Ch. XVI.] LOWER EOCENE FORMATIONS, ENGLAND. . 295 



this family a small fragment of a jaw with three molar teeth, found in 

 the Upper Eocene strata of the Swiss Jura. 



Plastic or mottled clays and sands (C. 2, p. 281). — The clays called 

 plastic, which lie immediately below the London clay, received their 

 name originally in France from being often used in pottery. Beds of 

 the same age (the Woolwich and Reading series of Prestwich) are 

 used for the like purposes in England.* 



No formations can be more dissimilar on the whole in mineral char- 

 acter than the Eocene deposits of England and Paris ; those of our 

 own island being almost exclusively of mechanical origin, — accumula- 

 tions of mud, sand, and pebbles ; while in the neighborhood of Paris 

 we find a great succession of strata composed of limestones, some of 

 them siliceous, and of crystalline gypsum and siliceous sandstone, and 

 sometimes of pure flint used for millstones. Hence it is by no meaus 

 an easy task to institute an exact comparison between the various 

 members of the English and French series, and to settle their respec- 

 tive ages. It is clear that, on the sites both of Paris and London, a 

 continual change was going on in the fauna and flora by the coming 

 in of new species and the dying out of others ; and contemporaneous 

 changes of geographical conditions were also in progress in conse- 

 quence of the rising and sinking of the land and bottom of the sea. 

 A particular subdivision, therefore, of the time was occasionally repre- 

 sented in one area by land, in another by an estuary, in a third by the 

 sea, and even where the conditions were in both areas of a marine 

 character, there was often shallow water in one, and deep sea in 

 another, producing the want of agreement in the state of animal life. 



But in regard to that division of the Eocene series which we have 

 now under consideration, we find an exception to the general rule, for, 

 whether we study it in the basins of London, Hampshire, or Paris, 

 we recognize everywhere the same mineral character. This uniformity 

 of aspect must be seen in order to be fully appreciated, since the beds 

 consist simply of mottled clays and sand, with lignite and well-rolled 

 flint pebbles, derived from the chalk, and varying in size from that of 

 a pea to an egg. These strata may be seen in the Isle of Wight in 

 contact with the chalk, or in the London basin, at Reading, Black- 

 heath, and Woolwich. In some of the lowest of them, banks of 

 oysters are observed, consisting of Ostrea bellovacina, so common in 

 France in the same relative position, and Ostrea edulina, scarcely dis- 

 tinguishable from the living eatable species. In the same beds at 

 Bromley, Dr. Buckland found one large pebble to which five frill- 

 grown oysters were affixed, in such a manner as to show that they had 

 commenced their first growth upon it, and remained attached to it 

 through life. 



In several places, as at Woolwich on the Thames, at Newhaven in 

 Sussex, and elsewhere, a mixture of marine and freshwater testacea 



* Prestwich, Waterbearing Strata of London, 1851. 



