ffered by the crust
over those particular tracts. On the other hand, the similarity of
volcanic lavas over wide regions is strong evidence that they are drawn
from one continuous magma, consisting of molten matter beneath the
solid exterior crust.

(_a._) _Lines of Volcanic Action._--It has been shown in a previous page
that volcanic action of recent or Tertiary times has taken place mainly
along certain lines which may be traced on the surface of a map or
globe. One of these lines girdles the whole globe, while others lie in
certain directions more or less coincident with lines of flexure,
plication or faulting. The Isle of Sumatra offers a remarkable example
of the coincidence of such lines with those of volcanic vents. Not only
the great volcanic cones, but also the smaller ones, are disposed in
chains which run parallel to the longitudinal axis of the island
(N.W.-S.E.). The sedimentary rocks are bent and faulted in lines
parallel to the main axis, and also to the chains of volcanic mountains,
and the observation holds good with regard to different geological
periods.[1] Another remarkable case is that of the Jordan Valley.
Nowhere can the existence of a great fracture and vertical displacement
of the strata be more clearly determined than along this remarkable line
of depression; and it is one which is also coincident with a zone of
earthquake and volcanic disturbances.

(_b._) _Such Lines generally lie along the Borders of the Ocean._--But
even where, from some special cause, actual observation on the relations
of the strata are precluded, the general configuration of the ground and
the relations of the boundaries between land and sea to those of
volcanic chains, evidently point in many cases to their mutual
interdependence. The remarkable straightness of the coast of Western
America, and of the parallel chain of the Andes, affords presumptive
evidence that this line is coincident with a fracture or system of
faults, along which the continent has been bodily raised out of the
waters of the ocean. Of this elevation within very recent times we have
abundant evidence in the existence of raised coral-reefs and oceanic
shell-beds at intervals all along the coast; rising in Peru to a level
of no less than 3,000 feet above the ocean, as shown by Alexander
Agassiz.[2] Such elevations probably occurred at a time when the
volcanoes of the Andes were much more active than at present. Considered
as a whole, these great volc