252 On the Laurentian Rocks of Donegal and of other parts of Ireland. 
PART II. 
PRoBABLE LauRENTIAN DisTRICTS OF OTHER PARTS OF IRELAND. 
(1.) Sheve Gamph and Ox Mountains.—This is a narrow belt of hilly, almost 
mountainous, ground, extending from the north of Castlebar by Loughs Conn and 
Cullen to Manor Hamilton, and bounded by Carboniferous rocks on both sides. This 
range of hills forms a high and rugged background to the wooded slopes which 
border Lough Gill, near Sligo. 
The rocks of this range consist of massive gray and reddish gneiss, often por- 
phyritic, together with beds of hornblende and micaceous schists, sometimes 
quartzose. Interstratified with the gneissic beds are those of crystalline limestone, 
seen near Lough Talt and Lough Alone.* In Correagh Glen, south of Lough Gill, 
a remarkable bed of dark green massive serpentine occurs in this series, underlaid 
by quartzites and quartz-schist, and overlaid by gneiss with red felspar. The 
serpentine is about 350 feet in thickness, and contains numerous bands of chrysotile 
and magnetite.t 
The massive gneiss bordering Lough Cullen, and extending southwards tor 
several miles, is well foliated, porphyritic, and is of a red or flesh colour from the 
large quantity of orthoclase it contains, along with which is black and white mica 
and quartz. This rock passes beneath a great series of micaceous and hornblendic 
schists with granite veins, and contains crystals of rutile and tourmaline.[ These 
old and highly metamorphosed rocks are bounded along the western edge by a 
large downthrow fault, ranging north and south in the meridian of Castlebar, and 
bringing into juxtaposition quartzites and schistose beds, not so highly metamor- 
phosedas those east of the fault.§ It may be assumed provisionally that the rocks on 
the east side of the fault are of Laurentian age, and those on the west of Lower 
Silurian age, as they belong to the great series of schists and quartzites which 
range from Nephin westwards and northwards through North Mayo.|| 
(2.) Belmullet, Co. Mayo.—For some time past the officers of the Geological 
Survey have entertained a strong impression that the remarkable beds of gneiss 
which are found in the central and northern portion of the promontory of Belmullet 
are of Laurentian age.] The recent determination of the age of the Donegal beds 
which those of Belmullet closely resemble adds fresh probability to this view. The 
beds are thus described by Mr. A. M‘Henry :—‘The rock (gneiss) varies in colour from 
pale pink or red to gray and light brown, and is generally thick bedded, highly, 
sometimes coarsely, crystalline and felspathic. It contains distinct crystals of pink 
* Expl. Mem. Sheet 65. By R. G. Symesand 8. B. Wilkinson. 
+ Pointed out to me by Mr. Hardman. 
¢tR.G. Symes. Expl. Mem. Geol. Survey. Sheet 75, p. 25-6. § Ibid, p. 24-5. 
|| Sheets 64 and 75 of the Geological Survey Map. At the time these maps were being surveyed there 
was no evidence to show to what geological period the rocks here described were referable. Griffith 
evidently considered them of greater antiquity than the rocks of N. W. Mayo, as shown on his map. 
q| See Expl. Mem. Geol. Survey. Sheets 39, 40, 51, and 52, p. 13. 
