actual phenomenon is a coast zone,
alternately covered and abandoned by the waters, varying in width from a
few inches to a few miles, according to the slope of the land, the range
of the tide and the direction of the wind. It has one breadth at the
minimum or neap tide, but increases often two or three fold at spring
tide, when the distance between ebb and flood is at its maximum. At the
mouth of Cook's Inlet on the southern Alaskan coast, where the range of
tides is only eight feet, the zone is comparatively narrow, but widens
rapidly towards the head of the inlet, where the tide rises twenty-three
feet above the ebb line, and even to sixty-five feet under the influence
of a heavy southwest storm. On flat coasts we are familiar with the wide
frontier of salt marshes, that witness the border warfare of land and
sea, alternate invasion and retreat. In low-shored estuaries like those
of northern Brittany and northwestern Alaska, this amphibian girdle of
the land expands to a width of four miles, while on precipitous coasts
of tideless sea basins it contracts to a few inches. Hence this boundary
zone changes with every impulse of the mobile sea and with every varying
configuration of the shore. Movement and external conditions are the
factors in its creation. They make something that is only partially akin
to the two contiguous forms. Here on their outer margins land and ocean
compromise their physical differences, and this by a law which runs
through animate and inanimate nature. Wherever one body moves in
constant contact with another, it is subjected to modifying influences
which differentiate its periphery from its interior, lend it a
transitional character, make of it a penumbra between light and shadow.
The modifying process goes on persistently with varying force, and
creates a shifting, changing border zone which, from its nature, cannot
be delimited. For convenience' sake, we adopt the abstraction of a
boundary line; but the reality behind this abstraction is the important
thing in anthropo-geography.

[Sidenote: Gradations in the boundary zone.]

All so-called boundary lines with which geography has to do have this
same character,--coastlines, river margins, ice or snow lines, limits of
vegetation, boundaries of races or religions or civilizations, frontiers
of states. They are all the same, stamped by the eternal flux of nature.
Beyond the solid ice-pack which surrounds the North Pole is a wide
girdle of almos