_London clay proper_ (3. _a_, Table, p. 197.).--This formation underlies the preceding, and consists of tenacious brown and blueish grey clay, with layers of concretions called septaria, which abound chiefly in the brown clay, and are obtained in sufficient numbers from the cliffs near Harwich, and from shoals of the Essex coast, to be used for making Roman cement. The principal localities of fossils in the London clay are Highgate Hill, near London, the island of Sheppey, and Bognor in Hampshire. Out of 133 fossil shells, Mr. Prestwich found only 20 to be common to the calcaire grossier (from which 600 species have been obtained), while 33 are common to the lits coquilliers (p. 196.), in which only 200 species are known in France. We may presume, therefore, that the London clay proper is older than the calcaire grossier. This may perhaps remove a difficulty which M. Adolphe Brongniart has experienced when comparing the Eocene Flora of the neighbourhoods of London and Paris. The fossil species of the island of Sheppey, he observes, indicate a much more tropical climate than the Eocene Flora of France, which has been derived principally from the "gypseous series." The latter resembles the vegetation of the borders of the Mediterranean rather than that of an equatorial region. Mr. Bowerbank, in a valuable publication on the fossil fruits and seeds of the island of Sheppey, near London, has described no less than thirteen fruits of palms of the recent type _Nipa_, now only found in the Molucca and Philippine islands. (See fig. 175.) These plants are allied to the cocoa-nut tribe on the one side, and on the other to the _Pandanus_, or screw-pine. Species of cocoa-nuts are also met with, and other kinds of palms; also three species of _Anona_, or custard-apple; cucurbitaceous fruits, also (the gourd and melon family), are in considerable abundance. Fruits of various species of _Acacia_ are in profusion; and, although less decidedly tropical, imply a warm climate. [Illustration: Fig. 175. _Nipadites ellipticus._ Bow. Fossil palm of Sheppey.]