210 MR. A. STRAHAN ON THE ORIGIN OF [May 1902,. 
escarpment of the limestone, nor is there any fracture along the 
gorges at Pontypool, Machen, or Risca. At Walnut Tree alone 
is there a displacement, the coincidence of gorge and fault here 
presumably being accidental. The parallelism of the valleys with 
the faults, moreover, is more apparent than real: the faults run 
a few degrees more east of south than the valleys, and the difference 
is maintained. Consequently the valleys cross the faults, but at so 
oblique an angle as to make it the more remarkable that they do 
not follow them. Lastly, we shall find that, although in the region 
about to be described the north-north-westerly system of faults is 
fully developed, the valleys take a different direction. I infer, 
therefore, that the north-north-westerly system of faulting had no 
more share in determining the river-system than the east-and-west 
folds. 
We come now to a series of valleys of a different character— 
they run about west 50° to 40° south, that is, almost at right-angles 
to those already described. The change of direction takes place in 
the region in which a set of powerful disturbances first manifests 
itself (A, B,C, D, & E, on the map, P1.V); and the direction assumed 
so closely coincides with that of the disturbances as to prove that 
it was determined by them. 
The Vale of Neath, or Nédd, forms one of the best examples. 
‘The sources of the river lie on the southern slopes of the Old Red 
Sandstone of Fforest Fawr; and from this upland region the Hepste, 
Mellte, and (upper) Neath make their way southward, crossing 
the Lower Carboniferous rocks, aud a number of north-north- 
westerly faults, in narrow and precipitous gorges with numerous 
waterfalls. The Hepste and Mellte unite, and reach the belt of 
plicated and overthrust strata which we have called the Vale-of- 
Neath disturbance (E on the map, Pl. V), at the Glyn Neath Powder- 
Mills; the Upper Neath reaches the same disturbance a mile farther 
west, at Pont Nedd Fychan. Once in the disturbed belt the water 
never leaves it, but follows it in a straight trench-like valley for 
12 miles to Neath. There the disturbance dies away, the trench 
comes to an end, and the river turns southward to the sea at 
Briton Ferry. The valley is about 1000 feet deep in the Pennant 
region, and contains a strip of alluvium from a third to half a 
mile in breadth. The margin of the alluvium, which marks the 
limits of the lateral swinging of the river, coincides with the 
margin of the disturbed strata, and the walls of the valley are 
formed by the unbroken strata of the north and south sides of the 
disturbance. The alluvial flat, however, ceases at the Powder-Mills, 
where limestone and Millstone Grit are thrown up along the axis 
of the disturbance. From that point eastward a ridge marks the 
axis of the disturbance, except at the spots where it is crossed by 
the Taff Fawr, Taff Fach, and a gap at Penderyn. 
Travelling 7 miles to the north-west over undisturbed strata, we 
reach a second belt cf plication and fracture, which we have called the 
Cribarth disturbance (D on the map, Pl. V), after a hill in which its 
