214 MR. A, STRAHAN ON THE ORIGIN OF [ May 1902, 
however, is due, not directly to the fault, but to the introduction by 
the fault of a strip of soft strata among the Silurian slates and 
grits. : 
The Charnian movement, as manifested in the north-north- 
westerly faults, was essentially one of subsidence. It is charac- 
terized by faults which hade normally to the downthrow, and each 
fault represents a gain in horizontal extent of the rocks that it 
shifts, proportionate to its throw and the lowness of its hade. 
In the Coalfield alone, this gain must amount to some hundreds of 
yards. 
(3) The age of the west-south-westerly, or Caledonian, 
movement in South Wales is more difficult to determine, for it did 
not extend to the area now occupied by Secondary rocks. The 
belts of disturbance attributable to this movement in the area 
under consideration may be enumerated as follows :— 
The Vale-of-Neath disturbance, which runs from the Old Red 
area north of Dowlais to Neath, a distance of 25 miles, in a 
general direction of west 27° south (E on the map, Pl. V). 
The Cribarth disturbance, which follows at a distance of 7 miles, 
runs from Fan Gihirych to near Swansea, a distance of 20 miles, 
in a general direction of west 40° to 50° south (D on the map). 
The Tair-Carn disturbance is an anticline which arches up the 
Millstone Grit 8 miles north of the Cribarth disturbance. It 
runs in about the same direction, but its course through the 
Coalfield is still under investigation (C on the map). 
The Cerig-Cennen disturbance runs about 2 miles north of Tair 
Carn, in a direction of west 20° south (B on the map). 
The Llandilo disturbance runs a mile or two farther north 
again, in a direction of about west 17° to 20° south, but curves 
northward to west 45° south (A on the map’). 
Direct evidence proves no more than that these disturbances were 
of post-Carboniferous age; and from the fact that they resemble the 
east-and-west folds in being due to a movement of compression, and 
do not differ widely from them in direction, I was disposed at first 
to regard them as contemporaneous with that system. That view 
is untenable, for the following reasons:—The two systems are 
developed in different regions, and do not amalgamate; the Cale- 
donian movement lost energy in the district in which the Armorican | 
movement was most intense, the one displaying itself in Central ; 
Wales, the other in Devon and Somerset. Secondly, the difference 
in direction, though slight, is maintained. Lastly, the Armorican 
disturbances were due to a movement or impulse from the south, and 
die gradually away northward in South Wales, while the Caledonian 
moyement records an impulse from the north, and dies gradually 
away southward in the same region. The detailed structure of 
the Caledonian disturbances, enumerated above, indicates this thrust 
2 The re-surveying of the district extends no farther north than Llandilo, 
and I am, therefore, unable to say what further disturbances occur to the 
north of that town. 
