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



APPENDIX. 



It may riot be uninteresting to insert here the following notices, 

 illustrative of the formation of the marsh lands immediately below 

 London, which, though not directly connected with our subject, the 

 plastic clay, yet forms a prominent feature in the physical history of 

 that part of the neighbourhood of the metropolis which we have 

 been describing. 



It is well known that at this time the waters of the Thames from 

 London to the sea are upheld by dykes or sea walls. Within these 

 dykes the river by its daily sediment of mud has so raised its bed, 

 that even in ordinary tides the water is above the level of the mea- 

 dows, as far up as Woolwich and the Isle of Dogs. This elevation 

 of its bed is precisely analogous to what has happened near the 

 mouths of the Po, the Rhine, and other large rivers, which have 

 been upheld for many centuries by embankments. The following 

 facts tend to illustrate the process that was going on before the pe- 

 riod at which these embankments were made. 



In the account given by Capt. J. Perry, about 100 years ago, of the 

 stopping of the breach made in the sea wall at Dagenham, about twelve 

 miles below London, that able engineer particularly describes what he 

 calls moor log. This, he says, was composed of vegetable matter 

 heaped together, but chiefly of brushwood, among which there ap- 

 peared to be a considerable quantity of hazel trees ; hazel nuts were 

 also found in the mass, but were easily crushed, the kernel being en- 

 tirely perished. There were also trunks of other trees, of which the 

 yews were the least decayed ; some of them measured 15 or 16 

 inches in diameter. There were also willows two feet and upwards 



