92 
THE GEOLOGIST. 
or the Swansea valley, followed by the Lloughor and Gwendraeth 
rivers, which have their respective outlets at Llanelly and Kidwelly. 
In whatever portion of the field we make our observations, we 
cannot help being at once struck with the effects of the enormous 
denudation to which it has been subject. The great pressure, or 
catastrophe, has been shown by Prof. Ramsay to have probably taken 
place at the close of the coal-measure period, and did not consist of a 
number of detached efforts, but of one gigantic contraction. The 
effects were to create tremendous flexures and contortions of the 
strata ; and, if such are not always visible to us, we must remember 
that the lower beds were saved from the shattering influence by the 
immense super-incumbent weight of strata wliich has been long ago 
worn away by denudation. 
The greater part of this denudation happened in Tertiary periods, 
and was almost exclusively the work of marine action, as Professor 
Ramsay shows that fluviatile forces or atmospheric influences, 
although doubtless contributing much to the general appearance of 
the country, had but little power to cause such extraordinary effects. 
We can well understand how it is that most of the tops of the hills are 
crowned with hard rock, such as Pennant grit, having from this 
cause been able to resist the denuding force of the sea. If we ex- 
amine the measures underground, we at once see the results of the 
pressure to which the field was subject. It is generally spoken of as 
an elongated basin, or trough ; but the real fact is that there are two 
troughs running east and west, the smaller one being parallel to the 
larger, and separated from it by a considerable ridge, or anticlinal 
axis. The main trough contains by far the greater portion of the 
measures, which of course basset north and south. The centre of 
the big trough runs underneath Newbridge (Monmouthshire), 
through the high ground into the Sirhowy valley, below Blackwood, 
then into the Rhymney valley, which it crosses at Craig Penalltau, 
and under the Gellygaer Hill, into the Taff Vale at Navigation. 
From thence it extends to Llanwonno, and through the Rhonnda 
valley, into that of the Afon. Curiously enough, as far as Gellygaer, 
the course of the trough tmderground corresponds closely vdth 
that of the Taff-Vale extension railway above ground. Newbridge in 
Momuouthshire is at the centre, or deepest part of the trough ; while 
