APPENDIX.
It may not 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 meadows, 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 period 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 appeared to be a considerable quantity of hazel trees; hazel nuts were also found in the mass, but were easily crushed, the kernel being entirely 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
