CHAP. L 
EPITOME OF THE TERTIARY VOLCANIC HISTORY 
463 
open fissures, sometimes from vents formed along the chasms. After the 
convulsions ceased which produced the earliest dykes, the communication 
that had been established between the magma-reservoir underneath and the 
air above would be maintained, and repeated eruptions might take place, 
either from the original fissures and vents or from others afterwards opened 
by the volcanic energy. 
As in the modern eruptions of Iceland, new fissures are successively 
opened through the older lava-sheets, so in the Tertiary volcanic areas, 
renewed ruptures of the earth’s crust allowed later dykes to be formed. The 
basalt-plateaux are traversed by such dykes, even up to their highest sheets. 
It is impossible to say how often the process of dyke-making may have been 
repeated. Not improbably it recurred again and again during the building 
of the basalt-plateaux, and we know that it was renewed even after the 
protrusion of the granophyre bosses which mark one of the latest phases of 
volcanism in the region. 
For a protracted geological period, with long intervals of quiescence, 
various basic lavas (basalts, dolerites, etc.), with occasionally some of inter- 
mediate composition (andesites, trachytes), and perhaps in Antrim acid 
rhyolites, flowed out from fissures and vents until they had filled up the 
hollows of the great valley, which then stretched from the south of Antrim 
northwards between the west coast of Scotland and the chain of the Outer 
Hebrides. In some places the accumulated pile of these ejections even 
now exceeds 3000 feet in thickness, but we cannot tell how much 
material has been bared away from its top by denudation. The volcanic 
discharges consisted mostly of lava, fragmentary materials being comparatively 
insignificant in amount and local in origin, though layers of fine tuff and 
basalt-breccias occur in all the. plateaux. None of the erupted materials 
thicken towards any centres that might be taken to mark volcanoes of the 
type of Vesuvius or Etna . On the contrary, the persistent flatness and 
uniformity of the volcanic series, and the thinning out of the separate beds 
in different directions, show that the lavas issued from many points all over 
the region. The positions of some of the actual vents can still be ascer- 
tained. They are now filled sometimes with dolerite, sometimes with coarse 
agglomerate. 
The surface over which the lava flowed seems to have been mainly 
terrestrial. Here and there, between the successive sheets of basalt, 
the leaves, stems, and fruit of land-plants, sometimes in most perfect 
preservation, may be observed, together with the remains of insects and 
fresh-water fish. Distinct relics of old river-channels can be recognized 
which have been buried under streams of lava. Among the deposits left 
by these streams the uppermost layers are commonly dark with decayed 
vegetation, while layers of coal are found here and there between the 
basalts. 
As the pile of erupted materials gradually thickened, and the subter- 
ranean energy possibly grew feebler, the ascending magma was forced 
between the layers of sedimentary strata underneath the basalts, or between 
