156 PROF. J. PRESTWICH ON A SOUTHEHX DRIFT IN THE THAMES 



the proportion and condition of those which are common to 

 both. Eor example, the Westleton Shingle is, as before men- 

 tioned, chaiacterized by the constant jDresence of numerous peb- 

 bles of white quartz with a few small pebbles of Lydian Stone, 

 Jasper, and others of old rock origin, and some peculiar larger 

 quartzite-pebbles ; whereas a similar abundance of quartz-pebbles 

 is wanting in the Southern Drift, which, on the other hand, is cha- 

 racterized by a large though variable proportion of worn frag- 

 ments of chert and ragstone — not that these are wanting in the 

 ^^'estleton Shingle, but they are in less abundance, more reduced, and 

 hold a more subordinate position. The materials common to both, 

 but present in different proportions, are subangular flints and flint- 

 pebbles ; while the absence of quartzite-pebbles derived from the 

 Triassic rocks of Central England equally stamps both, and is one 

 of the marks of distinction from the Northern Drift. The Westleton 

 beds also had a marine origin, whilst the other seems to ha\^e been 

 formed in great part under subaerial conditions. 



The following are the component materials of the Southern Drift, 

 taken in the order of their relative abundance, and with the sources 

 whence derived : — 



1. Ang-ular and subangular fragments of flint, some retainiDg 

 their natural colour, others much worn and stained uniformly of a 

 deep warm brown colour. The former are derived directly from 

 the Chalk, and the latter from an older Drift, the location of which 

 is still uncertain (probably Diestian). 2. Pebbles of flint derived 

 from the Lower Tertiary strata of Kent and Surrey, which formerly 

 extended over the Chalk-area more generally and also further 

 southward. 3. Subangular fragments in very variable proportions, 

 and more or less worn, of chert and ragstone, derived from the 

 Lower Greensand of Kent and Surrey. 4. Occasional small pebbles 

 of white quartz (smaller and more opaque usually than those of the 

 Westleton Shingle), derived, together with a few quartzites and 

 some old rock-pebbles, from the Lower Greensand or from the 

 W^ealden. 5. A few subangular fragments of ironstone-grit and 

 ironstone derived either from the Lower Tertiaries, the Diestian, 

 or the Lower Greensand. No organic remains have been found 

 in this Drift, of which the negative characters are the absence of 

 the white and often chalcedonic quartz-pebbles of the Westleton 

 type, of Triassic quartzite-pebbles, and of Jurassic debris. 



2. Distribution of the Southern Drift. 



This we \Vi\\ take, as in the former paper, in the order by counties. 



Kent. — There is a considerable spread of very ferruginous sandy 

 gravel on the Tertiary hills of East Kent, consisting of subangular 

 flints with a small proportion of flint-pebbles. Although fragments 

 of chert arc wanting, or extremely scarce, T think it probable that 

 the gravel on some of the higher grounds, such as Dunstead, the 

 hills west of Canterbury (250 to 300 ft.), and a few other of the 

 higher hills of the district, belongs to this Southern Drift, as it is 



