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VIII] ancient gravels, solid rock, or boulder clay, not on beds of silt.

We cannot speak confidently as to the time needed to form each thin layer of vegetable soil, marsh peat, or estuarine silt. On comparing the submerged land-surfaces, however, with similar accumulations formed within known periods, such as marsh soils grown behind ancient embankments, or forest-growth over flats silted up at known dates, we can learn something. No one of the land-surfaces alternating with the silts would necessarily require more than a century or two for its formation. Brushwood and swamp growth are the characteristic features of these deposits, and such growth accumulates and decays very rapidly. Possibly trees of older growth may still be found, but I have not succeeded in discovering a tree more than a century old in any one of the marsh deposits alternating with the estuarine silts. Oaks of three centuries may be observed rooted in the older deposits; but this, as above explained, is another matter.

It is useless to pretend to any exact calculations as to the time needed for the formation of these alternating strata of estuarine silt and marsh-soil; but looking at the whole of the evidence without bias either way, it seems that an allowance of 1000, or at most 1500, years would be ample time to allow. A period of 1500 years may therefore be taken to cover