Kj-i: PROP. J. PRESTNYICII OX A SOUTHERN DRIPT IN THE THAMES 



with a height of from 300 to 350 feet. These hills consist of London 

 Clay, with remnants of Bagshot Sands on the summits of some of 

 them. The Boulder-clay wraps round the east end of this range at 

 a level of about 150 feet below its summit, but on the northern 

 slope it rises much higher. The Pebble -beds which cap the hills 

 at Writtlepark, Beggarhill, Mill Green, Fryerning, and Norton 

 Heath seem to be little else than Bagshot Shingle more or less 

 disturbed and mixed with a few subangular flints, quartzite-pebbles, 

 and pieces of quartz. 



A more conspicuous range of hills is that which extends a few 

 miles further to the south-west, from Brentwood to Warley and 

 Billericay (see PI. VII. lig' 4). On Brentwood Common (352 feet) 

 the Pebble-bed is thin and very much disturbed, as shown in the 

 annexed section : — 



Fig. 3. — Section of disturbed Pebble-hed, toj) of Railway-cutting, 

 (luest end) Brentwood Common. 



a. Grey clay, with flints and pebbles i 



b. Pebke-bed disturbed Uto4ft. 



<?. Light-coloured sands J 



The railway-cutting exposed other features in connexion with the 

 Bagshot Beds, which require some notice. TheWarley Pebble-bed does 

 not extend so far as the cutting, or rather it is there hidden by a 

 bed of clay, which displaces the Bagshot strata, and soon acquires 

 a thickness of 30 feet. As it increases in thickness its true 

 Boulder-clay character is apparent ; it becomes darker, and contains 

 imbedded large subangular flints, flint- and chalk-pebbles, pebbles of 

 sandstone, and fragments of Jurassic rocks, with specimens of 

 Gryphcea incurva and a large Oyster. As this Clay trends further 

 eastward, it passes into a sandy brown and bluish clay with only a 

 few pebbles, and finally thins out as a greenish clay or marl (see 

 A, PL YII. fig. 5, and p. 152). 



At the base of this Clay is an irregular bed of sand and gravel, 

 thickening as the overlying clay thins out, and consisting chiefly 

 of flint-pebbles, while the way in which the Boulder-clay has 

 mounted the hill and displaced the strata of the Bagshot Beds de- 

 serves notice. 



The Bagshot Beds themselves, which are here 40 feet thick, will 

 be found described in the explanation of the section (PI. YII. fig. 5, 

 p. 152). There are no Pebble-beds here in sita. They were 



