OF RHYMNEY AND PEN-Y-LAN, CARDIFF. 495 
to the piling-up of some 2000 or 3000 feet of Old Red deposits upon 
the top of the barrier itself. It finally became completely sub- 
merged beneath the waters of the Carboniferous sea; and then con- 
ditions more like those of the Devonian prevailed over both the 
Devonshire and Welsh areas. Thus this central barrier seems to 
have anticipated in a general way the behaviour of the London-and- 
Harwich axis during the Neocomian and Cretaceous times. 
Ltelations of the Awes of the District with those of the 
surrounding Country. 
On referring to Sheet 36 of the Geological Survey, we shall find 
the Carboniferous Limestone, which forms the southern escarpment 
of the coal-basin, bending round southwards from Llanilid to Cow- 
bridge and then striking eastwards towards Leckwith; it forms, 
indeed, an east-to-west anticlinal, which ‘‘noses out” towards 
Bridgend. The conformable Old Red Sandstone, lying coaxially 
within it, “‘ noses out ” more to the east; so that the anticlinal axis 
rises in that direction, and thus brings in at length the Silurians of 
our area. Our area is therefore the lowest and easternmost exposure 
of a great Siluro-Carboniferous anticlinal which extends from bridg- 
end in the west to Rhymney in the east. The Old Red and Car- 
boniferous are dislocated by numerous north-and-south faults, from 
which the Silurian district appears to have escaped. ‘he anticlinal 
of Silurian near the Starting House is a difficult feature in the 
country; and some might explain it by a fault, for which, how- 
ever, there is no other evidence: I am inclined, myself, to regard 
it as a reappearance of the Cefncoedisaf-to-Pandy anticlinal, and 
except that traces of this fold will become more evident after a 
more searching examination of the country between Cefn-coed and 
Cefn-coed-fach. 
To the north of the Rhymney area we have the well-known Usk 
district, which with its principal anticlinal axis points north and 
south ; and though the axis seems to disappear south of Llandegfydd, 
yet its continued existence is indicated by patches of Silurian, 
which crop out about Llanfrechfa and elsewhere, while its influence 
on our region is shown. by the subordinate foldings which cross the 
latter from north to south. Between Usk and Llantrissant the 
Carboniferous Limestone lies in a generally N.E. to 8.W. curve as 
it bends from its north and south to its east and west strike; the 
Old Red Sandstone crops out in a parallel curve, as is shown by 
exposures §.W. of Newport. There can be little doubt that the 
Silurian turns round in a similar way below ground, and, I believe, 
to some extent above ground: at all events Mr. Storrie has met with 
fragments of Silurian beds near Ty-coch, Marshfield; and though 
he did not at the time succeed in finding the parent bed, there can 
be no doubt it was close at hand, and will be found on further search. 
Mr. Ross also writes me that he has evidence of outcrops of Silurian 
between Rhymney and Newport. 
Q.J.G.8. No, 139. 2M 
