We pass next to a very brief consideration of operations that originate
below the earth’s surface. The records of deep mines and artesian wells
show that the temperature of the ground always increases downwards from
the surface; and the much higher temperatures of hot springs and
volcanoes show that the heat continues to increase to a great depth, and
is not a merely superficial phenomenon. The observed rate of increase is
not uniform, but it seldom varies far from the average, which is about
1° Fahr. per 53 feet of vertical descent, or, in round numbers, 100° per
mile. This rate, if continued, would give a very high temperature at
points only a few miles below the surface; and until within a few years
the idea was generally accepted by geologists that the increase of
temperature is sensibly uniform for an indefinite distance downward;
that in the central regions of the earth the temperature is far higher
than anything we can conceive, and that everywhere below a depth of 20
to 40 miles the temperature is above the fusing-point of all rocks; and
hence that the earth is an incandescent liquid globe covered by a thin
shell or crust of cold, solid rock.
Our limited space will not permit us to enter into a discussion of the
condition of the earth’s interior, and I will merely point out in a few
sentences the position occupied by geologists at the present time. The
reasoning of Thompson has shown that the temperature cannot increase
downward at a uniform rate, but at a constantly and rapidly diminishing
rate; and that everywhere below a depth of 300 miles the temperature is
probably sensibly the same, and nowhere, probably, above 8000° to
10,000° Fahr.
Unlike water, all rocks contract on solidifying and expand on melting,
and consequently the high pressures to which they are subjected in the
earth’s interior—10,000,000 to 20,000,000 pounds per square inch—must
raise their fusing-points enormously, and the probabilities are that
they are solid, in spite of the high temperature. But Thompson and
Darwin have shown us farther that the phenomena of the oceanic tides
could not be what they are known to be if the earth were any less rigid
than a globe of solid steel; while Hopkins has proved that the
astronomical phenomena of precession and nutation could not be what they
are if the earth’s crust were less than 800 or 1000 miles thick. Putting
these considerations together, geologists are almost universally agreed
that, while the earth has an incandescent interior, it is still
continuously solid from centre to circumference, with the exception of a
thin plastic stratum at a depth not exceeding 40 or 50 miles, which
forms the seat of volcanic action.
The earth is not only a very hot body, but it is rotating through almost
absolutely cold space, and therefore must be a cooling body. But, except
at the very beginning of the cooling, the loss of heat has gone on
almost entirely from the interior; and since cooling means contraction,
the heated interior must be constantly tending to shrink away from the
cold external crust.
Of course no actual separation between the crust and interior or nucleus
can take place, but there is no doubt that the crust is left unsupported
to a certain extent, and it must then behave like an arch with a radius
of 4000 miles, and the result is an enormous horizontal or tangential
pressure.
This lateral pressure in the earth’s crust is one of the most important
and most generally accepted facts in geology, and lies at the bottom of
many geological theories. According to what seems to me to be the most
probable theory of the origin of continents and ocean-basins, they are
broad upward and downward bendings or arches into which the crust is
thrown by the tangential pressure. Finally, the strain becomes great
enough to crush the crust along those lines where it is weakest. When
the crust is thus mashed up by horizontal pressure, a mountain range is
formed, the crust becomes enormously thicker, and a weak place becomes a
strong one.
During the formation of mountains the stratified rocks, which were
originally horizontal, are thrown into folds or arches, and tipped up at
all possible angles; they are fractured and faults produced; and by the
immense pressure the structure known as slaty cleavage is developed. In
fact, a vast amount and variety of structures are produced during the
growth of a mountain range.
These great earth-movements are not always perfectly smooth and steady,
but they are accompanied by slipping or crushing now and then; and, as a
result of the shock thus produced, a swift vibratory movement or jar,
which we know as an _earthquake_, runs through the earth’s crust.
Extensive fissures are also formed, opening down to the regions where
the rocks are liquid or plastic, and through these the melted rocks flow
up to or toward the surface. That portion which flows out on the surface
builds up a volcanic cone, while that which cools and solidifies below
the surface, in the fissures, forms dikes. Thus among the igneous or
eruptive rocks we have two great classes,—the _dike_ rocks and the
_volcanic_ rocks.
It is important to observe that all these subterranean operations—the
formation of continents, of mountain-ranges with all their attendant
phenomena of folds, faults and cleavage, and every form and phase of
earthquake and volcanic activity—depend upon or originate in the
interior heat of the earth. And over against the _superficial_ or
_aqueous_ agencies, originating in the _solar_ heat and producing the
_stratified_ or _sedimentary_ rocks, we set the _subterranean_ or
_igneous_ agencies originating in the _central_ heat, and producing the
_unstratified_ or _eruptive_ rocks.
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