﻿Yol. 66.] THE ORDOVICIAN OF THE GLENSAUL DISTRICT. 267 



IV. The Field-Relations of the Crystalline Igneous Rocks. 

 (a) The Main Felsite Mass. 



In both the Glensaul and the Tourmakeady districts felsites are by 

 far the most important igneous rocks : while in the latter district, 

 however, they not only occur in extensive masses, but also in 

 numerous minor intrusions, in the former, besides those forming 

 the large masses near Tonaglanna and Greenaun, only two small 

 patches are met with. The latter mass has its western portion 

 shifted slightly northwards by a fault : and, though the Tonaglanna 

 mass is separated by sedimentary beds from the Greenaun mass, 

 there can be no doubt that all these portions were originally one 

 continuous intrusion. Each has a width of about 450 yards and 

 a thickness, judging by the dip of the adjacent beds, of about 

 1100 feet. The rock is almost everywhere very uniform in 

 appearance, having a more or less brown groundmass, through 

 which are scattered numerous conspicuous crystals of quartz and 

 small dark irregular patches representing pseudomorphs after 

 pyroxenes. 



As regards its structure, and especially in the size and promi- 

 nence of the quartz phenocrysts, this felsite bears a much greater 

 resemblance to the intrusive felsites of the Tourmakeady district 

 than to those which we there regard as contemporaneous flows ; 

 and for this, and for other reasons, the probabilities are strongly 

 in favour of its being also an intruded rock. 



But while the chief intrusive felsites of the Tourmakeady district 

 are clearly of the nature of bosses bearing no relation to the lie of 

 the strata, the felsite in the Glensaul district is evidently a sill in the 

 main following the bedding, although at one point near its south- 

 western border the Greenaun felsite mass appears to break across 

 the bedding of the Shangort Beds. 



(b) The Small Felsite Intrusions. 



(1) We have already described the occurrence, almost due north 

 of Greenaun summit, of a stream which runs northwards off the 

 felsite mass and exposes a band of limestone breccia and some 

 graptolitic beds. Between the main felsite mass and the limestone 

 breccia a small exposure of a green nodular felsite is seen in the 

 stream-bed : this may be intruded into the Shangort tuffs. 



(2) Immediately south of the road from Glensaul School to 

 Tonaglanna, and about half way between the two places, is a small 

 exposure of a light green felsite. Its specific gravity is 2*66. 



(3) About 250 yards to the east of this intrusion is another small 

 mass of a pale-grey quartz-felsite. Owing to surface-deposits, its 

 extent is not clear. Specific gravity = 2-69. 



