84 
SILURIA. 
[Chap. IV- 
the Llandeilo formation already ir 
mass is established. In the next 
dicated, and thus the age of the whole 
overlying accumulations, d, we already 
find many Caradoc fossils, although 
the original beds alternate rapidly 
with volcanic dejections of felspathic 
and other materials. 
For a graphic delineation of these 
grand igneous phenomena of North 
Wales, I refer the reader to Prof. 
Ramsay's description of the enor- 
mous outpourings and accumulations 
of volcanic materials in Merioneth 
and the neighbouring districts, where 
he finds that ' ash-beds ' are certainly 
underlain by porphyritic rocks over 
an area of at least 60 square miles, 
and were once probably overlain by 
an equally wide sheet of felspathic 
lava, — the igneous rocks of Snow- 
don itself being estimated to have a 
thickness of 3100 feet, without the 
intercalated beds of fossiliferous grits 
and slates. ('Geology of North 
Wales,' p. 133.) 
Truly grand as are these igneous 
features of North Wales (and si- 
milar phenomena, though on a less 
scale, occur in South Wales — Pem- 
brokeshire), they are, I repeat, pre- 
cisely of the same class as those of 
the western hills of Shropshire, de- 
scribed (pp. 65, 67) as the true Lower 
Silurian types, and the age of whose 
volcanic eruptions was established 
by the associated organic remains. 
In like manner, we now know, 
through their imbedded fossils, that 
the slaty and arenaceous masses, 
with their beds of igneous deposits, 
constituting the summit of Snowdon 
were formed at the very same time 
with the shelly sandstones of the 
humbler hills on the eastern flank 
of Caer Caradoc. 
