Vol. 6S.'] PALJiOZOIC EOCKS OF THE KILBRIDE PENINSULA. 89„ 



{b) The Lime-Bostonite.- 



A much-faulted band of this rock extends for nearly 3 miles 

 from the Pinny Eiver to Knocknamuck, where it is faulted against 

 felsite. A second band, which appears to be faulted off from the 

 northern end of the main band, extends along the eastern base of 

 Knocknamuck down to the shore of the lake. The southern and 

 western boundaries of this latter band are formed by faults. In 

 the Barnarinnia area four small patches of lime-bostonite occur. 

 Wherever the lime-bostonite occurs throughout the district, it is 

 overlain by red sandstone. The most noteworthy feature of the 

 lime-bostonite is its remarkable variability in width of outcrop. 

 South of the summit of Knock Kilbride the outcrop is only some 25 

 yards wide ; while, in the neighbourhood of the labradorite-por- 

 phyrite intrusion south-east of Lough Mweelaun, the width increases 

 to about 275 yards. This remarkable variation in width of outcrop 

 depends mainly on variation in thickness, not on difference of dip. 

 The maximum thickness is about 600 feet. The lime-bostonite is a 

 very resistant rock, and frequently stands up forming steep crags. 

 In view of the marked variation in the thickness of the lime-bostonite, 

 its n on- association with tuffs, and the absence of evidence in this 

 or any other Irish area (with the exception of the Clogher-Head 

 district of Kerry) of the existence of volcanic rocks of Silurian age, 

 it seems clear that the lime-bostonite, like the coarse porphyrite, is a 

 sill intruded at the base of the red sandstone. It is noteworthy 

 that the line of least resistance followed by the intrusions was 

 not always that of the unconformity, as the basal Silurian con- 

 glomerate, where it is seen, lies beneath the lime-bostonite. 



(c) The Labradorite-Porphyrite. 



This rock is entirely unrepresented in the Glensaul and 

 Tourmakeady areas. It forms four intrusive masses, all of which 

 occur in close relation to the lime-bostonite and near the base of the 

 Silurian strata. The principal intrusion is a sill, having a probable 

 thickness of about 100 feet, which occurs immediately south of the 

 top of Knock Kilbride. It strikes north-east and south-west, and 

 can be followed with slight breaks for rather over half a mile. 

 About 300 yards from its south-western end it is interrupted by a 

 fault, which alters its strike from south-west to south-south-west. 

 The north-eastern end is also separated by a fault from the main 

 mass. A second band of porphyrite of about the same thickness, 

 having a length of about a third of a mile, commences about 500 

 yards to the west of that just described. It resembles the first 

 band in its relations to the other rocks, and in the fact that it is 

 shifted by a fault. It is intersected by a dolerite dyke. 



1 [As British writers seem to confine the term keratophyre to lavas, we 

 have thought it advisable to call this rock a lime-bostonite, especially as it is 

 so similar both chemically and microscopically to the Abercastle bostonite. — 

 C. I. G. 4^ S. H. R., Feb. 16th, 1912.] 



