318 Scientific Proceedings, Royal Dublin Society. 
Hocene intrusion; their secondary characters, in so far as they 
indicate contact-metamorphism, may be due to the arrival of some 
later igneous mass, which still lies concealed beneath the surface. - 
The “ felstones” that are so common in the area of the 
Dalradian schists fall, as Dr. Hyland first pointed out, very 
largely in the vogesite and camptonite series of Rosenbusch. 
The officers of the Geological Survey have allowed us to examine 
their section of the rock of Aughagault,! which they have properly 
re-named camptonite, rather than vogesite, owing to the prevalence 
of plagioclastic felspar. It contains dull brown hornblende 
similar to that occurring in the porphyritic dyke at Convoy, which 
is about to be described. 
The Convoy “ felstone” is mapped in sheet 17? as a band 
some six miles long, running N.N.W., and broken up by faults. 
From the description in the memoir, its characters appear to vary 
considerably, and the detached strips are possibly not all portions 
of the same mass. ‘The band from which our specimens were 
collected cuts the Dalradian limestone about a mile north-east of 
Convoy. The rock is rich in hornblende, and has in its fresh 
condition, a compact grey phonolitic groundmass. This weathers 
to a browner colour; but the specimens gathered in the wood 
north of the main road from Convoy to Raphoe indicate that the 
brown earthy type is far from being an adequate representative 
of the rock. The same is true, no doubt, of the brown and 
reddish types so familiar among the lamprophyres as a group, 
whether in the Vosges, the Lake District, or the south of 
Scotland. 
The specific gravity of a fresh example of the Convoy dyke is 
2°73. The compact grey groundmass gives a fair potassium 
reaction (about 3 per cent. of potash) in a bead of sodium carbonate, 
but shows nothing definite even with a high power of the micro- 
scope. It is anisotropic, and trachytic in aspect; when the rock is 
weathered, the ferro-magnesian constituent becomes picked out 
in the ground, giving rise to chloritic specks. Minute amphibole 
and a felspar probably constitute the mass of the ground, which 
retains also abundant crystallites, belonging to its earliest phase 
1 Mem. Sheets 3, 4, 5, 9, &c., p. 141. The dyke occurs in sheet 16. 
* See also Mem. Sheet 17 (1889), p. 27. 
