Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/310

 may be seen at Sundridge Park, near Bromley, in Kent, where we find an immense deposit of shells, peculiar to the plastic clay formation, accumulated confusedly in a bed of loose sand and pebbles. Of these shells some are broken and others entire, and delicately preserved. They are also sometimes fixed together by a calcareous cement (derived apparently from the substance of the shells themselves) forming a hard breccia with the siliceous pebbles and sand in which they are imbedded. A similar breccia was sunk into in the workings of the Redriffe tunnel.

I have from this bed at Bromley a specimen, in which five oyster shells are so affixed to the opposite sides of a large kidney-shaped pebble, that they seem to have commenced their first growth on it, and to have been attached to it through life, without injury by friction from the neighbouring pebbles. We cannot but infer then that these pebbles received their form during a long period of agitation, which was succeeded by a period of repose; in which latter they were in a state of sufficient tranquillity for the shells in question to live and die undisturbed in the midst of them.

The enormous quantity of these completely rolled and rounded chalk flint pebbles It may be observed of these pebbles occurring in the plastic clay formation, that they are never calcareous, but composed almost entirely of oval or roundish and rather flat chalk flints, completely rolled down and slightly altered, sometimes to the centre, by decomposition; which beginning from without has produced, in some cases, a number of concentric zones, disposed in agate like rings, nearly parallel to the outer surface of the pebble, and resembling an agate in colour though inferior in purity. The fact that in these pebbles we occasionally find fragments of organic remains peculiar to the chalk formation, shews that they were not formed like agates in empty cavities. And the do composition of their iron commencing from the outer surface, is fully adequate to produce the concentric structure which they present; as may be seen in similar concentric zones resulting from the same cause in pebbles of sandstone, and many other rocks, of which the substance is compact and tolerably uniform in texture.

More frequently the pebbles are clouded with tints of red and yellow, presenting an indefinite variety of beautiful modifications, and assuming the irregular arrangement of the colours in an Egyptian pebble. The finest varieties of these colours are displayed to the best advantage in polished specimens of the Hertfordshire pudding-stone, so common in cabinets and ornamental jewellery. The pebbles of this pudding-stone appear to be no other than altered chalk flints of the same era with those found at Blackheath, and differing only in the accident of their being firmly united by a strong siliceous cement. Many of the purest varieties of the Blackheath pebbles if polished, are exactly similar to those of the Hertfordshire pudding-stone.

Large blocks of a coarse variety of the same siliceous pudding-stone are not uncommon on the surface of the chalk in the south of England. I have seen them at Bradenham, near High Wycombe, at Nettlebed, at Portesham, near Abbotsbury, and in Devonshire, lying insulated on the bare chalk. They have not yet I believe been found imbedded in their native stratum, which seems to have been destroyed extensively above the English chalk, and to have been a member of that series of irregular alternations of beds of clay, sand, and gravel, either separate or mixed together, which for reasons already stated, has been designated by the appellation of the plastic clay formation. that occur in the English plastic clay formation