﻿Vol. 66.' 



THE ORDOVICIAN OF THE GLENSAUL DISTRICT. 



255 



district : namely, coarse conglomerate, black graptolitic beds, black 

 or grey radiolarian chert, coarse and fine grits, and subordinate 

 tuft's. 



We will now describe the exposures of these beds. 



(1) On the southern slopes of the ridge running east- 

 wards from the top of Lettereeneen. — This is by far the 

 largest area occupied by the Mount Partry Beds, and the finest and 

 most complete section is here exposed. The succession is as follows,. 

 in descending order : — 



Thickness in feet. 



(4) Coarse grits 150 



(3) Fine grits and tuffs, associated with black radiolarian cherts 

 and graptolitic beds, and with a prominent band of coarse 



tuft' or breccia about 30 feet thick (?) 150 



(2) Coarse grits '. about 110 



(1) Coarse conglomerate about 600 seen 



The coarse conglomerate, the blocks in which are mainly of grit, 

 often reaching a length of 2 feet, is finely exposed in the stream- 

 courses and road-tracks lying north of the village of Lettereeneen ; 

 also in a deep little ravine close to the road, and nearly due south of 

 the top of Lettereeneen Hill, where it is seen faulted against cherts, 

 and grits. 



The coarse grits, which closely resemble those of the Mount 

 Partry neighbourhood, Tourmakeady, are seen at various points on 

 the hillside, and probably occupy the greater part of a stretch of 

 drift-covered country lying north of the conglomerate outcrop. 



None of the rocks forming Band 3 are exposed very continuously- 

 along the hillside; but they are readily traceable, the cherts by 

 fragments on the hillside and by occasional exposures, the breccia, 

 by a more continuous outcrop. The graptolitic beds are nowhere- 

 exposed in place, until a point nearly due south of the top of. 

 Lettereeneen Hill is reached. Here they are well seen in some- 

 watercourses which score the hillside, and are associated with a 

 much disturbed series of black and banded radiolarian chert and 

 fine tuff and grit. At the spot marked 334 on the map (PI. XX) 

 graptolites are very abundant, and the following have been very 

 kindly determined for us by Miss G. L. Elles, D.Sc. : — 



Clonograptus lapworthi, Rued. 

 Dictyonema. 



Didymograptus affinis, Nicb. 

 Didymograptus bifidus, Hall 



extensus, Hall 

 fasciculatus, Nich. 



(common). 

 Didy mograptu s 



(common). 

 Didymograptus 



(1 specimen). 

 Didymograptus filiformis, Tullberg. 

 Didymograptus gracilis, Tornquist. 

 Didymograptus nanus, Lapw. 



(common). 



Q. J. G. S. No. 262. 



Didymograptus nicholsoni, Lapw.. 



A Dendrograptid. 



Glyptograptus dentatus, Brongrr. 



(1 specimen). 

 Tetragraptus amii, Lapw. MS. 

 Tetragraptus fruticosus, Hall . 

 Tetragraptus pendens, Elles (three 



specimens). 

 Tetragraptus quadribrachiatus, Hall. 

 Tetragraptus serra, Brongn. 

 Thamnograptus sp. 

 ? Trichograptus fragilis, Nich. 



(common), 



