42 
TYPES OF VOLCANOES 
HOOK I 
In the descriptions which will subsequently be given of ancient British 
volcanoes of the Vesuvian type, it will be shown that at many successive 
periods during Paheozoic time, and at many distinct centres, lavas and tufts 
have been piled up to a depth of frequently more than 5000 feet — that is to 
say, higher than the height of Vesuvius. Sometimes the vent from which 
these materials were ejected can be recognized. In other places, it is still 
buried under later formations, or has been so denuded as to be represented 
now merely by the column of molten or fragmental rock that finally 
solidified in it. Examples will be quoted of such ancient vents, measuring 
not less than two miles in diameter, with subsidiary “ necks ” on their flanks, 
like the parasitic cones on Etna. 
I shall show that while the ejected volcaidc products have accumulated 
in greatest depth close to the vent that discharged them, they die away as 
they recede from it, sometimes so rapidly that a volcanic pile which is 
7000 feet thick around its source may entirely thin out and disappear in a 
distance of not more than ten or twelve miles. I shall point out how, as 
the lavas and tuffs are followed outwards from their centre, they not only 
get thinner, but are increasingly interstratified among the sedimentary 
deposits with which they were coeval , and that in this way their limits, 
their age, and the geographical conditions under which they were accumu- 
lated can be satisfactorily fixed. 
As illustrations of the Vesuvian type in the volcanic history of Britain, 
I may refer to the great Lower Silurian volcanoes of Cader Idris, Arenig, 
Snowdon and the Lake District, and to the Old Bed Sandstone volcanoes 
of Central Scotland. 
2. The ricUeau or Fmure type is, among modern volcanoes, best 
developed in Iceland, as will be more fully detailed in Chapter xl. In that 
island, during a volcanic eruption, the ground is rent open into long parallel 
fissures, only a few feet or yards in width, but traceable sometimes for 
many miles, and descending to an unknown depth into the interior. From 
these fissures lava issues — in some cases flowing out tranquilly in broad 
streams from either side, in other cases issuing with the discharge of slags 
and blocks of lava which are piled up into small cones set closely together 
along the line of the rent. It was from a fissure of this kind that the 
great eruption of 1783 took place — the most stupendous outpouring of lava 
within historic time. 
By successive discharges of lava from fissures, or from vents opening on 
lines of fissure, wide plains may be covered with a floor of rock hundreds or 
thousands of feet in thickness, made up of horizontal beds. The original 
topography, which might have been undulating and varied, is completely 
bur’ied under a vast level lava-desert. 
The rivers which drained the country before the beginning of the 
volcanic history will have their channels filled up, and will be driven to 
seek new courses across the lava-fields. Again and again, as fresh 
eruptions take place, these streams will be compelled to shift their line of 
How, each river-bed being in turn sealed up in lava, with all its gravels. 
