WHAT VOLCANOES TEACH US CONCERNING THE NATURE OF THE EARTH'S INTERIOR.
In entering upon any speculations or enquiries concerning the nature of
the interior of our globe, it is necessary before all things that we
should clearly realise in our minds how small and almost infinitesimal
is that part of the earth's mass which can be subjected to direct
examination. The distance from the surface to the centre of our globe
is nearly 4,000 miles, but the deepest mines do not penetrate to much
more than half a mile from the surface, and the deepest borings fall
far short of a mile in depth. Sometimes, it is true, the geologist
finds means for drawing inferences as to the nature of the rocks at
depths of ten or fifteen miles below the surface; but the last-named
depth must be regarded as the utmost limit of that portion of our
globe which can be made the object of direct observation and study.
This thin exterior film of the earth's mass, which the geologist is
able to investigate, we call the 'crust of the globe'; but it must be
remembered that in using this term, it is not intended to imply that
the outer part of our globe differs in any essential respect from the
interior. The term 'crust of the globe' is employed by geologists as
a convenient way of referring to that portion of the earth which is
accessible to their observation.
But if we are unable to make direct investigations concerning the
nature of the internal portions of the globe, there are nevertheless a
number of facts from which we may draw important inferences upon the
subject. These facts and the inferences based upon them we shall now
proceed to consider.
First in importance among these we may mention the results which have
been obtained by weighing our globe. Various methods have been devised
for accomplishing this important object, and the conclusions arrived
at by different methods agree so closely with one another, that there
is no room for doubt as to the substantial accuracy of those results.
It may be taken as proved beyond the possibility of controversy that
our globe is equal in weight to five and a half globes of the same size
composed of water, or, in other words, that the average density of the
materials composing the globe is five and a half times as great as that
of water.
Now the density of the materials which compose the crust of the globe
is very much less than this, varying from about two-and-one-third to
three times that of water. Hence we are compelled to conclude that the
interior portions of the globe are of far greater density than the
exterior portions; that, as a matter of fact, the mass of the globe is
composed of materials having twice the density of the rocks exposed at
the surface.
[Sidenote: DENSITY OF EARTH'S INTERIOR.]
It has been sometimes argued that as all materials under intense
pressure appear to yield to an appreciable extent, and to allow their
particles to be packed into a smaller compass, we may find in this fact
an explanation of the great density of the internal parts of the globe.
It has in fact been suggested that under the enormous pressure which
must be exerted by masses of rock several thousand feet in thickness,
the materials of which our earth is composed may be compelled to pack
themselves into less than one-half the compass which they occupy at the
surface. But the ascription of such almost unlimited compressibility to
solid substances can be supported neither by experiment nor analogy.
Various considerations point to the probability that solid bodies yield
to pressure up to a certain limit and no farther, and that when this
limit is reached an increase in pressure is no longer attended with a
reduction in bulk.
If then we are compelled to reject the idea of the unlimited
compressibility of solid substances, we must conclude that the interior
portions of our globe are composed of _materials of a different kind_
from those which occur in its crust. And this conclusion, as we shall
presently see, is borne out by a number of independent facts.
The study of the materials ejected from volcanic vents proves that even
at very moderate depths there exist substances differing greatly in
density, as well as in chemical composition. The lightest lavas have a
specific gravity of 2·3, the heaviest of over 3. And that materials of
even greater density are sometimes brought by volcanic action from the
earth's interior, we have now the clearest proofs.
[Sidenote: RELATION BETWEEN EARTH AND OTHER PLANETS.]
But in considering a question of this kind, it will be well to remember
that analogy may furnish us with hints upon the subject which may prove
to be by no means unimportant. There is no question upon which modern
science has wrought out a more complete revolution in our ideas, than
that of the relation of our earth to the other bodies of the universe.
We know, as the result of recent research, that our globe is one of
a great family of bodies, moving through space in similar paths and
in obedience to the same laws. A hundred years ago the primary and
secondary planets of the solar system could be almost numbered upon
the fingers; now we recognise the fact that they exist in countless
millions, presenting every variety of bulk from masses 1,400 times
as large as our earth down to the merest planetary dust. Between the
orbits of Mars and Jupiter, more than 200 small planets have been
recognised as occurring, and every year additions are made to the
number of these asteroids. Comets have now been identified with streams
of such planetary bodies, of minute size, moving in regular orbits
through our system. The magnificent showers of 'shooting-stars' have
been proved to be caused by the passage of the earth through such
bands of travelling bodies, and 'the zodiacal light' finds its most
probable explanation in the supposition that the sun is surrounded by
a great mass of such minute planets. Every increase in the power of
the telescope reveals to us the existence of new secondary planets
or moons, revolving about the primaries; and the wonderful system
of the Saturnian rings is now explained by the proved existence of
great streams of such secondary planets circling around it. The solar
system was formerly conceived of as a vast solitude through which a
few gigantic bodies moved at awful distances from one another. Now we
know that the supposed empty void is traversed by countless myriads of
bodies of the most varied dimensions, all moving in certain definite
paths, in obedience to the same laws, ever acting and reacting upon
each other, and occasionally coming into collision.
There are not wanting further facts to prove that the other planets are
like our own in many of their phenomena and surroundings. In some of
them atmospheric phenomena have been detected, such as the formation of
clouds and the deposition of snow, so that the external forces at work
on our globe act upon them also. And that internal forces, like those
we have been considering in the case of our earth, are at work in our
neighbours, is proved by the great solar storms and the condition of
the moon's surface.
But the results of spectrum-analysis in recent years have furnished new
facts in proof of the close relationship of our earth to the numerous
similar bodies by which it is surrounded. So far as observation has yet
gone we have reason for believing that not only the members of the
solar system, but the more distant bodies of the universe, are all
composed of the same elementary substances as those which enter into
the composition of our globe.
The most satisfactory information concerning the composition and nature
of other planetary bodies is derived from the study of those small
planets which occasionally come into collision with our globe, and
which have their own proper motion in space thereby arrested. These
meteorites, as such falling planetary bodies are called, have justly
attracted great attention, and their fragments are treasured as the
most valuable objects in our museums.
[Sidenote: COMPOSITION OF METEORITES.]
The first fact concerning these meteorites, which it is necessary
to notice, is that they are composed of the same chemical elements
as occur in the earth's crust. No element has yet been found in any
meteorite which was not previously known as existing in the earth, and
of the sixty-five or seventy known terrestrial elements no less than
twenty-two have already been detected in meteorites.
There are, however, a dozen elements which occur in overwhelming
proportions in the earth's crust. We shall probably not be going too
far in saying that these twelve elements--namely, oxygen, silicon,
aluminium, calcium, magnesium, sodium, potassium, iron, carbon,
hydrogen, sulphur, and chlorine--make up amongst them not less than 999
out of 1,000 parts of the earth's crust, and that all the other fifty
or sixty elements are 80 comparatively rare that they do not constitute
when taken altogether more than one part in 1,000 of the rocks of the
globe. Now all of these twelve common terrestrial elements occur in
meteorites, and the fact that the rarer terrestrial elements have not
as yet been found in them will not surprise anyone, who remembers how
small is the bulk of all the specimens of these meteorites existing in
our museums.
We have hitherto insisted on the points of resemblance in the chemical
composition of meteorites and that of the rocks of the globe, but we
shall now have to indicate some very important points in which they
differ.
While in the rocks composing the earth's crust oxygen forms one-half of
their mass, and silicon another quarter, we find that in the meteorites
these elements, though present, play a much less important part. The
most abundant element in the meteorites is iron; and nickel, chromium,
cobalt, manganese, sulphur, and phosphorus, are much more abundant in
these extra-terrestrial bodies than they are in the earth's crust.
We have already referred to the remarkable fact that in our earth's
crust nearly all the other elementary substances are found combined
in the first instance with oxygen, and that most rocks consist of the
oxide of silicon combined with the oxides of various metals. But this
is by no means the case with the meteorites. In them we find metals
like iron, nickel, cobalt, &c., in their uncombined condition, and
forming alloys with one another. The same and other metals also occur
in combination with carbon, phosphorus, chlorine, and sulphur, and
some of the substances thus formed are quite unknown among terrestrial
rocks. Compounds of the oxide of silicon with the oxides of the metals
such as form the mass of the crust of the globe do occur in meteorites,
but they play a much less important part than in the case of the
terrestrial rocks.
Among the substances found in meteorites are several which do not exist
among the terrestrial rocks--some, indeed, which it seems impossible
to conceive of as being formed and preserved under terrestrial
conditions. Among these we may mention the phosphide of iron and nickel
(Schreibersite), the sulphide of chromium and iron (Daubréelite), the
protosulphide of iron (Troilite), the sulphide of calcium (Oldhamite),
the protochloride of iron (Lawrencite), and a peculiar form of
crystallised silica, called by Professor Maskelyne 'Asmanite.'
[Sidenote: DIFFERENT KINDS OF METEORITES.]
There are other phenomena exhibited by meteorites which indicate that
they must have been formed under conditions very different to those
which prevail upon the earth's surface. Thus we find that fused iron
and molten slag-like materials have remained entangled with each other,
and have not separated as they would do if a great body like the earth
were near to exercise the varying force of gravity upon the two
classes of substances. Again, meteorites are found to have absorbed
many times their bulk of hydrogen gas, and to exhibit peculiarities in
their microscopic structure which can probably be only accounted for
when we remember that they were formed in the interplanetary spaces,
far away from any great attracting body.
But in recent years a number of very important facts have been
discovered which may well lead us to devote a closer attention to the
composition and structure of meteorites. It has been shown, on the
one hand, that some meteorites contain substances precisely similar
to those which are sometimes brought from the earth's interior during
volcanic outbursts; and, on the other hand, there have been detected,
among some of the ejections of volcanoes, bodies which so closely
resemble meteorites that they were long mistaken for them. Both kinds
of observation seem to point to the conclusion that the earth's
interior is composed of similar materials to those which we find in the
small planets called meteorites.
M. Daubrée has proposed a very convenient classification for
meteorites, dividing them into the following four groups:--
I. _Holosiderites_; consisting almost entirely of metallic iron, or of
iron alloyed with nickel, stony matter being absent; but sulphides,
phosphides, and carbides of several metals are often diffused through
the mass. The polished surfaces of these meteoric irons, when etched
with acid, often exhibit a remarkable crystalline structure.
II. _Syssiderites_; in which a network of metallic iron encloses a
number of granular masses of stony materials.
III. _Sporadosiderites_; which consist of a mass of stony materials,
through which particles of metallic iron are disseminated.
IV. _Asiderites_; containing no metallic iron, but consisting entirely
of stony materials.
There are, besides the meteorites belonging to these principal groups,
a few of peculiar and exceptional composition, which we need not notice
further for our present purpose.
From the above classification it will be seen that most meteorites
consist of a mixture in varying proportions of metallic and stony
materials. Sometimes the metallic constituents are present in greater
proportions than the stony, at other times the stony materials
predominate, while occasionally one or other of these elements may be
wholly wanting.
The stony portions of meteorites, upon careful examination, prove to be
built up of certain minerals, agreeing in their chemical composition
and their crystalline forms with those which occur in the rocks of the
earth's crust. Among the ordinary terrestrial minerals occurring in
the stony portions of meteorites, we may especially mention olivine,
enstatite, augite, anorthite, chromite, magnetite, and pyrrhotite.
[Sidenote: METEORITES AND ULTRA-BASIC ROCKS.]
The minerals which occur in meteorites are in every case such as are
found in the more basic volcanic rocks--quartz, and the acid felspars,
with the other minerals which occur in acid rocks, being entirely
absent in the 'extra-terrestrial' rocks.
Now, besides the three great classes of lavas which we have described
as being ejected from volcanic vents, there are some rarer materials
occasionally brought from the earth's interior by the same agency,
that present a most wonderful resemblance to the stony portions of
meteorites. These materials we may call 'ultra-basic rocks.' Their
specific gravity is very high, usually exceeding 3, and they contain
a very low percentage of silica; on the other hand, the proportion of
iron and magnesia is often much greater than in ordinary terrestrial
rocks. But the most remarkable fact about these ultra-basic rocks is,
that they are almost entirely composed of the minerals which occur in
meteorites; namely, olivine, enstatite, augite, anorthite, magnetite,
and chromite.
The ultra-basic rocks often occur under very peculiar conditions.
Sometimes they are found forming ordinary volcanic protrusions
through the sedimentary rocks. The rocks named pikrites, lherzolites,
dunites, &c., are examples of such igneous protrusions composed of
these ultra-basic materials, and probably all the true serpentines are
rocks of the same class which have absorbed water and undergone great
alteration. The ultra-basic rocks sometimes contain platinum and other
metals in the free or uncombined state. But not unfrequently we find
among the ordinary ejections of volcanoes, nodules and fragments of
such ultra-basic materials, which have clearly been carried up with the
other lavas from great depths in the earth's crust. Thus in Auvergne,
the Eifel, Bohemia, Styria, and many other volcanic districts, the
basaltic lavas and tuffs are found to contain nodules composed of
the minerals which are so highly characteristic of meteorites. Such
nodules, too, often form the centres of the volcanic bombs which are
thrown out of craters during eruptions.
We thus see that materials identical in composition and character with
the stony portions of meteorites, exist within the earth's interior,
and are thrown out on its surface by volcanic action. A still more
interesting discovery has been made in recent years; namely, that
materials similar to the metallic portion of meteorites, and consisting
of nickeliferous iron, also occur in deep-seated portions of the
earth's crust, and are brought to the surface during periods of igneous
activity.
In the year 1870, Professor Nordenskiöld made a most important
discovery at Ovifak, on the south side of the Island of Disko, off
the Greenland coast. On the shore of the island a number of blocks of
iron were seen, and the chemical examination of these proved that,
like ordinary metallic meteorites, they consisted of iron alloyed with
nickel and cobalt.
[Sidenote: IRON-MASSES OF OVIFAK.]
Now, when the facts concerning the masses of native iron of Ovifak were
made known, the first and most natural explanation which presented
itself to every mind was, that these were a number of meteorites which
at some past period had fallen upon the earth's surface.
Metallic iron.
Opaque crystals of magnetite (black oxide of iron).
Transparent crystals of felspar, augite, and olivine.
[Illustration: Fig. 87.--Section of basalt from Ovifak, Greenland, with
particles of metallic iron diffused through its mass.]
But a further examination of the locality revealed a number of facts
which, as Professor Steenstrup pointed out, it is very difficult
to reconcile with the theory that the Ovifak masses of iron are of
meteoric origin. The district of Western Greenland, where these masses
were discovered, has been the scene of volcanic outbursts on the
grandest scale during the Miocene period. In close proximity to the
great iron masses, there are seen a number of basaltic dykes; and, when
these dykes are carefully examined, the basaltic rock of which they are
composed is seen to be full of particles of metallic iron. In fig. 87,
we have a drawing made from a section of the Ovifak basalts magnified
four or five diameters. The rock-mass is seen to be composed of black,
opaque magnetite, and transparent crystals of augite, labradorite,
olivine, &c.; while, through the whole, particles of metallic iron are
found entangled among the different crystals in the most remarkable
manner.
It has been suggested that this singular rock might have been formed by
a meteorite falling, in Miocene times, into a lava-stream in a state
of incandescence. But the relation of the metallic particles to the
stony materials is such as to lend no support whatever to this rather
strained hypothesis.
A careful study of all the facts of the case by Lawrence Smith,
Daubrée, and others well acquainted with the phenomena exhibited by
meteorites, has led to the conclusion that the large iron-masses of
Ovifak, as well as the particles of metallic iron diffused through
the surrounding basalts, are all of terrestrial origin, and have been
brought by volcanic action from the earth's interior. It is probable
that, just as we find in many basaltic lavas nodules of ultra-basic
materials similar to the stony parts of meteorites, so in these basalts
of Ovifak we have masses of iron alloyed with nickel, similar to the
metallic portions of meteorites. Both the stony and metallic enclosures
in the basalt are in all probability derived from deeper portions of
the earth's crust. By the weathering away of the basalt of Ovifak, the
larger masses of metallic iron have been left exposed upon the shore
where they were found.
There are a number of other facts which seem to support this startling
conclusion. Thus it has been shown by Professor Andrews that certain
basalts in our own islands contain particles of metallic iron of
microscopic dimensions, and it is not improbable that some of the
masses of nickeliferous iron found in various parts of the earth's
surface, which have hitherto been regarded as meteorites, are, like
those of Ovifak, of terrestrial origin.
[Sidenote: MATERIALS FILLING METALLIC-VEINS.]
Another piece of evidence pointing in the same direction, is derived
from those great fissures communicating with the interior of our globe
which become filled with metallic minerals, and are known to us as
mineral-veins. In these mineral-veins the native metals, their alloys,
and combinations of these with sulphur, chlorine, phosphorus, &c., are
frequently present. But oxides of the metals, except as products of
subsequent alteration, occur far less frequently than in the earth's
crust generally. Hence we are led to conclude that the substances which
in the outer part of the earth's crust always exist in combination
with oxygen, are at greater depths in a free and uncombined condition.
Nor is it a circumstance altogether unworthy of attention that the
researches of Mr. Norman Lockyer and other astronomers, based on the
known facts of the relative densities of the several members of the
solar system, and the ascertained relations of the different solar
envelopes, have led to conclusions closely in accord with those arrived
at by geologists. These researches appear to warrant the hypothesis
that the interior of our globe consists of metallic substances
uncombined with oxygen, and that among these metallic substances iron
plays an important part. Our globe, as we know, is a great magnet,
and the remarkable phenomena of terrestrial magnetism may also not
improbably find their explanation in the fact that metallic iron forms
80 large a portion of the earth's interior.
The interesting facts which we have been considering may be made
clearer by the accompanying diagram (fig. 88). The materials ejected
from volcanic vents (lavas) are in almost all cases compounds of
silicon and the various metals with oxygen. In the lighter or acid
lavas oxygen constitutes one-half of their weight, and the proportion
of metals of the iron-group is very small. As we pass to the heavier
intermediate and basic lavas, we find the proportion of oxygen
diminishing, and the metals of the alkaline earths (magnesium and
calcium) with the metals of the iron-group increasing, in quantity. In
the small and interesting group of the ultra-basic lavas the proportion
of oxygen is comparatively small, and the proportion of magnesium and
iron very high. So much for the terrestrial rocks.
[Illustration: Fig. 88.--Diagram illustrating the relation between the
Terrestrial and the Extra-Terrestrial Rock.]
[Sidenote: TERRESTRIAL AND EXTRA-TERRESTRIAL ROCKS.]
Now let us turn our attention to the extra-terrestrial rocks or those
found in meteorites. The Asiderites are quite identical in composition
with the ultra-basic lavas of our globe, but in the Sporadosiderites
and the Syssiderites we find the proportion of oxygen rapidly
diminishing, and that of metallic iron increasing. Finally, in the
Holosiderites the oxygen entirely disappears, and the whole mass
becomes metallic.
From the Holosiderites at one end of the chain to the add lavas
at the other, we find there is a complete and continuous series;
the rocks of terrestrial origin overlapping, in their least
oxydized representatives, the most highly oxydized representatives
of the extra-terrestrial rocks. But the discovery at Ovifak of
the iron-masses, and the basalts with iron disseminated, has
afforded another very important link, placing the terrestrial and
extra-terrestrial rocks in closer relations with one another.
All these facts appear to point to the conclusion that the earth's
interior consists of metallic substances either quite uncombined
or simply alloyed with one another, and among these iron is very
conspicuous by its abundance. The outer crust, which is probably of no
great thickness, contains an enormous proportion of oxygen and silicon
combined with the materials which constitute the interior portions of
our globe. It may be, as has been suggested by astronomers, that our
earth consisted at one time of a solid metallic mass surrounded by a
vaporous envelope of metalloids, and that the whole of the latter, with
the exception of the constituents of the atmosphere and ocean, have
gradually entered into combination with the metals of the nucleus to
form the existing crust of the globe. But of this period the geologist
can take no cognisance. The records which he studies evidently
commenced at a long subsequent period, when the conditions prevailing
at the earth's surface differed but little, if at all, from those which
exist at the present day. Equally little has the geologist to do with
speculations concerning a far distant future when, as some philosophers
have suggested, the work of combination of the waters and atmosphere of
the earth's surface with the metallic substances of its interior shall
be completed, and our globe, entirely deprived of its fluid envelopes,
reduced to the condition in which we find our satellite, the moon.
* * * * *
[Sidenote: PHYSICAL CONDITION OF EARTH'S INTERIOR.]
There is another class of enquiries concerning the earth's interior to
which the attention of both geologists and astronomers has long been
directed--that, namely, which deals with the problem of the _physical
condition_ of the interior of our globe.
The fact that masses of molten materials are seen at many points of the
earth's surface to issue from figures in the crust of our globe, seems
at first sight to find a simple explanation if we suppose our planet
to consist of a fluid central mass surrounded by a solid crust. Hence
we find that among those who first thought upon this subject, this
hypothesis of a liquid centre and a solid crust was almost universally
accepted. This hypothesis was supposed to find further support in the
fact that, as we penetrate into the earth's crust by mines or boring
operations, the temperature is found to continually increase. It was
imagined, too, that this condition of our planet would best agree with
the requirements of the nebular hypothesis of Laplace, which explains
the formations and movements of the bodies of the solar system by the
cooling down of a nebulous mass.
But a more careful and critical examination of the question has led
many geologists and astronomers to reject the hypothesis that the earth
consists of a great fluid mass surrounded by a comparatively thin shell
of solid materials.
Volcanic outbursts and earthquake tremors, though so terrible and
destructive to man and his works, are but slight and inconsiderable
disturbances in a globe of such vast dimensions as that on which we
live. The condition of the crust of the globe is, in spite of volcanic
and earthquake manifestations, one of general stability; and this
general stability has certainly been maintained during the vast periods
covered by the geological record. Such a state of things seems quite
irreconcilable with the supposition that, at no great depth from the
surface, the whole mass of the globe is in a liquid condition. If, on
the other hand, it be supposed that the solid crust of the globe is
several hundreds of miles in thickness, it is difficult to understand
how the local centres of volcanic activity could be supplied from such
deep-seated sources.
There are other facts which seem equally irreconcilable with the
hypothesis of a fluid centre and a thin solid crust in our globe. If
all igneous products were derived from one central reservoir, we might
fairly expect to find a much greater uniformity of character among
those products than really exists. But in some cases, materials of
totally different composition are ejected at the same time from closely
adjoining volcanic districts. Thus in Hungary and Bohemia, as we have
seen, lavas of totally different character were being extruded during
the Miocene period. In the island of Hawaii, as Professor Dana has
pointed out, igneous ejections have taken place at a crater 14,000 feet
above the sea-level, while a closely adjoining open vent at a level
10,000 feet lower exhibited no kind of sympathy with the disturbance.
Whatever may be the cause of volcanic action, it seems clear that it
does not originate in a universal mass of liquefied material situated
at no great depth from the earth's surface.
The conclusions arrived at by astronomers and physicists is one quite
in accord with those which geologists have reached by totally different
methods. It is now very generally admitted that if the earth were not
a rigid mass, its behaviour under the attract live influences of the
surrounding members of the solar system would be very different to what
is found to be the case.
[Sidenote: ARGUMENTS AGAINST LIQUID INTERIOR.]
That the earth is in a solid condition to a great depth from the
surface, and possibly quite to the centre, is a conclusion concerning
which there can be little doubt; and in the next chapter we shall
endeavour to show that such a condition of thirds is by no means
incompatible with those manifestations of internal energy, the
phenomena of which we are considering in this work. The question,
therefore, of the complete solidity of our globe, or of its consisting
of a solid and a liquid portion, is one of speculative interest only,
and is in no way involved in our investigations concerning the nature
and origin of volcanic activity. We may conclude this chapter by
enumerating the several hypotheses which have at different times been
maintained concerning the nature of the interior of our globe.
_First._ It has been suggested that the earth consists of a fluid or
semi-fluid nucleus surrounded and enclosed in a solid shell. Some
have maintained this shell to be of such insignificant thickness, as
compared with the bulk of the interior liquid mass, that portions of
the latter are able to reach the earth's surface through movements
and fractures of the outer shell, and that in this manner volcanic
manifestations originate. Others, impressed with the general stability
and rigidity of the globe as a whole, have maintained that the outer
solid shell must have a very considerable thickness, amounting
probably to not less than several hundreds of miles. But through a
shell of such thickness it is difficult to conceive of the liquid
masses of the interior finding their way to the surface, and those who
have held this view are driven to suggest some other means by which
local developments of volcanic action might be brought about.
_Secondly._ Some physicists have asserted that a globe of liquid
matter radiating its heat into space, would tend to solidify both at
the surface and the centre, at the same time. The consequence of this
action would be the production of a sphere with a solid external shell
and a solid central nucleus, but with an interposed layer in a fluid
or semi-fluid condition. It has been pointed out that if we suppose
the solidification to have gone so far, as to have caused the partial
union of the interior nucleus and the external shell, we may conceive a
condition of things in which the stability and rigidity is sufficient
to satisfy both geologists and astronomers, but that in still
unsolidified pockets or reservoirs, filled with liquefied rock, between
the nucleus and the shell, we should have a competent cause for the
production of the volcanic phenomena of the globe. In this hypothesis,
however, it is assumed that the cooling at the centre and the surface
of the globe would go on at such rates that the reservoirs of liquid
material would be left at a moderate depth from the surface, so that
easy communication could be opened between them and volcanic vents.
[Sidenote: REVIEW OF THE SEVERAL HYPOTHESES.]
_Thirdly._ It has been maintained that the earth may have become
perfectly solid from the centre to the surface. Those who hold this
view endeavour to account for the phenomena of volcanoes in one of two
ways. It may be, they say, that the deep-seated rock-masses, though
actually solid, are in a state of _potential_ liquidity; that though
reduced to a solid state by the intense pressure of the superincumbent
masses, yet such is the condition of unstable equilibrium in the whole
mass, that the comparatively slight movements and changes taking place
at the earth's surface suffice to bring about the liquefaction of
portions of its crust and consequent manifestations of volcanic energy.
But It may be, as other supporters of the doctrine of the earth's
complete solidity have maintained, that the phenomena of volcanoes
have no direct connection with a supposed incandescent condition of
our planet at all, and that there are chemical and mechanical forces
at work within our globe which are quite competent to produce at the
surface all those remarkable phenomena which we identify with volcanic
action.
From this summary of the speculative views which have been entertained
upon the subject of the physical condition of the earth's interior,
it will be clear that at present we have not sufficient evidence for
arriving at anything like a definite solution of the problem. The
conditions of temperature and pressure which exist in the interior of
a globe of such vast dimensions as our earth, are so far removed from
those which we can imitate in our experimental enquiries, and it is so
unsafe to push the application of laws arrived at by the latter to the
extreme limits required by the former, that we shall do well to pause
before attempting to dogmatise on such a difficult question.
In the next chapter we shall endeavour to grapple with a somewhat more
hopeful task, to point out how far observation and experiment have
enabled us to offer a reasonable explanation of the wonderful series of
phenomena which are displayed during outbursts of volcanic activity.