102 



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



Marine series above the Neritina-bed, is some foxy-coloured sand- 

 rock, while lenticular layers of white sand are seen thinning out in 

 the grey clays, which also contain red-clay ironstone nodules, a dif- 

 ferent lithological condition from what occurs further north in the bay, 

 or to the south of "Warden Point. Above comes the richest part of the 

 Venus -bed with abundant Cytherea incrassata ; these fossils not only 

 strew the tumbled clays, but with other fossils are commingled by the 

 waves with recent shells on the strand. At this point of the bay Ostrea 

 velata is very abundant above the part richest in Cytherea. 



In the centre of the bay between Colwell and Bramble Chines 

 this oyster is in extraordinary abundance ; at one place it nearly 

 crowds out most of the other fossils, and forms a massive oyster- 

 bank about 21 ft. thick *, of which the lower 12 ft. is almost 

 made up of these shells. The ordinary character of the Venus- 

 bed is quite altered here, though its fossils occur mixed up with 

 the oysters ; we may notice as especially abundant Murex seooden- 

 tatus, Pisania labiata, Natica labellata, Nerita aperta, Cerithium 

 variabile, Ostrea velata, Nucula headonensis, Cytherea incrassata, &c. 

 On either side of this spot, near both How Ledge and Bramble Chine, 

 the excess of oysters has disappeared, and they are chiefly abundant in 

 a zone above the richest part of the Venus-bed, though they do occur 

 sparingly throughout. Cytherea incrassata occurs through several 

 feet, but the richest part of the Venus-bed consists of about 9 inches 

 of bluish-green sand ; in this the shells are found in the best state 

 of preservation. Above the grey and greenish-grey sandy clays 

 richest in marine forms are some pale bluish-green clayey sands, 

 between Bramble and Linstone (or Lynchen) Chines ; in these, at 

 the level of about 5 feet from the base, is a band very rich in Ceri- 

 thium ventricosum, C. variabile, Melania muricata, Corbicula obovata, 

 with, occasionally, Merita ; this bed is seen just beyond the spring at 

 Linstone Chine, it is only a few feet below the base of the Upper 

 Headon. We wish to draw attention to the first-named fossil, as it 

 occurs also near the top of the marine beds at Headon Hill, i. e. in 

 quite an analogous position. 



Upper Headon of Colwell Bay. — The slate-coloured grey clay with 

 Potamomya f (immediately succeeding the bluish-green sands) we 

 take as the base of the Upper Headon. A detail vertical section 

 is given (fig. 3), in order to show the lithological differences 

 existing between the series here and equivalent beds in Headon Hill. 

 We do not, however, think them greater than the differences already 

 noticed as occurring in the marine series in different parts of Col- 

 well Bay, while the resemblances are sufficiently great to allow of 

 their perfect correlation — not to mention their position between the 

 Osborne beds above (so identical with the Osborne beds of Headon 

 Hill) and the marine series below, which we have shown, on strati- 

 graphical grounds, is most certainly identical with the Middle Headon 

 of Headon Hill. The differences consist here in a greater develop- 

 ment of sand and a reduction of limestone : the sands at the base 



* Bed 12 of Dr. Wright (ib. p. 92), who, however, much underrates its thickness, 

 t Base of bed no. 5 of Dr. Wright {ib. p. 91). 



