76. There have been many speculations as to the condition of the
interior of the earth. Some have inferred that the external crust of the
globe incloses a fluid or molten mass; others think it more probable
that the interior is solid, but contains scattered throughout its bulk,
especially towards the surface of the earth, irregular seas of molten
matter, occupying large vesicles or tunnels in the solid honey-combed
mass. At present, the facts known would appear to be best explained by
the latter hypothesis. All that we know from observation is, that the
temperature increases as we descend from the surface. The rate of
increase is very variable. Thus, in the Artesian well at Neuffen, in
Wuertemberg, it was as much as 1 deg. F. for every 19 feet. In the mines of
Central Germany, however, the increase is only 1 deg. F. for every 76 feet;
while in the Dukinfield coal-pit, near Manchester, the increase was
still less, being only 1 deg. F. in 89 feet. Taking the average of many
observations, it may be held as pretty well proved that the temperature
of the earth's crust increases 1 deg. for every 50 or 60 feet of descent
after the first hundred.
77. The crust of the earth is subject to certain movements, which are
either sudden and paroxysmal, or protracted and tranquil. The former are
known as earthquakes, which may or may not result in a permanent
alteration of the relative level of land and sea; the latter always
effect some permanent change, either of upheaval or depression.
78. _Earthquakes_ have been variously accounted for. Those who uphold
the hypothesis of a fluid interior think the undulatory motion
experienced at the surface is caused by movements in the underlying
molten mass--an earthquake being thus 'the reaction of the liquid
nucleus against the outer crust.' By others, again, earthquakes are
supposed to be caused by the fall of large rock-masses from the roofs of
subterranean cavities, or by any sudden impulse or blow, such as might
be produced by the cracking of rocks in a state of tension, by a sudden
volcanic outburst, or sudden generation or condensation of steam. In
support of this latter hypothesis, many facts may be adduced. The
undulatory motion communicated to the ground during gunpowder
explosions, or by the fall of rocks from a mountain, is often propagated
to great distances from the scene of these catastrophes, and the
phenomena closely resemble those which accompany a true earthquake. When
the level of a district has been permanently affected by an earthquake,
the movement has generally resulted in a lowering of the surface. Thus,
in 1819, the Great Runn of Cutch, in Hindustan, was depressed over an
area of several thousand square miles, so as during the monsoons to
become a salt lagoon. Occasionally, however, we find that elevation of
the land has taken place during an earthquake. This was the case in New
Zealand in 1855, when the ground on which the town of Wellington stands
rose about two feet, and a cape in the neighbourhood nearly ten feet.
Sometimes the ground so elevated is, after a shorter or longer period,
again depressed to its former level. A good example of this occurred in
South America in 1835. The shore at Concepcion was raised a yard and a
half; and the Isle Santa Maria was pushed up two and a half yards at one
end, and three and a half yards at the other. But only a few months
afterwards the ground sank again, and everything returned to its old
position. The heaving and undulatory motion of an earthquake produces
frequently considerable changes at the surface of the ground, besides an
alteration of level. Rocks are loosened, and sometimes hurled down from
cliff and mountain-side, and streams are occasionally dammed with the
soil and rubbish pitched into them. Sometimes also the ground opens, and
swallows whatever chances to come in the way. If these chasms close
again permanently, no change in the physiography of the land may take
place, but sometimes they remain open, and affect the drainage of the
country.
79. _Movements of Upheaval and Depression._--Besides the permanent
alteration of level which is sometimes the result of a great earthquake,
it is now well known that the crust of the earth is subject to
long-continued and tranquil movements of elevation and depression. The
cause of these movements is at present merely matter for speculation,
some being of opinion that they may be caused by the gradual contraction
of the slowly cooling nucleus of the earth, which would necessarily give
rise to depression, while this movement, again, would be accompanied by
some degree of elevation--the result of the lateral push or thrust
effected by the descending rock-masses. It is doubtful, however, if this
hypothesis will explain all the appearances. The Scandinavian peninsula
affords a fine example of the movements in question. At the extremity of
the peninsula (Scania), the land is slowly sinking, while to the north
of that district gradual elevation is taking place at a very variable
rate, which in some places reaches as much as two or three feet in a
century. Movements of elevation are also affecting Spitzbergen, Northern
Siberia, North Greenland, the whole western borders of South America,
Japan, the Kurile Islands, Asia Minor, and many other districts in the
Mediterranean area, besides various islets in the great Pacific Ocean.
The proofs of a slow movement of elevation are found in old
_sea-beaches_ and _sea-caves_, which now stand above the level of the
sea. In the case of Scandinavia, it has been noticed that the pine-woods
which clothe the mountains are being slowly elevated to ungenial
heights, and are therefore gradually dying out along their upper limits.
The proofs of depression of the land are seen in submerged forests and
peat, which occur frequently around our own shores, and there is also
strong human testimony to such downward movements of the surface. The
case of Scania has already been referred to. Several streets in some of
its coast towns have sunk below the sea, and it is calculated that the
Scanian coast has lost to the extent of thirty-two yards in breadth
within the past hundred and thirty years. The coral reefs of southern
oceans also afford striking evidence of a great movement of depression.
Not long ago a theory was started by a French savant, M. Adhemar, to
account for changes in the sea-level, without having recourse to
subterranean agency. He pointed out that a vast ice-cap, covering the
northern regions of our hemisphere, as was certainly the case during
what is termed the glacial epoch, would cause a rise of the sea by
displacing the earth's centre of gravity. Mr James Croll has recently
strongly supported this opinion; and there can be no doubt that we have
here a _vera causa_ of considerable mutations of level. It is
unquestionably true, however, that great oscillatory movements, such as
described above, and which can only be attributed to subterranean
agencies, have frequently taken and are still taking place.
80. Such movements of the earth's crust cannot take place without
effecting some change upon the strata of which that crust is composed.
During _depression_ of the curved surface of the earth, the under strata
must necessarily be subjected to intense lateral pressure, since they
are compelled to occupy less space, and contortion and plication will be
the result. It is evident also that contortion will diminish from below
upwards, so that we can conceive that excessive contortion may be even
now taking place at a great depth from the surface in Greenland. During
a movement of _elevation_, on the other hand, the strata are subjected
to excessive tension, and must be seamed with great rents: when the
elevating force is removed, the disrupted rocks will settle down
unequally--in other words, they will be _faulted_, and their continuity
will be broken. But both contortion and faulting may be due, on a small
scale, to local causes, such as the intrusion of igneous rocks, the
consolidation of strata, the falling in of old water-courses, &c.
_Cleavage_ is believed to have been caused by compression, such as the
rocks might well be subjected to during great movements of the earth's
crust. The particles of which the rock is composed are compressed in one
direction, and of course are at the same time drawn out at right angles
to the pressure. This is observed not only as regards the particles of
the rock themselves, but imbedded fossils also are distorted and
flattened in precisely the same way.
81. _Volcanoes._--Besides movements of elevation and depression, there
are certain other phenomena due to the action of the subterranean
forces. Such are the ejection from the interior of the earth of heated
matters, and their accumulation upon the surface. The erupted materials
consist of molten matter (lava), stones and dust, gases and steam--the
lava, ashes, and stones gradually accumulating round the focus of
ejection, and thus tending to form a conical hill or mountain. Could we
obtain a complete section of such a volcanic cone, we should find it
built up of successive irregular beds of lava, and layers of stones and
ashes, dipping outwards and away from the source of eruption, but having
round the walls of the _crater_ (that is, the cavity at the summit of
the truncated cone) a more or less perceptible dip inwards. Fig. 25
gives a condensed view of the general phenomena accompanying an
eruption. In this ideal section, _a_ is the funnel or neck of the
volcano filled with lava; _b_, _b_, the crater. The molten lava is
highly charged with elastic fluids, which continually escape from its
surface with violent explosions, and rise in globular clouds, _d_, _d_,
to a certain height, after which they dilate into a dark cloud, _c_.
From this cloud showers of rain, _e_, are frequently discharged. Large
and small portions of the incandescent lava are shot upwards as the
imprisoned vapour of water explodes and makes its escape, and, along
with these, fragments of the rocks forming the walls of the crater and
the funnel are also violently discharged; the cooled bombs, angular
stones, and _lapilli_, as the smaller stones are called, falling in
showers, _f_, upon the exterior parts of the cone or into the crater,
from which they are again and again ejected. Most frequently the great
weight of the lava inside the crater suffices to break down the side of
the cone, and the molten rock escapes through the breach. Sometimes,
however, it issues from beneath the base of the cone. At other times,
finding for itself some weak place in the cone, it may flow out by a
lateral fissure, _g_. In the diagram, _i_, _i_ represents the lava
streaming down the outward slopes, jets of steam and fumaroles escaping
from almost every part of its surface. Forked lightning often
accompanies an eruption, and is supposed to be generated by the intense
mutual friction in the air of the ejected stones. The trituration to
which these are subjected reduces them, first, to a kind of coarse
gravel (_lapillo_); then to sand (_puzzolana_); and lastly, to fine dust
or ashes (_ceneri_).
[Illustration: Fig. 25.--Diagrammatic Section of Volcano.]
82. _Lava._--Any rock which has been erupted from a volcano in a molten
state is called _lava_. Some modern lava-streams cover a great extent of
surface. One of two streams which issued from the volcano of Skaptur
Jokul (Iceland) in 1783 overflowed an area fifty miles in length, with a
breadth in places of fifteen; the other was not much less extensive,
being forty miles in length, with an occasional breadth of seven. In
some places the lava exceeded 500 feet in thickness. Again, in 1855, an
eruption in the island of Hawaii sent forth a stream of lava sixty-five
miles long, and from one to ten miles wide. The surface of a stream
quickly cools and consolidates, and in doing so shrinks, so as to become
seamed with cracks, through which the incandescent matter underneath can
be seen. As the current flows on, the upper crust separates into rough
ragged scoriform blocks, which are rolled over each other and jammed
into confused masses. The slags that cake upon the face or front of the
stream roll down before it, and thus a kind of rude pavement is formed,
upon which the lava advances and is eventually consolidated. Thus, in
most cases, a bed of lava is scoriaceous as well below as above. Other
kinds of lava are much more ductile and viscous, and coagulate
superficially in glossy or wrinkled crusts. When lava has inclosed
fragments of aqueous rocks, such as limestone, clay, or sandstone, these
are observed to have undergone some alteration. The sandstone is often
much hardened, the clay is porcelainised, and the limestone, still
retaining its carbonic acid, assumes a crystalline texture. But the
aqueous rock upon which lava has cooled does not usually exhibit much
change, nor does the alteration, as a rule, extend more than a few feet
(often only a few inches) into the rock. A lava-current which entered a
lake or the sea, however, has sometimes caught up much of the sediment
gathering there, and become so commingled with it, that in some parts it
is hard to say whether the resulting rock is more igneous or aqueous.
Lava which has been squirted up from below into cracks and crevices, and
there consolidated so as to form _dykes_, sometimes, but not often,
produces considerable alteration upon the rocks which it intersects. The
basaltic structure is believed to be due to the contraction of lava
consequent upon its cooling. The axes of the prisms are always
perpendicular to the cooling surface or surfaces, and in some cases the
columns are wonderfully regular. There are numerous varieties of lava,
such as _basalt_, _obsidian_, _pitchstone_, _pearlstone_, _trachyte_,
&c.; some are heavy compact rocks, others are light and porous. Many are
finely or coarsely crystalline; others have a glassy and resinous or
waxy texture. Some shew a flaky or laminated structure; others are
concretionary. Most of the lava rocks, however, are granularly
crystalline. In many, a vesicular character is observed. These vesicles,
being due to the bubbles of vapour that gathered in the molten rock,
usually occur in greatest abundance towards the upper surface of a bed
of lava. They are also more or less well developed near the bottom of a
bed, which, as already explained, is frequently scoriaceous.
Occasionally the vesicles are disseminated throughout the entire rock.
As a rule, those lavas which are of inferior specific gravity are much
more vesicular than the denser and heavier varieties. The vesicles are
usually more or less flattened, having been drawn out in the direction
in which the lava-current flowed. Sometimes they are filled, or
partially filled, with mineral matter introduced at the time of
eruption, or subsequently brought in a state of solution and deposited
there by water filtering through the rock: this forms what is called
_amygdaloidal lava_. In volcanic districts, the rocks are often
traversed by more or less vertical dykes or veins of igneous matter.
These dykes appear in some cases to have been formed by the filling up
of crevices from above--the liquid lava having filtered downwards from
an overflowing mass. In most cases, however, the lava has been injected
from below, and not unfrequently the 'dykes' seem to have been the
feeders from which lava-streams have been supplied--the feeders having
now become exposed to the light of day either by some violent eruption
which has torn the rocks asunder, or else by the gradual wearing away of
the latter by atmospheric and aqueous agencies.