Bevan— Coal-basin of South Wales. 161 
acted on the Coal-basin from a westerly direction—in other words, 
from what is now Ireland—and that, as might naturally be expected, 
its effects would be strongest and most obvious the nearer to its 
source, and would decrease as the radius became wider. Accord- 
ingly, we find that the Pembrokeshire beds are contorted and 
displaced more than any others, while the disturbances are evidently 
lessened as we go eastward. If we suppose a large area of tolerably 
uniform surface of clays and shales, more or less recently deposited, 
and then apply to it a pressure (either sudden or long-continued) 
in any given direction, we shall find that a general rumpling or 
ridging of the surface would take place ; such ridges assuming a 
certain amount of parallelism to each other, and depending on the 
direction from whence the pressure was exerted. 
The general arrangement of the physical features of the South- 
Welsh Coal-basin, then, are probably due to the action of this south- 
westerly force during the deposition of the Coal-measures. I say 
during the deposition, because I cannot help thinking that it took 
place prior to the deposition of the Upper Measures, and subsequent 
to the Lower and Pennant beds. It is probable indeed that the 
whole chemico-dynamical force to which the lower beds were subject 
caused those alterations in the coal by which the beds have become 
steam-coal,—an alteration which has given South Wales such a pre- 
eminence as a steam-coal basin. Whether or not the Steam-coal- 
measures are due to this, there is no question but that the Upper 
Measures, where found, are of a totally different character to the 
Lower Measures. In Pembrokeshire, where the force was exerted 
most, we find trap-rocks in the immediate vicinity of the coal: also, 
we find that in Ireland, where the force is assumed to have 
originated, the same identical Lower Measures are all anthracitic. 
Jt may be, however, that the force originated in Pembrokeshire, or 
at a point half-way between that and Ireland, so as to cause the 
anthracitic qualities to be formed on either side of it ; just as a dis- 
turbance in water causes a ripple on every side equally. In Pem- 
brokeshire and Caermarthenshire, the anthracitic quality gradually, 
but surely, decreases as we get eastward ; and finally, in the neigh- 
bourhood of the Rhymney Valley it is lost altogether; the coals 
thence to the ‘ Kast crop’ being entirely bituminous. At Aberdare 
and the Vale of Neath, which is about half-way between Caermar- 
thenshire and Rhymney, we find, as we might expect, the highest 
and most profitable development of steam-coal, being neither too 
anthracitic nor too bituminous; where we do jind the upper Coal- 
measures, as in the neighbourhood of Swansea and Llanelly, they 
are bituminous, showing the very reverse of the anthracitic quality, 
and that they could not have been subjected to the same treatment 
as the Lower Measures. I therefore conclude that the great chemico- 
dynamical force which operated on the steam-coal, and in all proba- 
bility gave the coal-field its physical features, took place prior to the 
deposition of the Upper Measures. Almost every one, if not every 
one, of the parallel valleys has a main fault, generally known in 
the neighbourhood as the ‘big fault, running through the Lower 
VOL. II. — NO. X. M 
