THE WORK OF UNDERGROUND WATER.
We have already noted two means by which water finds its way
underground. The simplest and largest method by which this action is
effected is by building in the fluid as the grains of the rock are
laid down on the floors of seas or lakes. The water thus imprisoned is
firmly inclosed in the interstices of the stone, it in time takes up
into its mass a certain amount of the mineral materials which are
contained in the deep-buried rocks. The other portion of the ground
water--that with which we are now to be specially concerned--arises
from the rain which descends into the crevices of the earth; it is
therefore peculiar to the lands. For convenience we shall term the
original embedded fluid _rock water_, and that which originates from
the rain _crevice water_, the two forming the mass of the earth water.
The crevice water of the earth, although forming at no time more than
a very small fraction of the hidden fluid, is an exceedingly potent
geological agent, doing work which, though unseen, yet affords the
very foundations on which rest the life alike of land and sea. When
this water enters the earth, though it is purified of all mineral
materials, it has already begun to acquire a share of a gaseous
substance, carbonic acid, or, as chemists now term it, carbon dioxide,
which enables the fluid to begin its rôle of marvellous activities. In
its descent as rain, probably even before it was gathered in drops in
the cloud realm, the water absorbs a certain portion of this gas from
the atmosphere. Entering the realm of the soil, where the decaying
organic matter plentifully gives forth carbon dioxide, a further store
of the gas is acquired. At the ordinary pressure of the air, water may
take in many times its bulk of the gas.
The immediate effect of carbonic acid when it is absorbed by water is
greatly to increase the capacity which that fluid has for taking
mineral matters into solution. When charged with this gas, in the
measure in which it may be in the soil, water is able to dissolve
about fifty times as much limestone as it can in its perfectly pure
form take up. A familiar instance of this peculiar capacity which the
gas gives may often be seen where the water from a soda-water fountain
drips upon the marble slab beneath. In a few years this slab will be
considerably corroded, though pure water would in the same time have
had no effect upon it.
The first and by far the most important effect of crevice water is
exercised upon the soil, which is at once the product of this action,
and the laboratory where the larger part of the work is done.
Penetrating between the grains of the detrital covering, held in large
quantities in the coating, and continually in slow motion, the
gas-charged water takes a host of substances into solution, and brings
them into a condition where they may react upon each other in the
chemical manner. These materials are constantly being offered to the
roots of plants and brought in contact with the underlying rock which
has not passed into the state of soil. The changes induced in this
stony matter lead to its breaking up, or at least to its softening to
the point where the roots can penetrate it and complete its
destruction. Thus it comes about that the water which to a great
extent divides the rocks into the state of soil, which is continually
wearing away the material on the surface, or leaching it out through
the springs, is also at work in restoring the layer from beneath.
The greater part of the water which enters the soil does not
penetrate to any great depth in the underlying rocks, but finds its
way to the surface after no long journey in the form of small springs.
Generally those superficial springs do not emerge through distinct
channels, but move, though slowly, in a massive way down the slopes
until they enter a water course. Along the banks of any river, however
small, or along the shores of the sea, a pit a few inches deep just
above the level of the water will be quickly filled by a flow from
this sheet which underlies the earth. At a distance from the stream
this sheet spring is in contact with the bed rocks, and may be many
feet below the surface, but it comes to the level of the river or the
sea near their margins. Here and there the shape of the bed rocks,
being like converging house roofs, causes the superficial springs to
form small pipelike channels for the escape of their gathered waters,
and the flow emerges at a definite point. Almost all these sources of
considerable flow are due to the action of the water on the underlying
rock, where we shall now follow that portion of the crevice water
which penetrates deeply into the earth.
Almost all rocks, however firm they may appear to be, are divided by
crevices which extend from the soil level it may be to the depths of
thousands of feet. These rents are in part due to the strains of
mountain-building, which tend to disrupt the firmest stone, leaving
open fractures. They are also formed in other ways, as by the
imperfectly understood agencies which produce joint planes. It often
happens that where rocks are highly tilted water finds its way
downward between the layers, which are imperfectly soldered together,
or a bed of coarse material, such as sandstone or conglomerate, may
afford an easy way by which the water may descend for miles beneath
the surface. Passing through rocks which are not readily soluble, the
water, already to a great extent supplied with mineral matter by its
journey through the soil, may not do much excavating work, and even
after a long time may only slightly enlarge the spaces in which it
may be stored or the channels by which it discharges to the surface.
Hence it comes about that in many countries, even where the waters
penetrate deeply, they do not afford large springs. It is otherwise
where the crevice waters enter limestones composed of materials which
are readily dissolved. In such places we find the rain so readily
entering the underlying rock that no part of the fall goes at once to
the brooks, but all has a long underground journey.
In any limestone district where the beds of the material are thick and
tolerably pure--as, for instance, in the cavern district of southern
Kentucky--the traveller who enters the region notes at once that the
usual small streams which in every region of considerable rainfall he
is accustomed to see intersecting the surface of the country are
entirely absent. In their place he notes everywhere pitlike
depressions of bowl-shaped form, the sink holes to which we have
already adverted. Through the openings in the bottom of these the rain
waters descend into the depths of the earth. Although the most of
these depressions have but small openings in their bottom, now and
then one occurs with a vertical shaft sufficiently large to permit the
explorer to descend into it, though he needs to be lowered down in the
manner of a miner who is entering a shaft. In fact, the journey is
nearly always one of some hazard; it should not be undertaken save
with many precautions to insure safety.
When one is lowered away through an open sink hole, though the descent
may at first be somewhat tortuous, the explorer soon finds himself
swinging freely in the air, it may be at a point some hundred feet
above the base of the bottle-shaped shaft or dome into which he has
entered. Commonly the neck of the bottle is formed where the water has
worked its way through a rather sandy limestone, a rock which was not
readily dissolved by the water. In the pure and therefore easily cut
limestone layers the cavity rapidly expands until the light of the
lantern may not disclose its walls. Farther down there is apt to be a
shelf composed of another impure limestone, which extends off near the
middle of the shaft. If the explorer can land upon this shelf, he is
sure to find that from this imperfect floor the cavern extends off in
one or more horizontal galleries, which he may follow for a great
distance until he comes to the point where there is again a well-like
opening through the hard layer, with another dome-shaped base beneath.
Returning to the main shaft, the explorer may continue his descent
until he attains the base of this vertical section of the cave, where
he is likely to find himself delivered in a pool of water of no great
depth, the bottom of which is occupied by a quantity of small, hard
stones of a flinty nature, which have evidently come from the upper
parts of the cavern. The close observer will have noted that here and
there in the limestone there are flinty bits, such as those which he
finds in the pool. From the bottom of the dome a determined inquirer
can often make his way along the galleries which lead from that level,
though it may be after a journey of miles to the point where he
emerges from the cavern on the banks of an open-air river.
Although a journey by way of the sink holes through a cavern system is
to be commended for the reason that it is the course of the caverning
waters, it is, on the whole, best to approach the cave through their
exits along the banks of a stream or through the chance openings which
are here and there made by the falling in of their roofs. One
advantage of this cavity of entrance is that we can thus approach the
cavern in times of heavy rain when the processes which lead to their
construction are in full activity. Coming in this way to one of the
domes formed beneath a sink hole, we may observe in rainy weather that
the water falling down the deep shaft strikes the bottom with great
force; in many of the Kentucky caves it falls from a greater height
than Niagara. At such times the stones in the basin at the bottom of
the shaft are vigorously whirled about, and in their motion they cut
the rocks in the bottom of the basin--in fact, this cavity is a great
pot hole, like those at the base of open-air cascades. It is now easy
to interpret the general principles which determine the architecture
of the cavern realm.
When it first enters the earth all the work which the water does in
the initial steps of cavern formation is effected by solution. As the
crevice enlarges and deepens, the stream acquires velocity, and begins
to use the bits of hard rock in boring. It works downward in this way
by the mixed mechanical and chemical action until it encounters a hard
layer. Then the water creeps horizontally through the soft stratum,
doing most of its work by solution, until it finds a crevice in the
floor through which it can excavate farther in the downward direction;
so it goes on in the manner of steps until it burrows channels to the
open stream. In time the vertical fall under the sink hole will cut
through the hard layer, when the water, abandoning the first line of
exit, will develop another at a lower level, and so in time it comes
about that there may be several stories of the cave, the lowest being
the last to be excavated. Of the total work thus done, only a small
part is accomplished by the falling of the water, acting through the
boring action of its tools, the bits of stone before mentioned; the
principal part of the task is done by the solvent action of the
carbonated waters on the limestone. In the system of caverns known as
the Mammoth Cave, in Kentucky, the writer has estimated that at least
nine tenths of the stone was removed in the state of solution.
When first excavated, the chambers of a limestone cavern have little
beauty to attract the eye. The curves of the walls are sometimes
graceful, but the aspect of the chambers, though in a measure grand,
is never charming. When, however, the waters have ceased to carve the
openings, when they have been drained away by the formation of
channels on a lower level, there commonly sets in a process known as
stalactitization, which transforms the scene into one of singular
beauty. We have already noted the fact that everywhere in ordinary
rocks there are crevices through which water, moving under the
pressure of the fluid which is above, may find its way slowly
downward. In the limestone roofs of caverns, particularly in those of
the upper story, this ooze of water passes through myriads of unseen
fissures at a rate so slow that it often evaporates in the dry air
without dropping to the floor. When it comes out of the rocks the
water is charged with various salts of lime; when it evaporates it
leaves the material behind on the roof. Where the outflow is so slight
that the fluid does not gather into drops, it forms an incrustation of
limy matter, which often gathers in beautiful flowerlike forms, or
perhaps in the shape of a sheet of alabaster. Where drops are formed,
a small, pendent cone grows downward from the ceiling, over which the
water flows, and on which it evaporates. This cone grows slowly
downward until it may attain the floor of the chamber, which has a
height of thirty feet or more. If all the water does not evaporate,
that which trickles off the apex of the cone, striking on the floor,
is splashed out into a thin sheet, so that it evaporates in a speedy
manner, lays down its limestone, and thus builds another and ruder
cone, which grows upward toward that which is pendent above it.
Finally, they grow together, enlarged by the process which constructed
them, until a mighty column may be formed, sculptured as if by the
hands of a fantastic architect.
[Illustration: Fig. 13.--Stalactites and stalagmites on roof and floor
of a cavern. The arrows show the direction of the moving water.]
All the while that subterranean streams are cutting the caverns
downward the open-air rivers into which they discharge are deepening
their beds, and thereby preparing for the construction of yet lower
stories of caves. These open-air streams commonly flow in steep-sided,
narrow valleys, which themselves were caves until the galleries became
so wide that they could no longer support the roof. Thus we often find
that for a certain distance the roof over a large stream has fallen
in, so that the water flows in the open air. Then it will plunge
under an arch and course, it may be, for some miles, before it again
arrives at a place where the roof has disappeared, or perhaps attains
a field occupied by rocks of another character, in which caverns were
not formed. At places these old river caverns are abandoned by the
streams, which find other courses. They form natural tunnels, which
are not infrequently of considerable length. One such in southwestern
Virginia has been made useful for a railway passing from one valley to
another, thus sparing the expense of a costly excavation. Where the
remnant of the arch is small, it is commonly known as a natural
bridge, of which that in Rockbridge County, in Virginia, is a very
noble example. Arches of this sort are not uncommon in many cavern
countries; five such exist in Carter County, Kentucky, a district in
the eastern part of that State which abounds in caverns, though none
of them are of conspicuous height or beauty.[7]
[Footnote 7: It is reported that one of these natural bridges of Carter
County has recently fallen down. This is the natural end of these
features. As before remarked, they are but the remnants of much more
extensive roofs which the processes of decay have brought to ruin.]
At this stage of his studies on cavern work the student will readily
conceive that, as the surface of the country overlying the cave is
incessantly wearing down, the upper stories of the system are
continually disappearing, while new ones are forming at the present
drainage level of the country. In fact, the attentive eye can in such
a district find here and there evidences of this progressive
destruction. Not only do the caves wear out from above, but their
roofs are constantly falling to their floors, a process which is
greatly aided by the growth of stalactites. Forming in the crevices or
joints between the stones, these rock growths sometimes prize off
great blocks. In other cases the weight of the pendent stalactite
drags the ill-supported masses of the roof to the floor. In this way a
gallery originally a hundred feet below the surface may work its way
upward to the light of day. The entrance by which the Mammoth Cave is
approached appears to have been formed in this manner, and at several
points in that system of caverns the effect of this action may be
distinctly observed.
We must now go a step further on the way of subterranean water, and
trace its action in the depths below the plane of ordinary caves,
which, as we have noted, do not extend below the level of the main
streams of the cavern district. The first group of facts to be
attended to is that exhibited by artesian wells. These occur where
rocks have been folded down into a basinlike form. It often happens
that in such a basin the rocks of which it is composed are some of
them porous, and others impervious to water, and that the porous
layers outcrop on the high margins of the depression and have
water-tight layers over them. These conditions can be well represented
by supposing that we have two saucers, one within the other, with an
intervening layer of sand which is full of water. If now we bore an
opening in the bottom of the uppermost saucer, we readily conceive
that the water will flow up through it. In Nature we often find these
basins with the equivalent of the sandy layer in the model just
described rising hundreds of feet above the valley, so that the
artesian well, so named from the village of Artois, near Paris, where
the first opening of this nature was made, may yield a stream which
will mount upward, especially where piped, to a great height. At many
places in the world it is possible by such wells to obtain a large
supply of tolerably pure water, but in general it is found to contain
too large a supply of dissolved mineral matter or sulphuretted gases
to be satisfactory for domestic purposes. It may be well to note the
fact that the greater part of the so-called artesian wells, or borings
which deliver water to a height above the surface, are not true
artesian sources, in that they do not send up the water by the action
of gravitation, but under the influence of gaseous pressure.
Where, as in the case of upturned porous beds, the crevice water
penetrates far below the earth's surface or the open-air streams which
drain the water away, the fluid acquires a considerable increase of
temperature, on the average about one degree Fahrenheit for each
eighty feet of descent. It may, indeed, become so heated that if it
were at the earth's surface it would not only burst into steam with a
vast explosive energy, but would actually shine in the manner of
heated solids. As the temperature of water rises, and as the pressure
on it increases, it acquires a solvent power, and takes in rocky
matter in a measure unapproached at the earth's surface. At the depth
of ten miles water beginning as inert rain would acquire the
properties which we are accustomed to associate with strong acids.
Passing downward through fissures or porous strata in the manner
indicated in the diagram, the water would take up, by virtue of its
heat and the gases it contained, a share of many mineral substances
which we commonly regard as insoluble. Gold and even platinum--the
latter a material which resists all acids at ordinary
temperatures--enters into the solution. If now the water thus charged
with mineral stores finds in the depths a shorter way to the surface
than that which it descended, which may well happen by way of a deep
rift in the rocks, it will in its ascent reverse the process which it
followed on going down. It will deposit the several minerals in the
order of their solubilities--that is, the last to be taken in will be
the first to be crystallized on the walls of the fissure through which
the upflow is taking place. The result will be the formation of a vein
belonging to the variety known as fissure veins.
[Illustration: Fig. 14.--Diagram of vein. The different shadings show
the variations in the nature of the deposits.]
A vein deposit such as we are considering may, though rarely, be
composed of a single mineral. Most commonly we find the deposit
arranged in a banded form in the manner indicated in the figure (see
diagram 14). Sometimes one material will abound in the lower portions
of the fissure and another in its higher parts, a feature which is
accounted for by the progressive cooling and relinquishment of
pressure to which the water is subjected on its way to the surface.
With each decrement of those properties some particular substance goes
out of the fluid, which may in the end emerge in the form of a warm or
hot spring, the water of which contains but little mineral matter.
Where, however, the temperature is high, some part of the deposit,
even a little gold, may be laid down just about the spring in the
deposits known as sinter, which are often formed at such places.
In many cases the ore deposits are formed not only in the main channel
of the fissure, but in all the crevices on either side of that way. In
this manner, much as in the case of the growth of stalactitic matter
between the blocks of stone in the roofs of a cavern, large fragments
of rock, known as "horses," are often pushed out into the body of the
vein. In some instances the growth of the vein appears to enlarge the
fissure or place of the deposit as the accumulation goes on, the
process being analogous to that by which a growing root widens the
crevice into which it has penetrated. In other instances the fissure
formed by the force has remained wide open, or at most has been but
partly filled by the action of the water.
It not infrequently happens that the ascending waters of hot springs
entering limestones have excavated extensive caves far below the
surface of the earth, these caverns being afterward in part filled by
the ores of various metals. We can readily imagine that the water at
one temperature would excavate the cavern, and long afterward, when at
a lower heat, they might proceed to fill it in. At a yet later stage,
when the surface of the country had worn down many thousands of feet
below the original level, the mineral stores of the caverns may be
brought near the surface of the earth. Some of the most important
metalliferous deposits of the Cordilleras are found in this group of
hot-water caverns. These caverns are essentially like those produced
by cold water, with the exception of the temperature of the fluid
which does the work and the opposite direction of the flow.
In following crevice water which is free to obey the impulses of
gravitation far down into the earth, we enter on a realm where the
rock or construction water, that which was built into the stone at
the time of its formation, is plentiful. Where these two groups of
waters come in contact an admixture occurs, a certain portion of the
rock water joining that in the crevices. Near the surface of the
ground we commonly find that all the construction water has been
washed out by this action. Yet if the rocks be compact, or if they
have layers of a soft and clayey nature, we may find the construction
water, even in very old deposits, remaining near the surface of the
ground. Thus in the ancient Silurian beds of the Ohio Valley a boring
carried a hundred feet below the level of the main rivers commonly
discovers water which is clearly that laid down in the crevices of the
material at the time when the rocks were formed in the sea. In all
cases this water contains a certain amount of gases derived from the
decomposition of various substances, but principally from the
alteration of iron pyrite, which affords sulphuretted hydrogen. Thus
the water is forced to the surface with considerable energy, and the
well is often named artesian, though it flows by gas pressure on the
principle of the soda-water fountain, and not by gravity, as in the
case of true artesian wells.
The passage between the work done by the deeply penetrating surface
water and that due to the fluid intimately blended with the rock built
into the mass at the time of its formation is obscure. We are,
however, quite sure that at great depths beneath the earth the
construction water acts alone not only in making veins, but in
bringing about many other momentous changes. At a great depth this
water becomes intensely heated, and therefore tends to move in any
direction where a chance fissure or other accident may lessen the
pressure. Creeping through the rocks, and moving from zones of one
temperature to another, these waters bring about in the fine
interstices chemical changes which lead to great alterations in the
constitution of the rock material. It is probably in part to these
slow driftings of rock water that beds originally made up of small,
shapeless fragments, such as compose clay slates, sandstones, and
limestones, may in time be altered into crystalline rocks, where there
is no longer a trace of the original bits, all the matter having been
taken to pieces by the process of dissolving, and reformed in the
regular crystalline order. In many cases we may note how a crystal
after being made has been in part dissolved away and replaced by
another mineral. In fact, many of our rocks appear to have been again
and again made over by the slow-drifting waters, each particular state
in their construction being due to some peculiarity of temperature or
of mineral contents which the fluid held. These metamorphic phenomena,
though important, are obscure, and their elucidation demands some
knowledge of petrographic science, that branch of geology which
considers the principles of rock formation. They will therefore not be
further considered in this work.
VOLCANOES.
Of old it was believed that volcanoes represented the outpouring of
fluid rock which came forth from the central realm of the earth, a
region which was supposed still to retain the liquid state through
which the whole mass of our earth has doubtless passed. Recent
studies, however, have brought about a change in the views of
geologists which is represented by the fact that we shall treat
volcanic phenomena in connection with the history of rock water.
In endeavouring to understand the phenomena of volcanoes it is very
desirable that the student should understand what goes on in a normal
eruption. The writer may, therefore, be warranted in describing some
observations which he had an opportunity to make at an eruption of
Vesuvius in 1883, when it was possible to behold far more than can
ordinarily be discerned in such outbreaks--in fact, the opportunity of
a like nature has probably not been enjoyed by any other person
interested in volcanic action. In the winter of 1882-'83 Vesuvius was
subjected to a succession of slight outbreaks. At the time of the
observations about to be noted the crater had been reduced to a cup
about three hundred feet in diameter and about a hundred feet deep.
The vertical shaft at the bottom, through which the outbursts were
taking place, was about a hundred feet across. Taking advantage of a
heavy gale from the northwest, it was practicable, notwithstanding the
explosions, to climb to the edge of the crater wall. Looking down into
the throat of the volcano, although the pit was full of whirling
vapours and the heat was so great that the protection of a mask was
necessary, it was possible to see something of what was going on at
the moment of an explosion.
The pipe of the volcano was full of white-hot lava. Even in a day of
sunshine, which was only partly obscured by the vapours which hung
about the opening, the heat of the lava made it very brilliant. This
mass of fluid rock was in continuous motion, swaying violently up and
down the tube. From four to six times a minute, at the moment of its
upswaying, it would burst as by the explosion of a gigantic bubble.
The upper portion of the mass was blown upward in fragments, the
discharge being like that of shot from a fowling piece; the fragments,
varying in size from small, shotlike bits to masses larger than a
man's head, were shot up sometimes to the height of fifteen hundred
feet above the point of ejection. The wind, blowing at the rate of
about forty miles an hour, drove the falling bits of rock to the
leeward, so that there was no considerable danger to be apprehended
from them. Some seconds after the explosion they could be heard
rattling down on the farther slope of the cone. Observations on the
interval between the discharge and the fall of the fragments made it
easy to compute the height to which they were thrown.
At the moment when the lava in the pipe opened for the passage of the
vapour which created the explosion the movement, though performed in
a fraction of a second, was clearly visible. At first the vapour was
colourless; a few score feet up it began to assume a faint, bluish
hue; yet higher, when it was more expanded, the tint changed to that
of steam, which soon became of the ordinary aspect, and gathered in
swift-revolving clouds. The watery nature of the vapour was perfectly
evident by its odour. Though commingled with sulphurous-acid gas, it
still had the characteristic smell of steam. For a half hour it was
possible to watch the successive explosions, and even to make rough
sketches of the scene. Occasionally the explosions would come in quick
succession, so that the lava was blown out of the tube; again, the
pool would merely sway up and down in a manner which could be
explained only by supposing that great bubbles of vapour were working
their way upward toward the point where they could burst. Each of
these bubbles probably filled a large part of the diameter of the
pipe. In general, the phenomena recalled the escape of the jet from a
geyser, or, to take a familiar instance, that of steam from the pipe
of a high-pressure engine. When the heat is great, steam may often be
seen at the mouth of the pipe with the same transparent appearance
which was observed in the throat of the crater. In the cold air of the
mountain the vapour was rapidly condensed, giving a rainbow hue in the
clouds when they were viewed at the right angle. The observations were
interrupted by the fact that the wind so far died away that large
balls of the ejected lava began to fall on the windward side of the
cone. These fragments, though cooled and blackened on their outside by
their considerable journey up and down through the air, were still so
soft that they splashed when they struck the surface of cinders.
Watching the cone from a distance, one could note that from time to
time the explosions, increasing in frequency, finally attained a point
where the action appeared to be continuous. The transition was
comparable to that which we may observe in a locomotive which, when it
first gets under way, gives forth occasional jets of steam, but,
slowly gaining speed, finally pours forth what to eye and ear alike
seem to be a continuous outrush. All the evidence that we have
concerning volcanic outbreaks corroborates that just cited, and is to
the effect that the essence of the action consists in the outbreak of
water vapour at a high temperature, and therefore endowed with very
great expansive force. Along with this steam there are many other
gases, which always appear to be but a very small part of the whole
escape of a vaporous nature--in fact, the volcanic steam, so far as
its chemical composition has been ascertained, has the composition
which we should expect to find in rock water which had been forced out
from the rock by the tensions that high temperature creates.
Because of its conspicuous nature, the lava which flows from most
volcanoes, or is blown out from them in the form of finely divided
ash, is commonly regarded as the primary feature in a volcanic
outbreak. Such is not really the case. Volcanic explosions may occur
with very little output of fluid rock, and that which comes forth may
consist altogether of the finely divided bits of rock to which we give
the name of ash. In fact, in all very powerful explosions we may
expect to find no lava flow, but great quantities of this finely
divided rock, which when it started from the depths of the earth was
in a fluid state, but was blown to pieces by the contained vapour as
it approached the surface.
If the student is so fortunate as to behold a flood of lava coming
forth from the flanks of a volcano, he will observe that even at the
very points of issue, where the material is white-hot and appears to
be as fluid as water, the whole surface gives forth steam. On a still
day, viewed from a distance, the path of a lava flow is marked by a
dense cloud of this vapour which comes forth from it. Even after the
lava has cooled so that it is safe to walk upon it, every crevice
continues to pour forth steam. Years after the flowing has ceased, and
when the rock surface has become cool enough for the growth of
certain plants upon it, these crevices still yield steam. It is
evident, in a word, that a considerable part of a lava mass, even
after it escapes from the volcanic pipes, is water which is intimately
commingled with the rock, probably lying between the very finest
grains of the heated substance. Yet this lava which has come forth
from the volcano has only a portion of the water which it originally
contained; a large, perhaps the greater part, has gone forth in the
explosive way through the crater. It is reasonably believed that the
fluidity of lava is in considerable measure due to the water which it
contains, and which serves to give the mass the consistence of paste,
the partial fluidity of flour and rock grains being alike brought
about in the same manner.
So much of the phenomena of volcanoes as has been above noted is
intended to show the large part which interstitial water plays in
volcanic action. We shall now turn our attention again to the state of
the deeply buried rock water, to see how far we may be able by it to
account for these strange explosive actions. When sediments are laid
down on the sea floor the materials consist of small, irregularly
shaped fragments, which lie tumbled together in the manner of a mass
of bricks which have been shot out of a cart. Water is buried in the
plentiful interspaces between these bits of stone; as before remarked,
the amount of this construction water varies. In general, it is at
first not far from one tenth part of the materials. Besides the fluid
contained in the distinct spaces, there is a share which is held as
combined water in the intimate structure of the crystals, if such
there be in the mass. When this water is built into the stone it has
the ordinary temperature of the sea bottom. As the depositing actions
continue to work, other beds are formed on the top of that which we
are considering, and in time the layer may be buried to the depth of
many thousand feet. There are reasons to believe that on the floors of
the oceans this burial of beds containing water may have brought great
quantities of fluid to the depth of twenty miles or more below the
outer surface of the rocks.
[Illustration: Fig. 15.--Flow of lava invading a forest. A tree in the
distance is not completely burned, showing that the molten rock had
lost much of its original heat.]
The effect of deep burial is to increase the heat of strata. This
result is accomplished in two different ways. The direct effect
arising from the imposition of weight, that derived from the mass of
stratified material, is, as we know, to bring about a down-sinking of
the earth's crust. In the measure of this falling, heat is engendered
precisely as it is by the falling of a trip-hammer on the anvil, with
which action, as is well known, we may heat an iron bar to a high
temperature. It is true that this down-sinking of the surface under
weight is in part due to the compression of the rocks, and in part to
the slipping away of the soft underpinning of more or less fluid rock.
Yet further it is in some measure brought about by the wrinkling of
the crust. But all these actions result in the conversion of energy of
position into heat, and so far serve to raise the temperature of the
rocks which are concerned in the movements. By far the largest source
of heat, however, is that which comes forth from the earth's interior,
and which was stored there in the olden day when the matter forming
the earth gathered into the mass of our sphere. This, which we may
term the original heat, is constantly flowing forth into space, but
makes its way slowly, because of the non-conductive, or, as we may
phrase it, the "blanketing" effect of the outer rock. The effect of
the strata is the same as that exercised by the non-conductive
coatings which are put on steam boilers. A more familiar comparison
may be had from the blankets used for bedclothing. If on top of the
first blanket we put a second, we keep warmer because the temperature
of the lower one is elevated by the heat from our body which is held
in. In the crust of the earth each layer of rock resists the outflow
of heat, and each addition lifts the temperature of all the layers
below.
When water-bearing strata have been buried to the depth of ten miles,
the temperature of the mass may be expected to rise to somewhere
between seven hundred and a thousand degrees Fahrenheit. If the depth
attained should be fifty miles, it is likely that the temperature will
be five times as great. At such a heat the water which the rocks
contain tends in a very vigorous way to expand and pass into the state
of vapour. This it can not readily do, because of its close
imprisonment; we may say, however, that the tendency toward explosion
is almost as great as that of ignited gunpowder. Such powder, if held
in small spaces in a mass of cast steel, could be fired without
rending the metal. The gases would be retained in a highly compressed,
possibly in a fluid form. If now it happens that any of the strain in
the rocks such as lead to the production of faults produce fissures
leading from the surface into this zone of heated water, the tendency
of the rocks containing the fluid, impelled by its expansion, will be
to move with great energy toward the point of relief or lessened
pressure which the crevice affords. Where rocks are in any way
softened, pressure alone will force them into a cavity, as is shown by
the fact that beds of tolerably hard clay stones in deep coal mines
may be forced into the spaces by the pressure of the rocks which
overlie them--in fact, the expense of cutting out these in-creeping
rocks is in some British mines a serious item in the cost of the
product.
The expansion of the water contained in the deep-lying heated rocks
probably is by far the most efficient agent in urging them toward the
plane of escape which the fissure affords. When the motion begins it
pervades all parts of the rock at once, so that an actual flow is
induced. So far as the movement is due to the superincumbent weight,
the tendency is at once to increase the temperature of the moving
mass. The result is that it may be urged into the fissure perhaps even
hotter than when it started from the original bed place. In proportion
as the rocky matter wins its way toward the surface, the pressure upon
it diminishes, and the contained vapours are freer to expand. Taking
on the vaporous form, the bubbles gather to each other, and when they
appear at the throat of the volcano they may, if the explosions be
infrequent, assume the character above noted in the little eruption of
Vesuvius. Where, however, the lava ascends rapidly through the
channel, it often attains the open air with so much vapour in it, and
this intimately mingled with the mass, that the explosion rends the
materials into an impalpably fine powder, which may float in the air
for months before it falls to the earth. With a less violent movement
the vapour bubbles expand in the lava, but do not rend it apart, thus
forming the porous, spongy rock known as pumice. With a yet slower
ascent a large part of the steam may go away, so that we may have a
flow of lava welling forth from the vent, still giving forth steam,
but with a vapour whose tension is so lowered that the matter is not
blown apart, though it may boil violently for a time after it escapes
into the air.
Although the foregoing relatively simple explanation of volcanic
action can not be said as yet to be generally accepted by geologists,
the reasons are sufficient which lead us to believe that it accounts
for the main features which we observe in this class of explosions--in
other words, it is a good working hypothesis. We shall now proceed in
the manner which should be followed in all natural inquiry to see if
the facts shown in the distribution of volcanoes in space and time
confirm or deny the view.
The most noteworthy feature in the distribution of volcanoes is that,
at the present time at least, all active vents are limited to the sea
floors or to the shore lands within the narrow range of three hundred
miles from the coast. Wherever we find a coast line destitute of
volcanoes, as is the case with the eastern coast of North and South
America, it appears that the shore has recently been carried into the
land for a considerable distance--in other words, old coast lines are
normally volcanic; that is, here and there have vents of this nature.
Thus the North Atlantic, the coasts of which appear to have gone
inland for a great distance in geologically recent times, is
non-volcanic; while the Pacific coast, which for a long time has
remained in its present position, has a singularly continuous line of
craters near the shore extending from Alaska to Tierra del Fuego. So
uninterrupted is this line of volcanoes that if they were all in
eruption it would very likely be possible to journey down the coast
without ever being out of sight of the columns of vapour which they
would send forth. On the floor of the sea volcanic peaks appear to be
very widely distributed; only a few of them--those which attain the
surface of the water--are really known, but soundings show long lines
of elevations which doubtless represent cones distributed along fault
lines, none of the peaks of sufficient height to break the surface of
the sea. It is likely, indeed, that for one marine volcano which
appears as an island there are scores which do not attain the surface.
Volcanic islands exist and generally abound in the ocean and greater
seas; every now and then we observe a new one forming as a small
island, which is apt to be washed away by the sea shortly after the
eruption ceases, the disappearance being speedy, for the reason that
the volcanic ashes of which these cones are composed drift away like
snow before the movement of the waves.
If the waters of the ocean and seas were drained away so that we could
inspect the portion of the earth's surface which they cover as readily
as we do the dry lands, the most conspicuous feature would be the
innumerable volcanic eminences which lie hidden in these watery
realms. Wherever the observer passed from the centres of the present
lands he would note within the limits of those fields only mountains,
much modified by river action; hills which the rivers had left in
scarfing away the strata; and dales which had been carved out by the
flowing waters. Near the shore lines of the vanished seas he would
begin to find mountains, hills, and vales occasionally commingled with
volcanic peaks, those structures built from the materials ejected from
the vents. Passing the coast line to the seaward, the hills and dales
would quickly disappear, and before long the mountains would vanish
from his way, and he would gradually enter on a region of vast rolling
plains beset by volcanic peaks, generally accumulated in long ranges,
somewhat after the manner of mountains, but differing from those
elevations not only in origin but in aspect, the volcanic set of peaks
being altogether made up of conical, cup-topped elevations.
A little consideration will show us that the fact of volcanoes being
in the limit to the sea floors and to a narrow fringe of shore next
certain ocean borders is reconcilable with the view as to their
formation which we have adopted. We have already noted the fact that
the continents are old, which implies that the parts of the earth
which they occupy have long been the seats of tolerably continuous
erosion. Now and then they have swung down partly beneath the sea, and
during their submersion they received a share of sediments. But, on
the whole, all parts of the lands except strips next the coast may be
reckoned as having been subjected to an excess of wearing action far
exceeding the depositional work. Therefore, as we readily see,
underneath such land areas there has been no blanketing process going
on which has served to increase the heat in the deep underlying rocks.
On the contrary, it would be easy to show, and the reader may see it
himself, that the progressive cooling of the earth has probably
brought about a lowering of the temperature in all the section from
the surface to very great depths, so that not only is the rock water
unaffected by increase of heat, but may be actually losing
temperature. In other words, the conditions which we assume bring
about volcanic action do not exist beneath the old land.
Beneath the seas, except in their very greatest depths, and perhaps
even there, the process of forming strata is continually going on.
Next the shores, sometimes for a hundred or two miles away to seaward,
the principal contribution may be the sediment worn from the lands by
the waves and the rivers. Farther away it is to a large extent made up
of the remains of animals and plants, which when dying give their
skeletons to form the strata. Much of the materials laid down--perhaps
in all more than half--consist of volcanic dust, ashes, and pumice,
which drifts very long times before it finds its way to the bottom. We
have as yet no data of a precise kind for determining the average rate
of accumulation of sediments upon the sea floor, but from what is
known of the wearing of the lands, and the amount of volcanic waste
which finds its way to the seas, it is probably not less than about a
foot in ten thousand years; it is most likely, indeed, much to exceed
this amount. From data afforded by the eruptions in Java and in other
fields where the quantity of volcanic dust contributed to the seas can
be estimated, the writer is disposed to believe that the average rate
of sedimentation on the sea floors is twice as great as the estimate
above given.
Accumulating at the average rate of one foot in ten thousand years, it
would require a million years to produce a hundred feet of sediments;
a hundred million to form ten thousand feet, and five hundred million
to create the thickness of about ten miles of bed. At the rate of two
feet in ten thousand years, the thickness accumulated would be about
twenty miles. When we come to consider the duration of the earth's
geologic history, we shall find reasons for believing that the
formation of sediment may have continued for as much as five hundred
million years.
The foregoing inquiries concerning the origin of volcanoes show that
at the present time they are clearly connected with some process which
goes on beneath the sea. An extension of the inquiry indicates that
this relation has existed in earlier geological times; for, although
the living volcanoes are limited to places within three hundred miles
of the sea, we find lava flows, ashes, and other volcanic
accumulations far in the interior of the continents, though the energy
which brought them forth to the earth's surface has ceased to operate
in those parts of the land. In these cases of continental volcanoes it
generally, if not always, appears that the cessation of the activity
attended the removal of the shore line of the ocean or the
disappearance of great inland seas. Thus the volcanoes of the
Yellowstone district may have owed their activity to the immense
deposits of sediment which were formed in the vast fresh-water lakes
which during the later Cretaceous and early Tertiary times stretched
along the eastern face of the Rocky Mountains, forming a Mediterranean
Sea in North America comparable to that which borders southern Europe.
It thus appears that the arrangement of volcanoes with reference to
sea basins has held for a considerable period in the past. Still
further, when we look backward through the successive formations of
the earth's crust we find here and there evidences in old lava flows,
in volcanic ashes, and sometimes in the ruins of ancient cones which
have been buried in the strata, that igneous activity such as is now
displayed in our volcanoes has been, since the earliest days of which
we have any record, a characteristic feature of the earth. There is no
reason to suppose that this action has in the past been any greater or
any less than in modern days. All these facts point to the conclusion
that volcanic action is due to the escape of rock water which has been
heated to high temperatures, and which drives along with it as it
journeys toward a crevice the rock in which it has been confined.
We will now notice some other explanations of volcanic action which
have obtained a certain credence. First, we may note the view that
these ejections from craters are forced out from a supposed liquid
interior of the earth. One of the difficulties of this view is that we
do not know that the earth's central parts are fluid--in fact, many
considerations indicate that such is not the case. Next, we observe
that we not infrequently find two craters, each containing fluid lava,
with the fluid standing at differences of height of several thousand
feet, although the cones are situated very near each other. If these
lavas came from a common internal reservoir, the principles which
control the action of fluids would cause the lavas to be at the same
elevation. Moreover, this view does not provide any explanation of the
fact that volcanoes are in some way connected with actions which go on
on the floors of great water basins. There is every reason to believe
that the fractures in the rocks under the land are as numerous and
deep-going as those beneath the sea. If it were a mere question of
access to a fluid interior, volcanoes should be equally distributed on
land and sea floors. Last of all, this explanation in no wise accounts
for the intermixture of water with the fluid rock. We can not well
believe that water could have formed a part of the deeper earth in the
old days of original igneous fusion. In that time the water must have
been all above the earth in the vaporous state.
Another supposition somewhat akin to that mentioned is that the water
of the seas finds its way down through crevices beneath the floors of
the ocean, and, there coming in contact with an internal molten mass,
is converted into steam, which, along with the fluid rock, escapes
from the volcanic vent. In addition to the objections urged to the
preceding view, we may say concerning this that the lava, if it came
forth under these circumstances, would emerge by the short way, that
by which the water went down, and not by the longer road, by which it
may be discharged ten thousand feet or more above the level of the
sea.
The foregoing general account of volcanic action should properly be
followed by some account of what takes place in characteristic
eruptions. This history of these matters is so ample that it would
require the space of a great encyclopædia to contain them. We shall
therefore be able to make only certain selections which may serve to
illustrate the more important facts.
By far the best-known volcanic cone is that of Vesuvius, which has
been subjected to tolerably complete record for about twenty-four
hundred years. About 500 B.C. the Greeks, who were ever on the search
for places where they might advantageously plant colonies, settled on
the island of Ischia, which forms the western of what is now termed
the Bay of Naples. This island was well placed for tillage as well as
for commerce, but the enterprising colonists were again and again
disturbed by violent outbreaks of one or more volcanoes which lie in
the interior of this island; at one time it appears that the people
were driven away by these explosions.
In these pre-Christian days Vesuvius, then known as Monte Somma, was
not known to be a volcano, it never having shown any trace of
eruption. It appeared as a regularly shaped mountain, somewhat over
two thousand feet high, with a central depression about three miles in
diameter at the top, and perhaps two miles over at the bottom, which
was plainlike in form, with some lakes of bitter water in the centre.
The most we know of this central cavity is connected with the
insurrection of the slaves led by Spartacus, the army of the revolters
having camped for a time on the plain encircled by the crater walls.
The outer slopes of the mountain afforded then a remarkably fertile
soil; some traces, indeed, of the fertility have withstood the modern
eruptions which have desolated its flanks. This wonderful Bay of
Naples became the seat of the fairest Roman culture, as well as of a
very extended commerce. Toward the close of the first century of our
era the region was perhaps richer, more beautifully cultivated, and
the seat of a more elaborate luxury than any part of the shore line of
Europe at the present day. At the foot of the mountain, on the eastern
border of the bay, the city of Pompeii, with a population of about
fifty thousand souls, was a considerable port, with an extensive
commerce, particularly with Egypt. The charming town was also a place
of great resort for rich Egyptians who cared to dwell in Europe. On
the flanks of the mountain there was at least one large town,
Herculaneum, which appears to have been an association of rich men's
residences. On the eastern side of the bay, at a point now known as
Baiæ, the Roman Government had a naval station, which in the year 79
was under the command of the celebrated Pliny, a most voluminous
though unscientific writer on matters of natural history. With him in
that year there was his nephew, commonly known as the younger Pliny,
then a student of eighteen years, but afterward himself an author.
These facts are stated in some detail, for they are all involved in
the great tragedy which we are now to describe.
For many years there had been no eruption about the Bay of Naples. The
volcanoes on Ischia had been still for a century or more, and the
various circular openings on the mainland had been so far quiet that
they were not recognised as volcanoes. Even the inquisitive Pliny,
with his great learning, was so little of a geologist that he did not
know the signs which indicate the seat of volcanic action, though they
are among the most conspicuous features which can meet the eye. The
Greeks would doubtless have recognised the meaning of these physical
signs. In the year 63 the shores of the Bay of Naples were subjected
to a distinctive earthquake. Others less severe followed in subsequent
years. In an early morning in the year 79, a servant aroused the elder
Pliny at Baiæ with the news that there was a wonderful cloud rising
from Monte Somma. The younger Pliny states that in form it was like a
pine tree, the common species in Italy having a long trunk with a
crown of foliage on its summit, shaped like an umbrella. This crown of
the column grew until it spread over the whole landscape, darkening
the field of view. Shortly after, a despatch boat brought a message to
the admiral, who at once set forth for the seat of the disturbance. He
invited his nephew to accompany him, but the prudent young man relates
in his letters to Tacitus, from whom we know the little concerning the
eruption which has come down to us, that he preferred to do some
reading which he had to attend to. His uncle, however, went straight
forward, intending to land at some point on the shore at the foot of
the cone. He found the sea, however, so high that a landing was
impossible; moreover, the fall of rock fragments menaced the ship. He
therefore cruised along the shore for some distance, landing at a
station probably near the present village of Castellamare. At this
point the fall of ashes and pumice was very great, but the sturdy old
Roman had his dinner and slept after it. There is testimony that he
snored loudly, and was aroused only when his servants began to fear
that the fall of ashes and stones would block the way out of his
bedchamber. When he came forth with his attendants, their heads
protected by planks resting on pillows, he set out toward Pompeii,
which was probably the place where he sought to land. After going some
distance, the brave man fell dead, probably from heart disease; it is
said that he was at the time exceedingly asthmatic. No sooner were his
servants satisfied that the life had passed from his body than they
fled. The remains were recovered after the eruption had ceased. The
younger Pliny further relates that after his uncle left, the cloud
from the mountain became so dense that in midday the darkness was that
of midnight, and the earthquake shocks were so violent that wagons
brought to the courtyard of the dwelling to bear the members of the
household away were rolled this way and that by the quakings of the
earth.
Save for the above-mentioned few and unimportant details concerning
the eruption, we have no other contemporaneous account. We have,
indeed, no more extended story until Dion Cassius, writing long after
the event, tells us that Herculaneum and Pompeii were overwhelmed; but
he mixes his story with fantastic legends concerning the appearance of
gods and demons, as is his fashion in his so-called history. Of all
the Roman writers, he is perhaps the most untrustworthy. Fortunately,
however, we have in the deposits of ashes which were thrown out at the
time of this great eruption some basis for interpreting the events
which took place. It is evident that for many hours the Vesuvian
crater, which had been dormant for at least five hundred years, blew
out with exceeding fury. It poured forth no lava streams; the energy
of the uprushing vapours was too great for that. The molten rock in
their path was blown into fine bits, and all the hard material cast
forth as free dust. In the course of the eruption, which probably did
not endure more than two days, possibly not more than twenty-four
hours, ash enough was poured forth to form a thick layer which spread
far over the neighbouring area of land and sea floor. It covered the
cities of Herculaneum and Pompeii to a depth of more than twenty feet,
and over a circle having a diameter of twenty miles the average
thickness may have been something like this amount. So deep was it
that, although almost all the people of these towns survived, it did
not seem to them worth while to undertake to excavate their dwelling
places. At Pompeii the covering did not overtop the higher of the low
houses. An amount of labour which may be estimated at not over one
thirtieth of the value, or at least the cost which had been incurred
in building the city, would have restored it to a perfectly
inhabitable state. The fact that it was utterly abandoned probably
indicates a certain superstitious view in connection with the
eruption.
The fact that the people had time to flee from Herculaneum and
Pompeii, bearing with them their more valuable effects, is proved by
the excavations at these places which have been made in modern times.
The larger part of Pompeii and a considerable portion of Herculaneum
have been thus explored; only rarely have human remains been found.
Here and there, particularly in the cellars, the labourers engaged in
the work of disinterring the cities note that their picks enter a
cavity; examining the space, they find they have discovered the
remains of a human skeleton. It has recently been learned that by
pouring soft plaster of Paris into these openings a mould may be
obtained which gives in a surprisingly perfect manner the original
form of the body. The explanation of this mould is as follows: Along
with the fall of cinders in an eruption there is always a great
descent of rain, arising from the condensation of the steam which
pours forth from the volcano. This water, mingling with the ashes,
forms a pasty mud, which often flows in vast streams, and is
sometimes known as mud lava. This material has the qualities of
cement--that is, it shortly "sets" in a manner comparable to plaster
of Paris or ordinary mortar. During the eruption of 79 this mud
penetrated all the low places in Pompeii, covering the bodies of the
people, who were suffocated by the fumes of the volcanic emanations.
We know that these people were not drowned by the inundation; their
attitudes show that they were dead before the flowing matter
penetrated to where they lay.
It happened that Pompeii lay beyond the influence of the subsequent
great eruptions of Vesuvius, so that it afterward received only slight
ash showers. Herculaneum, on the other hand, has century by century
been more and more deeply buried until at the present time it is
covered by many sheets of lava. This is particularly to be regretted,
for the reason that, while Pompeii was a seaport town of no great
wealth or culture, Herculaneum was the residence place of the gentry,
people who possessed libraries, the records of which can be in many
cases deciphered, and from which we might hope to obtain some of the
lost treasures of antiquity. The papyrus rolls on which the books of
that day were written, though charred by heat and time, are still
interpretable.