536 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 8, 
origin is due to the bleaching-action of water holding organic matters 
in solution upon the red peroxide of iron, by reducing it to a soluble 
protoxide in the way pointed out by Bischoff*. The pipes of white 
gravel penetrating the red, and the black carbonaceous band separat- 
ing the two gravels, are usually full of root-fibres. Where, asin some 
sections, the white gravel seems to be interstratified in the red, it is 
probable that an unseen pipe communicates with a more sandy and 
porous seam, which has been bleached, while the less porous gravel 
over it has been unaffected. In some instances, however, it appears 
as if the white gravel were a subsequent deposit. In a section in a 
brick-yard to the east of Lymington the red gravel seems to have 
been cut away before the white was deposited on its flank. In the 
chines or bunnies to the west of Bournemouth there are sections at 
right angles to the coast showing the red gravel with brick-earth 
over it, ending against a slope of Bagshot sand on the land side 
and thinning out towards the sea, so that the white gravel which 
overlies both overlaps the brick-earth, and rests directly on the red 
gravel, reduced to less than half its thickness in the sea face of the 
cliff. In the east side of Allum Chine the white gravel is seen over- 
lapping both brick-earth and red gravel, and resting directly on the 
Bagshot sand. These appearances are not, however, inconsistent 
with the supposition that the white gravel is contemporaneous with 
the red, and has been bleached in the way above noticed in conse- 
quence of its porosity. 
The occurrence of white gravel over red in the south-west of 
Sussex is noticed by Mr. Martiny, and by Mr. Godwin-Austen +, both 
gentlemen considering the white gravel to be the newer, and a di- 
stinct deposit from the red gravel. 
The gravel varies much in thickness. On the plains at high levels 
sections are rare, but the depth does not generally appear to exceed 
5 or 6 feet. On the edges of the plains bordering on the valleys, 
and on the terraces which occur, a little below the level of the plains 
in the larger valleys, the thickness is greater. In the railway-cut- 
ting one mile south of Wimborne, through a plain 176 feet above the 
sea, the gravel is as thick as from 25 to 30 feet. Where the junction 
of the gravel with the underlying formation is exposed in long sec- 
tions, it is seen to be much more irregular than the surface. ‘There 
are instances of this in the railway-cutting near Wimborne, and in 
that through the level plain between Christchurch and Bournemouth, 
where the gravel varies from 2 to 12 feet in thickness in 20 yards. 
In the cliff-sections similar variations are observable; but the 
average thickness in them and in the gravel-pits in the neighbour- 
hood is about 10 feet. In the Barton and Hordwell Cliffs the gra- 
vel is 15 or 18 feet thick, but it thins out, as before noticed, to 8 or 
9 feet in pits near the coast, and to 5 or 6 feet more inland. On the 
plains about Beaulieu, and on the eastern side of Southampton Water, 
6 or 7 feet appears to be the average thickness, while at low levels, 
as in the cliff-section between Southampton Water and Gosport, it is 
* Chemical and Physical Geology, vol. i. p. 166. 
t+ Quart. Journ. Geol. Soe. vol. xii. p. 136, ¢ Ibid, vol. xiii. p, 48. 
