Vol. 65.] THE ROCKS OF THE TOTTRMAKEADY DISTRICT. 113 



(8) In the area extending from Gortbunacullin to a 

 point south-west of Shangort. — This is by far the most 

 extensive tract of Arenig rocks, the exposures being met with over 

 an area having a length of about a mile and a half, and showing an 

 uninterrupted strike of about a mile. 



Along the whole length of this tract the gritty and cherty rocks 

 are cut off on the west by the great green felsite-intrusion, while 

 their whole eastern boundary appears to be faulted, and the con- 

 tinuity of the exposures is further broken up by smaller felsite- 

 intrusions. Coarse grits, sometimes associated with cherts, are seen 

 at many points in the neighbourhood of Gortbunacullin, and at one 

 point become conglomeratic ; but the most instructive section of 

 these rocks is seen in the bed of Stream G, which, after cutting its 

 way through the great green felsite-mass, and through the grits and 

 one of the minor felsite-intrusions, turns north-north-eastwards and 

 for some distance follows the fault bounding the grits on the east. In 

 the stream-section the lowest beds are coarse, green, quartzose grits, 

 and these are overlain by fine grits with chert-bands. Grits, slates, 

 and cherts, in this case underlying, not overlying, coarse grits, can be 

 traced more or less continuously northwards from Stream G to a 

 point west of Shangort, that is, for a distance of about three-quarters 

 of a mile. At one point only have we found any fossils in this 

 exposure of the Arenig Series, namely about a third of a mile south- 

 west of Shangort, where the track from the hamlet crosses the 

 stream : here Diplograptus (Glyptog raptus) dentatus, Brongn., was 

 found in slate. 



The thick series of coarse grits overlying the cherts is finely 

 exposed in a prominent scarp, probably produced in former times by 

 Stream G. The scarp does not follow the outcrop of any particular 

 bed, but, commencing on the south in the green felsite, continues 

 in the coarse grits and a minor felsite-intrusion, ending eventually 

 in more grits at a point west of Shangort. At the southern end 

 of the scarp the coarse grits do not differ in any marked respect 

 from exposures of these rocks elsewhere in the area ; but, as one 

 approaches Shangort the beds become much disturbed, the under- 

 lying fine grits and slates being broken up and contorted, and 

 blocks of them being embedded in the coarse grit, which farther on 

 becomes conglomeratic, containing well-rounded blocks of felsite, 

 sometimes as much as 2 feet long. These disturbed strata have a 

 thickness of about 30 feet, and pass up into the ordinary quartzose 

 grits. It is probably to this bed that Sir Archibald Geikie refers 

 when he. writes * : — 



' Near Shangort I noticed in one of these breccias one block measuring 12 feet, 

 another 20 feet in length and 3 or 4 feet thick, composed of alternating bands 

 of grit and slate.' 



Sir Archibald regards the rock to which he refers as a volcanic 



1 ' Ancient Volcanoes of Great Britain ' vol. i (1897) p. 253. 

 Q.J.G.S. No. 258. ' i 



