Vol. 58.] THE RIVER-SYSTEM OF SOUTH WALES. 215 
from the north, for at the head of the Vale of Neath the Carboniferous 
Limestone has been thrust southward over the Millstone Grit °; 
while the relations of the Ordovician and Silurian rocks in the 
Llandilo neighbourhood shew that the fold takes the form of a great 
sigmoidal curve, supplemented in places by fracture and thrusting 
trom the north.” 
It being evident, therefore, that the Caledonian movement was 
distinct in character, distribution, and, presumably, age from the 
Armorican, it remained to compare it with the north-north-westerly 
faults of the Charnian movement. Theoretically the later move- 
ment would shift the structures due to the earlier; and this test, 
when applied to the Charnian and Armorican structures, gave a 
definite result, for the north-north-westerly faults maintain their 
direct courses across the east-and-west folds everywhere. As regards 
the relations of the Charnian with the Caledonian movements the 
evidence is as follows :— 
A considerable number of north-north-westerly faults have been 
mapped on either side of the Vale-of-Neath disturbance, but a few 
only could be traced across it. At Penderyn there is a cross- 
fracture, but the details of the structure are hidden by Drift. Of 
the many faults visible in the ravines of the Hepste, Mellte, and 
Upper Neath, not one could be recognized in the belt of disturbed 
ground. The Glyn Corrwg Fault reaches the disturbance on its 
south side at Blaengwrach. It has there a large westerly downthrow, 
but no fault of corresponding magnitude was to be found on the north 
side of the disturbance. The strata, however, are much plicated ; 
and it is possible that the fault continues, though it is certainly 
smaller. The behaviour of the Pen-y-Castell Fault scarcely admits 
of this explanation. It runs into the disturbance from the south 
3 miles below the Glyn Corrwg Fault, and has been proved at 
a point about 2 miles south of the intersection to have a westerly 
downthrow of 118 yards. The bold Pennant features on the north 
side of this part of the Neath Valley give facilities for a full 
investigation, but I was unable to find any trace of the Pen-y- 
Casteli Fault. Two explanations are possible: either the fault died 
away, or it turns abruptly, through more than a right-angle, along 
the Vale of Neath. The latter involves the supposition that a line 
of weakness already existed along the Vale, which is tantamount 
to saying that the Caledonian preceded the Charnian movement. 
I put forward this explanation in 1900 (while the Survey was in 
progress), and it seemed to be supported by the fact that a north-and- 
south fault crossed the Cribarth disturbance near Pen Wyllt’; but 
since then further evidence has been obtained, leading to an opposite 
conclusion. 
Both the Vale-of-Neath and the Cribarth disturbances are crossed 
1 Mem. Geol. Surv. Summary of Progress for 1898, pl. ii, facing p. 118. 
The illustration will be reproduced in the Memoir on Sheet 231 (Merthyr 
Tydfil), in preparation. 
? Mem. Geol. Surv. Summary of Progress for 1901. (in the press.) 
3 Ibid. 1898, pp. 118, 119. 
