116 PROCEEDINGS OF SOCIETIES. 



quin, in the county of Tyrone, to the neighbourhood of Pettigo, in the 

 county of Fermanagh, decidedly belongs to the millstone grit." This 

 rock occurs also on the high mountain group south-west of Lough Erne, 

 as shown on the " Geological Map." The base of this millstone grit, at 

 Portinode, between Kesh and Pettigo, on the north shore of Lough Erne, 

 stands at the level of the water, or about 150 feet above the sea. At 

 Shean Hill, west of Churchhill, on the south side of the lough opposite, 

 it stands at 1135 feet, that being the height of the trigonometrical point 

 on the top of the hill, on limestone, near the base of the millstone grit. 

 Here is a difference of 985 feet between two localities in the base of the 

 same group, showing the base of the millstone grit to be lower on the 

 north shore of Lough Erne than on the south by 985 feet, or, say, about 

 1000 feet; and the fact proves that a fault of great magnitude exists 

 between them, as there is no curvature of the strata, and the rock in 

 both places has a persistent dip southwards. 



The seat of this fault, as I interpret it, lies on the northern boun- 

 dary of a sandstone belt, seen on the "Map," which runs from the 

 north-west corner of Lough Erne to Bundoran, where it enters the sea. 

 In estimating the amount of the fault, these 985 feet of horizontal diffe- 

 rence would make half as much more, or about 1500 feet, if the dip at 

 Portinode be persistent under the lough southwards as far as the fault, 

 which is very probable, as the rocks visible in both places have the same 

 low, general dip to the south, as just mentioned. 



This fault I look upon to be the key to the geology of the district 

 round it. By means of it, all the phenomena in its vicinity can be ex- 

 plained. With the millstone grit, of course, go up or down all the sub- 

 divisions of the Caboniferous formation, at one side or the other, of a 

 fault which passes through them, and all the Carboniferous rocks north 

 of the line of fault are, therefore, on a lower level than on the south of 

 it by about 1500 feet (see Plate YL, Eig. 4). 



Again, the surface of the limestone country at Ashbrook, near Bally- 

 shannon, on the north side of the fault, stands at 150 feet above 

 the sea, and on the south side of it, the trigonometrical point on the 

 " Ordnance Map" is 1712 feet on Dartry mountain, making here, 

 near the meridian of Bundoran, a horizontal difference of 1562 feet, 

 or a real difference in the amount of the fault of more ; taking into 

 account the amount gained by the dip, which, as at Lough Erne, is 

 southward all through. This fact between Ashbrook and Dartry, on 

 the upper surface of the limestone, affords a corroboration as to the 

 amount of the fault, of the case, at Portinode and Shean Hill, both on 

 the millstone grit. 



In this particular fault, because it cuts through a formation, the 

 beds of which, are all parallel, there is provided at one side of it a du- 

 plicate for every band of rock on the other, though those duplicates 

 occur on levels far different ; and since there are in the district four 

 distinct bands of the Carboniferous formation, that is, millstone grit, 

 limestone, Carboniferous Slate, and Old Eed Sandstone, so there are in 

 the vicinity of the fault two millstone grits, two limestones, two Car- 



