92 



H. KEEPING AND E. B. TAWNEY ON THE BEDS AT 



of How Ledge, we may adduce that the former is very rich in 

 Planorbis, while in the latter these shells are comparatively rare, 

 the fossils being chiefly Limncea. 



As the Upper Headon limestone is the strongest bed in the 

 section and forms an escarpment through the greater part of Headon 

 Hill, in it we may obtain a datum line. We take as a point of 

 reference the spot where the top of the escarpment cuts the surface 

 of the ground or outline of the cliff ; this is seen on the 6-inch or 

 25-inch map to be about halfway between the 100 feet and 200 feet 

 contour-lines. From these points of known altitude, by the aid of 

 the barometer, we obtain the height of our point of reference : it is 

 about 140 feet above Ordnance datum. 



The beds below the Limncea-limestone are green clay, 1 foot, 

 with broken Paludina, then pale buff or whitish sands, varying from 

 6| to 8 feet, with occasional layers of lignitic matter, Potamomya, 

 and Melania muricata ; below, where it is sometimes grey, Unio 

 Solandri and Paludina lenta may occur. We take this bed with 

 the 3-inch lignite below as the base of the Upper Headon. The 

 boundary chosen is, of course, arbitrary ; but the fact of the next 

 bed being decidedly brackish inclines us to draw the division from 

 the Middle Headon here ; the Survey vertical section, sheet 25, places 

 it a few feet lower. 



Our estimate for the Upper Headon at Headon Hill amounts to 

 a thickness of 50 feet ; the thickness on the Survey vertical section 

 is given as 37 feet ; but if we read off the distance between the beds 

 which we have taken as boundaries, it becomes 48 feet. The com- 

 bined thickness of Osborne and Upper Headon beds, according to 

 the Survey section, is 119 feet, i. e. adopting the top of the O. ven- 

 tricosum bed as the boundary ; our estimate, taken at the north-east 

 end of the hill, is 120 feet. The agreement is sufficiently close to 

 render it probable that a thickness above the average of the cal- 

 careous portion is accompanied at the same spot by a diminution in 

 the clays ; so that the balance of average thickness is maintained at 

 both ends of the hill. As we have said, we think it convenient to 

 retain the name " Osborne Series" for the red and greenish mottled 

 clays and marls and pale greenish-white limestones, since these 

 physical characters distinguish them at once along this side of the 

 Solent. We must decline to accept the statement (op. cit. p. 169) that 

 " under this name beds lying below the Brockenhurst series, as at 

 Headon Hill, have been confounded with others on a totally diffe- 

 rent horizon, above the Brockenhurst series." 



We next come to the Middle Headon. E. Forbes relates (Mem. 

 p. 85) that the uppermost and lower portions at Headon Hill are 

 brackish-water beds abounding in Cerithium ventricosum, C. pseudo- 

 cinctum, O. concavum, Neritina concava, and Nematarce, the condi- 

 tions being less purely marine than at Colwell Bay. This is, no 

 doubt, true of the series as a whole ; for below the O. ventricosum bed 

 there are two freshwater Limnsea-limestones. But it appears to us 

 that too much has been made of this ; for instance, the lower Neri- 

 tina-bed is identical in Headon Hill with the similar bed at Warden 



