1850.] PRESTWICH ON THE LOWER TERTIARY STRATA. 255 



equally distinct, exhibiting a strong massive clay, reposing abruptly 

 and without passage on a thick series of yellow and ash-coloured 

 sands, with many subordinate beds of pebbles, and a few laminated 

 clays, and containing a varied and irregular fauna. This series 

 seems as independent of the London clay as do the sands and mot- 

 tled clays to the westward of London ; but it is a question whether 

 the thin basement conglomerate bed, which in the latter district merges 

 into the London clay, and has a character so entirely distinct from 

 that of the underlying beds, does not, as it trends eastward, assume a 

 lithological structure more entirely different from that of the London 

 clay and not passing into it, but, on the contrary, assimilating so 

 closely to the underlying sandy series, that in general appearance 

 it seems an upper and subordinate member thereof. (See Sections 

 10 & 1 1.) 1 believe, however, this bed to be part rather of the Lon- 

 don clay than of the so-called plastic clays with which it has been 

 grouped. But although belonging to the former rather than to the 

 latter, yet it forms in some of its characters a stratum separable from 

 both. I purpose therefore to describe this bed ("c" of the Sections) 

 before proceeding to an examination of the underlying deposits. 

 This will follow in natural order the general description I have before 

 given of the London clay ; and as this will of itself constitute a sub- 

 ject of some extent, which it is important to our argument to examine 

 in detail, I will subdivide the general question under different heads, 

 and begin with this first division of it. 



1st Division. On the Basement bed of the London Clay ("c"). 



In the fine section of Alum Bay there may be seen, at a distance of 

 about ninety-four feet from the chalk, a thin and insignificant layer, 

 not a foot thick, of rather large (egg-sized), rounded, black flint peb- 

 bles imbedded in a scanty yellow sand and brown clay, and separating 

 the important mass of the London clay on the one side from that of 

 the mottled clays on the other, and to the former of which it forms the 

 base*. This pebble-band contains a few organic remains, of which 

 the most common are the teeth of a species of Lamna ; the Ditrupa 

 plana also occurs, and traces of several species of shells. It reposes 

 upon a somewhat uneven and worn surface of the underlying stratum. 

 Small and unimportant, however, as this bed here is, it is nevertheless 

 remarkable for the extent of its range, the uniformity of its litholo- 

 gical characters, and the permanence of its organic remains, — con- 

 ditions of the more value from its position between the two main mem- 

 bers of the Eocene series. It forms an excellent base-line, and its 

 characters are so well marked, that it can be traced without much 

 difficulty from the Isle of Wight to Woodbridge in Suffolk, a di- 

 stance in a straight line of above 160 miles from S.W. to N.E. 



In a former paper, when discussing the connection of the London 



* See Section of Alum Bay, Quart. Journ. Geol. Soe. vol. ii. pi. 9. %. 1. I have 

 since applied the term "London Clay" to the division there called " Bognor 

 beds," and have abandoned it for the strata higher in the series (see Quart. Journ. 

 Geol. Soc. vol. iii. p. 355). 



VOL. YI. PART I. U 



