An Unconformity in the Carboniferous. 163 
Upper Avonian, the Basal Grit of the Millstone Grit, and some part 
of the Coal Measures. 
As previously mentioned sandstones are present to a greater or 
less extent in the Upper Avonian in the adjacent Bristol and Forest 
of Dean districts, and in the eastern part of the South Wales basin 
also. But these sandstones are invariably conformable with the 
underlying beds, and cannot, therefore, be compared with the Ifton 
sandstones. 
It is harder to choose between the Basal Grit and the Coal 
Measures. Both formations have been proved to rest uncon- 
formably on the Carboniferous Limestone in adjacent regions: in 
the South Wales basin, about 14 miles to the west, in the case of 
the Basal Grit ; and in the Forest of Dean,! at the same distance to 
the north-east, in that of the Coal Measures. It is probable, too, 
that the unconformities on both horizons have been preceded by 
“piping”? of the Carboniferous Limestone. This has certainly 
happened prior to the deposition of the Basal Grit, and appears 
also, from Dr. Sibly’s ? description, to have preceded the formation 
of the Coal Measures of the Forest of Dean. 
In support of a Coal Measure age we have the presence of Coal 
Measures close by, Pennant Sandstone and coals having been proved? 
in the Severn Tunnel near Southbrook, 24 miles east of ton. On the 
other hand, the Ifton sandstones resemble neither the Pennant, nor 
the Trenchard Upper Coal Measures, described by Sibly, that form 
the base of the Coal Measures in much of the Forest of Dean. 
It is possible, of course, that what have been spoken of as -two 
unconformities are really but one, accompanied by persistent 
overlap of the overlying basal beds between South Wales and the 
Forest of Dean. On this hypothesis the Forest of Dean would 
represent part of the margin of the South Wales basin, and the 
Ifton sandstones might be correlative with neither the Basal Grit, 
deposited, presumably soonaftertheuplift and subsequentsubsidence, 
in the interior part of the basin, nor with the Upper Coal Measures, 
laid down much later in the Forest of Dean. Simplicity has its 
charms, and the postulation of breaks in “ conformable’’ series 
should be regarded as an unpleasant duty rather than a light mental 
exercise. But in this case it happens that a break between Middle 
and higher Coal Measures has been proved in Wyre Forest at no 
sreat distance north of the Forest of Dean. The hypothesis of a 
persistent overlap between South Wales and the Forest of Dean 
therefore does not help us. As a fact the Ifton sandstones 
so closely resemble the Basal Grit of South Wales that, in default 
of conclusive evidence to the contrary, we must refer them to that 
horizon and suppose, as a corollary, that there has been no overlap 
17. F. Sibly, Grou. Mac., 1912, p. 420. 
2 Grou. Maa., 1918, p. 26. 
3 Charles Richardson, Proc. Bristol Nat. Soc., ser. m1, vol. v, 1887, p. 49, 
section. 
