278 
EOCENE PERIOD. 
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, 
3rdly, 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 lame 
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 it, 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 |. 
* Geol. Trans., First Series, vol. iv. p. 300. 
f See Mr. Webster's Memoir, Geol. Trans., vol. ii., First Series, and his Letters 
in Sir H. Englefield's Isle of Wight. 
+ See Mr. Webster's sections, plate 11. Geol. Traus., vol, ii., First Series. 
