We shall see later in these pages how united all that line of the
stream has been; how the great monasteries founded upon the Thames
were supported by possessions stretched all along the valleys; how
much of it, and what important parts, were held by the Crown; and how
strong was the architectural influence of towns upon one another up
and down the water, as also how the place names upon the banks are
everywhere drawn from the river; but before dealing with these it is
best to establish the main portions into which the Thames falls and to
see what would naturally be their limits.

It may be said, generally, that every river which is tidal, 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 difficulties are still formidable for