ndary into the form of a common district, as in the
vast, disputed Oregon country, accepted provisionally as a district of
joint occupancy between the United States and Canada from 1818 to 1846,
or that wide highland border which Norway so long shared with Russia and
Sweden. In South America, where land is abundant and population sparse,
this common boundary belt is not rare. It suggests a device giving that
leeway for expansion desired by all growing states. By the treaty of
1866, the frontier between Chile and Bolivia crossed the Atacama desert
at 24 deg. South Latitude; but the zone between 23 deg. and 25 deg. was left under
the common jurisdiction of the two states, for exploitation of the guano
deposits and mineral wealth.[362] A common border district on a much
larger scale is found between Brazil and the eastern frontier of French
Guiana. It includes a belt 185 miles (300 kilometers) wide between the
Oyapok and Arawary rivers, and is left as a neutral district till its
fate is decided by arbitration.[363] All these instances are only
temporary phases in the evolution of a political frontier from wide,
neutral border to the mathematically determined boundary line required
by modern civilized states.

[Sidenote: Tariff free zones.]

Even when the boundary line has been surveyed and the boundary pillars
set up, the frontier is prone to assert its old zonal nature, simply
because it marks the limits of human movements. Rarely, for instance,
does a customs boundary coincide with a political frontier, even in the
most advanced states of Europe, except on the coasts. The student of
Baedecker finds a gap of several miles on the same railroad between the
customs frontier of Germany and France, or France and Italy. Where the
border district is formed by a high and rugged mountain range, the
custom houses recede farther and farther from the common political line
upon the ridge, and drop down the slope to convenient points, leaving
between them a wide neutral tariff zone, like that in Haute Savoie along
the massive Mont Blanc Range between France and Italy.

Allied to this phase, yet differing from it, is the "Zona Libre" or Free
Zone, 12 miles broad and 1,833 miles long, which forms the northern hem
of Mexico from the Gulf to the Pacific. Here foreign goods pay only
18-1/2 per cent., formerly only 2-1/2 per cent., of the usual federal
duties. Goods going on into the interior pay the rest of the tariff at
the inner margin