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The South Wales coal-field is divided into three series, 
i.e., the Upper Pennant, 500ft. thick in some places, but as 
much as from 1,200 to 3,000 in others. The Lower Pennant 
series average about 1,500ft. thick in the neighbourhood 
between the Taff River and Llanelly. 
The third or White Ash series is estimated to have a 
thickness of 1,000ft. in the centre of the basin, but thins out 
to about 500ft. towards the Eastern side. If the distance 
between the South and North Wales and Bristol coal fields 
were too near the Forest of Dean to cause any difference in the 
vegetable growth for the formation of coal seams in the latter, 
then the explanation already given would have to be accepted, 
or some other theory sought for in order to agree with some of 
the phenomena presented to us. 
There are various well-defined and interesting faults in the 
South Wales coal-field, some of which seem to run generally in 
an Eastern and Western direction. The North basin synclinal 
portion has two such prominent dislocations of the strata, and 
the coal measures on the Northern side of the most Northern of 
these faults has been elevated ; but according to a section which 
has been consulted, on the opposite or Southern side of it they 
have been depressed as much as 450ft.; thus in remote Geological 
epochs some of the seams of coal in the Upper Pennant series were 
raised to the then surface, and afterwards denuded. More to the 
North on the line of section this effect is still more notable, for 
long before we come to the Dowlais great fault the Upper Pennant 
series of coal seams have vanished altogether. The Northern 
and North-Eastern portion of the South Wales coal field in the 
direction of the Forest of Dean has apparently suffered the 
most from denudation. The great fault, or anti-clinal, known 
to exist at Risca on the East, runs through the entire length 
of the coal field into Pembrokeshire, where it only affects the 
lower and older strata. Probably it had an augmented effect 
upon the measures to the North and North-East, and may have 
passed close to, or between the Bristol and Forest of Dean coal 
fields. 
