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]

[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:—

EnglandWalesScotlandIreland
Upper SilurianLudlow Group............
Wenlock Group.........---
Llandovery Group?.........
Lower SilurianBala and Caradoc Group------------
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.

I. THE ERUPTIONS OF ARENIG AGE

i. MERIONETHSHIRE

Placing the upper limit of the Cambrian system at the top of the Tremadoc group, we pass into the records of another series of volcanic eruptions which marked various epochs during the Silurian period over the area of the British Isles. The earliest of these volcanic episodes has left its memorials in some of the most impressive scenery of North Wales. To the picturesque forms sculptured out of the lavas and ashes of that early time, we owe the noble range of cliffs and peaks that sweeps in a vast semicircle through the heights of Cader Idris, Aran Mawddwy, Arenig, and Moel Wyn. To the east other volcanic masses, perhaps in part coeval with these, rise from amidst younger formations in the groups of the Berwyn and Breidden Hills, and the long ridges of the Shelve and Corndon country. Far to the south, traces of Silurian volcanoes are met with near Builth, while still more remote are the sheets of lava and tuff interstratified among the Lower Silurian rocks of Pembrokeshire, and those which extend into Skomer Island.

The most important of these districts is unquestionably that of Merionethshire. In this area, as was pointed out in the last chapter, the eruptions certainly began before the close of the Cambrian period, for traces of them occur in the Tremadoc and Lingula Flag groups. But below these strata, in the vast pile of grits and conglomerates of the Harlech anticline, there does not appear to be any trace of contemporaneous volcanic action.