212 MR. A. STRAHAN ON THE ORIGIN OF [May 1902, 
of the strike-valley is carried partly westward by the Cennen, and 
partly eastward by a stream flowing eastward along the same line 
of strike, the parting between the two streams being ill-defined. 
Both streams leave the line of strike as soon as the disturbance 
loses intensity, and turn northward into the Towy. 
The remaining rivers to be mentioned are the Taf and the Cleddau. 
Rising west of the region affected by the Llandilo disturbance, 
they pursue an uninterrupted course from their sources in the 
main watershed to the southern coast of Pembrokeshire. In so 
doing they ignore great east-and-west folds, which are in a direct 
line with the pre-Triassic folds of South Glamorgan ; the Western 
Cleddau, for example, crosses in succession an area of Archean 
rocks and the synclinal region of the Coalfield. In Milford Haven 
the valley pursues a westerly course along the strike of the beds, 
and thus gives, what is rare in South Wales, an example of a valley 
coinciding with the pre-Triassic strike. 
The influence of the group of south-westerly disturbances may 
be inferred also from a consideration of the catchment-basins of the 
rivers. The Vale-of-Neath disturbance dies away a short distance 
east of the Taff Valley ; the Usk consequently escapes its influence, 
and rises in the next belt of disturbance, namely, that which I 
have named after Llandilo. ‘The Llandilo disturbance broadens 
out in the neighbourhood of Builth; the Wye consequently, and 
the Upper Severn, go beyond it to the main water-parting of Wales 
for their sources. Similarly the Pembrokeshire Taf and the 
Cleddau rise in the main water-parting, and flow directly from it, 
both the Llandilo and the Cribarth disturbances having died out 
before entering their catchment-basins. All the rivers, in fact, have 
assumed a course at right-angles to the main water-parting, except 
those which have been deflected along local subsidiary disturbances. 
This prevalent, or normal, direction of flow locates the axis of the 
elevatory movement by which the drainage was initiated, and it is 
of the greatest interest to note that that axis is parallel to those 
subsidiary disturbances which have controlled the river-courses 
wherever they are developed. 
III. Tae Ace anp CHARACTER OF THE STRUCTURES. 
That the movements which have affected this region are charac- 
terized by their direction is apparent at once, and this fact alone 
suffices to distinguish them into the three systems, the directions of 
which are expressed by Prof. Lapworth under the names of 
Armorican, Charnian (or Hercynian of Suess), and Caledonian. 
These three systems, moreover, differ no less in character than in 
direction. 
(1) The east-and-west, or Armorican, movement, was one 
of compression from south to north. It reached its greatest intensity 
in Somerset and Devon, but extended so far north as to fold sharply 
the Lower Carboniferous rocks south of the Coalfield, and to throw 
the Coal-Measures themselves into great synclinal and anticlinal 
he 
