Vol. 70.] ORDOVICIAX XSB SILURIAX OF LOUGH NAFOOEIT. 115 



If miles, as one follows the outcrop of this band westwards from 

 the Finny River, no dolerites are seen. South and south-west 

 of Bencorragh five small dykes occur either in, or in contact 

 with, the Purple Shale. To the west of the big fault which shifts 

 the Silurian outcrop northwards, these dykes become still more 

 numerous, probably at least a dozen occurring between this fault 

 and the one north of Benbeg. At several points the Purple 

 Shales are much baked by these intrusions. Mica -dolerites are not, 

 however, confined to the Purple Shale ; thus a good example occurs 

 in the grey shales underlying the Doon Rock Grits near the southern 

 border of our map, half a mile east of Benbeg ; and another in 

 breccia west of the Finny River. 



VI. PETRO GRAPHICAL DETAILS. 



(a) The Felsites. 



The felsites resemble very closely those of Kilbride, especially 

 in the strong corrosion of the quartz-crystals and the frequent 

 association of plagioclase (albite) with orthoclase. Pseudomorphs 

 after pyroxene are, however, less characteristic. In many rocks 

 the ground-mass is wholly or partly spherulitic. In one case (60) 

 remarkable micropegmatitic groups representing twinned felspar 

 occur. Three samples of felsite had an average specific gravity of 

 264. 



(b) The Lime-Bostonite. 



Some of the lime-bostonites differ from those of the Kilbride 

 area in their relative richness in quartz. This is especially marked 

 in the case of the oblique d} r ke south-west of the top of Bencorragh, 

 as the marginal part includes abundant quartz-phenocrysts which 

 have not been found in any other lime-bostonite from Kilbride 

 or Lough Nafooey. Three samples of bostonite had an average 

 specific gravity of 2*65. 



(c) The Labradorite-Porphyrite. 



. We have nothing to add to the description of this rock given in 

 our Kilbride paper. 



(d) The Dolerites. 



These closely resemble the corresponding rocks from Kilbride, 

 especially in the prevalence of mica-dolerites. Certain of the mica- 

 dolerites (35), however, are exceptional in containing numerous 

 erystals of hornblende. The augite is sometimes fresh, sometimes 

 represented by pseudomorphs in carbonates ; it may occur in well- 

 formed crystals or in ophitic plates. No olivine was met with. 



The development of carbonates along the cleavage-planes of the 

 mica is characteristic. Eleven samples of augite-dolerite had an 

 average specific gravity of 2'81 ; one specimen of mica-dolerite gave 

 2-SO; 



