Geological Relations of the East of Ireland . 22 7 
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This island is principally composed of trap rocks ; massy unstra- 
tified thick beds of compact greenstone and greenstone porphyry, 
perfectly analogous, in composition and aspect, to the same rocks in 
Portrane headland. These beds include, and are interstratified with 
smaller beds of stratified rocks, composed of clay slate, clay slate 
conglomerate, greywacke, and greywacke slate, in alternation 
with each other. The trap and the slaty beds thus constitute seven 
or eight alternations on the large scale, the prevailing range of 
which is 20° north of west and south of east, with a dip of 65° to- 
ward the west of south ; and in one instance they appear to be 
intersected by a powerful vein, or dyke, of sandstone conglo- 
merate. 
From the Great Bay on the north side of the island to Seal-hole 
Bay on the south side of the eastern headland, the whole coast pre- 
sents rugged mural precipices of greenstone and greenstone por- 
phyry, which exhibit only occasionally a disposition toward a rude 
stratification, with a dip toward the west of south. The porphyry 
is visible in many parts of this island, but no where in greatef 
beauty than in the eastern headland, where the compact greenstone 
is thickly sown with crystals of compact felspar, of a yellowish 
white, greenish grey, or pale siskin green colour. To the south-- 
westward of this porphyry, the rock merges into amygdaloidal 
greenstone ; having a base of light greyish green compact felspar, 
closely crowded with kernels and small round grains of chalcedony, 
quartz, and calcareous spar; which being sometimes removed by 
decomposition, leave empty cavities, but in general they are com- 
pletely occupied. Toward the south-west, the amygdaloid gradu- 
ates again into compact greenstone, and this latter rock, on ap- 
proaching a body of clay slate conglomerate in Seal-hole Bay, 
acquires a fissile texture, and passes into greenstone slate, and ulti- 
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