176 
THE SILURIAN VOLCANOES 
BOOK IV 
r. THE EEUPTIONS OF AEENIG AGE 
i. MERIONETHSHIRE 
riacing 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 Horth 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 Coimdon country. 
Ear 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 Skoiner 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. 
At the time when the Geological Survey maps of this region were 
prepared, the Cambrian and Lower Silurian rocks had not been subdivided 
into the various palreontological groups which are now recognized. Nor 
had any attempt been made to separate the various kinds of contempor- 
aneous igneous masses from each other and from the tuffs in so extensive 
and complicated a mountain-region. The task undertaken by the Survey 
was beset with difficulties, some of which geologists, furnished with the 
advantages of a later time, can hardly perliaps realize. The imperfections 
of the mapping were long ago recognized by the original surveyors, and 
various corrections of them were made from time to time. First of all, the 
volcanic rocks, which originally had been all massed under one colour, were 
traced out separately on the ground, according to their structure and mode 
of origin, and were distinguished from each other on the maps.^ Subse- 
quently divisional lines were followed out between some of the larger 
stratigraphical groups, the maps and sections were still further modified, and 
the results were summed up in tlie volume on the Geology of North Walcsl^ 
1 iMcm. <!col. Surv. vol. iii. 2nd edit. p. 95, note. 
2 Some of tlie modifications introduced are. I think, to be regretted, for the earlier editions of 
the maps and sections are in certain respects more accurate than the later. On this point I 
concur with tlie criticism made by Messrs. Cole and Jennings, Quart. Journ. Gml, Soc. vol. xlv. 
(1889), p. 436. 
