162 PROF. J. PEESTWICH ON A SOUTHERN DRIFT IN THE THAMES 



Common), and continues past Silchester, Tadley, and Brimpton 

 Commons, to Greenham Common near jSFewbury. This gravel, which 

 is sandy, roughly stratified, and weathers white, consists essentially 

 of subangular iiints with Tertiary flint- pebbles, and some much- 

 worn fragments of a very fine-grained variety of Sarsen stone. 

 There is also a certain number of small blocks of Sarsen stone, or so- 

 called Druid Sandstone, scattered over the district ; and one well- 

 known large block (the Fymph Stone) of hard Lower Chalk lies on 

 Silchester Common. Though there is in this gravel an absence or 

 extreme rarity of Lower-Greensand debris, I think, from the level 

 of these plateaux (330 feet on Silchester Common to 402 feet near 

 Newbury), and from its relation to the valley-gravels, that it 

 belongs to the period of the Southern Drift. The Wealden anticlinal 

 did not bring the Lower Greensand to the surface to the south 

 of this district ; it only raised the Chalk. 



Wiltshire. — Westward of this point there are few traces of this 

 flint-gravel. I have found it capping Upper Kirby Green hill (573 

 feet), near Inkpen. A little also occurs on the high ground of Mar- 

 tinslee Hill, near Marlborough ; but I have not recognized it further 

 westward in this direction, though traces of it extend over some of 

 the Oolitic hills. 



. 3. Other P re-Glacial Hill- Gravels : the Warley and Brentwood 



Groivps. 



Besides the two distinctive Pre-Glacial Drifts here described, 

 there are two others not so easily assigned to a definite horizon, 

 although there can be no doubt of their Pre-Glacial age. One has 

 many of the characters of the Southern Drift ; and the other, which 

 is better known, has the essential characters of a Bagshot pebble- 

 bed *. 



The first of these groups caps Langdon and Eayleigh Hills in South 

 Essex, and Hampstead and Highgate Hills in Middlesex. They 

 are so few in number and are so nearly on the same plane as the 

 other two groups, with which they possess many characters in com- 

 mon, that I can scarcely doubt that they belong to one or the other 

 of them. 



Rayleigh. — This range of hills, which is a little to the north-west 

 of Southend, and rises to the height of 250 or 260 feet, is capped by 

 a thin gravel consisting of : — 1. Numerous flint-pebbles. 2. Some 

 white subangular fragments of flint. 3. A very few small white 

 quartz-pebbles. 4. Fragments of Tertiary Sandstone (one piece 

 measured 18 x 9 inches). 5. Subangular fragments of brown chert 



* See Mr. S. V. Wood, juur., "On the Pebble-beds of Middlesex, Essex, and 

 Herts," Quart. Journ. Geol. Soc. vol. xxiv. (1868), p. 464; Mr. W. Whitaker's 

 " Geology of the London Basin," Mem. Geol. Survey, vol. iv. (1872), pp. 320-328 ; 

 and ' Guide to the Geology of London,' 5th edit. 1884, p. ,55 ; and Messrs. H. 

 W. Monckton and R. S. Herries, " On some Bagshot Pebble-beds and Pebble 

 Gravel," Proc. Geol. Assoc, vol. ix. (1889), p. 1. 



