With the exception of the two isolated groups of the Thian Shan and
the Hawaiian Islands, nearly all the active volcanoes of the globe are
situated near the limits which separate the great land-and-water-masses
of the globe--that is to say, they occur either on the parts of
continents not far removed from their coast-lines, or on islands in
the ocean not very far distant from the shores. The fact of the general
proximity of volcanoes to the sea is one which has frequently been
pointed out by geographers, and may now be regarded as being thoroughly
established.
VOLCANOES PARALLEL TO MOUNTAIN CHAINS
Many of the grandest mountain-chains have bands of volcanoes
lying parallel to them. This is strikingly exhibited by the great
mountain-masses which lie on the western side of the American continent.
The Rocky Mountains and the Andes consist of folded and crumpled masses
of altered strata which, by the action of denuding forces, have been
carved into series of ridges and summits. At many points, however, along
the sides of these great chains we find that fissures have been opened
and lines of volcanoes formed, from which enormous quantities of lava
have flowed and covered great tracts of country.
This is especially marked in the Snake River plain of Idaho, in the
western United States. In this, and the adjoining regions of Oregon and
Washington, an enormous tract of country has been overflowed by lava in
a late geological period, the surface covered being estimated to have a
larger area than France and Great Britain combined.
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