132 
PRE-CAMBRIAN VOLCANOES 
BOOK II 
The geological age of this volcanic group is a question of much interest 
and importance in regard to the history of volcanism in this country. An. 
inferior limit to the antiquity of the group can at once be fixed by the fact 
that, as originally pointed out by Dr. Callaway, the quartzite which overlies 
the volcanic rocks passes under a limestone containing Cambrian fossils in 
which Professor Lapworth has since recognized Ohnellus, Faradoxides and 
other Lower Cambrian forms. The eruptions, therefore, must be at least as 
old as the earlier part of the Cambrian period. But it is affirmed that the 
quartzite rests with a complete unconformability on the volcanic rooks. If 
this be so, then the epoch of eruption must be shifted much farther back. 
The evidence adduced in favour of this great break appears to me to be 
threefold. In the first place, the quartzite contains fragments of the 
volcanic rocks. I do not think much stress can be laid on this fact. When 
I visited the ground, what struck me most in the composition of the 
(piartzite was its singularly pure quartzose character, and the comparative 
scarcity of felsite-pebbles in it. Any deposit laid down conformably upon 
the top of the breccias and ■ tuffs might obviously contain some of these 
materials, while, if laid down unconformably, it might reasonably be 
Fiq. 39. Section across tlie Uriconiaii series of Caer Caradoc. 
S3 Umier Silurian ; S2, Bala group; SI, Arenig group ; C, Cambrian; L, Loiigmyndian ; n, Uriconian 
’ ff. faults. 
expected to be full of them. In the second place, this quartzite is alleged 
to pass transgressively across the edges of successive sheets of the volcanic 
group, and thus to have a quite discordant dip and strike. I failed to find 
satisfactory evidence of this unconformability in the northern part of the 
district. But in the Caer Caradoc area the ipiartzite does appear to steal 
across the outcrops of the older rocks, which plunge at nearly right angles 
in an opposite direction. In the third place, the felsitic volcanic group is 
believed by Professor Lapworth to pass upwards into the Longinynd rocks. 
Obviously, if this group lies at the very bottom of the vast Longmynd 
series, the discordance between it and the quartzite must be enormous, and 
the date of the volcanic eruptions must be placed vastly farther back in 
geological antiquity. Though the evidence does not seem^ to me to amount 
to clear proof, I am disposed, in the meantime, to accept it as affording the 
most probable solution of the difficulties presented by the structure of the 
ground. 
The sequence of the rocks around Caer Caradoc is partly concealed by 
surface accumulations, but if these could be cleared away the structure of 
the ground would be, according to Messrs. Lapworth and Watts, as shown 
in Pig. 39.1 
' Proc. Geol, Assuc. vol. xiii. (1894), pp. 314, 315. 
