CotE—On the Geology of Slieve Gallion, in the County of Londonderry. 235 
broken up and penetrated by a groundmass of basic glass. The specific gravity 
of this rock is 2°73, a figure close to that of the mingled quartz-diorite of Letteran. 
Turning again to the extreme west of the mountain, we may note the delicate 
veins of pink eurite that penetrate the andesites in Mobuy, producing some of the 
most beautiful interminglings observable with the naked eye. The veins in the 
diorite bosses to the south are coarser and truly granitic; and both types occur, 
again, in the road-side quarries of Tatnagilta. 
IV.—Re ations oF THE Rocks oF Siieve GALLION TO THOSE WEST OF CooKSTOWN. 
As already pointed out, most authors have treated the ‘“ pyroxenic rocks” of 
the south-east of the county of Londonderry and the adjacent part of the county 
of Tyrone as belonging to the same series.* The schistose rocks between 
Lough Fea and Tatnagiltat form an apparent break upon the map, but are 
admittedly in close relations with the more crystalline masses on either hand. I 
have made one or two excursions into the moorland-parish of Kildress, in order to 
satisfy myself as to the similarity of the features presented there on the one hand, 
and in the massif of Slieve Gallion on the other. The whole area requires separate 
investigation; but the observations here given bear, I think, directly on the 
questions discussed in the present paper. 
The quarries of the townland of Tatnagilta open up the problems of Kildress, 
and suggest that we shall encounter a basic series, ranging from gabbro to compact 
diabase, and penetrated by granite and eurite—in fact, an extension of the pheno- 
mena seen on the steep hillsides of Mobuy.= To the south-west, the roads at once 
enter what Portlock has styled§ ‘a singularly wild and rugged district, in fact, a 
mere plain of rocks.” New roads have been cut through it since his time, and 
cultivation has spread around the feet of the rocky bosses ; but the area still remains 
a strange one, an undulating land some 600 feet above the sea, with gravel hills 
and long weird eskers, contrasting with the abrupt domes of the gabbro. 
On proceeding south to Oritor, we come at once on the typical landscapes of 
the district, and find far coarser igneous rocks than those of the summits of Slieve 
Gallion. A handsome dark diallage-gabbro, passing by uralitisation into horn- 
blende-diorite, is undoubtedly the prevailing rock. Even in its freshest condition, 
it contains a fair amount of actinolitic hornblende ; and the plagioclase is usually 
* Sir R. Griffith, ‘Geological Surveys of the Coal Districts of Tyrone and Antrim” p. 1; Portlock, 
‘Geol. of Londonderry,” pp. 530 and 538; Nolan, Mem. sheet 26, p. 12; &e. 
+ Mem. sheet 26, p. 15; Mem. sheet 27, p. 11. 
¢ See the end of division II. of this paper. 
§ ‘Geol. of Londonderry,” p. 533. 
