320 Scientific Proceedings, Royal Dublin Society. 
the Convoy dyke has many characters in common with the well- 
known augite-hornblende-andesites that occur near Wernstadt and 
Tichlowitz in the Tetschen district of northern Bohemia, where — 
some of them are extensively quarried for road-metal. These 
rocks, which occasionally pass into olivine-basalts, none the less 
contain sufficient potassium in their groundmass for the production 
of a fair flame-reaction. Rosenbusch’ has allied them with his 
leucite-monchiquites, owing to the presence of leucite in the 
groundmass of some members of the group. The rarity of 
porphyritic felspar, and the abundance of brown hornblende 
prisms in the groundmass, certainly connect them with the 
lamprophyres; but a slight decrease of magnesium and iron in 
the groundmass, and a substitution of porphyritic hornblende 
for augite, would give us a type closely allied to the dyke of 
Convoy in Donegal. This latter rock may well be placed with 
the camptonites; but its possible modifications at points to the 
south-east of Convoy have yet to be studied microscopically. 
Olivine has not been observed in it; but this mineral is not 
regarded by Rosenbusch as essential in his comprehensive campton- 
ite division.” 
A second dyke that we have examined is of interest, inasmuch 
as it has not been previously mapped, although it has been 
quarried for some time by the local road-contractor, to whom it 
furnishes excellent road-metal. The rock is traversed by numerous 
irregular joints, which render it very easily broken down to the 
size required for road-mending, while very difficult to reduce to a 
smaller size. It occurs on the promontory of Drumboy on the 
southern shore of Lough Swilly, two miles north-west of the 
village of Newtown Cunningham. ‘The locality lies near the 
south-west corner of Sheet 11 of the one-inch map of the Geological 
Survey of Ireland. The dyke cuts across the conspicuously 
exposed rocky summit to the south-east of the road, and can be 
1“ Mikrokoskop. Physiographie,”’ 3te Aufiage, p. 545. ‘The more recent views as 
to the character of the isotropic matter, analcim or glass, in the monchiquites are 
expressed by Loewinson-Lessing, ‘‘ Studien iiber die Eruptivgesteine,’”’ Congrés 
géol. internat., Compte rendu, session 1897 (pub. 1899), p. 289, and by J. 8. Flett, 
‘«Trap Dykes of the Orkneys,’’ Trans. Roy. Soc. Edinburgh, vol. xxxix. (1900), 
p. 889. 
2 Op. cit., p. 536; also ‘‘ Elemente der Gesteinslehre’’ (1898), p. 283. 
