BEMBKIDGE LIMESTOKE. 



161 



variable within short distances, and of a highly travertinous 

 character. Indeed, very much of the limestone in this locality is 

 a true travertine, or calcareous-tufa. Much of it has a peculiarly 

 brecciated appearance not presented by the Headon limestones, 

 and the poro»ity dependent on the presence of irregular confervoid 

 tubular cavities, so characteristic oi' the Bembridge limestone in all 

 its localities, and so strikingly comparable with a like appearance 

 exhibited by the travertines of the Paris basin, is very manifest in 

 the rock at Sconce, The cause of this structure, first noticed 

 by Von Buch, and afterwards laid stress upon by Cuvier and 

 Alex. Brongniart, has been frequently discussed by French geo- 

 logists, who are inclined to refer it to the effect of the disengage- 

 ment of gaseous vapours. I am inclined to refer some of these 

 appearances to the ancient and now obliterated presence of vege- 

 table bodies, such as chara stems and algae. The distinctive 

 palseontological feature of the Sconce locality for this limestone 

 is the remarkable abundance of land shells in it. These occur tor 

 the most part in the upper half of the beds, freshwater shells 

 being more frequent in the lower, but much of the strata here 

 seems entirely unfossiliferous. In some places the mass of land 

 shells seems to lie in irreo-ular tufaceous bands between harder 

 strata, the latter abounding in Limncea longiscatu, Planorhis discus, 



P. obtusus, and P. olujyratus, mostly 

 in the condition of casts, but never- 

 theless exceedingly well preserved 

 and easily extracted. Great blocks 

 of grey sandy limestone lie along the 

 shore, fallen from the hill crest, full 

 o£ Planorhidcs and LinmcBCB, mingled 

 with occasional Helices {H. occlusa, 

 H. D' Urbani, and H. vectensis being 

 most common), and the fine Paludina 

 orbicularis. These blocks are broken 

 up by the native collectors, who seek 

 especially for the last-named shell, and 

 for Bulimus ellipticus, Achatina cos- 

 tellata, and Helix globosa, all species of great size and beauty, that 

 find a ready sale among visitors. In a thin white band beneath a 

 belt of Limncsa longiscata I find here the little Paludina 

 globuloidcs occupy the same horizon as at Bembridge and Cowes, 

 and remarkable for its constancy of place. The most concre- 

 tionary and brecciated portion of these beds consists of a white 

 band from 6 inches to a foot thick not far from the uppermost 

 layer, and evidently comparable with the cap of the limestone at 

 Bembridge. Just below the top, every here and there, a hard 

 band of silex, often nodular, reminds us of the cherty layers near 

 the summit of this limestone at St. Helen's. Four or five inches 

 of soft calcareous marls, with small limestone pebbles (or pos.sibly 

 concretions), form the very uppermost portion. In the line of the 

 E 56786. T 



Fig. 53. 

 Plnnorbis discus, Edw. 



