1846.] PRESTWICH ON THE ISLE OF WIGHT TERTIARIES. 233 



the word. He merely gave them as two divisions of the same for- 

 mation, the former always underlying the latter and exhibiting a 

 distinct character ; and in this view I fully agree with him. He 

 was doubtful however where the line of separation should be drawn, 

 and hesitates where to place the brown clay stratum " d," In litho- 

 logical character he found it resemble the London clay, but its or- 

 ganic remains, which he distinctly notices, he considered as resembling 

 those of the Woolwich beds. In fact, in his accounts of the tertiary 

 formations of the Isle of Wight, both in the wovk of Sir Henry En- 

 glefield and in the Transactions of the Geological Society, he has 

 well and ably described the geology of Alum Bay and Headon Hill, 

 and soundly established such subdivisions as the then existing state 

 of our palaeontological and stratigraphical knowledge warranted. 

 He proposed no fixed and positive system, but rather pointed out 

 the analogies of the series with the classification then recently in- 

 troduced by Cuvier and Brongniart into the Paris basin, and left it 

 to be modified and completed as further observations might render 

 necessary. 



I purpose to restrict the term plastic clay solely to the mottled 

 clays (which term would probably be preferable) marked " h " at 

 Alum and White Cliff Bays. They are in both instances separated 

 from the chalk only by a layer of a few feet of sand, pebbles, and 

 large green-coated flints ; but this bed, though unimportant here, is 

 the representative, as I hope to show on a future occasion, of import- 

 ant fossiliferous beds at the east of the London basin, and at the north 

 and east of the Paris basin. This layer may be called the lower 

 sands. It is singular, that of all the varied beds of the tertiary 

 series of England and France, the most persistent in its range, the 

 most uniform in its composition, and the most regular in its organic 

 remains (at present, I believe, it is only known to exhibit, and that 

 rarely, traces of vegetable remains), are these mottled plastic clays. 

 The whole of the beds below stratum " B " at Alum Bay have been 

 considered as a great development of the limited series of sands and 

 chys exhibited under the London clay at Woolwich, Lewisham, &c. 

 I do not however think that their synchronism can be maintained; 

 it is limited, I conceive, to the lower part only of Alum Bay. Cu- 

 vier partly limited the term " Argile plastique " to the variable 

 and mottled clays without fossils occurring very generally below the 

 *' Calcaire grossier." The lignites and fossiliferous clays which oc- 

 casionally intervene between this and the calcaire grossier he con- 

 siders as subordinate, and designates them " fausses glaises." M. 

 d'Archiac makes the clays and lignites together subordinate in his 

 first and lowest group of the Eocene formations. 



In France the geological position of these clays is constant over 

 the larger portion of the I^aris basin. It is represented, I conceive, 

 solely by stratum " 6" in the Isle of Wight, and underlies the whole 

 of the London basin west of London, ranging north to Hertford and 

 Bishops Stortford. The uniform absence, or at all events, the ex- 

 treme rarity of animal remains in this division throughout its entire 

 range, is a singular feature. The reasons for assuming the synchro-. 



