CHAP. XIV 
THE LAST ERUPTIONS IN IRELAND 
255 
By far the most striking geological feature of this singularly interesting 
and impressive coast-line is to be found in the interstratification of lavas 
with hands of tuff among abundantly fossiliferous strata which, from their 
organic contents, are unmistakably of the age of the Weidock group. These 
lavas occur in a number of sheets, separated from each other by tuffs and 
other fragmental deposits. They thus point to a series of eruptions over 
a sea-bottom that teemed with Upper Silurian life. 1'hey consist for the 
most part of remarkably fine typical nodular felsites. The nodules vary in 
dimensions from less than a pea to the size of a hen’s egg. They are some- 
times hollow and lined with quartz-crystals. Ihej' vary greatly in numbei, 
some parts being almost free from them and others entirely made up of 
them. The matrix, where a fresh fracture can be obtained, is horny in 
texture, and often exhibits an exceedingly beautiful and fine fiow-structure. 
On weathered faces there may be seen thick parallel strips and lenticles of 
flow-structure like those of the Snowdon lavas. The upper portions of some 
of the sheets enclose fragments of foreign rocks. The microscopic examina- 
tion of a few slices cut from these lavas shows them to be true felsites 
(rhyolites) composed of a microcrystalliiie aggregate of quartz and felspar, 
with layers and patches of cryptocrystalline matter, and only occasional 
porphyritic crystals of orthoclase and plagioelase. 
The pyroclastic rocks associated with these lavas vary from exceedingly 
fine tuff to coarse agglomerate. Some of the finer tuffs contain puiniceous 
fragments and pieces of grey and red shale ; they pass into fine ashy sand- 
stones and shales, crowded with fossils, and into gravelly breccias made up 
of fragments of different volcanic rocks. 
But the most extraordinary of these intercalated fragmental strata is 
a breccia or agglomerate, about 1 5 feet thick, which lies in a thick gi’oup of 
fossiliferous dull-yellow, ashy and ochreous sandstones. The stones of this 
bed consist chiefly of blocks of different felsites, varying up to three feet in 
length. Some of them show most perfect flow-structure ; others are spongy 
and cellular, like lumps of pumice. The calcareous sandstone on the top of 
the breccia is crowded with fossils chiefly in the form of empty casts, and 
the same material, still full of brachiopods, crinoids, corals, etc., fills up the 
interstices among the blocks down to the bottom of the breccia, where 
similar fos.siliferous strata underlie it. 
Xowhere has the volcanic history of a portion of Pakeozoic time been 
more clearly and eloquently recorded than in this 1 emote line of cliffs swept 
by the gales of the Atlantic. We see that the ordinary sedimentation 
of Upper Silurian time was quietly proceeding, fine mud and sand being 
deposited, and enclosing the remains of the marine organisms that swarmed 
over the sea-bottom when volcanic eruptions began. First came discharges 
of fine dust and small stones, which sometimes fell so lightly as not seriously 
to disturb the fauna on the sea - floor, but at other times followed so rapidly 
and continuously as to mask the usual sediment and form sheets of tuff and 
volcanic gravel. Occasionally there would come more paroxysmal explosions, 
whereby large blocks of lava were hurled forth until they gathered in a 
