90 



ON TIEE BEDS AT HE A DON HILL AND COLWELL BAY. 



This brings us to the vertical esearpnient of the thick limestones, 

 so conspicuous a band along Headon Hill cliffs that it is indicated 

 on the Ordnance maps, both on the 25-inch and the 6-inch. We 

 pause awhile to draw attention to the fact that we have accounted 

 for about 110 feet of beds from the top of the Bembridge to the top 

 of the great limestone (Upper Headon) ; and the Brockenhurst series 

 does not exist here. There is not a single marine fossil to he found 

 in that interval ; nor is there any bed with the faintest resemblance 

 either lithologically or palceontologieally to the Oolwell-Bay Yenus-6^. 

 This is in opposition to the view (/. c. p. 176 et passim) that the 

 Colwell-Bay series exists here "entirety concealed" by some supposed 

 gravel talus*; yet it is upon the existence of such a second marine series, 

 thus supposed to be added a.bove the Yenus-bed that the presence of 

 a Brockenhurst series at the west end of the island is inferred. 



Next we turn to consider the thick limestone of the escarpment, 

 the Limnasa-limestone of the Upper Headon. It is in several beds, 

 of which details are given by the Survey ; we measured it by sus- 

 pending a tape, and found it 27 feet (fig. 1, p. 91). 



The only difficulty in correlating the Headon-Hill beds with those 

 of Colwell Bay is centred in this limestone : it might be a difficulty 

 to those who would have expected a priori that the limestone would 

 have maintained its thickness in direction of dip for a mile or two 

 to the north ; for we identify it with a limestone at Cliff End not 

 above 1 foot 8 inches thick. It would be equally a difficulty accor- 

 ding to the correlation which identifies it with the How-Ledge 

 limestone {op. cit. p. 144) ; in this case the 27 feet has thinned to 

 3 feet at How Ledge, a distance of 1-L mile in a straight line, while 

 in Warden Cliff (only 926 yards distant from the Headon-Hill bed) 

 it is about 5 feet; so that it must have thinned very rapidly at the 

 first stage. The limestone in the Upper Headon at Cliff End, with 

 which we identify it, is distant 1 mile 926 yards. In either case it 

 thins out considerably to the north, as noticed by E. Forbes (Mem. 

 p. 84). We shall be able to prove that it does not occupy the same 

 position as the How- Ledge bed ; for we have recognized that bed, 

 which forms the summit of the Lower Headon, at a lower position 

 in Headon Hill and in its natural position, viz. below the marine 

 series (Middle Headon), as in Warden Cliff, where it was last seen. 

 As a palasontological distinction between the Upper Headon limestone 

 of Headon Hill and Cliff End f and the Lower Headon limestone 



* With respect to the "inextricable difficulties and confusion " (I. c. p. 144) 

 in which the Survey is supposed to be involved by their not allowing the Col- 

 well Marine bed to come where the Osborne beds are placed, and which is sup- 

 posed to be shown by 48 feet of strata being classified in the letterpress (Forbes's 

 Mem. p. 81) as Osborne, while in the plate they are classed as Upper Headon, 

 this is merely a question of classification and the drawing of a boundary-line, 

 matters entirely subjective and not affecting the total thickness ; their vertical 

 section shows almost the same thickness of beds consistently, notwithstanding 

 certain irregularities in the boundaries and classification. 



t In the legend to the Survey Vertical Section, sheet 25, no. 4, this limestone 

 in the Upper Headon is said to form How Ledge ; this is plainly an oversight 

 or clerical error, as is also the statement in Forbes's Memoir, p. 132, that the 

 How-Ledge bed is faulted in Warden Cliff. The fourteen faults mentioned affect 

 the Upper Headon limestone at Cliff End : their section is fairly correct ; but 

 there seems to have been some confusion between Warden Point and Cliff-End 

 Point in the letterpress. 



