THE PRODUCTS OF VOLCANIC ACTION.
While Spallanzani was engaged in investigating the nature of the action
going on at Stromboli and other Italian volcanoes, his contemporary
Dolomieu was laying the foundation of another important branch of
vulcanology by studying the characters of the different materials of
which volcanoes are built up. Since the publication of Dolomieu's
admirable works on the rocks of the Lipari and Ponza Islands, science
has advanced with prodigious strides. The chemist has taught us how
to split up a rock into its constituent elements and to determine the
proportions of these to one another with mathematical precision; the
mineralogist has done much in the investigation of the characters and
mode of origin of the crystalline minerals which occur in these rocks;
and the microscopist has shown how the minute internal structure of
these rocks may be made clearly manifest. We shall proceed to give a
sketch of the present state of knowledge obtained by these different
kinds of investigations, concerning the materials which are ejected
from volcanic vents.
The most abundant of the substances which are ejected from volcanoes
is steam or water-gas, which, as we have seen, issues in prodigious
quantities during every eruption. But with the steam a great number
of other volatile materials frequently make their appearance. The
chief among these are the add gases known as hydrochloric acid,
sulphurous acid, sulphuretted hydrogen, carbonic add, and boracic acid;
and with these acid gases there issue hydrogen, nitrogen, ammonia,
the volatile metals arsenic, antimony, and mercury, and some other
substances. In considering the nature of the products which issue from
volcanic fissures, it must be remembered that many substances which
under ordinary circumstances do not exhibit marked volatility are
nevertheless easily carried away in fine particles when a current of
steam is passed over them. As we shall have to point out in the sequel,
different volatile substances have a tendency to make their appearance
at volcanic vents according as the intensity of the action going on
within it varies.
The volatile substances issuing from volcanic fissures at high
temperatures react upon one another, and many new compounds are thus
formed. We have already seen how, by the action of sulphurous acid
and sulphuretted hydrogen on each other, the sulphur so common in
volcanic districts has been separated and deposited. The hydrochloric
acid acts very energetically on the rocks around the vents, uniting
with the iron in them to form the yellow ferric-chloride. The rocks
all round a volcanic vent are not unfrequently found coated with this
yellow substance, which is almost always mistaken by casual observers
for sulphur. In many volcanoes the constant passage through the rocks
of the various acid gases has caused nearly the whole of the iron,
lime, and alkaline materials of the rocks to be converted into soluble
compounds known as sulphates, chlorides, carbonates, and borates; and,
on the removal of these by the rain, there remains a white, powdery
substance, resembling chalk in outward appearance, but composed of
almost pure silica. There are certain cases in which travellers have
visited volcanic islands where chemical action of this kind has gone on
to such an extent, that they have been led to describe the islands as
composed entirely of chalk.
[Sidenote: GASES EMITTED FROM VOLCANOES.]
Some of the substances issuing from volcanic vents, such as hydrogen
and sulphuretted hydrogen, are inflammable, and when they issue at a
high temperature, these gases burst into flame the moment that they
come into contact with the air. Hence, when volcanic fissures axe
watched at night, faint lambent flames are frequently seen playing over
them, and sometimes these flames are brilliantly coloured, through the
presence of small quantities of certain metallic oxides. Such volcanic
flames, however, are scarcely ever strongly luminous and, as we have
already seen, the red, glowing light which is observed over volcanic
mountains in eruption is due to quite another cause. The study by the
aid of the spectroscope of the flames which issue from volcanic vents
promises to throw much new light on the rarer materials ejected by
volcanoes. Spectroscopic observations of this kind have already been
commenced by Janssen, at Stromboli and Santorin.
Some of the volatile substances issuing from volcanic vents, are at
once deposited when they come in contact with the cool atmosphere,
others form new compounds with one another and the constituents of the
atmosphere, while others again attack the materials of the surrounding
rocks and form fresh chemical compounds with some of their ingredients.
Thus, there are continually accumulating on the sides and lips of
volcanic fissures deposits of sulphates, chlorides, and borates of
the alkalies and alkaline earths, with sal-ammoniac, sulphur, and the
oxides and sulphides of certain metals. The lips of the fissures from
which steam and acid gases issue in volcanoes are constantly seen to
be coated with yellow and reddish-brown incrustations, consisting of
mixtures, in varying proportions, of these different materials, and
these sometimes assume the form of stalactites and pendent masses.
[Sidenote: DEPOSITS AROUND VOLCANIC VENTS.]
Some of these products of volcanic action are of considerable
commercial value. At Vulcano regular chemical works have been
established in the crater of the volcano, by an enterprising Scotch
firm, a great number of workmen being engaged in collecting the
materials which are deposited around the fissures, and are renewed
by the volcanic action almost as soon as they are removed. In fig.
6, I have given a sketch of this singular spot, taken from the high
ground of the neighbouring Island of Lipari. From the village at
the foot of the volcano, where the workmen live, a zig-zag road has
been constructed leading up the side, and down into the crater of
the volcano. On this road, workmen and mules, laden with the various
volcanic materials, may be seen constantly passing up and down.
[Illustration: Fig. 6.--View of Vulcano, with Vulcanello in the
foreground taken from the south end of the Island of Lipari.]
Vulcano appears to have been frequently in a state of violent eruption
during the past 2,000 years--the last great outburst having taken place
in 1786. In 1873 the activity in the crater of Vulcano suddenly became
more pronounced in character, and the workmen hastened to escape from
the dangerous spot, but, before they could do so, several of them
were severely injured by the explosions. After this outburst, which
did not prove to be of very violent character, the quantity of gases
issuing from the fissures in the crater was for a time much greater
than before, and the productiveness of these great natural chemical
works was proportionately increased: but eventually the action died out
almost entirely. The chief products of Vulcano which are of commercial
value, are sal-ammoniac, sulphur, and boracic acid. At one time it was
even contemplated that great leaden chambers should be erected over the
principal fissures at the bottom of the crater of Vulcano, in which
chambers the volatile materials might be condensed and collected. The
change in the condition of the volcano has unfortunately prevented the
carrying out of this bold project.
Besides the volatile substances which issue from volcanic vents,
mingling with the atmosphere or condensing upon their sides, there are
also many solid materials ejected, and these may accumulate around the
orifices, till they build up mountains of vast dimensions, like Etna,
Teneriffe, and Chimborazo. Some of these solid materials are evidently
fragments of the rock-masses, through which the volcanic fissure has
been rent; these fragments have been carried upwards by the force of
the steam-blast and scattered over the sides of the volcano. But the
principal portion of the solid materials ejected from volcanic orifices
consists of matter which has been extruded from sources far beneath the
surface, in a highly-heated and fluid or semi-fluid condition.
[Sidenote: EJECTED ROCK-FRAGMENTS.]
The fragments torn from the sides of volcanic fissures consist of the
rocks through which the eruptive forces may happen to have opened their
way; pieces of sandstone, limestone, slate, granite, &c., are thus
frequently found in considerable numbers among materials which build up
volcanic mountains. Thus, some of the volcanic cones in the Eifel are
very largely made up of fragments of slate, which have been torn from
the sides of the vents by the uprushing currents of steam. At Vesuvius
masses of limestone are frequently ejected, and may be picked up all
over the slopes of the mountains. These limestone-fragments frequently
contain fossils, and Professor Guiscardi, of Naples, has been able to
collect several hundred species of shells, transported thus by volcanic
action from the rock-masses which form the foundation of the volcano
of Vesuvius. The action of water at a high temperature, and under
such enormous pressure as must exist beneath volcanic mountains, has
often produced changes in the rocks of which fragments are ejected
from volcanic vents. The so-called 'lava' ornaments, which are so
extensively sold at Naples, are not made from the materials to which
geologists apply that name, but from the fragments of altered limestone
that have been torn from the rocks beneath the mountain, and scattered
by the eruptive forces all over its sides. The chemical action of the
superheated and highly-compressed steam on the rocks beneath volcanoes
frequently results in the formation of beautifully crystallised
minerals. Such crystallised minerals abound in the rock-fragments
scattered over the sides of Vesuvius and other volcanoes, both active
and extinct. They have been formed in the great chemical laboratories
which exist beneath the volcano, and have been brought to the surface
by the action of the steam-jets issuing from its fissures.
Of still greater interest are those materials which issue from volcanic
orifices in an incandescent, and often in a molten, condition, and
which are evidently derived from sources far below the earth's surface.
It is to these materials that the name of 'lavas' is properly applied.
Lavas present a general resemblance to the slags and clinkers which
are formed in our furnaces and brick-kilns, and consist, like them, of
various stony substances which have been more or less perfectly fused.
When we come to study the chemical composition and the microscopical
structure of lavas, however, we shall find that there are many respects
in which they differ entirely from these artificial products.
Let us first consider the facts which are taught us concerning the
nature and origin of lavas, by a chemical analysis of them.
[Sidenote: CHEMICAL COMPOSITION OF LAVAS.]
Of the sixty-five or seventy chemical elements, only a very small
number occur at all commonly in lavas. Eight elements, indeed, make
up the great mass of all lavas--these are oxygen, silicon, aluminium,
magnesium, calcium, iron, sodium, and potassium. But even these
eight elements are present in very unequal proportions. Oxygen makes
up nearly one-half the weight of all lavas. Almost all the other
elements found in lavas exist in combination with oxygen, so that
lavas consist entirely of what chemists call 'oxides.' This is a most
remarkable circumstance, which, as we shall presently see, is of great
significance. The metalloid silicon makes up about one-fourth of the
weight of most lavas, and the metal aluminium about one-tenth. The
other five elements vary greatly in their relative proportions in
different lavas.
In all lavas the substance which forms the greatest part of the mass
is the compound of oxygen and silicon, known as silica or silicic
acid. In its pure form, this substance is familiar to us as quartz, or
rock-crystal and flint. Silica is present in all lavas in proportions
which vary from one-half to four-fifths of the whole mass. Now, this
substance, silica, has the property of forming more complex compounds
by uniting with the other oxides present in lavas--namely, the oxides
of aluminium, magnesium, calcium, iron, potassium, and sodium. Silica
is called by chemists an _acid_, the other oxides in lavas are termed
_bases_, and the compounds of silica with the bases are known as
_silicates_. Hence we see that lavas are composed of a number of
different silicates--the silicates of aluminium, magnesium, calcium,
iron, potassium, and sodium.
The above statements will perhaps be made clearer by the accompanying
table from which it will be seen that lavas are compounds in varying
proportions of six kinds of salts--namely, the silicates of alumina,
magnesia, lime, iron, potash, and soda.
Composition of Lavas.
Elements Binary Compounds Salts
Acid Bases
{ Silicon Silica--+
{ |
O { Aluminum +--Alumina Silicate of Alumina
x { |
y { Magnesium +--Magnesia " " Magnesia
g { |
e { Calcium +--Lime " " Lime
n { |
{ Iron +--Iron " " Iron
{ |
{ Potassium +--Potash " " Potash
{ |
{ Sodium +--Soda " " Soda
Now, in some lavas the acid constituent, or silica, is present in much
larger proportions than in others. Those lavas with a large proportion
of silica are called 'acid lavas,' those with a lower percentage of
silica, and therefore a higher proportion of the bases, are known as
the 'basic lavas.' It is convenient to employ the term 'intermediate
lavas' for those in which the proportion of silica is lower than in the
acid lavas, and the proportion of the bases is lower than in the basic
lavas.
The acid lavas contain from 66 to 80 per cent, of silica; they are poor
in lime, magnesia, and oxide of iron, but rich in potash and soda. The
basic lavas contain from 45 to 55 per cent, of silica; they are rich in
magnesia, lime, and oxide of iron, but poor in soda and potash. In the
intermediate lavas the proportion of silica varies from 55 to 66 per
cent.
As the basic-lavas contain a larger proportion of oxide of iron and
other heavy oxides than the acid-lavas, the former have usually a
higher specific gravity than the latter; it is, indeed, possible in
most cases to distinguish between these different varieties by simply
weighing them in water and in air.
[Sidenote: DIFFERENT KINDS OF LAVA.]
The basic lavas are usually of much darker colour than the add
lavas--the terms acid lavas, intermediate lavas, and basic lavas
correspond indeed pretty closely with the names trachytes, greystones
and basalt, which were given to the varieties of lavas by the older
writers on volcanoes, at a time when their chemical constitution had
not been accurately studied. Fresh lavas of acid composition are
usually nearly white in colour, intermediate lavas are of various
tints of grey, and basic lavas nearly black. It must be remembered,
however, that colour is one of the least persistent, and therefore
one of the least valuable, characters by means of which rocks can
be discriminated, and also that by exposure to the influence of the
atmospheric moisture the iron present in all lavas is affected, and
the lavas belonging to all classes, when weathered, assume reddish and
reddish-brown tints.
Geologists have devised a great number of names for the various kinds
of lava which have been found occurring round volcanic vents in
different parts of the world, and the study of these varieties is full
of interest. For our present purpose, however, it will be sufficient
to state that they nearly all fall into five great groups, known as
the Rhyolites, the Trachytes, the Andesites, the Phonolites, and the
Basalts. The Rhyolites are acid lavas, the Basalts are basic lavas,
and the Trachytes, Andesites, and Phonolites, different kinds of
intermediate lavas, distinguished by the particular minerals which they
contain.
Before we part from this subject of the classification of lavas
according to their chemical composition, it will be well to point
out that there exists a small group of lavas which stand quite by
themselves, and cannot be referred to either of the classes we have
indicated. They contain a smaller proportion of silica, and a much
larger proportion of magnesia and oxide of iron than the other lavas,
and may be made to constitute a small sub-group, to which we may apply
the term of 'ultra-basic lavas.' Although much less widely distributed
than the other varieties, they are, in some respects, as we shall
presently have to point out, of far greater interest to the geologist
than all the other kinds of lavas.
[Sidenote: MINUTE STRUCTURE OF LAVAS.]
We will now proceed to consider the facts which are brought to light
concerning the nature of lavas, when they are studied by the aid
of the microscope. Although most lavas appear at first sight to be
opaque substances, yet it is easy to prepare slices of them which are
sufficiently thin to transmit light. In such thin transparent slices
we are able to make out, by the aid of the microscope, certain very
interesting details of structure, which afford new and important
evidence bearing on the mode of origin of these rocks.
Host lavas are capable of being melted by the heat of our furnaces;
but the different kinds of lava vary greatly in the degree of their
fusibility. The basic lavas, or those with the smallest proportion of
silica, are usually much more easily fusible than those which contain a
high percentage of silica, the add lavas.
Now, it is a very noteworthy circumstance, that when a lava is
artificially fused it assumes on cooling very different physical
characters to those which were presented by the original rock.
If we examine the freshly-broken surface of a piece of lava, we
shall, in most cases, find that it contains a great number of those
regular-shaped bodies which we call crystals; in some cases these
crystals are so small as to be scarcely visible to the naked eye, in
others they may be an inch or more in length. Most lavas are thus seen
to be largely made up of crystals of different minerals. The minerals
which are usually contained in lavas are quartz, the various kinds of
felspar, augite, hornblende, the different kinds of mica, olivine, and
magnetite.
But when a piece of lava is melted in a furnace, all these crystalline
minerals disappear, and the resulting product is the homogeneous
substance which we call glass. If, as many suppose, lavas acquire the
fluidity which they possess when issuing from volcanic vents as the
result of simple fusion it is strange that artificially fused lavas do
not agree more closely in character with the natural products.
A careful examination of different kinds of lavas, however, will
show that they vary very greatly in character among themselves. Some
lavas are as perfectly glassy in structure as those which have been
artificially fused, while others contain great numbers of crystals,
which may sometimes be of very large size.
If we prepare thin transparent slices of these different kinds of
lavas, and examine them by the aid of the microscope, we shall find
that lavas are made up of two kinds of materials, a base or groundmass
of a glassy character, and distinct crystals of different minerals,
which are irregularly distributed through this glassy base, like the
raisins, currants, and pieces of candied peel in a cake. In some
cases the glassy base makes up the whole mass of the rock; in others,
smaller or larger numbers of crystals are seen to be scattered through
a glassy base; while in others again the crystals are so numerous that
the presence of an intervening glassy base or groundmass can only be
detected by the aid of the microscope.
[Sidenote: STUDY OF LAVAS WITH THE MICROSCOPE.]
If thin slices of the glassy materials of lavas be examined with high
magnifying powers, new and interesting facts are revealed. Through
the midst of the clear glassy substance cloudy patches are seen to be
diffused; and, if we examine them with a still higher power, these
cloudy patches resolve themselves into innumerable particles, some
transparent and others opaque, having very definite outlines. At the
same time fresh cloudy patches are brought into view, which can only
be resolved by yet higher powers of the microscope. In examining these
natural glasses by the aid of the microscope, we are forcibly reminded
of what occurs when the 'Milky Way' and some other parts of the heavens
are studied with a telescope. As the power of the instrument is
increased the nebulous patches are resolved into distinct stars, but
fresh nebulous masses come into view, which are in turn resolved into
stars, when higher powers of the instrument are employed.
In the Frontispiece, No. 1 illustrates the appearance presented by
these volcanic glasses when examined with a high power of a microscope.
Through a glassy base is seen a number of diffused nebulous patches,
which are in places resolved into definite particles.
These minute particles of definite form, which the microscope has
revealed in the midst of the glassy portions of lava, have received the
name of microliths, or crystallites. The study of the characters and
mode of arrangement of these microliths or crystallites has in recent
years thrown much new light on the interesting problems presented by
lavas.
In some glassy lavas the microliths or crystallites, instead of being
indiscriminately diffused through the mass of the base or groundmass,
are found to be collected together into groups of very definite form.
In No. 2 of the Frontispiece we have a section of a glassy rock in
which the crystallites have united together, so as to build up groups
presenting the most striking resemblance to fronds of ferns. Around
these groups spaces of dear glass have been left by the gathering up
of the crystallites, which in other parts of the mass are seen to be
equally diffused through it. In this formation of groups of microliths
we cannot but recognise the action of those crystalline forces, which
on frosty mornings cover our windows with a mimic vegetation composed
of icy particles.
In other cases, again, the crystallites scattered through the glassy
portions of lavas unite in radial groups about certain centres, and
thus build up globular masses to which the name of 'sphærulites' has
been given. No. 3 in the Frontispiece illustrates the formation of
these sphærulites.
Now, a careful study of the microliths or crystallites has proved that
they are the minute elements of which those wonderfully beautiful
objects which we call crystals are built up. In some cases we can
see that the crystallites are becoming united together in positions
determined by mathematical laws, and the group is gradually assuming
the outward form and internal structure of a crystal. In other cases
crystals may be found which are undergoing a disintegrating action,
and are then seen to be made up of minute elements similar to the
crystallites or microliths of glassy rocks.
[Sidenote: CRYSTALLITES AND CRYSTALS.]
The conclusion is confirmed by the fact that if we take an artificially
fused lava and allow it to cool slowly, it will be found that the
glassy mass into which it has resolved itself contains numerous
crystallites. If the cooling process be still further prolonged, these
crystallites will be found to have united themselves into definite
groups, and sometimes distinct crystals are formed in the mass; under
these circumstances the rock frequently loses its glassy appearance and
assumes a stony character.
In connection with this subject, it may be mentioned that some years
ago a very ingenious invention was submitted to trial in the Works
of the Messrs. Chance, of Birmingham. It had been suggested that if
certain lavas of easy fusibility were melted and poured into moulds,
we might thus obtain elaborately ornamented stone-work, composed of
the hardest material, without the labour of the mason. The molten rock
when quickly cooled was found to assume the form of a black glass, but
when very slowly cooled passed into a stony material. Unfortunately,
it was found that this material did not withstand the weather like
ordinary building stones, and, in consequence, the manufacture had to
be abandoned.
Now, the study of the products of volcanoes has led geologists to
recognise the true relations between glassy and crystalline rocks.
In the amorphous mixture of various silicates which compose a glass,
chemical affinity causes the separation of certain portions of
definite composition, and these form the microliths or elements of
which different crystalline minerals are built up. Under the influence
of the crystalline forces, there is a great shaking or agitation in
the mass, and the microliths of similar kind come together and become
united, like the fragments in Ezekiel's valley of dry bones.
Although we cannot see this process taking place under our eyes,
in a mass of lava, yet we may study specimens in which the action
has been arrested in its different stages. In order to understand
the development of an acorn into an oak-tree, it is not necessary
to watch the whole series of changes in a particular case. A visit
to an oak-thicket, in which illustrations of every stage of the
transformation may be found, will afford us equally certain information
on the subject.
In the same way by the examination of such a series of rock-sections
as that represented in the Frontispiece, we may understand how, in the
midst of a mass of mixed silicates constituting a natural glass, the
separation of microliths takes place; these unite into groups which are
the skeletons of crystals, and finally, by the filling up of the empty
spaces in these skeletons, complete crystals are built up. The series
of operations may, however, be interrupted at any stage, and this stage
we may have the chance of studying.
[Sidenote: GLASSY AND CRYSTALLINE LAVAS.]
We are able, as we shall show in a future chapter, to examine many
rock-masses that have evidently formed the reservoirs from which
volcanoes have been supplied, and others that fill up the ducts which
constituted the means of communication between these subterranean
reservoirs, and the surface of the earth. Now in these subterranean
regions the lavas have been placed under conditions especially
favourable for the action of the crystalline forces--they must have
cooled with extreme slowness, and they must have been under an enormous
pressure, produced in part by the weight of the superincumbent rocks,
and in part by the expansive force of the imprisoned steam. We are not,
therefore, surprised to find that in these subterranean regions, the
lavas, while retaining the same chemical composition, have assumed a
much more perfectly crystalline condition. In some cases, indeed, the
whole rock has become a mass of crystals without any base or groundmass
at all.
An examination of the Frontispiece will illustrate this perfect
gradation from the glassy to the crystalline condition of lavas. No.
1 represents a glass through which microliths or crystallites of
different dimensions and character are diffused. In Nos. 2 and 3,
these crystallites have united to form regular groups. In No. 4, which
may be taken as typical of the features presented by most lavas, we
have a glassy groundmass containing microliths (a 'crypto-crystalline
base'), through which distinct crystals are distributed. Nos. 5 and 6
illustrate the characters presented by lavas which have consolidated
at considerable depths beneath the surface; in the former we have
a mans of small crystals (a 'micro-crystalline base') with larger
crystals scattered through it; while the latter is entirely made up of
large crystals without any trace of a base or groundmass.
Now, as all lavas are found sometimes assuming the glassy condition at
the surface, so when seen in the masses which have consolidated with
extreme slowness, and under great pressure, in subterranean regions,
the same materials are found in the condition of a rock which is built
up entirely of crystals. Chemists have found that artificial mixtures
of silicates in which soda and potash are present in considerable
quantities, have a great tendency to assume the glassy condition on
cooling from a state of fusion, and glass manufacturers are always
careful to use considerable proportions of the alkalis as ingredients,
in making glass. It is found, in like manner, that those lavas which
contain the largest portion of the silicates of soda and potash (the
'acid lavas') most frequently assume the condition of a natural glass.
Geologists have given distinct names to the glassy and the perfectly
crystalline conditions of the different kinds of lavas, the glassy
varieties being found in masses which have cooled rapidly near the
surface, and the crystalline varieties in masses which have cooled
slowly at great depths. The names of these two conditions of the five
great classes into which we have divided lavas are as follows:--
[Sidenote: HIGHLY CRYSTALLINE IGNEOUS ROCKS.]
_Crystalline Forms._ _Lavas._ _Glassy Forms._
Granite Rhyolite }
Syenite Trachyte } Obsidian.
Diorite Andesite }
Miascite Phonolite }
Gabbro Basalt Tachylyte.
As vitreous rocks have little in their general appearance to
distinguish them from one another, the glassy forms of the first
four classes of lava have not hitherto received distinct names, but
have been confounded together under the name of obsidian. If we
determine the specific gravities of rocks having the same composition
but different structures, we shall find that they become heavier in
proportion as the crystalline structure is developed in them. Thus
gabbro is heavier, but tachylyte is lighter than basalt, bulk for bulk,
though all have the same chemical composition.
Nor are the crystals contained in lavas less worthy of careful study,
by the aid of the microscope, than the more or less glassy groundmass
in which they are embedded. Mr. Sorby has shown that the crystals found
in lavas, exhibit many interesting points of difference from those
which separate out in the midst of a mass of the same rock, when it has
been artificially melted and slowly cooled. There are other facts which
also point to the conclusion that, while the glassy groundmass of lavas
may have been formed by cooling from a state of fusion, the larger and
well-formed crystals in these lavas must have been formed under other
and very different conditions.
The larger crystals in lavas exhibit evidence of having been slowly
built up in the midst of a glassy mass, containing crystallites and
small crystals. We can frequently detect evidence of the interruptions
which have occurred in the growth of these crystals in the concentric
zones of different colour or texture which they exhibit; and portions
of the glassy base or groundmass are often found to have been caught up
and enclosed in these crystals during their growth.
But when we find, as in the porphyritic pitchstones, a glassy base
containing only minute crystallites, through which large and perfectly
formed crystals are distributed, we can scarcely doubt that the minute
crystallites and the larger crystals have separated from the base under
very different conditions. This is indicated by the bet that we detect
in these cases no connecting links between the embryo microliths and
the perfect crystals; and a confirmation of the conclusion is seen in
the circumstance that many of the crystals are found to have suffered
injury as if from transport, their edges and angles being rounded and
abraded, and portions being occasionally broken off from them.
Hence we are led to conclude that the larger crystals in lavas were
probably separated from the amorphous mass in the subterranean
reservoirs beneath the volcano, and were carried up to the surface in
the midst of the liquefied glassy material which forms the groundmass
of lavas. When we come to examine these crystals more closely, we find
that certain very curious phenomena are exhibited by them which lend
powerful support to this conclusion.
[Illustration: Fig. 7.--Minute Cavities, containing Liquids, in the
Crystals of Rocks.]
[Sidenote: LIQUID CAVITIES IN CRYSTALS.]
It is found convenient by geologists to designate those rocks which
have consolidated in deep-seated portions of the earth's crust
as Platonic Rocks, confining the name of Volcanic rocks to those
consolidating At the surface; but Plutonic and Volcanic Rocks shade
into one another by the most insensible gradations.
When the crystals embedded in granitic rocks, and in some lavas, are
examined with the higher powers of the microscope, they are frequently
seen to contain great numbers of excessively minute cavities. Each of
these cavities resembles a small spirit-level, having a quantity of
liquid and a bubble of gas within it. In fig. 7 we have given a series
of drawings of these cavities in crystals as seen under a high power
of the microscope. In No. 1 a group of such cavities is represented,
one of which is full of liquid, while two others are quite empty; the
remaining cavities all contain a liquid with a moving bubble of gas.
In No. 2 two larger cavities are shown, containing a liquid and a
bubble of gas; and it will be seen from these how varied in form these
cavities sometimes are. In Nos. 3, 4 and 6 the liquid in the cavities
contains, besides the bubbles, several, minute crystals; and in No. 6
we have a cavity containing two liquids and a bubble.
In the largest of such cavities the bubble is seen to change its
place so as always to lie at the upper side of the cavity, when the
position of the latter is altered, just as in a spirit-level. But in
the smallest cavities the bubbles appear to be endowed with a power of
spontaneous movement; like imprisoned creatures trying to escape, these
bubbles are seen continually oscillating from side to side and from end
to end of the cavities which enclose them. In fig. 8 a minute cavity
containing a liquid and bubble is shown, the path pursued by the latter
in its wonderful gyrations being indicated by the dark line. These
cavities are exceedingly minute, and so numerous that in some crystals
there must be millions of them present; indeed, in certain cases, as
we increase the magnifying power of our microscopes, new and smaller
cavities continually become visible. It has been estimated that in some
instances the number of these minute liquid-cavities in the crystals of
rocks amounts to from one thousand millions to ten thousand millions in
a cubic inch of space.
[Illustration: Fig. 8--Minute Liquid-cavity in a Crystal, with a moving
Bubble. (The path of the bubble is indicated by the dark line.)]
[Sidenote: NATURE OF LIQUIDS IN CAVITIES.]
What is the nature of the liquids which are thus imprisoned in these
cavities contained in the crystals of lavas and granites? Careful
experiments have given a conclusive answer to this question. In many
cases the liquid is water, usually containing considerable quantities
of saline matter dissolved in it. Sometimes the saline matters are
present in such abundance that they cannot all pass into solution, but
crystallise out, as in fig. 7--Nos. 3, 4, 5--where cubic crystals of
the chlorides of sodium and potassium are seen floating in the liquid;
in other cases the liquid is a hydrocarbon like the mineral oil which
is present in great abundance in deep-seated rocks in many parts of the
globe. But in some other cases the liquid contained in the cavities
of crystals is found to be one which could scarcely be anticipated
to occur under such circumstances--the gas known as carbonic add,
which under extreme pressure can be reduced to a liquid condition. In
cavities containing liquefied carbonic acid, if the rock be warmed up
to 86° or 90° Fahrenheit the bubble suddenly vanishes, sometimes with
an appearance like ebullition or boiling, as represented in fig. 9.
Now the temperature which we have indicated is the 'critical point' of
carbonic acid, and above that temperature it cannot exist in a liquid
condition, however great may be the pressure to which it is subjected.
The liquid has been converted into a gas which completely fills the
cavity. The carbonic acid in the cavities of crystals has frequently
been isolated and its nature placed beyond doubt by spectroscopic and
ordinary chemical tests.
The presence of these liquids in the cavities of crystals clearly
proves that the latter must have been formed under enormous pressure--a
pressure sufficiently great to reduce, not only steam, but also
volatile hydrocarbons and even gaseous carbonic acid, to the bulk of a
liquid.
[Illustration: Fig. 9.--Cavity in Crystal containing Carbonic-Acid Gas
at a temperature of 86° F., and passing from the liquid to the gaseous
condition.]
Such conditions of enormous pressure we may infer to exist in the
deep-seated reservoirs beneath volcanoes, where, besides the weight
of the superincumbent rock-masses, we have the compressing force of
great quantities of elastic vapour held in confinement. The crystals of
which granitic rocks are entirely built up exhibit clear evidence of
having been all formed under these conditions of enormous pressure. The
glassy base or groundmass of lavas, on the other hand, presents all the
characters of materials that have cooled from a state of fusion. Most
lavas consist in part of crystals, exhibiting fluid-cavities like those
present in granite, and in part of a base, which has evidently been
formed by the cooling of a fused mass. We are therefore justified in
concluding that the crystals have been formed in subterranean recesses,
and that the groundmass or base has consolidated at the surface. The
bearing of these conclusions upon some of the great problems presented
by volcanoes we shall have occasion to point out in the sequel.
[Sidenote: CAUSE OF MOVEMENT OF BUBBLES.]
One of the most interesting inquiries suggested by the study of
the liquid-cavities in volcanic rocks is that of the cause of the
apparently spontaneous movement of the bubbles which we have described
as taking place in some of the smaller of them. The ingenious
experiments of Mr. Noel Hartley have suggested to Professor Stokes
an explanation which is probably the true one. It appears that these
minute globes of vapour are in such a state of unstable equilibrium as
to be affected by the smallest changes of temperature, and that the
variations in the heat of the atmosphere, due to currents of air and
the movement of warm or cold bodies through it, are sufficient to cause
the oscillation of these sensitively poised bubbles.
The short account which we have been able to give in the foregoing
pages of the researches that have been carried on concerning the nature
of the materials ejected from volcanoes will serve to show that these
investigations have already made known many facts of great interest,
and that the farther pursuit of them is full of the highest promise.
To the scientific worker no subject is too vast for his research, no
object so minute as to be unworthy of his most patient study. In some
of our future inquiries concerning the nature of volcanic action, we
shall be led to an investigation of the phenomena displayed in the sun,
moon, comets and other great bodies of the universe; but another road
to truths of the same grandeur and importance is found, as we have
seen, in an examination of the mode of development of crystallites, and
a study of the materials contained in the microscopic cavities of the
minutest crystals.