CHAr. LI 
SUMMARY AND GENERAL DEDUCTIONS 
467 
time was marked by volcanic eruptions, sometimes over tracts hundreds of 
square miles in area and on a colossal scale. Alter a long period of 
quiescence during the Mesozoic ages, the renewed outbreak of volcanic energy 
in older Tertiary time, so marked over the western half ol Europe, reached 
its maximum of development along the Atlantic border, from the north ot 
England and Ireland through the chain of the Inner Hebrides to the laroe 
Islands, Iceland and Greenland. 
3. Not only has there been a remarkable persistence of volcanic activity 
over the comparatively limited area of the British Isles, viewed as a whole, 
but if we examine the different parts of this area we perceive that many of 
them, of relatively restricted extent, have been the sites of a recrudescence 
of volcanic action, again and again, through a vast succession of geological 
periods. While the whole region has been in different quarters and at 
different times affected, there have been districts where the volcanic fires 
have been rekindled after long intervals of quiescence, the new vents being 
opened among or near to the sites of earlier volcanoes. In the south-west 
of England, for example, the Middle Devonian tuffs and diabases were 
succeeded in the Carboniferous period by the eruptions of the Culm-measures, 
and in the very same tracts came last of all the lavas and tuffs of the 
Permian conglomerates. Still more astonishing is the record of volcanic 
energy in the south of Scotland, where, within a space of not many hundred 
square miles, there are the chronicles of the Arenig, Llandeilo and Bala 
eruptions of the Southern Uplands, the huge piles of lavas and tuffs of 
the Lower Old Bed Sandstone, the long succession of the plateaux and 
then of the puys of the Carboniferous period, the groups of tuff-cones ot the 
Permian period, and, lastly, the numerous dykes connected with the Tertiary 
volcanoes. . ... 
While some portions of the region have been specially liable to exhibitions 
of volcanic action, others have continuously escaped. Some of these “ horsts, 
or stationary and unaffected blocks of country, have been surrounded by 01- 
have risen close to the borders of this volcanic district, yet have maintained 
their immunity through a long series of ages. Thus the Central Highlands 
of Scotland, though they were flanked on the south and south-west by the 
active volcanoes of the Old Bed Sandstone, and again on the south by those of 
Carboniferous time, had no vents opened on their surface after the meta- 
morphism of their schists. Still more striking perhaps is the immunity of 
the Southern Uplands. Though they were in large measure surrounded by 
the volcanoes of the Lower Old Bed Sandstone, then by those of the 
Calciferous Sandstones and Carboniferous Limestone, and though the) 00 mc 
down on the Permian eruptions of Ayrshire and Nithsdale, which spread 
streams of lava and showers of ash along their flanks, these hills formed a 
solid block that seems to have resisted perforation by the volcanic iunne s. 
Ao-ain, the tracts covered with Carboniferous Limestone in England and 
Ireland almost entirely escaped from invasion by volcanic eruptions. 
We thus learn that even within comparatively restricted regions some 
portions of the terrestrial crust have been areas of weakness, liable to serve 
