THE DENUDATIONS OF NORTH WALES. 39 
At the close of the Lower Silurian period the volcanic out- 
bursts ceased, and have not since been renewed. The great 
terrestrial movements which I am about to describe have been 
unaccompanied by any outward manifestation of volcanic energy 
in this particular region. And this, perhaps, seems the more 
remarkable when it is remembered that in most of the forma- 
tions represented there were volcanic eruptions at no great 
distance ; in the Old Red Sandstone, for example, in the South 
of Scotland and in Ireland, and in the Carboniferous in the Isle 
of Man and North of England. 
The Lower Silurian formations were succeeded naturally by 
the Upper Silurian, but at the close of the latter period the first 
great break, of which there is any evidence in our district, 
occurs. The great unconformity between the Carboniferous 
and Upper Silurian rocks is to be seen in many places, but 
perhaps nowhere better than in the valley of the Clywedog, 
near Ruthin. The red conglomerates which form the base of 
Carboniferous Series are here seen resting on the edges of the 
steeply upturned Silurian rocks, the bedding of the one nearly 
at right-angles to that of the other. It is clear that the Upper 
Silurian rocks were tilted up and largely denuded, either by 
atmospheric erosion or by the planing action of an encroaching 
sea before the Carboniferous Basement Beds were deposited 
upon them. But more than this; here as elsewhere the Silurian 
rocks are metamorphosed, and shew slaty cleavage, yet the 
Carboniferous shew neither the one nor the other. Now it is 
well known that these structures, which are due to pressure, 
probably accompanied by heat, are only produced at a con- 
siderable depth under a great thickness of overlying rock. 
Where such ‘‘cover’’ is wanting contortion may be produced, 
but not cleavage or metamorphism. At first sight it would 
appear as though the “‘ cover” had been provided in the Carboni- 
ferous rocks themselves, which must, Coal-Measures and all, 
have once overspread all this area. That this was not so is 
proved beyond doubt by the. simple fact that the fragments of 
Wenlock Shale which may be picked out of the base of the 
limestone and of the conglomerates, are as much metamorphosed 
as the parent rock itself, shewing that the commencement of the 
Carboniferous epoch found the metamorphism ‘as complete as it 
is at the present day. The ‘‘cover” was therefore provided, not 
by the Carboniferous, but by some pre-existing strata, which 
must have been swept away, leaving apparently no trace behind. 
It is perfectly clear that such a denudation as this indicates 
that for a very long period intervening between the Upper 
Silurian and Carboniferous Epochs the rocks were elevated 
above the sea-level and subjected to atmospheric waste. But in 
past times, as at the present day, if there was dry land in one 
place there was sea in another; and, as wherever there is sea 
there must be some sort of deposition going on, so we should 
expect to find the gap filled up, and see what sort of beds they 
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