164 An Unconformity vm the Carboniferous. 
among the beds that have immediately overlain the Carboniferous 
Limestone for at least a dozen miles east of the main outcrop in 
South Wales. A determination of the age of the Coal Measures at 
the Severn Tunnel, which are exposed, so far as known, only in the 
half-tide rock called Lady Bench, would be both interesting and 
important ; their relationship to the so-called Millstone Grit that 
was proved below in the Tunnel works we cannot hope to learn. 
This Millstone Grit appears, from the descriptions and section made 
by the first engineer+ of the tunnel and by one of his assistants,” 
to have “ piped”’ the underlying Carboniferous Limestone as do 
the Ifton sandstones. 
A minor point is the question whether any of the Grit now present 
in Ifton quarry has been deposited in the light of day or whether 
it has all been laid down in cavities, the limestone-roof of which 
has been removed by the pre-Triassic denudation. The fact that 
some of the channels, such as that in Pl. II (a) and A in PI. II (6), 
which, from their relative positions in the quarry, are probably 
continuous one with the other, are large and widen upward may count 
for little, but the persistence of the Grit beneath the Trias for a 
distance of 50 yards, more or less at right angles to the trend of 
these channels, suggests deposition in the open. 
CoNCLUSIONS 
The Carboniferous Limestone was eroded suberially prior to the 
deposition of the Ifton sandstones, the Dibunophyllum Zone 
presumably being removed and the Seminula Zone below irregularly 
worn into deep channels with steep, even overhanging sides, into 
vertical “* pipes ’’ descending an unknown depth into the limestone, 
and into every conceivable intermediate form of cavern and passage. 
The sandstones and shales, with rare plant-remains, that were 
then deposited in and on the limestone were contemporaneous with 
the Basal Millstone Grit. 
My cordial thanks are due te the Ifton Limestone Company, Ltd., 
for their courtesy in readily permitting me to examine the quarry 
and to publish this account, to the Director of H.M. Geological Survey 
for the photographs for the plates, and to Messrs. T. C. Hall and 
G. Maunsell for help in the field. 
1 Charles Richardson, op. cit. 
2 EK. D. Jones, Proc. Geol. Assoc., vol. vii, 1881-2, p. 339. 
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