Vol. 58.] THE RIVER-SYSTEM OF SOUTH WALES, ZAllifl 
The fact that the Taff crosses the Vale-of-Neath disturbance needs 
further explanation. The effect of these subsidiary disturbances upon 
the drainage is attributable to their having been axes of elevation. 
The strata are crumpled up and overthrust along them. More than 
this, the Vale-of-Neath disturbance, where it so effectually traps 
the head-waters of the Neath, is accompanied by an uplift of the 
strata on its south side of about 600 feet, and would thus be hkely 
to throw a southward-flowing river out of its course. Where it is 
crossed by the Taff it assumes a different structure. There Mr. 
Gibson finds that the disturbance is not accompanied by an uplift of 
the south side, but that it takes the form of a faulted syncline about 
a mile broad, along which a wedge of Carboniferous Limestone is 
dropped into the heart of the Old Red country, to a level of about 
900 feet below its normal position. The disturbance dies out about 
3 miles to the north-east of the Taff Fach, and the absence of 
those evidences of compression which abound farther to the south- 
west, may be regarded as symptoms of diminishing energy. As a 
result no intercepting feature was formed, and the Taff Fawr and 
Fach kept their southward courses. 
The fact that the rivers ignore the structures produced by the 
Armorican and Charnian movements proves that the physical 
features produced by those structures had either been completely 
levelled by denudation, or, what is more probable, had been buried 
under later formations, after being partly levelled down. Of the 
elevation and vast amount of denudation which resulted from the 
pre-Triassic Armorican folding we have ample proof, as shown by 
Ramsay more than fifty years ago.!_ To mention one examp]e—the 
Keuper Marl rests directly upon Silurian strata near Cardiff. At 
that spot there must have been removed in pre-Triassic times about 
3500 feet of Old Red Sandstone, 1400 feet of Carboniferous Lime- 
stone and Millstone Grit, and not less than 2800 feet of Coal- 
Measures, or about 7700 feet of strata in all.? Evidence to this effect 
is yielded also by many rocky inliers of Carboniferous Limestone in 
the Keuper Marl, for much of the landscape produced at that early 
period still survives, though later erosion has only partly freed it 
from its envelopment of Secondary rocks. 
Of these inhers many sank beneath the water in Triassic time, 
but others survived as islands in the Liassic sea. In such cases the 
lias always overlaps the Trias; and it may be inferred that the 
same overlap took place along the main coast-line of the Liassic sea. 
Evidence exists (in the shape of veins filled with hematite, and 
even with marl, in the limestone) that the Trias extended farther 
and to a greater elevation than now, both to the west in Gower and 
to the north near Cardiff. Presumably the Lias extended still 
farther ; but the fact that it assumes a littoral type in parts of 
South Wales, proves that its limits were not far distant. In these 
* Mem. Geol. Surv. vol. i (1846) p. 297. 
ee vs . . 
I give here measurements recently obtained. Ramsay’s estimates were 
rather greater. 
