Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/738

536 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 separating the two gravels, are usually full of root-fibres. Where, as in 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 reel 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 overlappingboth 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 consequence of its porosity.

The occurrence of white gravel over red in the south-west of Sussex is noticed by Mr. Martin, and by Mr. Godwin-Austen , both gentlemen considering the white gravel to be the newer, and a distinct 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-cutting 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 sections, it is seen to be much more irregular than the surface. There are instances of this in the railway-cutting hear 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 neighbourhood is about 10 feet. In the Barton and Hordwell Cliffs the gravel 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