﻿Vol. 66.] THE ORDOVICTAN OF THE GLENSAUL DISTRICT. 269 



(j3 & y) The characters of these rocks, as observed in the field, 

 have been described already, and little need be said about their 

 microscopical characters. The matrix of the coarse felsite breccia 

 is identical in character with the finer type of tuff, and consists in 

 the main of quartz grains and of small felsite fragments. The same 

 constituents are present in the matrix of the limestone breccia 

 associated with abundant calcareous material. 



(2) The most noteworthy point about the tuffs from the 

 Mount Partry Beds is the tendency to silicification which 

 many of them show. 



VI. Comparison between the Glensaul and Tourmakeady Areas. 



The two areas are very closely related geologically, nearly all 

 the rock-types of Glensaul being common to the two. There are, 

 however, some interesting differences. Of the Mount Partry Beds 

 the coarse conglomerate, coarse and fine grits, graptolitic beds, 

 and radiolarian cherts are common to the two districts ; but the 

 tuff presents some differences, as the conspicuous band associated 

 with the cherts in the Glensaul district does not occur in that of 

 Tourmakeady. The dominant type of Shangort Beds, that is, gritty 

 more or less calcareous tuff, is the same in the two districts; but at 

 Glensaul it shows a greater development of coarse breccias than 

 near Tourmakeady, and there is also a greater tendency for the 

 Shangort Beds to pass locally into relatively pure limestone. In 

 addition to the type of calcareous deposit just referred to, the 

 peculiar limestone breccias, which occur so frequently but so 

 sporadically in the Tourmakeady district, are equally well seen 

 in that of Glensaul, and tend to form relatively definite bands 

 following the strike of the other rocks. We believe that the theory 

 which we adopted to explain the formation of these beds in the 

 Tourmakeady district, namely, that they are due to the disruption 

 of limestone bands by explosive action, is applicable to those found 

 at Glensaul. 



The most abundant fossils in the Glensaul district were obtained, 

 however, from limestone masses shattered, as we believe, not by 

 explosive action but by earth-movements along lines of fault. 



The conglomerates and grits of ?Bala age which bound the 

 Tourmakeady district on the west are also found along the western 

 border of the Glensaul district, but we have not studied them. 



The crystalline igneous rocks are represented almost solely by 

 the felsites of Tonaglanna and Greenaun, which are clearly parts of 

 the same mass. The rocks agree closely with the intrusive felsites 

 of the Tourmakeady district in petrographical character, and may 

 be regarded with considerable confidence as forming a great sill 

 intruded in the Shangort Beds. Three other small felsite intrusions 

 occur, but the Glensaul district differs markedly from that of 

 Tourmakeady in the absence of frequent small intrusions of felsite, 

 dolerite, hornblende-lamprophyre, and fine-grained andesitic rocks 

 or spilites. 



