Vol. 56.] IGNEOUS EOCKS OP COUNT? WATEEFOED. 673 



On both sides of Eon an 's Bay diabase is again found piercing 

 the felsitic rocks, and probably is the end of a similar sheet. It 

 leads on to a small neck of coarse greenish tuff composed prin- 

 cipally of fragments of diabase, situated near the edge of the cliffs 

 north of Little Island. The outline of the neck, which measures 

 several yards in diameter, can be traced in the grey felsites and 

 felsitic tuffs through which it bursts, and it appears to be of similar 

 character to that near Lady Elizabeth's Cove. Little Island itself 

 consists of an ophitic dark-green diabase which bursts through the 

 felsitic rocks, but has suffered in places much crushing ; and in the 

 small cove west of this island another mass of slightly different 

 type rises up through the same rocks, and also is seen to truncate 

 earlier intrusive veins. Another vent seems to be present on the 

 promontory called Great Island, about | mile farther west, where 

 a mass of diabase-tuff is exposed, associated with an intrusion 

 of solid diabase, as in the other instances. 



Fig. 10. — Section at the base of the cliffs on the west side of 

 Kilfarrasy Island. 



\^\ \ 



A = Sheet of intrusive diabase, showing columnar jointing. 

 B = Patch of diabase-agglomerate, representing a neck. 

 C = Greenish f'elsite, altered. 



The finest example of a diabase-sheet along the whole coast is 

 found on the west side of Kilfarrasy Island, where, close to the 

 waterfall, a great mass of dark-green diabase is seen breaking 

 through the greenish felsites which are intrusive in the black slates. 

 A portion of a neck of diabase-agglomerate is completely 

 enwrapped by the sheet, and probably marks the position of the 

 vent or of one of the vents connected with it (fig. 10). Eude 

 columnar jointing is noticeable in this sheet of diabase, which 

 is exposed, with only one small interruption, for fully 300 yards 

 continuously along the face of the cliffs, ending about | mile east of 

 Green Island. That it was intruded into the green felsites and 

 felsitic tuffs is plainly shown at its western extremity. In the 

 Geological Survey Memoir l this sheet is called the Whit efield 

 Greenstone. It is the westernmost of the large sheets of diabase ; 

 and the somewhat similar rocks which occur along the cliffs to the 

 west are of a different macroscopic or microscopic appearance, do 

 not form masses so extensive, have frequently suffered considerable 



1 Mem. G-eol. Surv. Irel. 1865, Explan. Sheets 167, 168, etc. p. 55. 

 Q. J. G. S. ffo. 224. 2 z 



