CHAPTER XI 
THE CA5IBKIAX VOLCANOES OF NOETH WALES, THE MALVERN HILLS 
AND AVAIIWICKSHIRE 
NORTH AVALES 
The Cambrian volcanic rocks in the northern part of the Welsh Principality 
have their main development in Caernarvonshire. Soutlnvards from that tract, 
though the Lower Cambrian strata form a vast pile of sedimentary material 
in the Harlech anticline, which is estimated by the Geological Survey to be 
from 6000 to 7000 feet thick, they have yielded no trace of any contem- 
poraneous volcanic rocks.^ The purple slates that rise along the centre of 
the anticline dip below the grits and conglomerates on either side Avithout 
disclosing a glimpse of the base of the system. This enormous accumula- 
tion of sedimentary deposits seems to diminish in thickness as it is traced 
northAvards, for towards the Menai Strait it does not reach more than a 
fourth part of the depth which it is said to display in the Harlech anticline.^ 
In the Pass of Llanberis the series of grits that overlies the purple slates is 
estimated to be about 1300 feet thick.* This gradual thinning away of 
the Cambrian series towards the north Avas, in the opinion of Sir AiidreAV 
Ramsay, accompanied by an increasing metamorphism of the lower portions 
of the system. In his view, the long ridge of quartz-porphyry which 
crosses the lower end of Llyn Padarn represents the result of the extreme 
alteration of the stratified rocks. He believed that he could trace an 
iusensible passage from the slates, grits and conglomerates into the 
porphyry, and he Avas led to the “ conviction that the solid porphyry itself 
is nothing but the result of the alteration of the stratified masses carried a 
stage further than the stage of porcellanite, into the condition of that 
kind of absolute fusion that in many other regions seems to have resulted 
^ Mam. Gaol. Sure, vol.'iii. 2iid edit. “Geology of North AA''ales,” p. 21. It i.s possible that 
this thiekuess has been somewhat overestimated. Dr. Hicks {Gaol. Mmj. 1880, p. 619) has re- 
ferred to certain “ highly fclsitio rocks, for the most part a luetamorphio .series of schists, 
'ilternating with harder felsitio bands, probably originally felsitic ashes,” lying at the bottom 
of the whole jiile, and he has claimed them as pre - Cambrian. But I have not found any 
evidence of such rocks, nor any trace of igneous materials save dykes and sills, acid and basic, 
such as are indicated on the Survey map. Ibid. p. 24. ® Ib-id. p. 173. 
