158 E. E. L. Diawon— 
an overstep at this horizon that had already been observed near 
Haverfordwest, at the other end of the South Wales basin, by 
Professor O. T. Jones! The value of the Hton section in 
demonstrating that this overstep was widespread was recognized, 
and photographs, two of which are here reproduced, were secured 
by H.M. Geological Survey. Since then the unconformity at the 
eastern end of the South Wales coalfield has been confirmed by the 
detailed examination of a large area by Mr. F. Dixey and Dr. T. F. 
Sibly,? and has been found by the writer, in the course of work not 
yet published, to reach its greatest known extent in the Abergavenny 
district. Nevertheless, it seems desirable to describe the Ifton 
section because it is the clearest exposure known, in the whole of 
the South-West Province, of the unconformity between the 
Carboniferous Limestone and the Millstone Grit, and also on account 
of the remarkable inter-relations of the two formations which it 
reveals. 
It should be observed that though the Ifton limestone-outerop is 
continuous with that surrounding the Forest of Dean coalfield, the 
sandstones spoken of as “ Millstone Grit”’ in the two areas, which are 
widely discontinuous, differ completely from one another in age and 
character. Those of the Forest of Dean have been shown by 
Dr. Vaughan® and by Dr. Sibly* to be Upper Avonian, the 
lower portion being correlated definitely by the latter worker with 
the upper Seminula subzone (S,); he therefore justly separates the 
sandstones of the Forest of Dean from other “ Millstone Grit” 
under the name of Drybrook Sandstone. 
The Millstone Grit of Ifton cannot be placed paleontologically 
within narrow limits, but lithologically it resembles the Basal Grit 
of South Wales, which is the horizon also of the beds resting un- 
conformably on the Carboniferous Limestone at Haverfordwest. 
There is reason to believe ® that the Basal Grit is correlative with 
part of the Pendleside Series. When it is remembered that other 
rocks of the South-West Province included in the Millstone Grit 
are certainly later than all Avonian and Pendleside zones—though 
they may not be MiddleCoal Measures as suggested by R. H. Goode’&— 
it can be seen how necessary it 1s to distinguish between the different 
components of the barren series, separating Limestone from Coal 
Measures, which has been given the useful name of Millstone Grit. 
The areal extent of the unconformity remains to be determined. 
The break exists at the eastern and western ends of the North Crop 
of the South Wales basin, but is presumably absent at those inter- 
mediate places along the outcrop where radiolarian cherts are 
1 In“ The Country around Haverfordwest ”’ (Mem. Geol. Oe. ), 1914, p. 151. 
2 Quart. Journ. Geol. Soc., vol. xxiii, 1918, p. 111. 
3 Quart. Journ. Geol. Soc , vol. Ixi, 1905, pp. 251-2 
4 GEOL. Mac., 1912, p. 417; Gor. Mac., 1918, p. 25. 
5 KE. Dixon, ae Country around Pembroke and Tenby ’’ (Mem. Geol. ° 
Surv.), 1921, chap. 
§ Quart. vee Geol. Soc., vol. |xix, 1913, pp. 272, 275. 
