﻿270 THE ORDOVICIAN OF THE GLENSAUL DISTRICT. [May IQIO, 



VII. General Succession of the Strata, and Conclusions. 



The general succession of the rocks in the Glensaul district is as 

 follows, in descending order : — 



3. ? Bala Beds. Conglomerates and sandstones. 

 These beds have not been studied. 



(Break.) 



2. Shangort and Tourmakeady Beds. Thickness in feet. 



(8) Calcareous gritty tuff of no great coarseness, sometimes 

 becoming so calcareous as to pass into fairly pure lime- 

 stone, enclosing also bands and patches of limestone- 

 breccia, and, more rarely, bands of bighly fossiliferous 

 limestone, which in some cases has been shattered by 

 earth-movements ? 



(7) Very coarse tuff or breccia, mainly composed of felsite 

 fragments, and associated with it are impersistent bands 

 of fine tuff 750 



(6) Tuff, coarse and fine, with occasional patches of calcareous 

 beds, and at one point a graptolitic bed indicating the 

 zone of Didymograjrtus hirundo 150 



(5) Great felsite sill of Tonaglanna and Greenaun about 1100 



(4) Coarse grit 20 



(3) Gritty tuff varying in thickness from 520-620 



(2) Coarse tuff or breccia, mainly composed of felsite fragments. 75 



(1) Fine banded tuff 55 



1. Mount Partry Beds. 



(4) Coarse grits 150 



(3) Fine grits and tuffs associated with black chert, graptolitic 



beds, and a prominent band of coarse tuff or breccia about 

 30 feet thick. The graptolites indicate the upper part of 

 the zone of Didymograptus extensus ?150 



(2) Coarse grits , 110 



(1) Coarse conglomerates (seen) about 600 



The succession includes the upper part of the zone of Didymo- 

 graptus extensus* to which the Mount Partry Beds are referable ; 

 the zone of Didymograptus hirundo, to which it appears that the 

 lower part of the Shangort and Tourmakeady Beds may be 

 assigned ; and an overlying series, perhaps also to be included in the 

 Didymograptus-liirundo Zone, but more probably representing that 

 of D. bifidus and possibly including part of that of I), murchisoni. 



In conclusion, we wish to tender our most sincere thanks to 

 Miss G. L. Elles, D.Sc, for examining our graptolites ; to Mr. F. U. 

 C. Reed, M.A., F.G.S., for studying the other fossils from the 

 district ; and also to the Director of the Geological Survey of Ireland 

 for the loan of maps. 



