CHAP. L 
EPITOME OF THE TERTIARY VOLCANIC HISTORY 
465 
the earth over the volcanic region was pushed upward and rent open by 
another system of parallel fissures. Again, from a reservoir or basin of 
basic lava underneath, molten rock was forced upwards into the rents, and 
thus another system of basic dykes was formed. These dykes are found 
crossing those of earlier date, and rising through the other volcanic rocks. 
They traverse the plateau-basalts from bottom to top ; they climb to the 
summits of the gabbro mountains, and they even pursue their undeviating 
course over the huge domes of granophyre. No proof has yet been found 
that from any of these dykes there was a superficial outflow of lava. But 
so great has been the subsequent denudation of the areas, that such outflows 
might quite well have taken place, and have subsequently been destroyed. 
Whether these basic dykes were the last manifestation of volcanic energy 
in our region cannot yet be decidedly affirmed. So far as the evidence 
at present goes, they are possibly older than another series of acid veins and 
dykes (pitchstone, felsite, and granophyre), which are found at many points 
from Antrim to the far end of the Inner Hebrides. These protrusions 
traverse every other member of the volcanic series, except some of the 
youngest basic dykes, and do not appear to be themselves cut by any. 
Since the close of the volcanic period considerable disturbance of the 
basalt-plateaux has taken place. The whole volcanic region has subsided, 
some districts having sunk more than others. In Britain the most striking 
evidence of such depression is supplied by the basin of Lough Neagh. But 
throughout the Inner Hebrides much of the lower portion of the terrestrial 
lava-plateaux is now below sea-level. In the Faroe Islands and in Iceland 
the subsidence has been still more marked. Dislocations, also, sometimes 
amounting to more than a thousand feet of displacement, have occurred 
among the volcanic masses. The bedded basalts, originally on the whole 
nearly flat, have thus been broken up into large blocks of country wherein 
the sheets are now inclined in various directions. 
One of the most important lessons taught by the Tertiary volcanic series 
of the north-west of Europe is the extent of the denudation of the land since 
the* close of the volcanic period. The horizontal or gently inclined layers of 
bedding among the basalts afford datum-lines from which the minimum 
amount of material removed may be measured. As a reasonable estimate 
it may be inferred that in the case of the Mull plateau, for example, the 
average amount by which its surface has been lowered since the close of the 
volcanic period cannot be less than 2000 feet. If the rate of lowering of 
the land-surface in western Europe by subaerial denudation be taken as 
-- ( Ljy of a foot in a year, then the lapse of time required for the degradation 
of the Mull plateau must amount to about twelve millions of years. Some 
such interval has therefore elapsed since the last Tertiary volcanoes became 
extinct. 
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VOL. II 
