/ 
An Unconformity in the Carboniferous. 157 
(Barton) section is the clearly defined pebble-bed towards the higher 
part of the Highcliffe Sands of the Survey. The elauconitic 
clays above this bed are strongly contrasted with the pale sands 
below. The Nummulites Prestwichii band is met with a few feet 
above, and so the pebble-bed and the few feet of clays between it 
and the Nummulite band are usually grouped in the upper part 
of the Bracklesham Beds. This seems curious, since the zonal 
Nummulite does occur, though sparingly, below its special band. 
Moreover, there is no lithological change at the Nummulite band, 
but a very obvious one at the pebble-bed below. This pebble-bed 
may be the same as the one seen in Alum Bay at the base of the 
47 feet of doubtful beds. 
The marine Bartonian beds (Barton Clay and Barton Sands) 
thicken steadily eastwards from 192 feet at Highcliffe (Barton) to 
338 feet in Alum Bay and 368 feet in Whitecliff Bay. The first two 
figures are increased to 206 and 385 feet respectively if the pebble- 
bed is taken as the base of the Bartonian. The continental Bartonian 
(Lower Headon Beds) consists of freshwater limestones with 
Limnea, marls, etc., and thins, as one would expect, from west to 
east; 821 feet at Barton, from 61 to 82 feet in Alum Bay and on 
Headon Hill; it is reduced to 28 feet in Whitecliff Bay, and has 
become more estuarine. 
Commencing with the Middle Headon Beds, the Oligocene deposits 
of the Hampshire Basin also seem to group themselves into cycles. 
The distribution of the zones of the Middle Headon agrees with the 
theory of a gradual transgression at that period, similar to the trans- 
gression observed in the Landenian or Lutetian. It is hoped to deal 
later with these beds. 
(To be concluded.) 
The Unconformity between the Millstone Grit and the 
Carboniferous Limestone at Ifton, Mon. 
By E. H. L. Dixon, A.R.C.S8c., B.Se., F.G:S. 
(PLATE Ii) 
INTRODUCTION 
i pee object of the present communication is to place on record a 
section, apparently unique, in which the unconformity between 
the Millstone Grit and Carboniferous Limestone that exists in parts 
of South Wales and Monmouthshire is clearly displayed. The 
exposure was first examined at the suggestion of the brilliant worker 
who threw so much light on Lower Carboniferous problems, the 
late Dr. Vaughan. The writer had explained’ the attenuation of the 
Carboniferous Limestone Series in the Newport (Mon.) district 
‘as due to unconformable overstep by the Millstone Grit, similar to 
1 Tn “‘ The Country around Newport ”’ (Mem. Geol. Surv.), ed. 2, 1909, p. 20. 
