THE DENUDATIONS OF NORTH WALES. 48 
on the other hand, the strata were preserved from denudation, 
and it was in this way that the tongue of Carboniferous Lime- 
stone and Millstone Grit, which occupy the Vale of Clwyd, was 
preserved; for thrown down here by a synclinal fold and by 
great faults, these strata occupied a position far below the 
general level of the Carboniferous rocks, which were being 
wasted away on the uplands of Denbighshire and Flintshire. 
The Vale of Clwyd then first began to come into existence at the 
close of the Carboniferous period; occupied subsequently by an 
arm of that inland sea, in which the sandstone and salt-bearing 
marls of Cheshire were deposited, it must from that time to this 
have remained distinguished by its structure and scenery from 
the rest of North Wales. As a direct result of these post- 
carboniferous movements, was commenced that network of 
valleys, cut so deeply down into the old table-land of Silurian 
rocks, and occupied by the Elwy, the Clywedog, the Clwyd, and 
the Dee, and at the same time the principal features of the 
Snowdon range may have begun to be blocked out. 
It may at first sight appear unreasonable to assume so great 
a denudation of the Carboniferous rocks to have taken place. 
That it is not so, a few observations on the effects of denudation, 
on rocks of equal hardness, but of much later date, will amply 
prove. During the Miocene period there was great volcanic 
activity in many parts of Europe, but more particularly in the 
region intervening between the North of Ireland and the West 
of Scotland. Great sheets of lava were poured out, extending 
from Antrim, where they are 600 to goo feet thick, through the 
islands of Mull, Rum, Eigg, Skye, through the Faroe Islands, 
and Iceland. Speaking on this subject, Ramsay says, ‘‘ The 
** whole region has, by denudation, been changed into a line of 
‘fragmentary islands, the high sea-cliffs of which attest the 
‘* greatness of the waste they have in time undergone.” Again, 
in Iceland, James Gerxir has observed that there rise from a 
broad undulating plain lofty plateaus of lava, deeply gashed 
with gorges and abruptly terminated, so as to present cliffs to 
the low ground at the base; these are obviously mere fragments 
of a once continuous sheet of lavas about 3,000 feet thick. 
Through the valleys cut in these Miocene lavas the modern 
volcanic rocks have poured themselves, so as to produce a 
striking instance of unconformity between igneous rocks. More- 
over, since this comparatively recent Miocene period, some of the 
greatest mountain ranges in the world have for the most part 
been elevated, as the Alps, the Himalayas, and the Rocky 
Mountains, so that beds of mud and clay that look like recent 
depositions in other parts, are found in the flanks of the 
mountains shewing the contortions and dislocations commonly 
seen in the older rocks. In the face of such evidences as these 
of upheaval and denudation at so late a period, I think we are 
not likely to err on the side of over-estimating the changes that 
conti taken place since the very beginning of the secondary 
epoch. 
