258 Cotr—On the Geology of Slieve Crallion, in the County of Londonderry. 
gravity of 2°70; but they range from 2°66 to 2°75, owing to their unsatisfactory 
state of decomposition. 
The whole of the rocks at Oritor have received thrusts, which have in some 
cases produced partial brecciation, and in others a foliated condition. But the 
great dyke of rhyolite has not been broken through, nor have the relations of the 
other rocks been obscured. The evidence shows that a series of compact apha- 
nites, or even andesites, were here invaded by a gabbro of medium grain, which 
was more felspathic in some parts than in others; subsequently the granite, which 
underlies so much of this district, sent up offshoots into the basic complex. 
After examining these deep-seated masses, characterising so wide an area, I 
was hardly prepared to meet an apparently volcanic series, identical with that of 
Slieve Gallion, during a traverse of the country to the west. Gravel covers 
much of the surface as we follow either bank of the Ballinderry River; and, 
about Charles Town and Killeenan school, we enter the region of true micaceous 
eneiss. But, on the other side of this axis, we meet the sheer boss of Dungate 
Rock, accurately described by Portlock* as a dense rock of a green colour, with 
epidote and pyrite, and somewhat amygdaloidal. This mass obviously corre- 
sponds to the ‘‘ pyroxenic rocks ” of Slieve Gallion ; even in its baked and flinty 
character, it recalls the andesites of Drummuck. In section, it proves to be an 
andesite, with porphyritic plagioclase felspars and green pseudomorphs of well 
developed pyroxene. _Its_peculiarity lies in the copious development of quartz 
in the groundmass, apparently filling up minute cavities, into which small rod- 
like felspars project. This quartz is no doubt secondary in origin; but it may be 
a product of contact-metamorphism, in which case the felspars of the groundmass 
erew out into the quartz areas as they formed. If it merely represents the infilling 
of a minutely scoriaceous rock, the angular cavities, and the projection of these 
felspars into them, seem at any rate unusual features. 
In the open country on the north side of the road, good exposures of the 
diabase series occur, lying in the south of the townland of Beaghbeg. Some of 
these rocks are flinty and bedded, probably representing altered ash; others are 
vesiculart ; others are of the dark type familiar on Tintagh Mountain. At the 
base of the series, in a ridge where the successive layers dip S8.E., an aphanite 
or dolerite appears. 
A very compact grey eurite, weathering white, and striking N.E. and 
S.W., comes up as a band in this series, and bakes the bedded rocks below 
and the andesite that overlies it. Veins of a similar rock stand vertically in the 
dark andesite of a neighbouring boss. Two-thirds of a mile to the west, well 
* Op. cit., p. 545. 
+ Mr. Nolan describes vesicular rocks from near Creggan, and comments on their resemblance to those of 
volcanic origin (Mem. sheet 26, p. 14). 
