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 Carrick. In Ireland they abound all down the east side of the island, and 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 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, Ramsay, 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.[131] [Footnote 131: For references to the older literature see _ante_, p. 142.] The amount of material being so ample 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 statement 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| Wales |Scotland|Ireland| +---------------------------------------+-------+-------+--------+-------+ | { Ludlow Group | ... | ... | ... | ... | |Upper Silurian { Wenlock Group | ... | ... | ... | --- | | { Llandovery Group | ? | ... | ... | ... | | | | | | | | { Bala and Caradoc Group| --- | --- | --- | --- | |Lower Silurian { Llandeilo Group | --- | --- | --- | --- | | { Arenig Group | --- | --- | --- | --- | +---------------------------------------+-------+-------+--------+-------+