CHAPTER XII.

THE ATTEMPTS WHICH HAVE BEEN MADE TO EXPLAIN THE CAUSES OF VOLCANIC

ACTION.

Every completed scientific investigation must consist of four series

of operations. In the first of these an attempt is made to collect the

whole of the facts bearing on the question, by means of observation

and experiment; the latter being only observation under conditions

determined by ourselves. In the second stage of the enquiry, the

attention is directed to classifying and grouping the isolated facts,

so as to determine their bearings upon one another, and the general

conclusions to which they appear to point. In the third stage, it is

sought to frame an hypothesis which shall embrace all the observed

facts, and shall be in harmony with the general conclusions derived

from them. In the fourth stage, this hypothesis is put to the most

rigid test; comparing the results which must follow, if it be true,

with the phenomena actually observed, and rejecting or amending our

hypothesis accordingly. Every great scientific theory has thus been

established by these four processes--observation, generalisation,

hypothesis, and verification.

The enquiry concerning the nature and causes of volcanic action is far

from being a completed one. It is true that many hypotheses upon the

subject have been framed, but in too many instances these have not been

based on accurate observations and careful generalisations, and can be

regarded as little better than mere guesses. Indeed, the state of the

enquiry at the present time would seem to be as follows. Although much

remains to be done in the direction both of observation and experiment,

the main facts of the case have been established upon irrefragable

evidence. The classification and comparison of these facts have led to

the recognition of certain laws, which seem to embrace all the known

facts. To account for these facts and their demonstrated relations to

one another, certain tentative hypotheses have been suggested; but in

no case can it be truly said that these latter have so far stood the

test of exact enquiry as to deserve to rank as demonstrated truths. A

complete and consistent theory of volcanic action still remains to be

discovered.

[Sidenote: VALUE AND LIMITS OF HYPOTHESES.]

In accordance with the plan which we have sketched out for ourselves

at the commencement of this work, we shall aim at following what has

been the order of investigation and discovery in our study of volcanic

action; and in this concluding chapter we shall indicate the different

hypotheses by which it has been proposed to account for the varied

phenomena, which we have discussed in the preceding pages, and their

remarkable relations to one another. We shall endeavour, in passing,

to indicate how far these several hypotheses appear to be probable,

as satisfying a larger or smaller number of those conditions of the

problem which have been established by observation, experiment, and

careful reasoning; but we shall at the same time carefully avoid such

advocacy of any particular views as would tend to a prejudgment of

the question. Hypothesis is, as we have seen, one of the legitimate

and necessary operations in scientific investigation. It only becomes

a dangerous and treacherous weapon when it is made to precede rather

than to follow observation and experiment, or when being regarded

with paternal indulgence, an attempt is made to shield it from the

relentless logic of facts. Good and bad hypotheses must be allowed to

'grow together till the harvest;' such as are unable to accommodate

themselves to the surrounding conditions imposed by newly-discovered

facts and freshly-established laws will assuredly perish; and in this

'struggle for existence' the true hypothesis will in the end survive,

while the false ones perish.

It may well happen, however, that among the hypotheses which have up to

the present time been framed, none will be found to entirely satisfy

all the conditions of the problem. New discoveries in physics and

chemistry have suggested fresh explanations of volcanic phenomena in

the past, and may continue to do so in the future; and the true theory

of volcanic action, when it is at last discovered, may combine many of

the principles which now seem to be peculiar to different hypotheses.

Let us, in the first place, enquire what are the facts which must be

accounted for in any theory of volcanic action. We have already been

led to the conclusion that the phenomena exhibited by volcanoes were

entirely produced by the escape of imprisoned water and other gases

from masses of incandescent and fluid rock. Our subsequent examination

of the problem confirmed the conclusion that in all cases of volcanic

outburst we have molten rock-materials from which water and other

gases issue with greater or less violence. The two great facts to

be accounted for, then, in any attempted explanation of volcanic

phenomena, are the existence of this high temperature at certain points

within the earth's crust, and the presence of great quantities of water

and gas, imprisoned in the rocks. We shall perhaps simplify the enquiry

if we examine these two questions separately, and, in the first place,

review those hypotheses which have been suggested to account for high

temperatures in the subterranean regions, and, in the second place,

examine those which seek to explain the presence of large quantities of

imprisoned water and gases.

[Sidenote: INCREASE OF TEMPERATURE WITH DEPTH.]

That a high temperature exists in the earth's crust at some depth from

the surface is a £act which does not admit of any doubt. Every shaft

sunk for mining operations, and every deep boring made for the purpose

of obtaining water, proves that a more or less regular increase of

temperature takes place as we penetrate downwards. The average rate

of this increase of temperature has been estimated to be about 1°

Fahrenheit for every 50 or 60 feet of depth.

Now if it be assumed that this regular increase of temperature

continues to great depths, a simple calculation proves that at a depth

of 9,000 feet a temperature of 212° Fahrenheit will be found--one

sufficient to boil water at the earth's surface--while at a depth of 28

miles the temperature will be high enough to melt cast-iron, and at 34

miles to fuse platinum.

So marked is this steady increase of temperature as we go downwards,

that it has been seriously proposed to make very deep borings in order

to obtain supplies of warm water for heating our towns. Arago and

Walferdin suggested this method for warming the Jardin des Plantes at

Paris; and now that such important improvements have been devised in

carrying borings to enormous depths, the time may not be far distant

when we shall draw extensively upon these supplies of subterranean

heat. At the present time the city of Buda-Pesth is extensively

supplied with hot-water from an underground source. Should our

coal-supply ever fail it may be well to remember that we have these

inexhaustible supplies of heat everywhere beneath our feet.

But although we may conclude that at the moderate depths we have

indicated such high temperatures exist, it would not be safe to

infer, as some have done, that at a distance of only 40 or 50 miles

from the surface the materials composing our globe are in a state

of actual fusion. Both theory and experiment indicate that under

increased pressure the fusing point of solid bodies is raised; and

just as in a Papin's digester we may have water retained by high

pressure in a liquid condition at a temperature far above 212° F.,

so in the interior of the earth, masses of rock may exist in a solid

state, at a temperature far above that at which they would fuse at

the earth's surface. We may speak of such rock-masses, retained in a

solid condition by intense pressure, at a temperature far above their

fusing point at the earth's surface, as being in a 'potentially liquid

condition.' Upon any relief of pressure such masses would at once

assume the liquid state, just as the superheated water in a Papin's

digester immediately flashes into steam upon the fracture of the strong

vessel by which it is confined. We have already seen how the action

at volcanic vents often appears to indicate just such a manifestation

of elastic forces, as would be exhibited by the relief of superheated

masses from a state of confinement by pressure.

In reasoning upon questions of this kind, however, we must always be

upon our guard against giving undue extension to principles and laws

which seem to be clearly established by experiment at the earth's

surface. It is well to remember how exceedingly limited is our command

of extreme pressures and high temperatures, when compared with those

which may exist within a body of the dimensions of our globe.

[Sidenote: EFFECT OF PRESSURE ON FUSION-POINT.]

If we were to imagine a set of intelligent creatures, who were able to

command only a range of temperatures from 50° to 200° F., engaged upon

an investigation of the properties of water, we shall easily understand

how unsafe it may be to extend generalisations far beyond the limits

covered by actual experiment. Such beings, from their observation of

the regular changes of volume of water at all the temperatures they

could command, might infer that at still higher and lower temperatures

the same rates of expansion and contraction would be maintained. Yet,

as we well know, such an inference would be quite wide of the truth;

for a little above 200° F. water suddenly expands to 1,700 times its

volume, and not far below 50° F. the contraction is suddenly changed

for expansion.

It has been argued by the late Mr. David Forbes and others that,

inasmuch as experiment has shown that--though the fusing points of

solids are raised by pressure, yet that this rise of the fusing

points goes on in a diminishing ratio as compared with the pressures

applied--a limit will probably be reached at which the most intense

pressure will not be sufficient to retain substances at a high

temperature in their solid state. The fact that gases cannot be

retained in a liquid condition by the most intense pressure at a

temperature above their critical point, may seem by analogy to favour

the same conclusion. Hence, David Forbes, Dana, and other authors, have

argued in favour of the existence of a great liquid nucleus in our

globe covered by a comparatively thin, solid crust. And if we accept

the supposed proofs of a constant increase of temperature from the

surface to the centre of the globe, such a conclusion appears to be at

least as well founded as that which regards the central masses of the

earth as maintained in a solid condition by intense pressure.

A little consideration will, however, convince us that the facts which

have been relied upon as proving the intensely heated condition of the

central masses of our globe, are by no means so conclusive as has been

supposed.

The earth's form, which mathematicians have shown to be exactly that

which would be acquired by a globe composed of yielding materials

rotating on its axis at the rate which our planet does, has often

been adduced as proving that the latter was not always in a rigid and

unyielding condition. In the same way, all the remarkable facts and

relations of the bodies of the solar system, which have been shown by

astronomers to lend such support to the nebular hypothesis, have been

thought, at the same time, to favour the view that our earth is still

in a condition of uncompleted solidification.

But it is quite admissible to accept the nebular hypothesis and the

view that our globe attained its present form while still in a state

of fluidity, and at the same time to maintain that our earth has long

since reached its condition of complete solidification. And there are

not a few facts which appear to lend support to such a conclusion.

[Sidenote: SUPPOSED PROOFS OF LIQUID NUCLEUS.]

If the rapid rate of increase in temperature which has been

demonstrated to occur at so many parts of the earth's surface be

maintained to the centre, then, as argued by David Forbes and Dana, it

is difficult to conceive of our earth as being in any other condition

than that of a liquid mass covered by a comparatively thin crust. The

objection to this view, both upon geological and astronomical grounds,

we have pointed out in the previous chapter.

Before accepting as a demonstrated conclusion this notion of a constant

increase of temperature from the surface to the centre of our globe, it

may be well to re-examine the facts which are relied upon as proving it.

That there is a general increase of temperature so far as we are able

to go downwards in the earth's crust, there can, as we have seen,

be no doubt whatever. Yet it may be well to bear in mind how very

limited is the range of our observation on the subject. The deepest

mines extend to little more than half-a-mile from the surface, and the

deepest borings to little more than three-quarters of a mile, while the

distance from the earth's surface to its centre is nearly 4,000 miles.

We may well pause before we extend conclusions, derived from such very

limited observations, to such enormous depths.

But when we examine critically these observations themselves, we

shall find equal grounds for caution in generalising from them. There

is the greatest and most startling divergence in the results of the

observations which have been made at different points at the earth's

surface. Even when every allowance is made for errors of observation,

these discrepancies still remain. In some places the increase of

temperature as we go downwards is so rapid that it amounts to 1°

Fahrenheit for every 20 feet in depth, while in other cases, in order

to obtain the same increase in temperature of 1° Fahrenheit, we have to

descend as much as 100 feet.

Now if, as is so often assumed, this increase of temperature as we

go downwards be due to our approach to incandescent masses forming

the interior portions of the globe, it is difficult to understand

why greater uniformity is not exhibited in the rate of increase in

different areas. No difference in the conducting powers of the various

rock-materials is sufficient to account for the fact that in some

places the rate of increase in temperature in going downwards is no

less than five times as great as it is in others.

[Sidenote: VARIATIONS IN UNDERGROUND TEMPERATURES.]

Again, there are some remarkable facts concerning the variation in

the rate of increase in temperature with depth which seem equally

irreconcilable with the theory that the heat in question is directly

derived from a great, central, incandescent mass. M. Walferdin, by a

series of careful observations in two shafts at Creuzot, proved that

down to the depth of 1,800 feet the increase of temperature amounted to

1° Fahrenheit for every 55 feet of descent, but below the depth named,

the rate of increase was as much as 1° Fahrenheit for every 44 feet. On

the other hand, in the great boring of Grenelle at Paris, the increase

in temperature down to the depth of 740 feet amounted to 1° Fahrenheit

for every 50 feet of descent, but from 740 feet down to 1,600 feet,

the rate of increase diminished to 1° for 75 feet of descent. The

same remarkable fact was strikingly shown in the case of the deepest

boring in the world--that of Sperenberg, near Berlin, which attained

the great depth of 4,052 feet. In this case, the rate of increase in

temperature for the first 1,900 feet, was 1° Fahrenheit for every 55

feet of descent, and for the next 2,000, it diminished to 1° Fahrenheit

for every 62 feet of descent. In the deep well of Buda-Pesth there was

actually found a decline in temperature below the depth of 3,000 feet.

Perhaps the most interesting fact in connection with this question

which has been discovered of late years, is that in districts which

have recently been the seat of volcanic agencies, the rate of increase

in temperature, as we go downwards in the earth's crust, is abnormally

high. Thus at Monte Massi in Tuscany, the temperature was found to

increase at the rate of 1° Fahrenheit for every 24 feet of descent.

In Hungary several deep wells and borings have been made, which prove

that a very rapid increase of temperature occurs. The deep boring at

Buda-Pesth penetrates to a depth of 3,160 feet, and a temperature of

178° Fahrenheit has been observed near the bottom. The rate of increase

of temperature in this boring was about 1° for every 23 feet of

descent. In the mines opened in the great Comstock lode, in the western

territories of the United States, an abnormally high temperature has

been met with amounting in some cases to 157° Fahrenheit. Although

this is the richest mineral-vein in the world, having yielded since

1859, when it was first discovered, 60,000,000_l._ worth of gold and

silver, this rapid increase in temperature in going downwards threatens

in the end to entirely baffle the enterprise of the miner. The rate

of increase in temperature in the case of the Comstock mines has been

estimated at 1° Fahrenheit for every 46 feet of descent, between 1,000

and 2,000 feet from the surface, but as much as 1° Fahrenheit for every

25 feet, at depths below 2,000 feet.

The facts which we have stated, with others of a similar kind, have

led geologists to look with grave feelings of doubt upon the old

hypothesis which regarded the increase of temperature found in making

excavations into the earth's crust as a proof that we are approaching a

great incandescent nucleus. They have thus been led to enquire whether

there are any conceivable sources of high temperatures at moderate

depths--temperatures which would be quite competent to produce locally

all the phenomena of volcanic action.

There are not wanting other facts which seem to point to the same

conclusion: namely, that volcanic action is not due to the existence

of a universal reservoir of incandescent material occupying the central

portion of our globe, but to the local development of high temperatures

at moderate depths from the surface.

[Sidenote: DEPTHS AT WHICH EARTHQUAKES ORIGINATE.]

The close connection between the phenomena of volcanoes and earthquakes

cannot be doubted. It is true that some of those vibrations or tremors

of the earth's crust, to which we apply the name of earthquakes,

occur in areas which are not now the seat of volcanic action; and it

is equally true that the stratified rock-masses of our globe, far

away from any volcanic centres, exhibit proofs of violent movement

and fracture, in the production of which, concussions giving rise

to earthquake vibrations, could scarcely fail to have occurred. But

it is none the less certain that earthquakes as a rule take place

in those areas which are the seats of volcanic action, and that

great earthquake-shocks precede and accompany volcanic outbursts.

Sometimes, too, it has been noticed that the manifestation of

activity at a volcanic centre is marked by the sudden decline of the

earthquake-tremors of the district around, as though a safety-valve had

been opened at that part of the earth's surface.

Mr. Mallet has shown that by the careful study of the effects

produced at the surface by earthquake-vibrations, we may determine

with considerable accuracy the point at which the shock or concussion

occurred which gave rise to the vibration. Now it is a most remarkable

fact that such calculations have led to the conclusion that, so far

as is at present known, earthquake shocks never originate at greater

depths than thirty miles from the surface, and that in some cases

the focus from which the waves of elastic compression producing an

earthquake proceed is only at the depth of seven or eight miles.

As we have already seen, there can be no doubt that in the great

majority of instances the forces originating earthquake-vibrations and

volcanic outbursts are the same, and independent lines of reasoning

have conducted us to the conclusion that these forces operate at very

moderate distances from the earth's surface.

Under these circumstances, geologists have been led to enquire

whether there are any means by which we can conceive of such an

amount of heat, as would be competent to produce volcanic outbursts,

being locally developed at certain points within the earth's crust.

Recent discoveries in physical science which have shown the close

relation to one another of different kinds of force, and their mutual

convertibility, have at least suggested the possibility of the

existence of causes by which such high temperatures within certain

portions of the earth's crust may be originated.

[Sidenote: DAVY'S CHEMICAL THEORY.]

When, at the commencement of the present century, Sir Humphry Davy

discovered the remarkable metals of the alkalies and alkaline earths,

and at the same time demonstrated the striking phenomena which are

exhibited if these metals be permitted to unite with oxygen, he at

once perceived that if such metals existed in an uncombined condition

within the earth's crust, the access of water and air to the mass might

give rise to the development of such an amount of heat, as would be

competent to produce volcanic phenomena at the surface. It is true that

at a later date Davy recognised the chemical theory of volcanoes as

being beset with considerable difficulties, and was disposed to abandon

it altogether. It was argued, with considerable show of reason, that if

the heat at volcanic centres were produced by the access of water to

metallic substances, great quantities of hydrogen would necessarily be

evolved, and this gas ought to be found in prodigious quantities among

the emanations of volcanoes. The fact that such enormous quantities of

hydrogen gas are not emitted from volcanic vents has been held by many

authors to be fatal to the chemical theory of volcanoes.

But the later researches of Graham and others have made known facts

which go far towards supplying an answer to the objections raised

against the chemical theory of volcanoes. Various solids and liquids

have been shown to possess the power of absorbing many times their

volume of certain gases. Among the gases thus absorbed in large

quantities by solids and liquids, hydrogen is very conspicuous. In

some cases gases are absorbed by metals or other solids in a state of

fusion, and yielded up again by them as they cool.

It is a very remarkable circumstance that some meteorites are found

to have absorbed large quantities of hydrogen gas, and this is given

off when they are heated in vacuo. Thus it has been demonstrated that

certain meteorites have contained as much as forty seven times their

own volume of hydrogen gas.

We have already pointed out that there are reasons for believing the

internal portions of our globe to be composed of materials similar to

those found in meteorites. If such be the case, the access of water

to these metallic substances may result in the formation of oxides,

attended with a great local development of heat, the hydrogen which

is liberated being at once absorbed by the surrounding metallic

substances. That this oxidation of the metallic substances in the

interior of our globe by the access of water and air from the surface

is continually going on, can scarcely be doubted. We may even look

forward to a far-distant period when the whole of the liquid and

gaseous envelopes of the globe shall have been absorbed into its

substance, and our earth thereby reduced to the condition in which we

now find the moon to be.

There is a second method by which high temperatures might be locally

developed within the earth's crust, which has been suggested by Vose,

Mallet, and other authors.

We have good grounds for believing that the temperature of our globe

is continually diminishing by its radiation of heat into space. This

cooling of our globe is attended by contraction, which results in

movements of portions of its crust. It may at first sight appear that

such movements would be so small and insignificant as to be quite

unworthy of notice. But if we take into account the vast size of our

earth it will be seen that the movements of such enormous masses may be

attended with the most wonderful results.

It has been shown that if a part of the earth's crust fifty miles in

thickness were to have its temperature raised 200° Fahrenheit, its

surface would be raised to the extent of 1,000 or 1,500 feet Le Conte

has pointed out that if we conceive the conduction of heat to take

place at slightly different rates along different radii of our globe,

we should at once be able to account for the existing inequalities of

the earth's surface, and for all those continental movements which can

be shown to have taken place in past geological periods.

[Sidenote: DYNAMICAL THEORIES.]

But if we admit, as we have good grounds for doing, that the loss of

heat from the external portions of our globe goes on more rapidly than

in the case of the central masses, we have thereby introduced another

powerful agent for the production of high temperatures within the

earth's crust. The external shell of the globe will tend to contract

upon the central mass, and in so doing a series of tangential strains

will result which will be capable of folding and crumpling the rocks

along any lines of weakness. That such crushing and crumpling has

during all geological periods taken place along lines of weakness

in the earth's crust, is proved, as we have seen, by the phenomena

presented by mountain-ranges. Now these crushings, crumplings, and

other violent movements of great rock-masses must result in the

development of a vast amount of heat, just as the forcing down of a

break upon a moving wheel produces heat. This conclusion is strikingly

confirmed by the well-known geological fact that nearly all rocks which

have undergone great movement and contortion are found to present

evidence of having been subjected to such chemical and crystalline

actions, as would result from the development of a high temperature

within their mass.

[Sidenote: RECAPITULATION OF SEVERAL THEORIES.]

Let us sum up briefly the various methods which have been suggested to

account for the high temperatures within certain parts of the earth's

crust by which volcanic phenomena are produced.

Our globe may be conceived of as an incandescent liquid mass surrounded

by a cooler, solid shell. If we regard this liquid interior mass as

supplying directly the various volcanic vents of the earth, it must be

conceded that the outer shell is of comparatively slight thickness.

But astronomers are almost universally agreed that such a thin outer

shell and inner liquid mass are quite incompatible with that rigidity

which our planet exhibits under the attractions of its neighbours.

Geologists are almost equally unanimous in regarding this hypothesis

of a liquid nucleus and thin, solid shell as contradicted by the

stability of the conditions which have been maintained during such

long past periods, and which exist at the present day. The extent and

character of volcanic action do not indicate a condition of general

instability in our earth, but one of stability subject to small and

local interferences The grandest volcanic disturbances appear small and

insignificant, if we take into account the vast dimensions of the globe

upon which they are displayed.

If, on the other hand, we consider the outer solid shell to be of

great thickness, we are met by the difficulty of accounting for the

upheaval of liquid matter through such vast thicknesses of a solid

shell. The differences in character of lavas extruded from closely

adjoining volcanic districts seem equally difficult of explanation on

any theory of a central, fluid nucleus and a solid, outer shell. Nor is

the distribution of heat within the earth's crust so uniform as might

be anticipated, if the source of that heat be a great central mass of

highly heated materials.

Under these circumstances, geologists and physicists have enquired

whether any other conditions can be imagined as existing in the earth's

interior, which would better account for the observed phenomena than

does the hypothesis of a liquid nucleus and a solid outer shell. Two

such alternative hypotheses have been suggested.

Mr. Hopkins, adopting the theory that the earth has solidified both at

the centre and its outer surface, endeavoured to explain the occurrence

of volcanoes and earthquakes by supposing that cavities of liquid

material have been left between the solid nucleus and the solid shell,

and these cavities full of liquid material constitute the sources from

which the existing volcanoes of the globe draw their supplies. But

this hypothesis is found to be beset with many difficulties when we

attempt to apply it to the explanation of the phenomena of volcanic

action. It entirely fails, among other things, to account for the

remarkable fact that during past geological periods the scene of

volcanic action has been continually shifting over the surface of the

earth, so that there is probably no considerable area of our globe

which has not at one time or other been invaded by the volcanic forces.

By some other theorists, who have felt the full force of this last

objection, an attempt has been made to explain the phenomena of

volcanoes by supposing that the globe is solid from its surface to its

centre, but that the internal portions of the globe are at such a high

temperature that they are only retained in a solid condition by the

enormous pressure to which they are subjected. The central masses of

the globe are thus regarded as being in an _actually_ solid, but in a

_potentially_ liquid condition, and any local relief of pressure is at

once followed by the conversion of solid to liquefied materials, in the

district where the relief takes place, resulting in the manifestation

of volcanic phenomena at the spot. It may be granted that this

hypothesis better accords with the known facts of Vulcanology than any

of those which we have previously described, but it is impossible to

shut our eyes to the fact that not a few serious difficulties still

remain. Thus it is based upon the assumption that the law of the

elevation of the point of fusion by pressure is true at temperatures

and pressures almost infinitely above those at which we are able to

conduct observations; but neither experiment nor analogy warrant this

conclusion, for the former shows that the elevation of the point of

fusion by pressure goes on in a continually diminishing ratio, and the

latter famishes us with the example of volatile liquids which, above

their critical points, obstinately remain in a gaseous condition under

the highest pressures. Nor is it easy upon this hypothesis to account

for the very irregular distribution of temperatures within the earth's

crust, as demonstrated by observations in mines, wells, and borings.

The hypothesis further requires the assumption that, at such very

moderate depths as are required for the reservoirs of volcanoes, the

effects of pressure and temperature on the condition of rock-materials

are so nicely balanced that the smallest changes at the surface lead to

a disturbance of the equilibrium.

[Sidenote: DIFFICULTIES NOT YET EXPLAINED.]

It is the weight of these several objections that has led geologists

in recent years to regard with greater favour those hypotheses which

seek to account for the production of high temperatures within parts of

the earth's crust, without having recourse to a supposed incandescent

nucleus. If it can be shown that there are any chemical or mechanical

forces at work within the crust of the globe which are capable of

producing local elevations of temperature, then we may conceive of a

condition of things existing in the earth's interior which is free

from the objections raised by the astronomer on the score of the

earth's proved rigidity, and by the geologist on the ground of its

general stability, and which at the same time seems to harmonise better

with the observed facts of the distribution of temperature within the

earth's crust. How far the existence of such chemical and mechanical

agencies capable of producing high temperatures within the crust of the

globe have been substantiated, we have already endeavoured to point out.

It must be admitted, then, that the questions of the nature of the

earth's interior and the cause of the high temperatures which produce

volcanic phenomena, are still open ones. We have not yet got beyond the

stage of endeavouring to account for the facts observed by means of

tentative hypotheses. Some of these, as we have seen, agree with the

facts, so far as they are at present known, much better than others;

but the decision between them or the rejection of the whole of them in

favour of some new hypothesis, must depend on the results of future

observation and enquiry.

It may be well, before leaving this subject, to remark that they are

all equally reconcilable with the nebular theory of Kant and Laplace.

Granting that the matter composing our globe has passed successively

through the gaseous and liquid conditions, it is open to us to imagine

the earth as now composed of a liquid nucleus with either a thick or

a thin solid shell; of a solid nucleus and a solid shell with more or

less liquid matter between them; or, lastly, to conceive of it as

having become perfectly solid from the centre to the surface.

[Sidenote: CAUSE OF THE PRESENCE OF WATER IN LAVAS.]

But it is not upon the existence of a high temperature within certain

parts of the earth's crust that the production of volcanic activity

alone depends. The presence of water and other liquid and gaseous

substances in a state of the most intimate admixture with the fused

rock-masses, is, as we have seen, the main cause of the violent

displays of energy exhibited at volcanic centres. And We shall now

proceed to notice the hypotheses which have been suggested to account

for the presence of these liquid and gaseous bodies in the midst of the

masses of incandescent materials poured out from volcanic vents.

There is an explanation of this presence of water and various gases in

the masses of molten rock-materials within the earth's crust which at

once suggests itself, and which was formerly very generally accepted.

Volcanoes, as we have seen, are usually situated near coast-lines, and

if we imagine fissures to be produced by which sea-water finds access

to masses of incandescent rock-materials, then we can regard volcanic

outbursts as resulting from this meeting of water with rock-masses in a

highly healed condition. This supposition has been thought to receive

much support from the fact that many of the gases evolved from volcanic

vents are such as would be produced by the decomposition of substances

present in sea-water.

But it frequently happens that an explanation which at first sight

appears to be very simple and obvious, turns out on more critical

examination to be quite the reverse, and this is the case with the

supposed origination of volcanic outbursts by the access of sea-water

to incandescent rock-material by means of earth-fissures. It is

difficult to understand how, by such means, that wonderfully intimate

union between the liquefied rock and the water, evolved in such

quantities during volcanic outbursts, could be brought about; and

moreover, we can scarcely regard the production of fissures in the

earth's crust as being at the same time both the cause and the effect

of this influx of water to the deep-seated rock-masses at a high

temperature.

[Sidenote: ABSORPTION OF GASES BY LIQUIDS AND SOLIDS.]

During recent years the attention of both geologists and physicists

has been directed to a remarkable property exhibited by many liquids

and solids, as supplying a possible explanation of the phenomena of

volcanic action. The property to which we refer is that whereby some

liquid and solid substances are able to absorb many times their volume

of certain gases--which gases under different conditions may be given

off again from the liquids or solids. This power of absorption is a

very remarkable one; it is not attended with chemical combination, but

the amount of condensation which gases must undergo within the solid or

liquid substances is sometimes enormous. Water may be made to absorb

more than 1,000 times its volume of ammonia, and more than 500 times

its volume of hydrochloric acid. Alcohol may absorb more than 300

times its volume of sulphurous acid. Charcoal may absorb 100 times its

volume of ammonia, 85 times its volume of hydrochloric acid, 65 times

its volume of sulphuretted hydrogen, 55 times its volume of sulphurous

acid, and 35 times its volume of carbonic acid. Platinum-black absorbs

many times its volume of oxygen and other gases.

This power of absorption of gases varies in different solids and

liquids according to the conditions to which they are subjected. Dr.

Henry showed it to be a general law in liquids that, as the pressure is

augmented, the weight of the gas absorbed is proportionately increased.

Sometimes this absorption of gases takes place only at high

temperatures. Thus silver in a state of fusion is able to absorb 22

times its volume of oxygen gas. When the metal is allowed to cool this

gas is given off, and if the cooling takes place suddenly a crust is

formed on the surface, and the phenomenon known as the 'spitting of

silver' is exhibited. Sometimes during this operation miniature cones

and lava-streams are formed on the surface of the cooling mass, which

present a striking resemblance to those formed on a grand scale upon

the surface of the globe. Similar phenomena are exhibited by several

other metals and by the oxide of lead.

The researches of Troost and others have shown that molten iron and

steel possess the property of absorbing considerable quantities of

oxygen, hydrogen, carbonic acid, and carbonic oxide, and that these

gases are given off in the operation known as 'seething,' when either

the pressure or the temperature is diminished.

Hochstetter has shown that in the process of extracting sulphur from

the residues obtained during the manufacture of soda, some very

interesting phenomena are manifested. The molten sulphur is exposed

to a temperature of 262° Fahrenheit, and a pressure of two or three

atmospheres, in the presence of steam; under these circumstances it

is found that the sulphur absorbs a considerable quantity of water,

which is given off again with great violence from the mass as it

undergoes solidification. The hardened crust which forms on the surface

of the molten sulphur is agitated and fissured, miniature cones and

lava-streams being formed upon it, which have a striking resemblance to

the grander phenomena of the same kind exhibited upon the crust of the

globe.

The observations which we have described prove conclusively that many

liquids and solids in a molten condition have the power of absorbing

many times their volume of certain gases, and that this action is aided

by heat and pressure.

That the molten materials which issue from volcanic vents have

absorbed enormous quantities of steam and other gases, we have the

most undisputable evidence. The volume of such gases given off

during volcanic outbursts, and while the lava-streams are flowing

and consolidating, is enormous, and can only be accounted for by

supposing that the masses of fluid rock have absorbed many times their

volume of the gases. But we have another not less convincing proof

of the same fact in the circumstance that volcanic materials which

have consolidated under great pressure--such as granites, gabbros,

porphyries, &c.--exhibit in their crystals innumerable cavities

containing similar gases in a liquefied state.

It is to the violent escape of these gases from the molten rock-masses,

as the pressure upon them is relieved, that nearly all the active

phenomena of volcanoes must be referred; and it was the recognition of

this bet by Spallanzani, while he was watching the phenomena displayed

in the crater of Stromboli, which laid the foundations of the science

of Vulcanology.

[Sidenote: SOURCE OF THE ABSORBED GASES.]

But here another question presents itself to the investigator of the

phenomena of volcanoes: it is this. At what period did the molten

rock-masses issuing from vents absorb those gaseous materials which

are given off so violently from their midst during eruptions? Two

different answers to this question have been suggested. It may be that

the original materials of which our globe was composed consisted of

metallic substances in a state of fusion which had absorbed many gases,

and that, in the fluid masses below the solid crust, vast quantities of

vapour and gas are stored up, which are being gradually added to the

atmosphere during volcanic outbursts. The fact that meteorites, which,

as we have seen, in all probability closely resemble the materials

forming the earth's interior, sometimes yield many times their volume

of hydrogen and other gases, may be thought to lend some support to

this idea. If it be the correct one, we must regard our globe as

gradually parting with its pent-up stores of energy, in those absorbed

gases and vapours held in bondage by the solid and fluid materials of

its interior.

But there is another hypothesis which is, to say the least, equally

probable. Water containing various gases in solution is continually

finding its way downwards by infiltration into the earth's crust.

Much of this water, after passing through pervious beds, reaches

some impervious stratum and is returned to the surface in the form

of springs. But that some of this percolating water penetrates to

enormous depths is shown by the fact that the deepest mines and borings

encounter vast underground supplies of water. When we remember that

nearly three-fourths of the earth's surface is covered by the waters

of the ocean, and that the average depth of these oceanic waters is

more than 10,000 feet, we may easily understand how great a portion of

the earth's crust must be penetrated by infiltrating waters which can

find no outlet in springs. The penetration of the waters of the ocean

into the earth's crust will be aided, too, by the enormous pressure

amounting to not less than several tons to the square-inch upon the

greater part of the ocean-floor. It might be thought that this downward

penetration of water would be counteracted by the upward current of

steam that would be produced as these subterranean waters reach the

hotter portions of the earth's crust. But the experiments of Daubrée

have conclusively shown that the penetration of water through rocks

takes place in opposition to the powerful pressure of steam in the

contrary direction. Hence, we may assume that certain quantities of

water, containing various gases and solids in solution, are continually

finding their way by capillary infiltration from the surface to

the deeply seated portions of the earth's crust, there to undergo

absorption by the incandescent rock-masses and to produce oxidation of

some of their materials.

[Sidenote: POSITION OF THE ISOGEOTHERMS.]

The deep-sea soundings of the 'Challenger' have shown that the floor of

the ocean is constantly maintained at a temperature but little above

that of the freezing point of water. This low temperature is probably

produced by the absorption of heat from the earth's crust by the waters

of the ocean, which distribute it by means of convection currents on

the grandest scale. Hence, the isogeotherms, or lines indicating the

depths at which the same mean temperature is found within the earth's

crust, are probably depressed beneath the great ocean-floors, and rise

towards the land-masses. It is to this circumstance, combined with

that of the enormous pressure of water on the ocean-beds, that we must

probably ascribe the general absence of volcanoes in the deep seas and

their distribution near coast-lines.

We have thus briefly reviewed the chief hypotheses which have been

suggested in order to account for the two great factors in all volcanic

phenomena--namely, the presence of highly heated rock-masses within

the earth's crust, and the existence of various vapours and gases in a

state of most intimate mechanical, but not chemical, union with these

incandescent materials. It must be admitted that we do not at present

appear to have the means for framing a complete and consistent theory

of volcanic action, but we may hopefully look forward to the time when

further observation and experiment shall have removed many of the

existing difficulties which beset the question, and when by the light

of such future researches untenable hypotheses shall be eliminated and

the just ones improved and established.

But if we are constrained to admit that a study of the observed

phenomena and established laws of volcanic action have not as yet

enabled us to frame any complete and satisfactory theory on the

subject, we cannot lose sight of the fact that all modern speculation

upon this question appears to be tending in one definite direction. It

is every day becoming more and more clear that our earth is bound by

ties of the closest resemblance to the other members of that family of

worlds to which it belongs, and that the materials entering into their

constitution, and the forces operating in all are the same.

We have had occasion in a previous chapter to point out that there

are the strongest grounds for believing the interior of our globe to

consist of similar materials to those found in the small planetary

bodies known as meteorites. That the comets are merely aggregations

of such meteorites, and that the planets differ from them only in

their greater dimensions, may be regarded as among the demonstrated

conclusions of the astronomer. The materials found most abundantly in

meteorites and in the interior of our globe are precisely the same as

those which are proved to exist in an incandescent state in our sun.

Hence we are led to conclude that the whole of the bodies of the solar

system are composed of the same chemical elements.

[Sidenote: ERUPTIVE ACTION IN THE SUN.]

That the forces operating in each of these distant bodies present

striking points of analogy is equally clear. The sun is of far greater

dimensions than our earth, and is still in great part, if not entirely,

in a gaseous condition. The great movements in the outer envelopes of

the sun exhibited in the 'sun-spots' and 'solar prominences,' recall

to the mind the phenomena of volcanic activity upon our globe. But

the vast energy still existing in the intensely heated mass of the

sun, and the wonderful mobility of its gaseous materials, give rise to

appearances beside which all terrestrial outbursts seem to sink into

utter insignificance. Vast cavities of such dimensions that many globes

of the size of our earth might be swallowed up in them are formed

in the solar envelopes in the course of a few days or hours. Within

these cavities or sun-spots incandescent vapours are observed, rushing

upwards and downwards with almost inconceivable velocity.

The drawings made by Secchi, and reproduced in figs. 89 and 90, will

give some idea of the appearances presented by these great holes in the

solar envelopes.

[Illustration: Fig. 89.--A group of Sun-spots. (After Secchi.)]

In fig. 89 a group of sun-spots is represented and, in their circular

outlines and tendency to a linear arrangement, they can scarcely fail

to remind anyone familiar with volcanic phenomena of terrestrial

craters, though their dimensions are so much greater.

In fig. 90 the sun-spot represented shows the presence of large

floating masses of incandescent materials rushing upwards and downwards

within the yawning gulf.

[Sidenote: PHENOMENA OF SUN-SPOTS.]

[Illustration: Fig. 90.--A Sun-spot, showing the great masses of

incandescent vapour rising or falling within it. (After Secchi.)]

[Illustration: Fig. 91.--The edge of a Sun-spot, showing a portion of

the prominent masses of incandescent gas (A), which detached itself at

E and floated into the midst of the cavity.]

From fig. 91, taken from a drawing by Mr. Norman Lockyer, we may

understand the movements of these great protuberances of incandescent

gas which are seen on the sides of the sun-spots.

The so-called solar prominences present even more striking resemblances

to the volcanic outbursts of our globe.

Two drawings made by Mr. Norman Lockyer will serve to give some idea

of the vast dimensions of these solar prominences, and of the rapid

changes which take place in their form.

[Illustration: Fig. 92.--Drawing of a Solar prominence, made by Mr.

Norman Lockyer on March 14, 1869, at 11 H. 5 M. A.M.]

The masses of incandescent gas were estimated as being no less than

27,000 feet in height, yet in ten minutes they had totally changed

their form and appearance, as shown in fig. 93.

Even still more striking are the changes recorded by Professor Young,

of New-Haven, in a solar prominence, which he observed on September 7,

1871.

[Illustration: Fig. 93.--The same object, as seen at 11 H. 15 M. on the

same day.]

[Sidenote: SOLAR PROMINENCES.]

That astronomer described a mass of incandescent gas rising from

the surface of the sun to the height of 54,000 miles. In less than

twenty-five minutes he saw the whole mass torn to shreds and blown

upwards, some of the fragments being in ten minutes hurled to the

height of 200,000 miles above the sun's surface. The masses of

incandescent gas thus hurled upwards were of enormous dimensions, the

smallest being estimated as having a greater area than the whole of the

British Islands, and the force with which they were urged upwards was

so great that they acquired a velocity of 166 miles per second. The

accompanying woodcut shows the successive appearances presented by this

grand eruptive outburst on the surface of the sun.

[Illustration: Fig. 94.--Drawings of a Solar prominence at four

different periods on Sept. 7, 1871. (After Young.)]

[Sidenote: EXTINCT VOLCANOES OF THE MOON.]

The moon, which is of far smaller size than our earth, exhibits on

its surface sufficiently striking evidences of the action of volcanic

forces. Indeed the dimensions of the craters and fissures which cover

the whole visible lunar surface are such that we cannot but infer

volcanic activity to have been far more violent on the moon than it

is at the present day upon the earth. This greater violence of the

volcanic forces on the moon is perhaps accounted for by the fact that

the force of gravity on the surface of the moon is only one-sixth of

that at the surface of the earth; and thus the eruptive energy will

have a much less smaller resistance to overcome in bursting asunder the

solid crust and accumulated heaps of ejected materials on its surface.

But the volcanic action on the moon appears now to have wholly ceased,

and the absence of both water and atmosphere in our satellite suggests

that this extinction of volcanic energy may have been caused by the

complete absorption of its gaseous envelope. The appearance presented

by a portion of the moon's surface is shown in fig. 95.

The sun and the moon appear to exhibit two widely separated extremes

in the condition assumed during the cooling down from a state

of incandescence of great globes of vaporised materials. The

several planets, our own among the number, probably exhibit various

intermediate stages of consolidation.

[Illustration: Fig. 95.--A group of Lunar craters (Maurolycus,

Barocius, etc.), the largest being more than 60 miles in diameter.]

[Sidenote: ERUPTIVE ACTION IN THE SUN, EARTH AND MOON.]

Our earth is, as we have seen, closely allied to the other bodies of

the solar system in its movements, its relations, and its composition;

and a true theory of terrestrial vulcanicity, when it is discovered,

may be expected not only to afford an explanation of the phenomena

displayed on our own globe, but to account for those displays of

internal energy which have been manifested in other members of the same

great family of worlds.

INDEX.

[The subjects illustrated in the engravings are indicated by _italics_,

the names of authors are in Capitals.]

ABICH, cited, 122

-- researches of, 4

Absorption of gases by liquids and solids, 354, 355

Acid lavas, 48

Æolian Islands. _See_ Lipari Islands

Æolus, origin of myth, 35

Africa, volcanoes of, 227

-- South, diamonds of, 147

Agates, formation of, 150

Allport, Mr., cited, 259

Alps, formation of, 292

Altered lavas, names given to, 261

America, volcanoes of, 227

Amygdaloids, formation of, 140, 141

Andesites, 50, 59

Andesite-volcanoes, 126

Andrews, Professor, cited, 321

Anne Boleyn and Etna, 3

Armstrong, Sir W., hydro-electric machine, 29

Arthur's Seat, 275

Artificial stone, 55

Asia, volcanoes of, 227

Asiderites, 316

Asmanite, 314

Astroni, crater-ring of, 170

Atlantic, volcanoes in, 223

Auvergne, _breached cones of_, 123, fig. 40

-- _denuded cones in_, 124, fig. 42

-- incrusting springs of, 184

-- puys of, 152, 212

-- volcanic cones in, 79

BALL-AND-SOCKET structure in basaltic columns, 107

Barrancos, formation of, 209

Basalt, controversy concerning origin of, 249

Basalts, 49, 50, 59

Basaltic columns of Bohemia, 107

-- -- of Central Germany, 107

-- -- of Monte Albano, 107

-- -- _from the Giant's Causeway_, 107, fig. 29

Basic lavas, 48

Bath, hot spring of, 219

Ben Nevis, 274

Bohemia, volcanoes of, 126

-- lavas of, 103

Boiling. _See_ Ebullition

Bonney, Professor, cited, 69, 109, 259

Boracic acid at volcanic vents, 216

_Bourbon, volcano of_, 176, figs. 74, 75

_Bracciano, crater-lake of_, 178, fig. 77

_Breached cones_, 123, fig. 40

Babbles of steam, escape from lava, 21

_Bubbles, spontaneous movement of, in liquid cavities_, 62, fig. 8

-- -- cause of, 65

Buch, Von, researches of, 4

Buda-Pesth, deep well of, 335, 341

Büdos Hegy, Transylvania, 215

Bunsen, cited, 201

Burning, does not take place at volcanoes, 2

CADER IDRIS, 274

'Calderas,' formation of, 180

Caldera of Palma, 209

Cambro-Silurian volcanoes of British Islands, 274

_Campi-Phlegræi, map of_, fig. 11

-- -- volcanoes of, 79

-- -- tuff-cones of, 118

-- -- fissures in, 197

Carbonic acid in cavities of crystals, 63

Carboniferous volcanoes of British Islands, 275

Carlsbad, Strudel of, 218

-- Strudelstein of, 184

Caspian Sea, mud-volcanoes of, 182

Catacecaumene, volcano cones in, 79

Cause of proximity of volcanoes to sea, 239

Central Asia, volcanoes of, 236

-- America, mud-volcanoes, 182

-- Pacific, volcanoes of, 236

'Challenger,' H.M.S., voyage of, 73

-- -- soundings of, 359

Chance, Messrs., of Birmingham, 55

Charnwood Forest, ancient volcanic rocks of, 259

Chemical deposits at Vulcano, 44

-- -- on surfaces of lavas, 110

-- elements present in lavas, 46

-- theory of volcanoes, 344, 346

Chiaja di Luna, 108

Chimborazo, size of, 44

-- 151

Chodi-Berg, Hungary, 161

_Citlaltepetl, view of_, 169, fig. 69

Coast-lines, proximity of volcanoes to, 228

Cole, Mr. Grenville, 110

Colours of lavas, 49

Columns in lava, 105

-- -- dimensions of, 105

-- radiating in intrusive masses, 136

Columnar structure in lavas, 104

-- -- origin of, 105

_Columnar lava-stream on the Ardèche_, 107, fig. 28

Combustion, does not take place at volcanoes, 2

Composite cones, 128, 161

Comstock mines, temperature of, 342

_Concentric jointing in lava_, 108, fig. 30

_Cones composed of viscid lava_, 129, fig. 43

-- miniature on lava-streams, 100, 101, figs. 25, 26

-- natural sections of, 129

-- shifting of axis in, 167

Coolin Hills, Skye, 144

Cotopaxi, volcanic dust of, 69

-- _view of_, 168, fig. 68

Craters, formation of, 82

-- origin of, 167

-- position of, 167

-- fissuring of sides, 180

Crater of Stromboli, aperture at bottom of, 15

Crater-lakes, formation of, 171

-- of Agnano, 171

-- of Albano, 171

-- of Avernus, 171

-- _of Bagno_, 171, fig. 71

-- of Bolsena, 171

-- of Bracciano, 171

-- of Frascati, 173, 175

-- _of Gustavila_, 171, fig. 72

-- of Laach, 171

-- of Nemi, 171

Crater-rings, formation of, 170

_Crater-ring of Somma_, 177, fig. 76

Crater-ring of Pianura, 174

-- -- of Piano di Quarto, 174

-- -- of Vallariccia, 174

Creuzot, shafts at, 340

'Critical point' of liquids, 63

Crust of globe, definition of, 308

Crystals in lavas, 51

-- -- formed of crystallites, 54-57

-- -- formed in subterranean reservoirs, 60

-- -- interruption in growth of, 60

-- pressure under which formed, 65

-- deposited on surface of lava, 110

-- porphyritic, origin of, 256

Crystalline minerals formed beneath volcanoes, 146, 147

-- -- ejected from volcanoes, 147

Crystallised minerals of volcanoes, 46

Crystallites, aggregates of, 54, _Frontispiece_

Crystallites in lavas, 53, _Frontispiece_

Crypto-crystalline base, 57

'Cupolas,' 135

Corral of Madeira, 209

DACITES, 198

Dana, Professor, J. D., cited, 100, 159, 291, 301, 327, 338, 339

Darwin, Mr., cited, 245, 246, 271, 289

Daubeny, cited, 182

Daubrée, M., cited, 147, 315, 320, 358

Daubréelite, 314

Davy, Sir Humphry, chemical theory of volcanoes, 344, 345

Deccan of India, 103

Density of the earth, 306

_Denuded cones and craters_, 158, fig. 59

Denudation, effects of, on volcanoes, 114

Deposits about volcanic fissures, 42

Detonations at Vesuvius, 26

Devonian volcanoes of British Islands, 274

Diorite, 59

Dolomieu, cited, 4, 39

Durocher, cited, 201

Dykes, formation of, 116, 117, 209, 210

-- structure of rock in, 211

-- pseudo-, 119

Dynamical theory of volcanoes, 347, 348

EARTH'S interior, nature of, 309

-- -- physical condition of, 325

-- -- hypothesis concerning, 328-330

-- relation to other planets, 310, 311

Earthquakes, depth of origin of, 343, 344

-- connection with volcanoes, 343

-- accompanying Vesuvian eruption of 1872, 27

Ebullition, compared to volcanic eruptions, 19, 20

Eifel, volcanic cones of, 45

Ejected blocks, 45

-- materials, height to which thrown, 72

-- -- stratification of, 117-119

Elements, pyroxenic and trachytic, theory of, 201

Elevation-craters, theory of, 135, 200

Erroneous opinions, sources of, in regard to volcanoes, 2

Eruptions, feeble and violent compared, 31

-- prediction of, not possible, 32

-- intervals between, 33

-- of varying intensity, 33

-- and barometric pressure, 36

-- effects of repetition of from same fissure, 80

Eruptive action in sun and moon, 360-369

Etna, ideas of ancients concerning, 3

-- and Anne Boleyn, 3

-- observatory on, 37

-- size of, 44

--, 151

-- eruptions at summit and on flanks, 207

_Etna, dyke and lava-stream in_, 133, fig. 54

_Etna, views of_, 162, 163, figs. 62, 63

Euganean Hills, 139

-- -- volcanoes of, 201

Europe, volcanoes of, 227

Extra-terrestrial rocks, 316

_Extra-terrestrial rocks, relation to ultra-basic rocks_, 322, fig. 88

FELSTONES, 263

Ferric-chloride, mistaken for sulphur, 41

_Fissure on flanks of Etna_, 194, fig. 84

Fissure-eruptions, 188

Fissures, volcanic cones on, 194

-- systems of, 198

Fingal's Cave, 106

Flames, phenomena mistaken for, 2

-- at volcanoes, feebly luminous, 17

-- false appearance of, in volcanoes, 17

Flames at volcanic vents, 41

Flashing lighthouse, compared to Stromboli, 10

Floods, accompanying volcanic outbursts, 30

Forbes, Mr. David, cited, 337, 339

Fossils, from beneath Vesuvius, 45

-- supposed in basalt, 250

Fouqué, M., cited, 110, 213

Fumaroles, gases emitted from, 213

Fusiyama, form of, 90, 166

_Fusiyama_, 178, fig. 77

GABBRO, 59

Gardiner's river, travertine terraces of, 185

Gases emitted from volcanoes, 40

-- -- volcanic vents, 212-216

Geanticlinals, formation of, 297

Gems, formation of, 147

-- mode of occurrence, 148

Geological continuity, doctrine of, 247

Geosynclinals, formation of, 294

Geysers, formation of, 217

-- intermittent action of, 218

-- of Colorado, 184, 217

-- of Iceland, 184, 217

Giant's Causeway, 108

Gilbert, Mr. G. K., cited, 208

Girgenti, mud-volcanoes of, 182

Glass, formed by fusion of lavas, 52

Glasses, composed of certain silicates, 58

Glassy base, 57

Goethe, cited, 112

_Graham Isle_, 178, 179, fig. 78

Graham, cited, 345

Grand Sarcoui, Auvergne, 161

Granite, 59

Granite of Secondary and Tertiary ages, 254

Granitic rocks, position beneath volcanoes, 145

Great earth movements, nature of, 286

Great volcanic bands of the globe, 232-234

Grenelle, boring of, 341

Greystones, 49

Groundmass of lavas, 52

Grotto del Cane, 215

Guevo Upas, Java, 215

Guiscardi, Professor, referred to, 45

_Gustavila, crater-lake of_, 172, fig. 72

HAMILTON, Sir W., researches of, 4, 75, 84

-- -- observations on Vesuvius, 80

Hannay, Mr., referred to, 147

Hartley, Mr. Noel, referred to, 65

Hawaii, volcanoes of, 100, 125

-- -- lava-masses of, 159

-- -- volcanic eruptions at different levels, 327

Hebrides, volcanoes of, 271

Henry, Dr., cited, 355

Henry Mountains, Southern Utah, 208

Hephæstus, forge of, 3

Hochstetter, cited, 135, 356

Holosiderites, 315

Hopkins, Mr., cited, 349

Hot springs, numbers of, 219

Humboldt, researches of, 4

Hungary, lavas of, 96, 103

-- volcanoes of, 126, 201

-- deep wells of, 341

_Hverfjall, Iceland_, 178, fig. 77

Hydro-electric machine of Sir W. Armstrong, 29

Hypothesis, value of, 331-333

ICE under lava of Vesuvius in 1872, and of Etna, 110

Iceland, volcanic dust of, carried to Norway, 72

Indian Ocean, volcanoes in, 229

_Insel Ferdinandez_, 178, 179, fig. 78

Intermediate lavas, 48

Intervals between Eruptions, 33

Ireland, north-east of, 103

_Iron in Ovifak-basalts_, 319, fig. 87

Iron, seething of, 356

-- of Ovifak, terrestrial origin of, 320

Ischia, eruption in 1301, 164

-- _crater-lake of Bagno in_, 172, fig. 71

-- _plan of_, 163, fig. 64

-- _parasitic cones in_, 164, fig. 65

Island of Bourbon, 93

_Isle Julie_, 178, 179, fig. 78

Isogeotherms, 359

JANSSEN, referred to, 42

Joint-structures in lava, 104-110

_KAMMERBÜHL_, 112-114, fig. 33

-- _section of_, 114, fig. 34

-- _section in side of_, 118, fig. 36

Kant, nebular hypothesis of, 352

Kilauea, volcano of, 71, 138

-- crater of, 181

King, Mr. Clarence, cited, 301

LAACHER SEE, minerals ejected at, 149

_Lac Paven, Auvergne_, 171, fig. 70

'Laccolites,' formation of, 208

Lago di Bolsena, 173, 175

Lago di Bracciano, dimensions of, 172, 173

Lake Avernus, 215

Lapilli, 70

Laplace, nebular hypothesis of, 325, 352

Lavas, action of acid gases on, 41

-- resemblance to slags, 46

-- chemical elements in, 46

-- oxygen in, 47

-- silicon in, 47

-- proportion of silica and other oxides in, 47

-- silicates in, 47

-- acid, intermediate, basic, 48

-- specific gravities of, 49

-- colours of, 49

-- microscopic study of, 50

-- fusibility of, 51

-- minerals in, 51

-- artificially fused, 51

-- crystals in, 51, 93

-- ground mass of, 52

-- crystalline forms of, 59

-- of Bohemia, 103

-- of Hungary, 96, 103

-- of Kilauea, 95

-- of Lipari, 96

-- of Niedermendig, 103

-- of Vesuvius, 104

-- of Volvic, 95

-- of Volcano, 95

-- presence of water In, 102

-- chemical deposits on, 110

-- different fluidity of, 204

-- augite and hornblende in, 267

_Lava, cascade of_, 93, fig. 18

Lava-cones, composed of liquid lava, 125

-- -- of viscid lava, 126, 127

-- characters of, of liquid lava, 159

-- -- of viscid lava, 160

_Lava-cones, outlines of_, 160, fig. 60

Lava, in deep-seated reservoirs, 138

-- consolidation of, at great depths, 139

Lava-fountains, 94

_Lava-sheets, intrusive_, 136, 137, fig. 56

'Lava' ornaments of Naples, 45

'Lava,' slow-cooling of, 110

-- a bad conductor of heat, 110

-- ice under, 110

Lava-streams, nature of movements, 92

-- difference in liquidity of, 92

-- miniature cones on, 100, 101

-- vast dimensions of, 102

-- structure of, 103

-- position of columns in, 106

-- sinking of surface of, 111

'Lave di fango,' 30

'Lave di fuoco,' 30

Lawrencite, 314

Laws of volcanic action, 38

Le Conte, cited, 347

Leucite, absence from ancient lavas, 268

Lightning, accompanying volcanic outbursts, 28

Linear arrangement of volcanic vents, 191

-- -- of volcanoes, 231

Lipari Islands, 3, 39

-- -- fissures in, 197

-- -- pumice-cones in, 154

-- -- order of appearance of lavas in, 200

-- -- _breached pumice-cones in_, 124, fig. 41

-- -- _map of_, 192, fig. 81

-- -- _lavas of_, 96, figs. 20, 21

Liquids in cavities of crystals, 63

_Liquid cavities in lavas_, 60, fig. 7

-- -- _spontaneous movement of bubbles in_, 62, fig. 8

-- -- spontaneous movement of bubbles in, cause of, 65

Lockyer, Mr. Norman, cited, 322, 363, 364

_Lunar craters_, 368, fig. 95

Lyell, Sir Charles, cited, 135, 167, 197

MACCULLOCH, cited, 207, 208

_Madeira, cliff-section in_, 128, fig. 47

Magmas, theory of, 201

-- objections to, 202, 203

Mallet, Mr., cited, 269, 343, 346

_Mamelons of Bourbon_, 126, 127, figs. 45, 46

Maskelyne, Professor, cited, 314

Massa di Somma, destruction of, 26

Mauna Loa, 138

Metamorphism around volcanic vents, 145

Meteorites, nature of, 312

-- composition of, 313

-- minerals of, 314

-- classification of, 315

Melaphyres, 262

Miascite, 59

Michel Lévy, M., 110

Micro-crystalline base, 58

Microliths. _See_ Crystallites

Microscopic study of lavas, 50

Minerals in lavas, 51

-- of Vesuvius, 46

Mineral-veins, formation of, 149

-- connection with volcanoes, 220

-- nature of materials in, 321

_Misenum, Cape of, section of tuff-cone of_, 121, fig. 38

Modena, mud-volcanoes of, 182

_Mont Dore, section at_, 130, fig. 48

Monte Cerboli, Tuscany, 216

Monte Massi, Tuscany, well at, 341

Monte Nuovo, history of formation of, 76

-- -- _description of_, 77, 78, fig. 10

-- -- 152

-- -- crater of, 168

-- -- production of fissure at, 190

Monte Rotondo, Tuscany, 216

Moon, effect of internal forces on, 305

Mountains, all volcanoes not, 2

Mountain-chains, formation of, 291

-- -- all of recent date, 292

Mud-streams at volcanoes, 30

Mud-volcanoes, formation of, 181, 182

_Mull, dissected volcano of_, 142-4, figs. 57, 58

Muscovite, absence of, from modern lavas, 268

NEBULAR hypothesis of Laplace, 325, 352

-- -- of Kant, 352

New Zealand, geysers of, 217

-- -- volcanoes of, 135

-- -- volcanic cones in, 79

Niedermendig, lava of, 103

Nordenskiöld, Professor, cited, 318

OBSERVATORY on Vesuvius, 24, 37

-- on Etna, 37

Obsidian, 59

Oceans, depth of, in volcanic areas, 242

Oceanic islands, volcanoes in, 228

Oliver, Capt. S. P., 92

Oldhamite, 314

_Outlines of Vesuvius_, 87, fig. 17

Ovifak, iron-masses of, 319

Oxidation of materials of globe, 324

Oxygen, proportion in lavas, 47

PACIFIC, volcanoes in, 229

Palmieri, Professor, cited, 25, 37

Papandayang, eruption of, 169

Papin's digester, nature of action in, 22

_Parasitic cones, formation of_, 161, 162, fig. 61

Pele's Hair, 71

Perlitic structure, 109

Phillips, Mr. J. A., cited, 220

Phonolites, 50, 59

Phonolite-volcanoes, 126

_Photograph of Vesuvius eruption_, 24, fig. 5

'Pine-tree, appendage of Vesuvius, 29

Pitchstones, porphyritic, 60

Plateaux formed of lava-sheets, 270

Pliny, Elder, death of, 7

Plombières, hot springs of, 147

Plutonic rocks, 61

Pompeii, nature of materials covering, 117

Ponza Islands, 39

_Ponza, sections in_, 131, 132, figs. 51, 52

Porphyrites, 263

Porphyritic pitchstones, 60

Potentially liquid rock, 250

Pre-Cambrian volcanoes of British Islands, 274

Presence of water in lavas, 353

Pressure under which crystals were formed, 65

_Predazzo, ancient volcano of_, 165, fig. 67

Propylites, 199

Pseudo-dykes, 119

Pumice, how formed, 68

-- cause of white colour of, 71

-- floating on ocean, 73

-- on ocean-beds, 73

Pumice-cones, 154

_Puy de Pariou, Auvergne_, 193, 194, figs. 82, 83

Puzzolana, 89

RAIN, accompanying volcanic outbursts, 30

Rate of movement of lava-streams, 97

Rath, Professor Vom, 72

Red clay of ocean-beds, 74

Red Mountains, Skye, 144

Reservoirs beneath volcanoes, 145

Reyer, Dr. Ed., experiments of, 125, 160

Reykjanes, eruption of, in 1783, 102

Rhyolites, 50, 59

Richthofen, Von, cited, 196, 199, 200, 205

_Rocca-Monfina_, 178, fig. 77

-- --, 204

Rock-masses, movements of, 288

Rocky Mountains, 103

-- -- volcanoes of, 201

Rotomahana, sinter-terraces of, 185

_Ropy-lavas_, 98, fig. 24

_SALINA, section in_, 132, fig. 53

Sandwich Islands, lavas of, 125

San Sebastiano, destruction of, 26

_San Stephano, section in_, 131, fig. 50

Santorin, 42

_Sarcoui, Grand Puy of_, 126, fig. 44

Sciarra del fuoco, 13

Scoria, how formed, 68, 70

Scoria-cones, altered by acid gas, 155

-- breached, 156

-- characters of, 153

-- preservation of, 155

-- red colour of, 154

_Scoria-cone in Vesuvius_, 122, fig. 39

_Scoria-cone near Auckland, N. Z._, 1656, fig. 66

Schmidt, referred to, 153

Schreibersite, 314

Scrope, Mr. Poulett, cited, 5, 69, 106, 135, 198, 205, 212, 238, 289

Sea of Azof, mud-volcanoes of, 182

Secchi, Father, cited, 362

Shiant Isles, 105

Silica, presence in lavas, 47

Silicates in lavas, 47

Silicon, proportion in lavas, 47

Siliceous sinter, deposits of, 220

Silver, spitting of, 355

Silvestri, Professor, cited, 230

Similarity of lavas of different ages, 260

_Sinter-cones, forms of_, 183, fig. 79

Skye, dissected volcano of, 144

Slags, compared with lavas, 46

Smith, Lawrence, cited, 320

Smoke, appearance of, due to steam, 2

Snowdon, 274

_Solar prominences_, 364-366, figs. 92, 93, 94

Solfatara of Naples, 214

Solfatara-stage of volcanoes, 215

Somma, 133

-- crater-ring of, 83

Sorby, Mr. H. C., referred to, 59, 252

Spallanzani, early researches of, 4

-- observations on Stromboli, 8

-- cited, 39, 367

Specific gravities of lavas, 49

-- -- of glassy and crystalline rocks, 59

Spectroscope in vulcanology, 41

Spectrum-analysis, results of, 311

Specular-iron, deposited on lava-streams, 110

Sperenberg, boring of, 341

_Sphærulites_, 54, _Frontispiece_

Sporadosiderites, 316

Stability of crust of globe, 326

Staffa, Isle of, 106

Steenstrup, cited, 319

Steam-engine compared to volcano, 8

Steam, emitted by lava of Vesuvius, 27

Sternberg, referred to, 113

St. Kilda, 181

Stokes, Professor, 65

St. Paul, Island of, 180

Stromboli, 42, 158

-- apertures at bottom of crater, 15

-- appearances in crater of, 16

-- -- at night, 10

-- compared with Vesuvius, 23

-- crater of, 13

-- dependence of eruptions on atmospheric conditions, 34

-- eruption of, 14, fig. 4

-- general features of, 11

-- map of, 11, fig. 2

-- observations by Spallanzani, 8

-- resemblance to flashing light, 10

-- _section of_, 13, fig. 8

-- soundings around, 12

-- vapour-cloud above, 9

-- violent eruptions of, 23

Strombolian stage, 23

Stufas, nature of, 217

Submarine volcanoes, 179

Subterranean forces, beneficial effects of, 303

Subsidence in centre of volcanoes, 165

Sulphur, absorption of water by molten, 356

-- deposited on lava-streams, 110

-- how formed at volcanoes, 18

-- not the cause of volcanic outbursts, 18

_Surfaces of lava-streams_, 97-99, figs. 22, 23

_Sun-spots_, 361-363, figs. 89, 90, 91

Syenite, 59

Syssiderites, 316

Szabo, Professor, cited, 199

TACHYLYTE, 59

Tertiary volcanoes of British Islands, 276

_Terraces, sinter- and travertine-formation of_, 185, fig. 80

Temperature, increase in deeper parts of earth's crust, 335

-- rate of increase in different areas, 340

Teneriffe, 44, 151

_Teneriffe_, 178, fig. 77

-- _peak of_, 175, fig. 73

Tenon-and-mortise structure in basaltic columns, 107

Theodosius and Vulcano, 3

Thunder, accompanying volcanic outbursts, 28

Trachytes, 49, 50, 69

Trap-rocks, origin of, 241

Trass, 90

Travertine or Tibur-stone, 184

-- deposits of, 220

Triassic volcanoes of British Islands, 275

Tridymite deposited on lava-streams, 110

Troilite, 314

Troost, cited, 355

Tufa, or tuff, 90

Tuff-cones, character of, 157

-- denudation of, 157, 158

Typhon, fable of, 3

ULTRA-BASIC lavas, 50, 66

-- rocks, 317

VAL DEL BOVE, Etna, 133, 180, 209

-- -- _dykes in_, 134, fig. 55

Vapour-cloud over Vesuvius, 26, 29

-- -- Stromboli, 9

Ventotienne, Island of, section at, 130, fig. 49

Vesuvius, 37

-- changes in form of, 81

-- compared with Stromboli, 23

-- _crater of in 1756_, 84, fig. 14

-- -- _of in 1767_, 85, fig. 16

-- -- _of in 1822_, 82, fig. 13

-- -- _of in 1843_, 86, fig. 16

-- detonations at, 26

-- early history of, 83

-- eruption of year 79, 84

-- -- of 1822, 69

-- -- of April 1872, 24

-- -- of October 1822, 24

-- ejected blocks of, 45

-- first eruption of, 7

-- form of, 166

-- fossils of, 45

-- growth of cone of, 80

-- history of, 204

-- last eruption of, 7

-- lava-stream of 1855, 101

-- lava-streams of 1858, 1872, 97

-- lavas of, 104

-- minerals of, 46

-- -- ejected at, 149

-- observatory on, 24, 37

-- outlines of, 87

-- pine-tree appendage of, 29

-- scoria-cones in lava, 122

-- -- on lava of 1855, 153

-- steam emitted by lava of, 27

-- vapour-cloud over, 26, 29

Vesuvian stage, 23

-- _eruption, photograph of_, 24, fig. 5

Viscid lavas of Lipari Islands, 94-96

Vitreous lavas, devitrification of, 259

Volcanic action, laws of, 32

-- bombs, 70, 71

-- cycles, nature of, 221, 222

-- -- duration of, 223

-- cones, internal structure of, 115-122

-- -- _experimental illustration of formation of_, 120, fig. 37

-- -- limits to height of, 166

-- -- form of, 152

-- -- dimensions of, 152

-- -- irregular development of, 90

-- -- slopes of sides of, 91

-- -- composed of ejected rock-fragments, 156

-- -- curved slopes of, 156

-- _débris_ on sea-bottom, 240

-- dust, fineness of, 69

-- districts, areas of upheaval, 245

-- ejections, alteration of, 258

-- eruptions, compared to ebullition, 19, 20

-- forces, compensate for denudation, 283

-- -- intensity at former periods, 278

-- -- necessity for action of, 285

-- -- shifting of from one area to another, 277

-- mountains, origin of conical forms, 89

-- -- mode of growth, 89

-- phenomena of the past similar to those at present, 273

-- products, order of appearance of, 198, 199

Volcanic rocks, 61

-- -- similarity of ancient and modern, 253

Volcano, origin of name, 3

-- craters of, 167

-- Island of. _See_ Vulcano.

-- compared to steam-engine, 8

Volcanoes, blocks, ejected from, 45

-- built up of ejected fragments, 74

-- destruction caused by, 281

-- dissected by denudation, 115, 139

-- erroneous ideas concerning, 1

-- ejection of different materials from, 205

-- known to ancients, 3

-- life-history of, 186

-- number of, 224, 225

-- of Africa, 227

-- of America, 236

-- of Asia, 236

-- of Bohemia, 126

-- of Central Asia, 236

-- of Central Pacific, 236

-- of Europe, 227

-- of Hungary, 126

-- position in relation to mountain chains, 243

-- popular ideas concerning, 1

-- reservoirs beneath, 145

Volvic lava of, 103

Vose, cited, 346

Vulcan, forge of, 3

Vulcano, island of, 3, 158

Vulcano and Theodosius, 3

_Vulcano_, 178, fig. 77

-- _and Vulcanello, view of_, 43, fig. 6

-- chemical deposits at, 44

-- eruption in 1786, 43

-- -- in 1873, 43

-- -- _lava-stream in_, 95, fig. 19

-- --, 103, fig. 27

-- _plan of_, 195, fig. 85

-- _section of volcanic cone in_, 116, fig. 35

-- section in, 129

-- shifting of centre of eruption in, 196

_Vulcanello, craters of_, 197, fig. 86

Vulcanology, origin of the science, 4

-- earliest treatise on, 5

Walferdin, M., cited, 340

Water in lavas, 353

-- penetration through rocks, 358

-- presence of in lavas, 102

-- and saline solutions in cavities of crystals, 63

Werner, cited, 201

Western Isles of Scotland, 103, 139, 142

-- -- volcanoes of, 212

Whymper, Mr., 69

Woodward, Mr., experiments of, 119

Wrekin, ancient volcanic rocks of, 259

YOUNG, Professor, cited, 365

ZEOLITES, formation of, 150

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Transcriber Note

Minor typos corrected. Listing of "The International Scientific Series"

was split between the front and end of book but are here moved to the

end. The list was also reformatted. The captions for the map key in

Figure 58 were reformatted into a table. Figure 41 was relocated as it

followed Figure 42 in the printed version.