Vol. 65.] ROCKS OF THE TOT/RHAKEADY DISTRICT. 12& 



cherts, grits, and tuffs lying about one mile south-west of Drum- 

 coggy Rectory. Here the red felsite is seen resting without any 

 apparent disturbance upon the cherts. Several small isolated patches 

 of felsite occur at intervals west of the main outcrop. 



North of Stream P the width of the felsite rapidly narrows. The 

 eastern outcrop from that stream to Shangort follows a fairly 

 straight line, and forms a well-marked scarp overlooking an alluvial 

 area where no exposures except occasional intrusions of dolerite 

 are seen ; the western outcrop is more sinuous. At Shangort the 

 felsite is overlapped by the basal Carboniferous conglomerate ; 

 but it commences again near Derrassa, and forms an irregular mass 

 extending as far as Derrindaffderg. 



(c) The augite-felsite. — Although augite has been detected 

 in the felsites at a number of points, only at one spot does this rock 

 form a well-marked and distinctive mass. This is at a point south- 

 south-west of Shangort, where an oval mass of augite-felsite, having 

 a length of about 150 yards, intrudes into the Shangort Beds. 



As regards the nature of the various felsites, there can be 

 no doubt that the principal green and brown felsite-masses are 

 intrusive. The massive boss-like character of the green felsite and 

 the way in which it truncates the ends of the coarse Arenig grits 

 south-west of Derrassa clearly point to its intrusive nature. It is, 

 however, noteworthy that little or no sign of metamorphism can be 

 detected round this great mass of felsite, but the type of rock that 

 it penetrates is not one which readily lends itself to metamorphic 

 change. 



The two long felsite-tongues south-west of the district described 

 in this paper, and the smaller patches of felsite which occur in the 

 Shangort Beds along the western margin of the main red felsite- 

 mass, are all clearly of an intrusive nature. 



It is, however, with regard to the main red felsite-mass, which 

 extends northwards from Tourmakeady stream to near Shangort, that 

 the chief difficulty arises. Its most noteworthy features are : — 



(1) The extraordinarily sinuous outcrop. 



(2) The remarkable variations in the width of the outcrop, from about 



250 yards near Tourmakeady to nearly 1200 yards at a point north 

 of Drumcoggy Eectory. 



Along its eastern border it is seen at various points in close 

 relation with the Mount Partry beds of Arenig age, and along its 

 western border with the Llandeilo grits and tuffs. Only at two 

 spots, one near Gortbunacullin Farmhouse, and a second at a point 

 near the south-western end of the triangular area of Arenig Beds, 

 1 mile west-south-west of Drumcoggy Rectory, have we found it 

 in actual contact with any other rock, and at both these spots it 

 rests upon the Arenig cherty series. It is nowhere seen in relation 

 to the coarse (?) Bala conglomerate. 



Q.J.G.S. No. 258. k 



