[Sidenote: Shifting of the inner edge.]

With the recent increase in the size of vessels, two contrary effects
are noticed. In the vast majority of cases, the inner edge, as marked by
ports, moves seaward into deeper water, and the zone narrows. The days
when almost every tobacco plantation in tidewater Virginia had its own
wharf are long since past, and the leaf is now exported by way of
Norfolk and Baltimore. Seville has lost practically all its sea trade to
Cadiz, Rouen to Havre, and Dordrecht to Rotterdam. In other cases the
zone preserves its original width by the creation of secondary ports on
or near the outer edge, reserved only for the largest vessels, while the
inner harbor, by dredging its channel, improves its communication with
the sea. Thus arises the phenomenon of twin ports like Bremen and
Bremerhaven, Dantzig and Neufahrwasser, Stettin and Swinemuende, Bordeaux
and Pauillac, London and Tilbury. Or the original harbor seeks to
preserve its advantage by canalizing the shallow approach by river,
lagoon, or bay, as St. Petersburg by the Pantiloff canal through the
shallow reaches of Kronstadt Bay; or Koenigsberg by its ship canal,
carried for 25 miles across the Frisches Haff to the Baltic;[417] or
Nantes by the Loire ship canal, which in 1892 was built to regain for
the old town the West Indian trade recently intercepted by the rising
outer port of St. Nazaire, at the mouth of the Loire estuary.[418] In
northern latitudes, however, the outer ports on enclosed sea basins like
the Baltic become dominant in the winter, when the inner ports are
ice-bound. Otherwise the outer port sinks with every improvement in the
channel between the inner port and the sea. Hamburg has so constantly
deepened the Elbe passage that its outport of Cuxhaven has had little
chance to rise, and serves only as an emergency harbor; while on the
Weser, maritime leadership has oscillated between Bremen and
Bremerhaven.[419] So the whole German coast and the Russian Baltic have
seen a more or less irregular shifting backward and forward of maritime
importance between the inner and the outer edges.

[Sidenote: Artificial extension of inner edge.]

The width of the coast zone is not only prevented from contracting by
dredging and canaling, but it is even increased. By deepening the
channel, the chief port of the St. Lawrence River has been removed from
Quebec 180 miles upstream to Montreal, and that of the Clyde from Port
Glasgow 16