The Rev. W BucklAnd o» /Aif Plastic Clay Formation. 301 



on the south of London, corroborate the arguments adduced by M. M. 

 Cuvier and Brongniart, from the irregular projections and furrowed 

 surface of the French chalk, and from the fragments of chalk forming 

 a breccia with the plastic clay at Meudon, to prove the consolida- 

 tion of the chalk to have been completed before that partial de- 

 struction of its upper strata by the force of water, to which they 

 justly attribute these furrows and the Meudon breccia. These 

 English beds of chalk flint pebbles (the wreck of strata thus destroyed) 

 afford additional evidence of the immense scale on which this aque- 

 ous destruction was carried on, and confirm also the conjecture (which 

 by them is chiefly grounded on the total difference of the organic 

 remains in the two formations) that a long period of time has pro- 

 bably intervened between the deposition of the chalk and the plastic 

 clay. 



More frequently the pebbles aro clouded with tints of red and yellow, presenting art 

 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 differ- 

 ing 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 uncom- 

 mon on the surface of the chalk in the south of England. I have seen them at Braden- 

 ham, near High Wycombe, at Nettlebed, at Portesham, near Abbotsbury, and in Devon- 

 shire, lying insulated on the bare chalk. They have not yet I believe been found im- 

 bedded 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. 



Vol. iv. 2 q 



