Cotr—On the Geology of Slieve Gallion, in the County of Londonderry. 229 
fresh specimen could be obtained, would repeat some of the characters of that of 
Letteran. In that case, its idiomorphic hornblendes would be seen to be due to 
the action of the invading magma upon one of the more crystalline members of 
the local diabase series. 
Two dykes of dolerite, which I have not myself traced out, have been mapped, 
running north-west and south-east, one on each side of Slieve Moyle ; they cut the 
granite, and are no doubt correctly assigned by Mr. Egan to the Cainozoic era.* 
The long flat capping of basalt, protecting the Cretaceous strata, and forming the 
summit of Slieve Gallion North, may thus have flowed from some local source. 
Even if not an outlier of the eastern plateaux, it is one of the most remarkable 
evidences of the denudation which the district has undergone. 
IIJ.—Txue GRanrre Series. 
It must have been already recognised that the great part of the massif of 
Slieve Gallion consists of granite. A section drawn across it in almost any 
direction would at once show the importance of the intrusive mass, and the 
probability that the diabase series has been bent into an elongated dome, into 
which the granite found its way (Pl. xm.). The division of the mass in the 
Survey Memoir into ‘‘ Granite” and “ Granitoid Porphyry or Elvanite” is not 
a very happy one, as all forms of the granite that are poor in mica seem to have 
been included under the latter designation,t even if the rock is ‘as largely 
erystalline as the granite.” Moreover, the area coloured as elvanite on the map 
covers some exposures of highly typical biotite-granite, such as that immediately 
east of Tintagh post-office. 
There is, however, a distinction in the field between the true granite and the 
‘“‘quartz-porphyry ” or eurite type, the latter having a characteristic compact 
groundmass, which seems to have been formerly taken for continuously erystal- 
lised felspar. ‘The eurites come in exactly where one would expect to find them, 
near the contact with other rocks in the top of the great granite dome. 
The granite has been carefully described, in successive Survey publications, as 
consisting typically of quartz, pink orthoclase felspar, and a greenish mica. A 
second felspar, regarded as oligoclase, has been noticed.t South of Glenarudda 
Mountain, the rock becomes hornblendic, and forms the “ syenite” of the Survey 
Memoirs. In the area separately coloured as syenite (7c. hornblende-granite),§ 
there is a good deal of the ordinary biotite-granite. The hornblendic type 
is, however, well seen in the bosses immediately south-west of Ballybriest 
Bridge. 
* Mem. sheet 27, p. 33. Also J. Nolan, Mem. sheet 26, p. 17. + Mem. sheet 27, pp. 18 and 14. 
t{ Mem. sheet 26, p. 10. § /bid., pp. 12 and 10. 
TRANS. ROY. DUB. SOC., N.S. VOL. VI., PART IX. 2M 
