324 Scientific Proceedings, Royal Dublin Society. 
The groundmass of one of these, from the quarry east of Six- 
Mile-Cross, crowded as it is with minute dark needles of amphi- 
bole, must possess a composition not far removed from that of the — 
eamptonites. The magma from which the lamprophyres arose 
may, whatever theory we adopt, have produced andesitic rocks 
when it reached the surface. We are inclined, then, to connect 
the series of lamprophyric and allied dykes in northern Ireland 
with the concealed caldrons that played so considerable a part in 
the British Isles in Lower Devonian times. Northern Ireland in the 
Oid Red Sandstone period can hardly have escaped the disturbing 
influences that worked their will in southern Scotland. We are 
strengthened in this view by the fact that Sir Archibald Geikie* 
states that the ‘‘ dykes of felsite, minette, lamprophyre, vogesite, 
and other varieties,’ which cut the Silurian and Ordovician rocks of 
the Southern Uplands, “may also be connected with the volcanic 
phenomena of the Lower Old Red Sandstone.” 
EXPLANATION OF PLATES. 
Pratt XIX. 
Fic. 
1. Microscopic section of glassy olivine-basalt from small dyke in 
stream north of Laghy, showing corroded crystals of olivine, and 
a few infilled steam-vesicles. x 37. 
2. Microscopic section of camptonite of Convoy, showing porphyritic 
brown amphibole in a compact grey ground. A ‘ glomero- 
porphyritic’? group is included, with colourless interstitial 
matter between the crystals of amphibole. x 12. 
Prars XX. 
View of Quarry opened near Drumboy. The vogesite dyke forms a 
dark mass crossing the foreground, and cutting the structural 
planes of the paler schists, which appear above. 
1 Ancient Volcanoes of Great Britain,’’ vol. i., p. 298. 
