Kennard 8f Warren — Tufa Dejmsit in Totland Bay. 19 



limestone quite similar to those still met with in large quantity ; no 

 source of abundant silica can be found in these rocks, so that we are 

 driven to conclude that it has been introduced from without. It is 

 simplest to suppose that the silica was introduced in solution in the 

 waters of hot springs. It may have been deposited at first in the 

 colloid form and subsequently have become chalcedonic in parts ; or 

 the two forms of silica may have been deposited more or less simul- 

 taneously. It seems likely, however, that, at least to some extent, 

 there has been a transformation from opal to chalcedony. 



In conclusion, the cherts described represent a secondary condition 

 of a rock originally different, viz. crystalline limestone; the silica 

 has been introduced from without, and is of inorganic origin ; the 

 silica has been chemically deposited, chiefly in the colloid form, and 

 replaces the carbonates which have been removed in solution. 



VI. — On the Recent Tufaceous Deposit of Totland Bay, 

 Isle of Wight. 



By A. Santer Kennard and S. Hazzledine Warren, F.G.S. 



^N the top of the cliff between Headon Hill and Widdick Chine, 

 in Totland Bay, there is a Eecent tufaceous deposit containing 

 land and fresh- water shells. It extends along the cliff for nearly 

 350 yards in a north-easterly direction from the base of Headon 

 Hill, and is about 60 feet above the sea-level. 



It was first described by Mr. Joshua Trimmer,^ and subsequently 

 by Professor Edward Forbes ^ and Mr. H. W. Bristovv.^ In the 

 more recent memoir on the Isle of Wight * the earlier descriptions 

 are quoted, but the section is described as being then almost entirely 

 overgi'own. 



The deposit is described as being of very variable character, as 

 the following details will show. At the base of Headon Hill 

 Mr. Trimmer states that it presented the following section : — 



feet. 

 e. "Warp -drift : brown sandy loam without lamination, containing 

 fragments of flint and Tertiary limestone. Filling furrows 



in the bed below 1 to 3 



a-c. Alternations of cream-coloured marl, calcareous tufa, and sand 

 and clay blackened by organic matter ; the calcareous tufa 

 being in beds 6 inches to 2 feet thick, and the sand and 

 clay forming bands of 2 to 6 inches in thickness 12 or more 



This author also states that Professor Edward Forbes obtained 

 shells of the genus TJnio in a layer of flint gravel which occurs ia 

 places beneath the tufaceous deposit. Possibly this should be 

 Anodonta rather than TJnio, but no fresh specimens have been found 

 to settle the point. Not far from the termination of the deposit 



1 Quart. Journ. Geol. See, 1854, vol. x, p. 53. 



* " On the Tertiary Fluvio-Marine Formation of the Isle of Wight"; Mem. 

 -Geol. Survey, 1856, p. 8. 



3 Ibid., p. 105. 



* " The Geology of the Isle of Wight," by Messrs. H. W. Bristow, Clement 

 Eeid, and Aubrey Strahau : Mem. Geol. Survey, 1889, p. 229. 



