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Show 278 :EOCENE PERIOD· [Ch; XX. Eocene deposits are chiefly marine, and have generally been divided into three groups: lst, the Plastic clay and sand, which is the lowest group;" 2dly, the London clay; and, 3rdly, the Bagshot sand. Of all these the mineral compoJ 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 sometimes attains a thickness of from 400 to 500 feet, consists principally 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 tranquil sea. Dr. Buckland mentions that he observed a large pebble in part of this formation at Bromley, to which five fullgrown oyster-shells were affixed, in such a manner as to show that they had commenced their £rst growth upon it, and remained 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 t, 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 t· * Geol. Trans., First Series, vol. iv. p. 300. . t See Mr. Webster's Memoir, Geol. Trans., vol. ii., First Series, and h1s Letters in Sir H. Englefit!ld's Isle of Wight. l See Mr. Webster's sections, plate 11. Geol. Trans., vol. ii., First Series. Ch.XX.J LONDON CLAY; 279 The arenaceous beds are chiefly laid open on the connnes of the basins of London and Hampshire, in following which we discover at many places great beds of perfectly rounded flints. Of this description, on the southern borders of the basin of London, are the hills of Comb Hurst and the Addinuton hills 0 , which form a ridge stretching from Blackheath to Croydon. Here they have much the appearance of banks of sand and shingle formed near the shores of the tertiary sea; but whether they were really of littoral origin cannot be determined for want of a sufficient number of sections which might enable us to compare the tertiary strata at the edges with those in the central parts of each basin. We have ample opportunities in the basin of Paris of examining steep cliffs of hard rock which bound many of the valleys, and innumerable excavations have been made for building-stone, limestone, and gypsum; but when we attempt to obtain a connected view of any considerable part of the tertiary series in the basin of London, we are almost entirely limited to a single line of coast-section; for in the interior the regular beds are much concealed by an alluvial covering of flint gravel spread alike over the summits and gentle slopes of the hills, and over the bottoms of the valleys. Organic remains are extremely scarce in the Plastic clay ; but when any shells occur they are of Eocene species. Vege. table impressions and fossil wood sometimes occur, and even beds of lignite, but none of the species of plants have, we believe, as yet been ascertained. London clay.-This formation consists of a bluish or blackish. clay, sometimes ?assing ~nto a calcareous marl, rarely into a sohd rock. Its thickness Is very great, sometimes exceedino- 500 feet*· It contains many layers of ovate or flattish masse: of argillac~ous li.mesto~e, w.hich, in their interior, are generally traversed m Yarwus d1rectwns by cracks, partially or wholly filled .by calcar.eous spar. These masses, called septaria, are sometimes contmued through a thickness of 200 feet t. * Con. and Phil, Outlines of Geo1.1 P• 33. t Outlines of Geol., p. 27. |