316 Scientfiiec Proceedings, Royal Dublin Society. 
at Clondermot, and a considerable group of minettes and ker- 
santites on the coast of the county of Down. ‘The latter are com- 
pared with similar rocks cutting the Ordovician strata in the 
Southern Uplands of Scotland. 
Our own observations on certain of the felstone dykes may be 
worth putting on record, if only to confirm the excellent diagnoses 
drawn up by the late Dr. Hyland. We were led to look into 
the matter during an examination of southern Donegal, since we 
were anxious to discriminate between rocks that have shared in 
the folding and deformation of the Dalradian series, and those of 
later date. Certain dykes, also described as “‘ felstone,” are repre- 
sented on the Survey map, sheet 32,1 as cutting Carboniferous strata. 
These cannot be included in the second series mentioned above, and 
it seemed probable that they might be classed, with the glassy 
rhyolites of Lough Eask, as of Cainozoic Age. We regret that a 
search along the Waterfoot River, south-west of Pettigo, where 
the banks are greatly overgrown, failed to reveal the ‘‘ dyke of 
compact bluish black felstone” recorded by Mr. Symes in 1891. 
A careful examination of the actual bed of the stream is probably 
required. North of Laghy, however, we were more fortunate. 
On wading for some distance down the stream, we found a thin 
vein of grey-green igneous rock, its joints richly coated with 
pyrite. Lower down, at a little waterfall, a dark grey, compact, 
and more massive dyke occurs, which has baked the surrounding 
Carboniferous limestone for more than a metre from its visible 
margins. This rock has a remarkably phonolitic aspect. It also 
has films of pyrite on some of its joint-surfaces. Its specific 
gravity is 2°90. 
Both these dykes seem to have suffered in their turn from 
contact-metamorphism, the source of which is not apparent in the 
field. A glance at them shows that they are very unlike any 
Cainozoic highly siliceous rocks in Ireland. Their flinty mode of 
fracture, due, as we believe, to secondary causes, probably led 
to their inclusion under the general name of “felstone.” In 
microscopic section, the smaller dyke proves to be a tachylytic basalt, 
very rich in altered porphyritic crystals of olivine (Plate XIX., 
fig. 1). These crystals have been corroded by the brown glass of 
—— 
1 See also Mem. Sheets 31 and 32, pp. 20 and 21. 
