CHAP. XII 
THE SILURIAN SYSTEM IN BRITAIN 
175 
geological history they continued to be comparatively feeble until they 
broke out into full activity in the succeeding epoch. There is no hiatus 
or essential difference between the volcanic phenomena, any more than 
there is between the sedimentary deposits, of the two periods. 
Although it may be only owing to 'the fact that the Silurian formations 
come much more extensively to the surface of the land than the underlying 
Cambrian are permitted to do, yet it is at least noteworthy that the relics 
of Silurian volcanoes are spread over a far wider area of the British Isles 
than those of the earlier period. Throughout a large part of Wales they 
form some of the most prominent mountains, such as Cader Idris, the Arans, 
Arenig Fawr, Moel Wyn, Moel Siabod, and Snowdon. They rise into the 
picturesque hill-groups of the Lake District, they appear at many detached 
places throughout the south of Scotland, and form conspicuous eminences in 
Garrick. In Ireland they abound all down the east side of the island, aird 
even reappear on the far western headlands of the Dingle coast-line. 
To the same pioneers, by whom the foundations of our knowledge of the 
Cambrian volcanoes were laid, we are indebted for the first broad outlines V 
of the history of volcanic action in Silurian time. The writings of 
Sedgwick and Murchison, but still more the detailed mapping of De la 
Beche, Eamsay, Selwyn, Jukes, and the other members of the Geological 
Survey, have given to the Silurian volcanic rocks of Wales a classic interest 
in the history of geology. To these labours further reference will be made 
in subsequent pages.^ 
The amount of material being so amide for the compilation of a record 
of volcanic action in Britain during Silurian time, it will be desirable to 
arrange it in stratigraphical order. For this purpose invaluable assistance 
is afforded by the evidence of organic remains, whereby the whole Silurian 
system has been subdivided into sections, each characterized throughout the 
whole region by certain distinctive fossils. The following tabular state- 
ment exhibits the chief stratigraphical divisions of the system, and the 
short black lines in it mark the positions of separate volcanic platforms in 
each of the three kingdoms : — 
England 
Wale,s 
Scotland 
Ireland 
f Ludlow Group .... 
Upper Silurian-; tVenlock Group .... 
( Llandovery Group .... 
1 ' Bala and Caradoc Group 
Lower Silurian J Llandeilo Group .... 
( Arenig Group .... 
? 
— 
— 
— 
It will be most convenient, following the combined stratigraphical and 
geographical arrangement of this table, to discuss first the volcanic history 
of the Lower Silurian period as recorded in each of the three kingdoms, and 
then that of the Upper Silurian. 
^ For references to the older literature see ante, p. 1451. 
