The Evansville daily journal. [volume] (Evansville, Ia. [i.e. Ind.]) 1848-1862, 1857-01-12

ORIGIN OF VOLCANIC ACTIoN.-Respecting
the action of pyrogenous rocks, ofigneous ac-
ton with all its attendent phenomena of vol
canoes. earthquakes, and other subterranean
-r~oyements, geologists are by no means agreed.
The two most prevalent hypbothesis are what
have been termed the chemical and .leckantc1l
the former ehdeavoring to account for the
lph snomena on chemical principles, the latter
aScrlbing to them to some great source within
the interior of the planet. BY the latter n.y.
pothesis it is assumed, chiefly or the ground
OF increasing temperature as We descend into
the crust, that the interior of the earth is
in a state of high incandescence of molten
tlui lity; that the cooled or rocky exterior is
of inconsiderable and varying thickness; and
that this crust is extensively cavernous rent
and primarily by unequal contrac-
ton from cooling, and subsequently 10y sub
terranean agitations AS this cooling process
must still be going on, however slowly, the
least contraction of the crust-even to the frac-
ton Of an inch-would be sufficient to squirt
molten rock matter from hundred pores or
cratcrs. Water also finds its way through
the assures Of the crust and, coming in con-
tact with the heated mass within, generates
steam and other gases, and these exploding
and struggling expand, produces earthquakes
and agitations, which are rendered more
precepuble by the cavernous condition Of the
crust, and the yielding material upon which
it rests-the thin rocky film undulating and
rendlng like sheet of ice on the surface of
agitated water. OccaSionly the heatea~yapors
make their way through the assures and
openings, as gaseous exhalations, or as hot
springs and jets ofsteam and water, carrying
with them various suhlinationS and solutions
of the rocks through which they pass. On
the other hand, when the expansive forces be-
come so powerful as to break through the
earth's crust discharges. melted rock matter,
Oava) red hot stones, ashes, steam, and other
vapors follow; and repetltions of such dis
charges at longer or shorter intervals gradually
form volcanic cones; and lines and centres of
such activity produces in the course OF ages,
mountain groups and ranges 1t does not ol-
Tow, however, that the volcanic discharges
must always take place at that point where
the greatest external contraction or internal
pressure is exerted; on the contrary, the mol
ten mass will obey the law of hydrostatic
pressure, and be propelled to whatever cIaters
or assures that may already exist. This So
called "mechanical'' theory Of central heat is
further supported by the occurrence of igne-
~ss phenomena in every region of the globe
and by the fact that most volcanic centres are
in intimate connection with each 0ther-a
commotion in one district being usually ac
companied by simular disturbance in another
1t is sometimes objected to this hypothesis
that if all the ignenous rocks proceed from a
common source, there ought to be a greater
uniformity among them IN composition and
aspect; bur, on the whole, there is really great
similarity in composition, and there is nothing
in their difference that may net be accounted
for by pressure, rapidity or slowness Of tOol'
Ing, fusion or re-fusion, and comminglenient
with the melted matter of the stratifed rocks
through which the eruption takes place.