and whose stream is so slow as to be easily navigable in either direction, divides itself naturally, when one is regarding it as a means of communication, into three main divisions.
There will first of all be the tidal portion which the tide usually scours into an estuary. As a general rule, this portion is not considerably inhabited in the early periods of history, for it is not until a large international commerce arises that vessels have much occasion to stop as they pass up and down the maritime part of the stream; and even so, settlements upon its banks must come comparatively late in the development of the history of the river, because a landing upon such flooded banks is not easily to be effected.
This is true of the Dutch marshes at the mouths of the Rhine, whose civilisation (one exclusively due to the energy of man) came centuries after the establishment of the great Roman towns of the Rhine; it is true of the estuary of the Seine, whose principal harbour of Havre is almost modern, and whose dificulties are still formidable for ocean-going craft; and it is true of the Thames.
The estuary of the Thames plays little or no part in the very early history of England. Invaders, when they landed, landed on the sea-coast at the very mouth, or appear to have sailed right up into the heart of the country.
It is, nevertheless, true that the last few miles of tidal water,
