486 MESSES. J. R. KILROE AND A. McHENRY OX [Aug. 19OI, 



mass, continued eastward across Annestown Baj'^, sends veins and 

 sills into the sedimentary strata forming Green Island, and may be 

 seen at low water surrounding a mass of black slate between the 

 island and the mainland, A little farther east the coarse frag- 

 mental rock, described as xenolithic felsite by Mr. Cowper Reed, 

 contains and gradually merges into distinctly stratified rock which 

 closely resembles tuff. The origin of the whole mass is perplexing, 

 for near to the spot where stratification is so distincth' seen a vein of 

 black slate, only an inch or two in thickness, is traceable for a long 

 distance in the face of the cliff. This we believe to be a rem- 

 nant of the sedimentary rock which the tuff-like mass invaded, and 

 not at all a deposit contemporaneous with the mass. The direc- 

 ition of this vein, moreover, is not parallel to the structure of the 

 stratified portion above mentioned, but is more nearly at right 

 angles to the strike of the apparent stratification. A similar thin 

 vein of black slate is to be observed near Balljmoney Fishery, in 

 Wexford, as shown in fig. 7 (p. 485). 



At Arklow Ilock and Duffcarrick, on the coast in the counties of 

 Wicklow and Wexford, the intrusive nature of those massive, 

 apparently bedded, tuff-like rocks is most impressively exhibited. 

 This structure is well exhibited on the coast, 1| miles south of the 

 town of Arklow, where tongues from the tuff-like rock penetrate 

 black slate of Llandeilo age. A similar disposition of these igneous 

 rocks is to be seen in the neighbourhood of EalJymoney Coastguard 

 Station, and that they cannot be regarded as in any sense con- 

 temporaneous with the slate is proved by their containing pieces 

 of limestone of Bala age as well as pieces of the black slate 

 (Llandeilo). 



Portions of the Arklow intrusive rock were examined micro- 

 scopically, both by Mr. Teall and Mr. Seymour, and were found to 

 yield the usual indications of rocks hitherto regarded as unques- 

 tionable tuffs. 



Farther south, fine-grained felsitic rocks, arranged in layers, 

 occur at Duffcarrick on the Wexford coast. These rocks, because 

 of their stratified appearance, and because under the microscope 

 they present a thoroughly clastic appearance, have been regarded 

 ^s still more unquestionably tufaceous in origin. They nevertheless 

 invade the adjoining sedimentary rocks, a peculiar banded slate — 

 the prevailing rock of the country, known as ' ribbon-slate.' The 

 sections and plans selected for publication by no means exhaust the 

 evidence bearing uipon the important question at issue obtainable 

 in the localities referred to ; they are submitted to the Society as 

 a selection, to illustrate the nature of the evidence for the intrusive 

 character of these tuff-like isrneous masses. 



■^o' 



The sections at Sheep Island, on the Waterford coast, 2 miles 

 west of the entrance to Tramore Bay, are equally instructive, as 

 showing that microscopic rock-structure cannot be relied upon, apart 

 from field-evidence, to afford the ground of decision regarding the 



