Tea\ 
/ 
An Unconformity in the Carboniferous. 159 
developed at the base of the Millstone Grit. In Gower and South 
Pembrokeshire the junction is almost certainly conformable. 
DESCRIPTION OF THE [FTON SECTION. 
The exposures at Iton, which lie 54 miles south-west of Chepstow, 
are disused parts of a quarry north-west of the Junction of the roads 
from Caerwent and Magor. The formations present in the 
quarry are :— 
3. Trias.—Brown, buff, or red friable sandrock and loose sand, up to 
6 feet in thickness, the lowest few inches littered with angular fragments, 
chiefly of Carboniferous Limestone, Millstone Grit and impure limonite. 
They rest on an even platform eroded across Limestone and Grit alike, 
and doubtless represent a sandstone that forms the base of the Keuper 
Marl in this district, resting immediately on the Dolomitic Conglomerate 
where the latter is present. 
2, Millstone Grit.—Hard sandstone, partly quartzitic and occasionally 
somewhat pebbly, with soft shales weathering to clay, both rocks raddled 
by ferruginous waters, the shales to purplish tones. The few moulds of 
plant-fragments present are generally small and indeterminable, but a large 
stem,! found in setu, has been identified by Dr. Kidston as a Carboniferous 
type, probably a Lepidophloios. The beds fill steep-sided channels and 
underground cavities, erodedin the Carboniferous Limestone, in the manner 
to be described later. 
1. Carboniferous Limestone.—Limestones, chiefly oolites or amorphous- 
looking calcite-mudstones, with some ‘‘ subsequent ’’ (vein-) dolomite ; 
in colour white, light-grey, or various red tones owing to raddling. All the 
beds appear to be free from contemporancously-deposited sand. The 
fossils, of which Seminula ficoides Vaughan is the most abundant, and the 
rock-types show that they belong to the Seminula Zone, a conclusion 
to which Dr. Vaughan had come prior to my visit. The Dibunophyllum 
Zone, which in its normal development is characterised by distinctive 
fauna and rock-types, is certainly not represented by any part of the 
limestone. 
The relations of the formations are illustrated by Pl. II (a and b) 
and Figs. 1 and 2; the general structure shown by PI. I (b), the 
photographs for which and for Plate IT (a) were taken in 1909, may 
still be seen in the quarry. 
In Pl. II (a) a single Grit-filled channel in the limestone 1s cut 
across, practically at right angles to its trend, by the quarry-face , 
it is 22 feet deep from the base of the Trias, which rests at both 
sides on the enclosing limestone, to the bottom (marked by a 
hammer). The dark material above the broken white line is the 
Trias ; its base, though somewhat uneven, crosses from limestone to 
Grit without any indication of post-Triassic subsidence. The sides 
of the channel are shown by a continuous white line. The lower 
two-thirds of the infilling is fine sandstone, in which bedding-planes 
are obscure except for some along the bottom, parallel to the broad 
base of the channel. The sandstone is hard, and unbroken by any 
subsidence since its consolidation. That its filling the channel dates 
from the Carboniferous is shown by its containing (at the point 
ties In the Geol. Surv. Coll. at Jermyn Street Museum (Regd. No. E.D. 
1). 
