252 COURSE OF PEBBLES FROM OXFORD TO LONDON. 
with the gravelly wreck of the neighbouring hills, in each successive 
district along the line of the Thames, from the vale of Oxford down¬ 
wards to the gravel-beds of London, their quantity decreasing with 
the distance from their source; so that in Hyde-Park, and the 
Kensington gravel-pits, they are less abundant than at Oxford. 
I have seen them on the summit of the chalk hills round Henley, 
Maidenhead, High Wycomb, and Eeaconfield. They have been 
noticed also by Lord Grenville in his park at Dropmore, and in 
the gravel-pits at Burnham; in all these last-named places the 
great mass of the gravel is composed of imperfectly rolled flints 
derived from the neighbouring chalk. They are found also mixed 
with chalk-flints, and slightly rounded oolitic gravel, in the valley of 
the Cherwell, and the plains adjacent to it, from its source at 
Claydon and Cherwelton, to Banbury and Oxford, e. g. at Steeple 
Aston, Heyford, Bowsham, Kirtlington, and Kidlington. At Abing¬ 
don, they occur not only in the gravel-beds of the valley, but are 
scattered loosely over the plains composed of various strata around 
that town, as well as on the low hills round Kewnham, Dorchester, and 
Wallingford. Among these pebbles, especially at Abingdon and in 
Bagley Wood, there are many of porphyritic green-stone and green¬ 
stone slate, which cannot have come from any nearer source than 
Charnwood Forest, in Leicestershire. 
The occurrence of quartzose pebbles in such high situations as 
the top of Henley Hill and Cumnor Hill, and again on the highest 
summits of Witchwood Forest, and generally on the elevated plains 
that flank the valleys of the Evenlode, the Cherwell, and the Thames, 
(see Plate XXVII.) goes far to prove the recent origin of the valleys 
