228 Scientific Proceedings, Royal Dublin Society. 



along a distance of about four miles by some subterranean agency, resuming 

 their original course and direction again. Slight "jumps" are frequently 

 observed in dykes in the ' ' countrj^ ' ' rock, but there appears to be no recorded 

 case of displacement of dykes alone to such a great extent. 



The second explanation is that the whole mass of z'ock-shales, sandstones, and 

 limestones, about four miles in width from east to west, with the dj'kes contained 

 in them, have been displaced en bloc to the south, subsequent to the intrusion of 

 the igneous rock, thus deflecting the dykes from their course. A study of the 

 principal faults in the district, and other considerations detailed below, favour 

 the second view. 



There are two small, unimportant faults in the shales on the west. Then 

 there is a N.-S. fault between Carrickpatrick sandstones and the land at Donagh 

 (see fig. 2, D). This is evidently a considerable fault under the sea, displacing 

 the dykes about a quarter of a mile to the south. 



The largest fault in the district is visible at Aughris Head. This is marked 

 by the Survey as probably ninning obliquely inland, though the fault where 

 visible is north and south. It would rather appear to be a very great N.-S. 

 fault, not oblique, but almost parallel to the fault in the sandstones, and dis- 

 placing the PoUachurry dykes to the south for a distance of about three-quarters 

 of a mile. 



Further considerations of the structure of the bay confirm the evidence that 

 displacement of strata has taken place. The sandstone and micaceous "flag- 

 stones" at Donaghintraine are hard and compact, whereas the shales at 

 Oughmore and Pollarone are very soft. The greater resistance to denudation 

 shown by the sandstone is evident from a comparison between the weathering 

 of tlie dykes in" the two kinds of "countiy" rocks. In the sandstones the 

 dykes form the floors of long deep channels in the rocks. In the shales, on the 

 other hand, the dykes form durable walls. Yet the sandstones form the inner 

 part of the bay, and the shales form one side. 



A displacement of the sandstones to the extent of about a quarter of a mile 

 would explain this curious feature. Thus it is probable that this displacement 

 of the harder strata to the south was one of the agents in the formation of the' 

 bay. 



There is a remarkable structure found further inland, south of Aughris 

 Head. At the base of the Ox Mountains the Carboniferous rocks rest uncon- 

 formably on metamorphic rocks — mica-schist in this region. At the junction 

 between the two there is a fault running east and west, separating the 

 Carboniferous rocks on the north from the mica-schist on the south. Directly 

 south of the fault at Aughris Head previously described there is a large fault 

 north and south, cutting the east and west fault. This separates Carboniferous 

 rocks on the west from mica-schist on the east for a distance of about three- 

 quarters of a mile towards the shore, and continues further with limestone on 

 either side (fig. 1). 



Thus immediately south of the fault at Aughris Head, which displaces the 

 dolerite dykes three-quarters of a mile to the south, is this fault which appears 

 to displace the limestone over the mica-schist for the same distance. 



It is undoubtedly the same fault ; and the whole mass of rock — four miles in 

 width — must have been displaced to the south, and carried over the underlying 

 mica-schist for about three-quarters of a mile on the east and a quarter of a mile 

 on the west. 



In conclusion, I wish to express my indebtedness to Mr. L. B. Smyth for his 

 kind assistance and advice, and for undertaking the communication of this 

 paper to the Society in my absence. 



