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 onr 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 which 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 

 tlie 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 undergTOund, 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 Graig Penalltau, 

 and under the Gellygaer Hill, into the Taff Vale at Navigation. 



From thence it extends to Llanwonno, and throuR-h the Rhonnda 

 valley, into that of the Afon. Cuiiously enough, as far as GeUygaer, 

 the course of the trough underground corresponds closely with 

 that, or tlic Taff- Vale extension railway above ground. Newbridge in 

 IMoninouthshirc is at the centre, or deepest part of the trough ; while 



