i9<^)4- Cole. — Sligo Conference, Geology. 215 



are more rouuded and irregular, with white slabs of bare rock gleaming 

 among the heather and the highland farms. This ridge obviously forms 

 the north-east end of the hummocky masses of the Ox Mountains, and 

 consists of pale fluidal gneiss, often crowded with small garnets. The 

 head of the Slishwood (Rockwood) valley, leading south from the lake, 

 is a good point for the study of these rocks. A later dyke of grey-green 

 serpentine here cuts the main range, and, by its easy weathering, has 

 been worn away and has formed the valley. The Carboniferous strata 

 rest on the gneissic series, or are faulted down against it ; and there is 

 every probability, from its general trend, that the axis of the Ox Moun- 

 tains is occupied by one of the intrusions of granite that occurred in 

 Ireland at the close of Silurian times. At Ballydawley Lake, and in the 

 Slish and Drumahair area, and again near Manorhamilton, dark inclu- 

 sions of hornblendic rocks occur as lumps in the gneiss ; and the latter 

 becomes streaked and banded where these are common. These lumps 

 are so similar to the basic igneous rocks in the old " Dalradian " schists 

 of north-west Ireland, that we ma}- safely regard them as parts of the 

 old floor of the country, caught up by the invading gneiss. Some authors 

 refer the Dalradian rocks to the Precambrian era, others to the Silurian 

 period. A comparison with Connemara makes the latter view difficult 

 to maintain. The gneissic ridge, about 1,000 feet high, affords the most 

 pleasing scenic contrasts with the limestone around the east end of 

 Lough Gill. (Plate 4.) Away in the north, the Dalradian rocks them- 

 selves may be seen from Sligo, forming the magnificent sea-cliffs on the 

 southern fringe of Donegal. 



Dublin. 



Note on Pi^atbs 3, 6, 8, and 13. 



These plates convey an excellent impression of the plateau-structure 

 and fringing cliffs referred to by Prof. Cole. The horizontal bedding is 

 shown on a large scale on Plates 6 and 13, and may be studied in 

 detail on Plate 8. Plate 3, (Ben Whiskiu) shows a hill on which denuda- 

 tion has proceeded fiirther, the eastern side being worn to a uniform 

 grassy slope, while the western plunges down in a gigantic cliff, the 

 whole being a very striking and picturesque surviving remnant of the 

 ancient plateau. Plate 8 (Swiss Valley) illustrates a remarkable ravine 

 running apparently along a line of fault at the foot of the great cliff above 

 Glencar Lake, and furnishing one of the most striking pieces of scenery 

 in the Sligo district. The remarkable fissure on Knockuarea, illustrated 

 in Plate 11, which runs along the slope of the hill for a distance of over a 

 mile is apparently to be referred to a dislocation and slipping of the land 

 along a strong vertical east-and-west joint-plane. 



R. Li.OYD Praegkr. 



