Cotr—On the Geology of Slieve Gallion, in the County of Londonderry. 237 
Portlock.* A compact and almost black diabase occurs in the upper part of the 
quarry, penetrated by the felspathic diorite. In section, this rock is seen, as 
above hinted, to be in the condition of hornblende-schist. 
The most startling feature in the quarry-face is the occurrence of a massive 
dyke of rhyolite, a pink-red compact rock, of an almost translucent ‘ hornstone ” 
type, cutting through the basic series at an angle of about 40°(PI. xtv., figs. 2 & 5). 
Its specific gravity is 2°65, owing to secondary devitrification ; but in microscopic 
section it shows an excellent flow-structure. Quartz-grains and porphyritic un- 
twinned felspars form “eyes” in the fluidal and banded groundmass, and have 
been sometimes broken up and faulted during the intrusion of the mass. An 
attempt has been made to shear the dyke subsequently at an angle to the direction 
of flow, and the small flakes of biotite have sometimes been dragged out along 
the new structural planes. 
‘The occurrence of this rhyolite makes one expect some intermingling with the 
diorite series; and the rock with pink felspar certainly attracts attention. Mr. 
Nolant probably had this in his mind when he wrote of the occurrence of ortho- 
clase in the gabbros; and we may well be influenced by his descriptions of the 
composite masses of Copney, Loughmacrory, and Craiggrena. But microscopic 
sections, aided by other observations, do not justify us in regarding the pink 
felspar of Oritor as derived from a granitic rock. It has a specific gravity of 2°63 
to 2°65, which does not, it is true, remove it from the altered orthoclases of Slieve 
Gallion; but it is in places fairly preserved, and shows repeated twinning, both of 
the albite and pericline types. Few of the crystals in the two sections examined 
are suitable for the test of symmetrical extinctions; but the results obtainable 
favour labradorite as the species. This pink felspar is intergrown with masses of 
green secondary hornblende. Earth-pressures have deformed the rock, have bent 
the felspars, and have driven the chloritised amphibole into cracks running through 
the mass; but the fundamental and original structure has not been entirely oblite- 
rated. For comparison, a section was made of the ordinary diorite, with white 
‘‘saussuritic” felspars, of the Oritor quarry. It is richer in hornblende, and the 
ophitic structure has a tendency to run into a pegmatitic one, as in the gabbros of 
Rum; but in this latter point it represents all the more closely the undeformed 
condition of the pmker rock. The latter must be regarded as having originally been 
a gabbro with much felspar, the opposite extreme being found in the diallage-rocks 
of Athenree. Its specific gravity is 2°87, as opposed to the 2-98 of the gabbro of 
Tullycall. 
The white felspars of the ordinary Oritor diorite have an average specific 
* Op. cit., p. 550. 
} ‘‘Metam. Rocks of Tyrone,” Geol. Mag., 1879, p. 157; alsoMem. sheet 26, p. 15; compare 
pp. 12 and 13. 
TRANS. ROY. DUB. SOC., N.S. VOL. VI., PART IX. 2N 
