298 E. B. POULTON O'S MAMMALIA]!^ REMAINS AND TREE- 



beds of the London Clay) altered and rearranged by fluviatile agency and 

 interpenetrated by river-gravels and with bones of land Mammalia and tree- 

 trunks. At either end, east and west, of this north face, reconstructed ■ 

 clays intervene between a thick bed of sand and the superimposed gravels. 

 Centrally, however, the clays are absent, and the gravel and sand come into 

 contact. Here, however, in the lower part of the gravel slight indications 

 of the clay may often be found, but no regular layer. To the east end of 

 the section many layers of intermixed clay, sand, and gravel intervene 

 between the gravels and sand, forming the group of beds marked a (1, 2, 

 3, 4) in fig. 1, while the west gravels and sand are separated by only one 

 subdivision, the equivalent of the eastern uppermost layer, and therefore 

 marked a (1). The characters of the reconstructed clays (beds a) are as 

 follows : — 

 (1)2 feet thick, east and west side. Large fragments of mottled clays from 

 the upper part of the Woolwich and Reading beds. These are entirely 

 vmstratified, and there is an irregular intermixture of rounded flints 

 (from waste of Tertiaries) and subangular ones (from chalk). Above, 

 this layer passes into the gravel, with no definite line of demarcation, by 

 the gradual cessation of the clay- masses. Additional proof of the Post- 

 glacial and fluviatile origin of this layer is found in a feAV rounded 

 quartzite masses, from the high-level gravels, which were taken from 

 its lowest part. In one part of this bed on the west side the clay 

 masses more resemble those of (2) layer in being finely broken and 

 slightly laminate. 



(2) 1 foot 6 inches, east side. Finely broken mottled clays, slightly stratified 

 and containing fragments of bivalve shells. The stratification and shell- 

 fragments are entirely quaternarily imposed characters, both completely 

 absent in the Tertiary clay from which the bed was derived. The shells 

 are so fragmentary that identification is impossible ; their general ap- 

 pearance, however, is such as to render probable their origin from the 

 basement beds of the London Clay, which crops out higher up the slope. 

 The last 4 inches of this layer graduate into a yellow sand overlying the 

 white sand (3). Scattered flint pebbles occur thinly throughout the 

 layer. ' ' 



(3) 6 inches thick, east side. Fine white sand. 



(4) 6 inches thick, east side. Coarse fragments of mottled clay, roughly 



stratified, and with white sand between the layers. Gravel is present, 

 especially in the lowest part, overlying the thick bed of sand (b). 



These four beds, above described, tMcken out eastward ; while to 

 the west the lower ones rise to the surface of the sand (5), and they 

 all die out between it and the gravel. Each of the four thins away 

 at this point ; and there is a tendency towards their coalescence or 

 interlamination, especially in (2) and (3). These beds indicate 

 powerful and rapid currents transporting the materials from the 

 Tertiary clays higher up the slope, and cutting away the sands pre- 

 viously deposited by gentler aqueous agency. The lowest bed (4) 

 on the east side, and (1) on the west, overlie the sands quite uncon- 

 formably, the laminae of the latter being cut through very sharply. 

 The currents must have been very powerful to remove masses of the 

 extremely tough and stiff mottled clays. They cannot have been 

 long continued, or the clay masses would not have been angular 

 but would have been further disintegrated and redeposited else- 

 where, losing the readily apparent characters by which their origin 

 is at once seen. The broken lumps are of exactly the same colour 

 and structure as the undisturbed beds higher up the slope. The 



