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SUMMARY AND GENERAL DEDUCTIONS 
BOOK VIII 
continued, we may expect it to have been connected with important 
terrestrial movements affecting extensive regions of the earth. The Tertiary 
volcanic history seems to afford a remarkable instance of this connection. A 
wide area of the European continent is dotted over with old centres of volcanic 
activity which were in eruption at successive epochs throughout the Tertiary 
period. Of all these centres the most important was that of the north- 
western basalt-plateaux, where floods of lava were discharged over many 
thousand square miles from Ireland to Greenland. The geological date of 
these outpourings probably coincides with the last great orographic move- 
ments that gave to the mountain-chains of Europe their latest elevation and 
dimensions. 
But. without entering into what must be for the present a field of 
speculation, we can be assured of one important fact in the connection of 
ancient volcanoes with movements of the terrestrial crust. A study of the 
lecords of volcanic action in Britain proves beyond dispute that the volcanoes 
of past time have been active on areas of the earth’s surface that were 
sinking and not rising. We usually associate volcanic action with elevation 
lather than subsidence, and there are certainly abundant proofs of such 
elevation around active or recently extinct volcanoes. Many of the active 
vents, of the present time, like Vesuvius and Etna, began with submarine 
eruptions and have been gradually upraised into land, it may be, however, 
that such uprise is merely a temporary incident, and that if we could survey 
the whole geological period of which human history chronicles so small a 
part, we might find that subsidence, and not upheaval, is ultimately the rule 
over volcanic areas. 
Be. this as it may, there can be no question that with the one solitary 
exception of the Tertiary volcanoes, which were terrestrial and not submarine, 
all the British vents were carried down and eventually buried under aqueous 
sediments. Even the Tertiary lava-fields have in many places sunk down 
below sea-level since their eruptions ceased. 
That there are any Palteozoic volcanic rocks now visible at the surface is 
obviously due to subsequent movements not immediately connected with 
then original conditions of eruption, and to gigantic denudation. The 
amount of subsidence which followed on a volcanic episode was sometimes 
enoimous, even within the same geological period, as one may see by observ- 
ing the prodigious piles of sedimentary material heaped' over the lavas and tuffs 
of Arenig time, or over those of the Lower Old Bed Sandstone. I do not wish 
to maintain that the downward movement was necessarily a consequence of 
volcanic ejections, for we know that it took place over tracts remote from 
centres of eruption. But I have sometimes asked myself whether it was not 
possibly increased as a sequel to vigorous volcanic action ; whether, for 
instance, the great depth of the Palaeozoic sedimentary rocks in some regions, 
as compared with their feeble development in others, may not have been due 
to an acceleration of subsidence consequent upon volcanic action. 
6.. A review of the geological history of Britain cannot but impress the 
geologist with a conviction of the essential uniformity of volcanism in its 
