216 MR. A. STRAHAN ON THE ORIGIN OF [May 1902, 
by the extremely deep trough-faults known as the Rhydding and 
the Dyffryn Faults. The Rhydding Fault has a westerly downthrow 
of 800 yards at its maximum, and of 580 yards at its intersection 
with the disturbance. The Dyffryn Fault has an easterly downthrow 
of about 300 yards at the intersection. Both faults hade normally 
to the downthrow, and therefore in opposite directions. If, then, they 
were crossed by a later fracture, their surface-positions should be 
shifted in opposite directions; but, so far from this being the 
case, both are shifted, or rather curved, considerably to the south 
in crossing to the north side of the Cribarth disturbance. The 
same effect is produced on other faults, notably the Gardener’s 
Fault at Clydach; and it may be the expianation of a remarkable 
twist in the Gnoll Fault at Neath, the tracing of which has puzzled 
successive generations of coal-miners. It would seem that the 
whole block of country north of the disturbance has moved south- 
westward relatively to the block south of it—a movement which 
would account also for the difference in Jevel noticeable in strata 
on either side of the disturbance. ‘The north-north-westerly faults 
suffered the shift, and therefore preceded the Vale-of-Neath and 
Cribarth disturbances. In the case of the Llandilo disturbance, 
Messrs. Cantrill and Thomas have traced several considerable north- 
north-westerly faults through the upturned Lower Carboniferous 
and Silurian rocks which flank the axis, but have found no clear 
case of one of these faults cutting clean through the axis itself, 
along which the overthrusting sets in. 
LV. DETERMINATION OF THE River-Systrm BY A CALEDONIAN 
MoveMENt. 
The behaviour of the rivers now becomes explicable. They were 
initiated by the latest of the three systems of movement, and during 
a period of elevation, which is evidenced by the character of 
that system. An axis, presumably of maximum elevation, formed 
the water-parting, and from it the rivers flowed east and west on 
its two sides respectively, but some subsidiary axes of elevation 
formed minor water-partings, or locally diverted the rivers in their 
courses from the major axis. Thus the Severn, Wye, Towy, Taf, 
and Cleddau rise in the main water-parting, and take a normal 
course from it; but the Towy, encountering the Llandilo disturbance, 
is deflected along it to the south-west, so far as the disturbance 
runs. The Usk rises in the Llandilo disturbance, assumes the 
normal direction and keeps to it. The Tawe, rising in the same 
subsidiary water-parting, encounters the Cribarth disturbance, and 
is deflected along it to the south-west. The Neath and the Laff rise 
in an elevated region which is in the line of the Cribarth disturbance, 
and assume the normal course; but the Neath is caught in the Vale- 
ot-Neath disturbance, and is deflected by it. The Tatf encounters 
the same disturbance, but, for a reason explained subsequently, 
crosses it. The group of rivers, including the Afon Lwyd on the 
east and the Ogmore on the west, rise south of the belt affected by 
the subsidiary disturbances and pursue a normal course. 
