Vol. 6S.'] PALEOZOIC BOCKS OP THC KILBETDE PENINSULA. t7 



€ontainiug Arenig graptolites. In the south-eastern corner of the 

 peninsula is an area of gneissic rocks, prcsumabl}^ Archaean, against 

 which the highest member of the Silurian series is faulted. At 

 the north-eastern corner of this gneissic area a second development 

 of the lower members of the Silurian System is met with, the dip 

 here being northerly and easterly. 



II, The Sedimentaey and Volcanic Aeenig Eocks. 



Under this head we group the great series of breccias and spilites, 

 together with a relatively small development of grits, shales, and 

 cherts, these sedimentary beds being confined to the northern part 

 of the district. 



Only at one point have Arenig fossils been found ; but we 

 consider that we are justified in regarding the whole series of rocks 

 alluded to above, and in addition the intrusive felsites, as of 

 Arenig age, on account of the general resemblance which they bear 

 to those of the Tourmakeady and Glensanl districts. There are, 

 however, many important differences, which will be summarized in 

 the sequel. The most marked feature of the Kilbride 

 series is the immense development of pillow-lavas 

 (spilites). 



The general structure of the area north-west of the Silurian 

 outcrop, and west of the great felsite-mass of Glenbeg, is difficult 

 to make out, and there has clearly been much faulting. Throughout 

 a belt stretching westwards along the crest of the ridge from the 

 top of Knock Kilbride to the neighbourhood of Lough Mweelaun, 

 the strike of the Arenig rocks is east-north-easterly, in conformity 

 with that of the neighbouring Silurian rocks ; and the same strike 

 is met with in the rocks immediately to the west of the Glenbeg 

 felsite. But two important faults cut these rocks off from those 

 in the western part of our map, occupying the country between 

 Finny and the end of Derry Bay, and throughout this part of the 

 area the strike when ascertainable is found to be south-easterly. 

 Of these faults, one, the exact position of which is not ascertainable, 

 lies to the east of Lough Mweelaun and strikes in a north-north- 

 easterly direction, meeting a second which we propose to call the 

 Oak Island Eault : this latter lies not far west of the great Glenbeg 

 felsite-m^ass, and strikes the coast to the south of Oak Is] and. 



(a) The Sedimentary Arenig Rocks. 



(1) A small but interesting exposure is seen at a point nearly 

 half a mile north-east of the top of Knock Kilbride. Here thinly- 

 bedded black cherts, with shaly partings, are overlain by quartzose 

 grits. Unfortunately, the exposure is entirely surrounded by peat, 

 and consequently the relations of the rocks are not clear. The 

 strike, however, is east-north-east, in conformity with that of all 

 the Arenig rocks (so far as it is possible to observe this point) east 

 of the Oak Island Fault. The cherts and grits probably form 



