162 Bevan— Coal-basin of South Wales. 
Measures, but which does not particularly affeet the Upper Measures, 
where they are found. For instance, in Monmouthshire, the Upper 
Measures are represented by only one seam of coal, all that is left 
from the effects of denudation. This is the ‘Mynyddwslwyn vein 
of red ash,’ or ‘ house-coal ;’ and although it is riddled with faults, 
they are merely local ones, and have no connection, I imagine, .with 
those of the Steam-measures, which lie several hundred yards lower, 
the Middle or Pennant Measures intervening. These main or big 
faults running down the valleys have formed, or at all events 
influenced, the drainage lines, and have thus given a decided groove 
for denudation to work upon afterwards. ‘The depression running 
at right angles across the field appears, with great probability, to be 
connected with the great saddle or anticlinal which commences in 
Monmouthshire at Newbridge (a little below Crumlin), and runs 
across the basin to the Rhondda Valley, thence to Maesteg in the 
Llynvi Valley, finally dying out at Lilanelly in Caermarthenshire ; 
having the effect, as Sir Henry De la Beche has shown, of bringing 
the Lower Measures, which should be at their deepest, to the 
surface, and thus enabling the coals to be easily won at Maesteg, 
where a large ironwork town has arisen in consequence. 
The presence of this anticlinal is manifested by the appearance of 
the Cockshot-rock (although it is not known in. Monmouthshire 
under that name); a white quartzose sandstone, which has evidently 
been altered and thrown up in connection with the anticlinal and the 
disturbed superficial cross valleys that I have named, and at the 
same time, I imagine, as when all the parallel north and south 
valleys were formed ; and this was the principal event in the 
geological history of the Coal-field, which we may therefore suppose 
to be summed up as follows :— 
1. The deposition of the coal-beds, with all their attendant clays, 
shales, and sandstones. 
2. The repeated subsidence of the strata thus formed, so as to 
allow a repetition of the process. 
3. The occurrence of the grand force which gave the change to 
the Measures already formed, and the outline of the topographical 
features. 
4, The deposition of the Upper Measures. 
5. The gradual elevation of the Coal-field by slow and successive 
stages,—the proof of this being seen in the uniformity of height and 
outline, and the occurrence of terrace-beaches in the Pennant Hills, 
each terrace marking a period of rest. ‘ 
6. The denudation which has carried off on the east nearly all the 
Upper Measures, and over the whole of the basin, has left them in 
a fragmentary condition. From the discovery by Sir W. Logan of 
coal-pebbles and detritus in Carboniferous beds of evidently more 
recent formation, it is not likely that denudation has been limited 
to one period ; but has taken place at different times. It is easy to 
conceive how the harder material of the Pennant rocks escaped, 
while the softer shales of the river-valleys were washed away; each 
denudation-period contributing to scoop out and deepen the valley, - 
the direction of which had already been given by the great pressure. 
