1854.] PRESTWICH, LONDON CLAY AND BRACKLESHAM SANDS. 435 



thin bands of twigs, leaves, and seeds of plants, with beetle remains, 

 lying inclined at an angle of about 15° towards the north. 



Borings in the vicinity have been made ; and at a few feet beneath 

 the bottom of the excavation a hard calcareous rock Avas reached con- 

 taining Cyrence. 



A similar deposit of peaty and argillaceous matter, with stems of 

 trees, occurs at the excavations now making for Victoria Docks*, about 

 a mile lower down the river. I have also seen something like it at 

 "Woolwich ; and it is probably continuous over all this part of the 

 valley of the Thames. At Victoria Docks a similar shelly bed occurs, 

 containing most of the shells above enumerated, vdth one or two 

 additions, viz. : — 



Limnseus auricularis. Helix rufescens. 



Neritina fluviatilis. . Planorbis nitidus. 



3. On the Distinctive Physical and Pal^ontological Fea- 

 tures of the London Clay and the Bracklesham Sands ; 

 and on the Independence of these two Groups of Strata. 

 By Joseph Prestwich, Jun., F.R.S., F.G.S. 



§ 1 . On the difference of the Species, and of the physical structure. 



The fact of the London Clay proper having been considered, until 

 within a comparatively recent period, synchronous with the clays 

 and sands of Bracklesham and Barton, gave rather naturally greater 

 weight and prominence to the resemblances than would attach to the 

 diiferences, in the character of the fauna of these several beds. Not 

 that the differences were overlooked, but they were, owing to this 

 presumed synchronism, referred to changes dependent upon geogra- 

 phical distribution, depth of water, and variations of sediment ; and 

 their true value was in consequence hardly sufficiently allowed This 

 supposition, which levelled the distinction of age, led to the union of 

 the three distinct faunas of the above-named deposits ; and conse- 

 quently, both here and abroad, the comparison of this associated 

 group with the continental tertiary faunas almost necessarily caused 

 the three, as a whole, to be considered synchronous with that deposit 

 with which it possessed a majority of fossils, i. e. the Calcaire gras- 

 sier ; whilst by a natural reaction the differences in the faunas thus 

 wrongly parallelled were inevitably referred to such causes as had been 

 used to account for the anomalies arising from associating together 



* The following is the section at Victoria Docks : — 



ft. in. 



Brown clay, with some land and freshwater shells 6 



Peat 4 



Sandy clay and sand, with roots of trees .5 



Gravels and sands, false-bedded, unfossiliferous, about 20 



Blue clay, apparently unfossiliferous. 



