1846.] PRESTWICH ON THE ISLE OF WIGHT TERTIARIES. '227 



which the tertiary beds repose is as usual much worn, and overlaid 

 by a few feet of sand with green-coated slightly rolled flints, and 

 some small flint pebbles. To this succeeds also, in both places, 

 a considerable thickness of bright-coloured mottled clays, and then 

 an important bed ("c?") of sandy brown clay with Septaria. So 

 far the sections are nearly identical in character, but above this bed 

 a considerable difference of lithological structure exists. At Alum 

 Bay we find a series of variously coloured siliceous sands with 

 some greensands and carbonaceous clay, the whole almost free 

 from calcareous matter. At White Cliff" Bay, on the contrary, 

 we have much less siliceous sand and carbonaceous clay, and more 

 greensands, marls and calcareous sands. In the overlying fresh- 

 water, or rather, as they might probably be better termed, the fluvio- 

 marine series, limestones are, on the contrary, more developed at 

 Headon Hill than at White Cliff' Bay. At Alum Bay as many as 

 ten or eleven layers of pebbles may be traced in the vertical beds ; 

 they are marked " P " in the sections (Plate IX. fig. 1 & 2). In White 

 Cliff" Bay they are also numerous, but their number cannot be well 

 ascertained owing to the many breaks in the cliffs. An unusual 

 condition connected with this subject may be here noticed. In the 

 lower limestones and sandstones, which, thin and unimportant at 

 White Cliff" Bay, swell out and become in places conglomeratic at 

 Bembridge, not only do we find the common small round black 

 flint pebbles imbedded in the rock, but also numerous angular flints, 

 but little rolled, just as they are found in the common flint gravel. 

 The following is a section of a block of this calcareous sandstone. 



— 1. 



.-.=c-w->^"^■T5••*ay?■=?^fe-^^^->=^4. Angular flints, but little rolled, 

 as^^=i-^S^^&£:^i;fe£^^- and small round flint pebbles. 



-^ ^s Freshwater shells, layer of. 



■■^^O^B€H3£=zs^E^s^I:^^^~~ZZS. Angular flints resembling the upper bed. 



Ditto. 



Even within small distances the lithological changes of some of the 

 strata are considerable. The short projecting outcrop of the verti- 

 cal beds precludes the exhibition of this fact at Alum and White Cliff" 

 Bays, but in the horizontal strata it may be constantly seen. Thus 

 the thick bed of earthy limestone (No. 39) at Headon Hill passes 

 in its range northward into a bed of sand ; the thin limestones below 

 this bed appear, on the contrary, to become more important in their 

 range north-east. In the upper part of the series at Headon Hill, 

 eighteen feet of hard limestone and indurated marl (71 and 72) al- 

 most entirely thin out before reaching Colwell Bay, so that the 

 upper part of the freshwater or fluvio-marine series in its range 

 northward and eastward loses much of its limestone, and presents 

 thick masses of marls and clays — to this Professor Sedgwick has 

 alluded. 



At Hordwell Cliff", on the top of the London clay, there is a bed of 

 sand fifteen feet thick overlaid by a few inches of grey clay ; tracing 



