258 
ELEMENTS OF GEOLOGY. 
drawn between Miocene and Eocene. But if we were to at¬ 
tach importance to such occasional passages, >we should soon 
find that no lines of division could be drawn anywhere, for 
in the present state of our knowledge of the Tertiary series 
there will always be species common to beds above and be¬ 
low our boundary-lines. 
Barton Series {Sands and Clays)^ A. 4, Table, p. 252.— 
Both in the Isle of Wight, and in Hordwell Cliff, Plants, the 
Headon beds, above-mentioned, rest on white sands usually 
devoid of fossils, and used in the Isle of Wight for making 
glass. In one of these sands Dr. Wright found Chama squa¬ 
mosa^ a Barton Clay shell, in great plenty, and. certain im¬ 
pressions of marine shells have been found in 
sands supposed to be of the same age in White- 
cliff Bay. These sands have been called Upper 
Bagshot in the maps of our Government Survey, 
but this identification of a fossiliferous series in 
Fig. 181. 
the Isle of Wight with an unfossiliferous forma¬ 
tion in the London Basin can scarcely be de- 
pended upon. The Barton Clay, which imme- 
«a,Eichvv. Bar- diately Underlies these sands, is seen vertical in 
Alum Bay, Isle of Wight, and nearly horizontal 
in the cliffs of the mainland near Lymington. This clay, to¬ 
gether with the Bracklesham beds, presently to be described, 
has been termed Middle Bagshot by the Survey. In Barton 
Cliff, where it attains a thickness of about 300 feet, it is rich 
in marine fossils. 
It was formerly confounded with the London Clay, an old¬ 
er Eocene deposit of very similar mineral character, to be 
mentioned below, p. 263, which contains many shells in com¬ 
mon, but not more than one-fourth of the whole. In other 
words, there are known at present 247 species in the London 
Clay and 321 in that of Barton, and only 70 common to the 
two formations. Fifty-six of these have been found in the 
intermediate Bracklesham beds, and the reappearance of the 
other 14 may imply a return of similar conditions, whether 
of temperature or depth or of a muddy argillaceous bottom, 
common to the two periods of the London and Barton Clays. 
According to M. Hebert, the most characteristic Barton Clay 
fossils correspond to those of the Gres de Beauchamp, or Sa¬ 
bles Moyens, of the Paris Basin, but it also contains many 
common to the older Calcaire Grossier. 
SHELLS OF THE BAETON CLAY. 
Certain foraminifera called Nummulites begin, when we 
study the Tertiary formations in a descending order, to make 
