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JOURNAL OF THE GEOLOGICAL SOCIETY OF DUBLIN. 
sandy. On tlie day on which it was most particularly examined, 
the Paraca was blowing fiercely outside, and yet this channel was 
nearly as calm as a mill-pond. Its depth varies from 2 to 7 fathoms, 
and in many places, even in the centre of the channel, there is no more 
than 6 fathoms. The bottom is either fine white sand, small gravel, 
or rocks ; in one part the rock can be traced continuous across the 
channel. 
The rock of the island is a porphyritic granite, intersected by 
numerous veins and dykes of serpentine trap, varying from 2 inches 
to as many feet in thickness; greenstone protrusions, some of them 
of considerable extent, also occur. 
The granite is in part broken up by a system of parallel joints, so 
as to appear like beds, and exhibit evidence of a great disturbing 
force, being twisted and contorted in various parts ; the overlying 
systems often being unconformable with those beneath, the one set 
running horizontally, the other curving over them in a strange 
manner. 
The greenstone protrusions are best seen on the shores of the 
bays, generally overlying the granite, and associated with a coarse 
sandstone grit. 
The trap occurs through all the islands; in the Middle Island in 
very large veins,—the cliffs on the south side of this island being 
almost altogether composed of it, in enormous square masses piled 
one on top of the other; the dykes here also run horizontally, the 
granite both underlying and overlying it. 
Owing to the peculiar formation of the granite, the sea has much 
effect on it, and all the cliffs and detached rocks around the islands 
are more or less hollowed out into caves, some of them extremely 
picturesque, and many communicating with each other, and thus 
forming fine bridges. In one of these bridges, in a detached rock off 
the Middle Island, the abutments of the bridge are formed externally 
of granite, while the arch and inner part of the abutments are trap 
with squared j’oints, which in the distance might well be taken for a 
bridge artificially formed. 
Many traces of recent upheaval of these islands are to be found 
on them, evidences of a raised sea-beach being nearly universal; even 
the cliffs 200 feet high are capped with a layer of large rolled pebbles 
of trap and granite cemented together ; this layer of conglomerate 
varying from one to several feet in thickness, and traces of it being 
