462 
THE TERTIARY VOLCANOES 
BOOK VIII 
peiiod, vast though it be, does not carry us back even as far as the begin- 
ning of Tertiary time. 
I 11 concluding this lengthened discussion of the Tertiary volcanic history 
of Britain, I may, perhaps, usefully add a brief summary of the leading features 
of the long record. 
The region within which volcanic activity displayed itself during older 
Tertiary time in the British Isles, if our estimate of its area is restricted to 
those parts of the country where igneous rocks, probably of that age, now 
appear at the surface, embraces the North of England and of Ireland, the 
southern half and the west coast of Scotland — a total area of more than 
40,000 square miles. Over that extensive region volcanic phenomena 
were displayed during an enormously protracted interval of geological time. 
The earliest beginnings of disturbance may possibly have started in the 
Eocene, and the iinal manifestations may not have ceased until the Miocene 
period. So prolonged was the duration of the eruptions, that enormous 
topographical changes from denudation, and probably also considerable 
variation in the fauna and flora., alike of land and sea, may have been 
effected. 
Owing to some cause which has not yet in this relation been investigated, 
but which is probably referable to secular terrestrial contraction, the volcanic 
region underwent elevation, while, at the same time, a vast subterranean 
lake or sea of molten rock existed underneath it. Enormous horizontal 
tension thus arose, and at last the stretched terrestrial crust gave wav. A 
system ol approximately parallel fissures opened in it, having a general direc- 
tion towards north-west. The rapid and simultaneous production of such a 
gigantic series ot rents must have given rise to earthquakes of enormous 
magnitude and destructive force. The great majority of the fractures, 
doubtless, did not reach to the surface of the ground, though probably not a 
few did so. Such was the potency of this development of terrestrial energy, 
that the fissures ran through the most varied kinds of rocks and the most 
complicated geological structures, crossing even earlier lines of powerful 
dislocation, and yet retaining their direction and parallelism for sometimes 
50 or 100 miles. 
Into the fissures thus formed the molten magma from underneath was 
forced for many hundreds or even thousands of feet above the surface of the 
subterranean lava-reservoir. Solidifying between the fissure walls, it formed 
the crowd of basic dykes that stand out as the most widespread and 
distinctive feature of the volcanic region. 
Where the fissures reached the surface or near to it, the molten rock 
would seek relief by egress in streams of lava. This probably occurred in 
many places from which subsequent denudation has removed all vestige of 
superficial volcanic manifestations. But, in the great range of basalt-plateaux, 
from Antrim northwards through the chain of the Inner Hebrides, there are 
still left abundant remains of the surface-outflows. Like the modern lavas 
of Iceland, the molten material probably flowed out sometimes from the 
