22 
PROCEEDINGS OF SOCIETIES. 
rounded. Notwithstanding the completely porphyritic appearance of 
this rock where it was first seen, there was a certain indefinable cha¬ 
racter about it that reminded me of other rocks both in Wales and Wa¬ 
terford, which had been at first taken for porphyry, hut were eventually 
found to be of mechanical formation, and I therefore searched carefully 
along the band in order to arrive at a true estimate of its character. 
When traced down to the cliffs at the north end of the headland, the 
rock was well exposed, and part of it was there clearly seen to he con- 
glomeritic in structure, containing small rounded pebbles of vesicular 
trap, and rounded and angular fragments of felstone and slate. In one 
part these were arranged in distinct layers, exhibiting a well-marked 
lamination striking N.N. E., parallel to the strike of the country, and 
evidently the result of stratification, and even the white opaque crystals 
or crystalline fragments of feldspar were in another part likewise ar¬ 
ranged in lines and layers, having the same strike as if they had been, 
not innate crystals produced where they are now found, but crystals 
brought either by water or air and deposited along with the paste in 
which they were embedded. Some parts of this rock in the quarries on 
the beach lost altogether the brecciated and conglomeritic character, con¬ 
sisting of dark gray felstone, quite smooth and compact, with little facets 
of innate crystals scattered here and there. The rock, however, retained 
a streaky or grained structure, in consequence of the parallel arrange¬ 
ment of small layers of different colours and slightly different texture. 
This grain often exists in truly molten rocks being caused by the flow¬ 
ing of the mass while in a pasty condition; or it may be the mark of 
an altered ash. 
Immediately over these rocks, or to the east of this band, on the 
coast, is a distinctly stratified rock, about ten feet thick, an ashy shale 
with interstratified fine-grained grits, or thin layers of felstone (I could 
not quite determine which), that dipped to E. S. E. at 50°; above that 
occurred a band about twenty yards wide, of columnar greenstone, the 
columns of which lay at right angles to the stratification of the slate 
below, and had both above and below them a thin band of earthy-look¬ 
ing greenstone, and over that were white felstones and slates, apparently 
interstratified, but much twisted and contorted, large roundish masses 
of felstone being partly enveloped by beds of indurated shale. Other 
masses of greenstone appeared obscurely connected or intertangled with 
them. These irregular and confused rocks occur on each side of the 
Arch Rock, an overhanging mass which has now fallen down, and beyond 
them, along the shore to the southward, we come upon a large mass of 
pure white felstone. 
Tracing the porphyritic band at first alluded to, from the coast over 
the hill, I found exactly in its strike, on the north slope of Arklow Rock, 
not far from a cottage occupied by George Prestwich, a mass of very 
coarse conglomerate, the base of which resembled the porphyry, and had 
both the opaque feldspar crystals and those which seemed certainly innate 
(or produced in the mass), while the whole rock was crowded with peb¬ 
bles of many other rocks, principally felstone, from the size of the first 
