BOOK IV 
THE SILUEIAN VOLCANOES 
CHAPTER XII 
CHARACTERS OF THE SILURIAN SYSTEM IN BRITAIN. THE ARENIG VOLCANOES 
The Land and Sea of Silurian time — Classification of the Silurian System — General 
Petrography of the Silurian Volcanic Rocks — I. The Eruptions of A renig Age. 
The next great geological period, to whicli Murchison gave the name of 
Silurian, has in Britain a fuller record than the period which preceded it. 
The rocks that tell its history are more varied in origin and structure. 
They are displayed at the surface over a far wider area, and, what gives 
them special interest and value, they contain a much larger assemblage of 
organic remains. For the immediate subject of the present volume, they 
have likewise the additional attraction that they include a singularly com- 
plete and widespread volcanic chronicle. 'I'hey display in many admirable 
sections the piled-up lavas and tuffs of scores of volcanoes, scattered all 
over the three kingdoms, from the headlands of Kerry to the hills of 
Lammermuir. They thus enable ns to form a truer conception of what the 
early Balaiozoic volcanoes were than is possible from the more limited 
evidence furnished by the Carahrian system. 
At the beginning of the Silurian period most of the area of the British 
Isles lay under the sea. But if we may judge from the sedimentary strata 
which represent the floor of that sea, the water, during most of the time, 
was of no great depth. There is evidence, indeed, that during a part of 
the period the sea was deep enough to admit of the accumulation of wide 
tracts of radiolarian ooze, with but little admixture of mechanical sediment. 
But, for tlie most part, sand and mud were drifted from neighbouring 
lands, the more important of which probably lay to the north, over w'hat 
are now the Highlands of Scotland and the north and north-west districts 
of Ireland. No general change in topography or in physical conditions 
took place at the close of Cambrian time. The older era glided insensibly 
