CHAP. XI 
THE MALVERN HILLS 
169 
everything older than themselves as they are traced northwards.’^ But 
this explanation will not entirely remove the difficidties ot the case. The 
inosculation of the volcanic group of Bangor with the base ot the Lower 
Silurian series cannot be accounted for by any such overlap ; it seems only 
explicable on the supposition that the volcanic activity, which ceased in the 
Llyn Padarn district about the time that the Llanberis Slates were deposited, 
was continued in the Bangor area until Arenig time, or was then renewed. 
The thick volcanic group of Bangor would thus he the stratigraphical 
equivalent not only of the thin volcanic group of Llyn Padarn, but ol the 
overlying mass of strata up to the Arenig rocks. In confirmation of this 
view, I shall show in a later chapter that A'olcanic action seems to have 
been prolonged in Anglesey to a still later geological period, that it 
appeared duriirg the deposition of the Arenig strata, and that it attained a 
great deA'elopment tliroughout the time of the Bala group. That a series 
of volcanic rocks, with associated cherty strata, may be the stratigraphical 
equivalent of a great thickness of ordinary sediments in other districts 
will be cBvelt upon in the description of the Lower Silurian volcanic geolog}' 
of the Southern Uplands of Scotland.’^ 
In the areas of North Wales which have now been described, A’olcanic 
action appears to have begun and ended within the limits of the Cambrian 
period. Southwards, in the district of Dolgelly, another distinct and, in 
some respects, very different development of Cambrian volcanic activity may 
be recognized. In that district there is evidence that the volcanoes which 
distinguished the earlier part of the Silurian period had already Iregun 
their eruptions during Cambrian time. As their records, however, are 
intimately linked with those of Silurian age, an account of them is deferred 
to the next chapter. 
THE MALVEKS HILLS 
Although the chief sui’viving records of Cambrian A'olcanic action in 
Britain are found in Wales, there is no evidence that the volcanoes of the 
period lay chiefly in that region. It is certainly a suggestive fact that, in 
the few districts where Cambrian strata apjiear from under younger forma- 
tions in England, they are generally accompanied with igneous rocks, though 
the age of the latter may he older or later than the Cambrian period. If 
the oldest Pakeozoie rooks could be uncoAcred over the English counties, a 
more abundant dcA'clopment of Amlcanic materials might be laid bare than 
is now to be seen in Wales. 
Taking, hoAvever, the extremely limited exposures of Camlirian strata, 
We find tAvo tracts that specially deserve attention. Eelerence has already 
been made to the ancient eruptive rocks of the Mah'ern Hills, the 
' 3/c7/j. Geol. Surv. vol. iii. 2ncl edit. p. 252. 
^ A group of cherts and mudstones not more than 60 or 70 teet thick appear in that region 
to be stratigrapliically erpiivalent to the great depth of sedimentary material whicli elsewliere 
constitutes the Upper Arenig and Loiver and Middle Llandeilo formations. ^See Anmwl Jtc 2 >ort of 
ihe Geological Siirve;/ for 1895, p. 27 of reprint. 
