94 



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



Towards the base My a angustata and Corbicula obovata were parti- 

 cularly abundant. 



Next below the Venus-hed is the Trigonocoelia-bed, chocolate- 

 brown sands, 3| feet, with sometimes a blackish tint ; we so call the 

 bed from the principal fossil which occurs at precisely the same 

 horizon in Warden Cliff and Colwell Bay ; other fossils are Ceri- 

 thium pseudocinctum, Melanopsis fusiformis, Natica labellata. Below 

 are blackish-brown sands, 6 inches ; at base is enough carbonaceous 

 matter almost to amount to a lignite band ; this may be called the 

 Neritina-hed, the chief fossils are Neritina concava, Melanopsis fusi- 

 formis, Corbicula obovata, the latter in perfect condition, very large, 

 and showing concentric colour-bands. The Neritina-bed occurs in 

 the same position towards the base of the series in Warden Cliff 

 and Colwell Bay. This can only be explained by admitting that 

 the marine series in Totland Bay and Colwell Bay are identical ; 

 the Ventricosum bed at the top, and the Neritina and TrigonocoeUa 

 beds at the base, identical in physical and fossil characters, are 

 strong presumptive proof of this. 



Below is very stiff dark-grey clay, 1 foot to 1 foot 3 inches ; 

 fossils occur in patches, Neritina concava, CeritJiium pseudocinctum, 

 Melania muricata, Limncea, Corbicula obovata. This is the lowest 

 bed of the Middle Headon here. Summing up, we obtain a thickness 

 of from 31 feet 9 inches to 33 feet for the Middle Headon of Headon 

 Hill at the N.E. end. Beading off the Survey vertical section by 

 scale, we obtain 35 feet for it between the boundaries adopted by 

 us * for the thickness towards the west end. 



The height of the base of the Middle Headon above the sea at 

 this point, viz. about 120 yards in horizontal distance west of our 

 reference-point, is by subtraction 72 feet. Direct barometric obser- 

 vations gave about 70 feet. We have already used these figures 

 when alluding to the position assigned to this series. 



The first bed of the Lower Headon is a Limnsea -limestone of the 

 usual buff colour : it is 2 feet thick at this point ; but a little further 

 west we obtained a measure of 4 feet. This is in our opinion the 

 well-known bed which forms the top of the Lower Headon in Warden 

 Cliff, where it is quite a marked feature. It has there and in Col- 

 well Bay precisely the same position in the series, supporting the 

 Middle Headon — recognized by the Neritina-hed with all its charac- 

 teristic features, the Trigonocoelia-bed, and so on. From Warden 

 Cliff it is traceable uninterruptedly to How Ledge, where it disappears 

 beneath the sea-level ; we therefore speak of it as the " How-Ledge 

 limestone." It is correctly drawn on the Survey vertical sections, 

 sheet 25, nos. 4 and 5, where in the legend is a clerical error, to 

 which we have already alluded. This bed is so distinctly lacustrine, 



* Some irregularity in the boundaries of the Survey vertical section is to be 

 noticed : viz. in the Headon-Hill Section, no. 5, the boundary is placed below 

 the How-Ledge Limestone ; in the Colwell-Bay section, no. 4, the boundary is 

 placed at one bed above the How-Ledge Limestone ; this seems an error of the 

 engraver, and of course does not affect the thickness of the beds. 



