Saint Mary's beacon., 1877-03-22
From Bsllou's Monthly Magasine)
Earthquakes And How CsuseG.


BY PROFESSOR JAMES MACINTOSH.
Earthquake action is nearly related to
volcanic, and frequently one Of these
natural forces may pass into the other,
Long-conunued experiments in deep
mines, in all kinds OF rocks, have plain
ly proved that the heat increases Is we
descend. This increase Of heat cannot
be ascribed to the heavier column of At
which must press DOVE 1t the
bottom Of deep mines. III We ascend
some mountain-sid from the sea-level,
The cold increases about one degree for
every 800 feet. We might, therefore,
conclude that the heat in aeep mines.
If i! were due 10 atmospherical pres
sure, would increase at the So ne rate.
II gains. however. much more rapidly
as much as one degree forev-
cry forty-fve feet Of descent, and seldom
less than one degree for every seventy.
II the heat goes on increasing con-
tinuously at the same rate. it becomes
an easy matter to calculate to what
depth We should have to go to reach a
point where all the known solid SUBJ
stances are in state of fusion. Chem
lists can tell US exactly what heat is re-
quired to melt iron, copper, ETC., so that
it would not be difficult to estimate the
depth in the earth where, ifthe heat in
creases at the average rate Of one de-
gree for about sixty feet, these WELL
known substances are constantly in
molten condition. For various reasons
Of mathematical character, it is be-
lieved that the thickness Of the earth's
crust is much greater than IL would be
IL the heat continued to increase as
aforementioned. II is thought that as
we go deeper through the strata, the in
creased pressure which ensues with
stands the tendency TO a molten condi-
tion; 20 that at great depths the enor-
nous pressure OF the overlving rocks
resists the tendency OF the great heat to
melt them. According TO the calcula-
tion that heat increases one degree Fab
renheit every sixty feet of descent, il
there were no counteracting agency, all
solid substances would be in a molten
state at the depth Of about thirty miles.
This is known to b. too thin a crust,
however, for planet 5000 miles in di-
amcter, and so il j. usually regarded as
not less than 100 miles in thickness.
owing to the pressure withstanding the
liq"efyi influence Of the earth's inter
nal heat.
The oblate shape Of the earth, and OF
all the planets OF the solar system, is
explained on the ground that they were
originally in molten condition. Since
then they have lost heat and cooled
down. The external strat,fed crust,
however, still cncloses the greater part
OF the originally molten planet. A loss
of heat is vet slowly going on, known
"a the secular cooling OF the earth"
The heat from the interior is conducted
upwards through the rocky crust, and
thence radiated into space. We have
not TO consider the siorness Of this OPP
Grafton, but the fact that il is taking
place. With loss of heat comes shrink
ing or contraction Of the earth's interior.
The hardened crust everywhere envel-
oping il cannot shrink in the same de-
gree, and 1t is necessary it should be
fitted to the contracting inte-
rior, otherwise there would be a broken
space between il and the crust.
II is along the weakest part of the
earth's crust that fractures usually hap
pen. Hence it is that earthquakes and
volcanoes generally run in lines extend
ing over enormous areas OF the earth's
surface. The force generated by the
fracturing or folding is that usually des-
'gnated as earthquake shocks" MIR.
Mallet has shown that the friction which
ensues during fracturing of fdeep-seated
solid rocks, and when the walls are rub
bed against each other, will develop
heat enough 10 melt rocks. In this way
earthquakes pass into volcanic action.
These two forces extend over the same
areas, and are often transmitted from
the same spots, or foci. Volcanic ac-
ton is nearly always preceded by earth
quake shocks, whose intensity increases
until volcanic energy is developed, af-
ter which the former subsidies. On
MIR. Mallet's theory, this near relation
IS easily understood. aud he has shown
that the amount Of heat which must be
annually developed by the secular COOL
ing OF the earth's interior, is more than
Is required 10 produce all the volcanic
energy. MIR. Mallet, however. does
not think the interior of the earth is
molten, but in heated solid state. the
molten lava which issues from the vol
ranic craters having been formed bv
the fusion of the solid matter where
hea, has been developed during shrink
age. In either case, however wheth-
er we acknowledge a molten interior or
hly-heated solid one, we have
equally to explain the origin Of earth
quakes and volcanoes on the theory of
the secular cooling and contraction of
the earth as a whole.
The shocks propagated through the
earth's crust will vary in their degree
NJ intensity in proportion to the amount
of fracture or contraction. In every
case the shock Is propagated in waves,
analogous to those which ripple the sur
lace OF . smooth pond when stone has
been thrown in, only vertical instead OF
These are termed ssearth-
quake wsves:.a and whea powerfully
lelt, the surface OF the earth will actu-
ally be thrown into series of tempora-
~ andula.ions. like that Of iield OF
corn when gentle Sreeze Is passing
over H. Not unfrequen,ly. if the sur
iare rocks he nf n nafnre that L.. -.