepatrick churchyard. Larger 
porphyritic grains of quartz and orthoclase, mostly rounded and corroded, lie 
scattered among the little ones, and are easily seen by the naked eye. Round 
these, the microlites of the groundmass have sometimes settled down, forming 
a thin and delicate granular zone, the elements of which are not in optical 
continuity with the enveloped crystal. Plagioclase is unusually rare. 
Fragments broken from the lithoidal rhyolites of the agglomerate, and 
examined under high powers of the microscope, are very similar to the pebble 
above described; they are almost completely crystalline, and a microgranular 
groundmass, speckled with magnetite dust, is prevalent. Biotite, however, seems 
somewhat rare. 
The relations of the rhyolites of Carnearny, Tardree, and Barnish, and all 
their outlying spurs, to the basalts which everywhere surround them are, 
unhappily, of the most obscure character in the field. Berger + expressed 
uncertainty in the matter, but was inclined to believe that the rhyolites underlie 
the basalts. Hardman,t on insufficient grounds, held that the rhyolites were the 
older series. Professor Hull, § in the Survey Memoi, placed the rhyolites as older 
than all the other igneous rocks of the area, and stated that the lower basaltic 
series, ‘‘at Brown Dod, Scolboa, and Carnearny, have remained as isolated 
patches capping the trachytic rocks.” On the next page the Scolboa basalt is 
* Compare ‘‘The Volcano of Tardree,” Geol. Mag., 1895, p. 306. 
t Op. cit. (1816), pp. 191, 192. t Op. cit. (1871), p. 82. 
§ Mem. to Sheets 21, 28 and 29 (1876), p.9, &e. 
