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20 would serve no useful purpose and would be merely tedious. We need only say that the floor of Eocene or Cretaceous strata on which these ancient subaerial deposits rest is constantly found at depths of 50 or 60 feet below the level of the existing salt-marsh. But where, as in the estuary of the Thames and Humber, an older channel underlies a modern channel, the floor sinks about 30 feet lower. From present marsh-level to ancient marsh-level is about 60 feet; from present river-bottom to old river-bottom is also about 60 feet. This, therefore, is the extent of the former elevation, unless we can prove that the sea was then so far away that the river once had many miles to flow before reaching it. This is the point we have now to consider as we trace the submerged forests northward and towards the deeper seas.

Before we leave the southern part of the North Sea basin it will be well to draw attention to a few of the half-tide exposures which for one reason or another may tend to mislead the observer. The mere occurrence of roots below tide marks is not sufficient to prove that the land-surfaces seen are all of one date.

Not far from Tilbury is found the well-known geological hunting ground of Grays, where the brickyards have yielded numerous extinct mammalia and several land and freshwater shells now extinct in Britain. These deposits lie in an old channel of the