Cotu—The Rhyolites of the County of Antrim ; with a Note on Bauzite. 91 
those collected by myself, epidote is rare on the whole, but is distinctly visible as 
vivid granules, as a coating to some of the cavities, and in the green bands that 
run across some altered portions of the rock. I have not come across zones of this 
mineral round the quartz crystals, such as von Lasaulx observed in his microscopic 
sections. Its occurrence as a crust upon the tridymite has been noted above in 
the microscopic description of the rock. 
The distance to which the type-rock of Tardree Mountain extends from the 
central dome is a point difficult to settle in the field. The whole boundary of the 
rhyolitic area is conjectural,* owing to the dense and continuous layer of peat and 
heather that covers this undulating moorland. Even where farming has reclaimed 
portions of the surface, the roads are still of the steep highland character, and 
seldom cut deeply enough into the hillsides to expose the underlying rock. But 
the rhyolites have a marked fluidal structure, and strongly suggest lava-flows, as 
we recede from the denuded core of Tardree Mountain. This feature has been 
noticed at Browndod (14 miles distant) on the south-east; at Scolboa (14 miles) 
on the south ; emphatically in the hollow east of Carnearny (1+ miles south-south- 
west of T'ardree); and equally strikingly in the townland of Barnish, only # mile 
from the central mass. If these rhyolites are lava-flows, the Tardree type of rock 
may still be found underlying them, and even intruding into them. Away in the 
south-west (2+ miles), nearly half-a-mile south of the Covenanters’ Meeting-House, 
on the hill-road from Antrim to Kells, there is an exposure of compact grey 
rhyolite, decomposing to brown, seen in the low cliff of a disused quarry. This 
rock, in microscopic section, shows an almost microcrystalline groundmass, full 
of minute transparent rods of felspar, and stained by oxides of iron and manganese, 
which have spread inward from the numerous joint-surfaces. The porphyritic 
crystals are quartz, orthoclase, and plagioclase, the last-named being unusually 
prevalent aud greatly corroded by the once glassy groundmass. <A ferromag- 
nesian mineral has been sparsely present, but has decomposed to mere chloritic 
patches. ‘Tridymite lines the minute cavities in the section. The characters of 
this rock resemble those of the central mass at Tardree rather than of the 
surrounding lavas, and it may be merely a westerly offshoot from the neck. 
The rocks at the cross-roads south of Tardree Cottage seem to represent flows 
both of glassy and lithoidal rhyolite; lamination is also recordedt from the braes 
east of Carnearny, where the compact grey and brown rhyolite rises as a con- 
siderable hill, actually higher than Tardree Mountain. Certainly, a continuously 
fissile structure is very apparent in this mass, though it might be overlooked in 
* Compare Hull, ‘‘ Phys. Geol. and Geogr. of Ireland,” 2nd ed., p.98. The area is fairly repre- 
sented on the map accompanying Berger’s Paper (Trans. Geol. Soc. London, vol. iii., 1816, pl. 8), though 
the name ‘‘ Brown Dod” is misplaced. 
} Mem. to Sheets 21, 28 and 29, p. 19. 
