278 EOCENE PERIOD, [Ch. XX. 



Eocene deposits are chiefly marine, and have generally been 

 divided into three groups: 1st, the Plastic clay and sand, 

 which is the lowest group;' 2dly, the London clay; and, 

 Srdly, the Bagshot sand. Of all these the mineral compo- 

 sition is very simple, for they consist almost entirely of clay, 

 sand, and shingle, the great mass of clay being in the middle, 

 and the upper and lower members of the series being more 

 arenaceous. 



Plastic clay and sand. The lowest formation, which some- 

 times attains a thickness of from 400 to 500 feet, consists prin- 

 cipally of an indefinite number of beds of sand, shingle, clay, 

 and loam, irregularly alternating, some of the clay being used 

 in potteries, in reference to which the name of Plastic clay has 

 been given to the whole formation. The beds of shingle are 

 composed of perfectly rolled chalk flints^ with here and there 

 small pebbles of quartz. Heaps of these materials appear 

 sometimes to have remained for a long time covered by a tran- 

 quil sea. Dr. Buckland mentions that he observed a large 

 pebble in part of this formation at Bromley, to which five full- 

 grown oyster-shells were affixed, in such a manner as to show 

 that they had commenced their first growth upon h } and re- 

 mained attached through life *. 



In some of the associated clays and sand, perfect marine shells 

 are met with, which are of the same species as those of the 

 London clay. The line of separation, indeed, between the 

 superincumbent blue clay last alluded to, and the Plastic clay 

 and sand, is quite arbitrary, as any geologist may be convinced 

 who examines the celebrated section in Alum Bay, in the Isle 

 of Wight f , where a distinct alternation of the two groups is 

 observable, each marked with their most characteristic pecu- 

 liarities. In the midst of the sands of the lower series a mass 

 of clay occurs 200 feet thick, containing septaria, and replete 

 with the usual fossils of the neighbourhood of London J. 



* Geol. Trans., First Series, vol. iv. p. 300. 



J- See Mr. Webster's Memoir, Geol. Trans,, vol. ii., First Series, and his Letters 

 in Sir H. Englefield's Isle of Wight. 

 I See Mr, Webster's sections, plate 11. Geol. Trans., vol. ii., First Series. 



