300 The Rev. W. Buckland on the Plastic Clay Formation. 



may be seen at Sundridge Park, near Bromley, in Kent, where we 

 find an immense deposit of shells, peculiar to the plastic clay for- 

 mation, accumulated confusedly in a bed of loose sand and pebbles. 

 Of these shells some are broken and others entire, and delicately pre- 

 served. They are aslo sometimes fixed together by a calcareous 

 cement (derived apparently from the substance of the shells them- 

 selves) 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 fric- 

 tion 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 * that occur in the English plastic clay formation 



* 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 agaie 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 Ave occasionally find fragments of organic remains peculiar to the chalk 

 formation, shews that they were not formed like agates in empty cavities. And the de- 

 composition of their iron commencing from the outer surface, is fully adequate to pro- 

 duce 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 aud tolerably uniform in texture. 



