VOLCANIC ACTION AT DIFFERENT PERIODS OF THE EARTH'S HISTORY.
It is only in comparatively recent times that the important doctrine of
geological continuity has come to be generally accepted, as furnishing
us with a complete and satisfactory explanation of the mode of origin
of the features of our globe. The great forces, which are ever at
work producing modifications in those features, operate so silently
and slowly, though withal so surely, that without the closest and
most attentive observation their effects may be easily overlooked;
while, on the other hand, there are so many phenomena upon our globe
which seem at first sight to bear testimony to the action of sudden
and catastrophic forces, very different to any which appear to be at
present at work, that the tendency to account for all past changes by
these violent actions is a very strong one. In spite of this tendency,
however, the real potency of the forces now at work upon the earth's
crust has gradually made its way to recognition, and the capability of
these forces, when their effects are accumulated through sufficiently
long periods of time, to bring about the grandest changes, is now
almost universally admitted. The modern science of geology is based
upon the principle that the history of the formation and development
of the earth's surface-features, and of the organisms upon it, has
been continuous during enormous periods of time, and that in the study
of the operations taking place upon the earth at the present day, we
may find the true key to the changes which have occurred during former
periods.
In no branch of geological science has the doctrine of continuity
had to encounter so much opposition and misconception as in that
which relates to the volcanic phenomena of the globe. For a long time
students of rocks utterly failed to recognise any relation between the
materials which have been ejected from active volcanic vents and those
which have been formed by similar agencies at earlier periods of the
earth's history. And what was far worse, the subject became removed
from the sphere of practical scientific inquiry to that of theological
controversy, those who maintained the volcanic origin of some of the
older rocks being branded as the worst of heretics.
[Sidenote: CONTROVERSY CONCERNING ORIGIN OF BASALT.]
With the theological aspects of the great controversy concerning the
origin of basalt and similar rocks--a controversy which was carried
on with such violence and acrimony during the latter half of the
eighteenth century--we have here nothing to do. But it may not be
uninstructive to notice the causes of the strange misconceptions which
for so long a period stood in the way of the acceptance of rational
views upon the subject.
At this period but little had been done in studying the chemical
characters of aqueous and igneous rock-masses respectively; and while,
on the one hand, the close similarity in chemical composition between
the ancient basalts and many modern lavas was not recognised, the
marked distinction between the composition of such materials and most
aqueous sediments remained, on the other hand, equally unknown. Nor
had anything been yet accomplished in the direction of the study of
rock-masses by the aid of the microscope. Hence there could be no
appeal to those numerous structural peculiarities that at once enable
us to distinguish the most crystalline aqueous rocks from the materials
of igneous origin.
On the other hand, there undoubtedly exist rocks of a black colour
and crystalline structure, sometimes presenting a striking similarity
in general appearance to the basalts, which contain fossils and are
undoubtedly of aqueous origin. Thus on the shore near Portrush, in the
North of Ireland, and in the skerries which lie off that coast, there
occur great rock-masses, some of which undoubtedly agree with basalt in
all their characters, while others are dark-coloured and crystalline,
and are frequently crowded with _Ammonites_ and other fossils. We now
know that the explanation of these facts is as follows. Near where
the town of Portrush is now situated, a volcanic vent was opened in
Miocene times through rocks of Lias shale. From this igneous centre,
sheets and dykes of basaltic lava were given off, and in consequence
of their contact with these masses of lava, the Lias shales were baked
and altered, and assumed a crystalline character, though the traces of
the fossils contained in them were not altogether obliterated. In the
last century the methods which had been devised for the discrimination
of rocks were so imperfect that no distinction was recognised between
the true basalt and the altered shale, and specimens of the latter
containing _Ammonites_ found their way to almost every museum in
Europe, and were used as illustrations of the 'origin of basalt by
aqueous precipitation.'
Another source of the widely-spread error which prevailed concerning
the origin of basalt, was the failure to recognise the nature of
the alterations which take place in the character of rock-masses in
consequence of the passage through them, during enormous periods of
time, of water containing carbonic acid and other active chemical
agents. The casual observer does not recognise the resemblance which
exists between certain ornamental marbles and the loose accumulations
of shells and corals which form many sea-beaches; but close examination
shows that the former consist of the same materials as the latter,
bound together by a crystalline infilling of carbonate of lime, which
has been deposited in all the cavities and interstices of the mass. In
the same way, as we have already seen, the vesicles and interstices
of heaps of scoriæ may, by the percolation of water through the mass,
become so filled with various crystalline substances, that its original
characters are entirely masked.
But the progress of chemical and microscopic research has effectually
removed these sources of error. Many rocks of aqueous origin, formerly
confounded with the basalts, have now been relegated to their proper
places among the different classes of rocks; while, on the other
hand, it has been shown that the chemical and physical differences
between the ancient basalts and the modern basic lavas are slight
and accidental, and their resemblances are of the closest and most
fundamental character.
[Sidenote: VOLCANIC ORIGIN OF 'TRAP ROCKS.']
The notion of the aqueous origin of basalt, which was so long
maintained by the school of Werner, has now been entirely abandoned,
and the so-called 'trap-rocks' are at the present day recognised as
being as truly volcanic in their origin as the lavas of Etna and
Vesuvius.
There is, however, a vestige of this doctrine of Werner, which still
maintains its ground with obstinate persistence. Many geologists in
Germany who admit that volcanic phenomena, similar to those which are
going on at the present day, must have occurred during the Tertiary and
the later Secondary periods, nevertheless insist that among the earlier
records of the world's history we find no evidence whatever of such
volcanic action having taken place. By the geologists who hold these
views it is asserted that while the granites and other plutonic rocks
were formed during the earlier periods of the world's history, true
volcanic products are only known in connection with the sediment of the
later geological periods.
Some geologists have gone farther even than this, and asserted that
each of the great geological periods is characterised by the nature
of the igneous ejections which have taken place in it. They declare
that granite was formed only during the earliest geological periods,
and that at later dates the gabbros, diabases, porphyries, dolerites
and basalts, successively made their appearance, and finally that the
modern lavas were poured out.
A little consideration will suffice to convince us that these
conclusions are not based upon any good evidence. The plutonic rocks,
as we have already seen, exhibit sufficient proofs in their highly
crystalline character, and in their cavities containing water,
liquefied carbonic acid, and other volatile substances, that they must
have been formed by the very slow consolidation of igneous materials
under enormous pressure. Such pressures, it is evident, could only
exist at great depths beneath the earth's surface. Mr. Sorby and
others have endeavoured to calculate what was the actual thickness of
rock under which certain granites must have been formed, by measuring
the amount of contraction in the liquids which have been imprisoned
in the crystals of these rocks. The conclusions arrived at are of a
sufficiently startling character. It is inferred that the granites
which have been thus examined must have consolidated at depths varying
from 30,000 to 80,000 feet beneath the earth's surface. It is true
that in arriving at these results certain assumptions have to be made,
and to these exception may be taken, but the general conclusion that
granitic rocks could only have been formed under such high pressures as
exist at great depths beneath the surface, appears to be one which is
not open to reasonable doubt.
If, then, granites and similar rocks were formed at the depth of some
miles, it is evident that they can only have made their appearance at
the surface by the removal of the vast thickness of overlying rocks;
and the sole agency which we know of that is capable of effecting the
removal of such enormous quantities of rock-materials, is denudation.
But the agents of denudation--rain and frost, rivers and glaciers, and
sea-waves--though producing grand results, yet work exceeding slowly;
and almost inconceivably long periods of time must have elapsed before
masses of rock several miles in thickness could have been removed, and
the subjacent granites and other highly crystalline rocks have been
exposed at the surface.
[Sidenote: ANCIENT AND MODERN VOLCANIC ROCKS.]
It is an admitted fact that among the older geological formations, we
much more frequently find intrusions of granitic rocks than in the case
of younger ones. It is equally true that among the sediments formed
during the most recent geological periods, no true granitic rocks have
been detected. But if, as we insist is the case, granitic rocks can
only be formed at a great depth from the surface, the £acts we have
described are only just what we might expect to present themselves
under the circumstances. The older a mass of granitic rock, the greater
chance there is that the denuding forces operating upon the overlying
masses, will have had an opportunity of so far removing the latter
as to expose the underlying crystalline rocks at the surface. And,
on the other hand, the younger crystalline rocks are still, for the
most part, buried under such enormous thicknesses of superincumbent
materials that it is hopeless for us to search for them. Nevertheless,
it does occasionally happen that, where the work of denudation has been
exceptionally rapid in its action, such crystalline rocks formed during
a comparatively recent geological period, are exposed at the surface.
This is the case in the Western Isles of Scotland and in the Pyrenees,
where masses of granite and other highly crystalline rocks are found
which were evidently formed during the Tertiary period.
The granites which were formed in Tertiary times present no essential
points of difference from those which had their origin during the
earlier periods of the earth's history. The former, like the latter,
consist of a mass of crystals with no imperfectly crystalline base or
groundmass between them; and these crystals include numerous cavities
containing liquids.
Between the granites and the quartz-felsites every possible
gradation may be found, so that it is impossible to say where the one
group ends and the other begins; indeed, many of the rocks called
'granite-porphyries' have about equal claims to be placed in either
class. Nor is the distinction between the quartz-felsites and rhyolites
any more strongly marked than that between the former class of rocks
and the granites; some of the more crystalline rhyolites of Hungary
being quite undistinguishable, in their chemical composition, their
mineralogical constitution, and their microscopic characters, from
the quartz-felsites. The more crystalline rhyolites are in turn found
passing by insensible gradations into the glassy varieties and finally
into obsidian.
[Sidenote: RELATIONS BETWEEN GRANITE AND PUMICE.]
A piece of granite and a piece of pumice may at first sight appear to
present so many points of difference, that it would seem quite futile
to attempt to discover any connection between them. Yet, if we analyse
the two substances, we may find that in ultimate chemical composition
they are absolutely identical. There is nothing irrational, therefore,
in the conclusion that the same materials under different conditions
may assume either the characters of granite on the one hand, or of
pumice on the other; the former being consolidated under circumstances
in which the chemical and crystalline forces have had the freest play
and have used up the whole of the materials to form crystallised
minerals, while the latter has cooled down and solidified rapidly at
the surface, in such a way that only incipient crystallisation has
occurred, and the glassy mass has been reduced to a frothy condition
by the escape of steam-bubbles from its midst This conclusion receives
the strongest support from the fact that examples of every stage of
the change, between the glassy condition of pumice and the crystalline
condition of granite, may be detected among the materials of which the
globe is built up.
There is still another class of facts which may be adduced in
support of the same conclusion. Many lavas, as we have seen, contain
crystals of much larger dimensions than those constituting the mass
of the rock, which is then said to be 'porphyritic' in structure.
The porphyritically embedded crystals, when carefully examined, are
often seen to be broken and injured, and to exhibit rounded edges,
with other indications of having undergone transport. When examined
microscopically, too, they often present the cavities containing
liquids which distinguish the crystals of plutonic rocks. All the
facts connected with these porphyritic lavas point to the conclusion
that while the crystals in their groundmass have separated from the
liquefied materials near the surface, the large embedded crystal, have
been floated up from great depths within the earth's crust, where they
had been originally formed.
[Sidenote: GRANITIC REPRESENTATIVES OF OTHER LAVAS.]
The careful consideration of all the facts of the case leads to the
conclusion that where pumice, obsidian, and rhyolite are now being
ejected at the surface, the materials which form these substances
are, at various depths in the earth's interior, slowly consolidating
in the form of quartz-felsite, granite-porphyry and granite. It may
be that we can nowhere point to the example of a mass of rock which
can be traced from subterranean regions to the surface, and is, under
such conditions, actually seen to pass from the dense and crystalline
condition of granite to the vesicular and glassy form of pumice;
but great granitic masses often exhibit a more coarsely crystalline
condition in their interior, and the offshoots and dykes which they
give off not infrequently assume the form of quartz-felsite; while,
on the other hand, the more slowly consolidated rocks found in the
interior of some rhyolite masses are not distinguishable in any way
from some of the true quartz-felsites.
That which is true of the lavas of acid composition is equally true
of the lavas of intermediate and basic character. The andesites,
the trachytes, the phonolites, and the basalts have all their exact
representatives among the plutonic rocks, and these have a perfectly
crystalline or granitic structure. The plutonic and the volcanic
representatives of each of these groups are identical in their chemical
composition, and numerous intermediate gradations can be found between
the most completely granitic and the most perfectly vitreous or glassy
types. In illustration of this fact, we may again refer to the series
of microscopic sections of rocks given in the frontispiece.
Another objection to the conclusion that the volcanic products of
earlier periods of the earth's history were identical in character with
those which are being ejected at the present day is based on the fact
of the supposed non-existence of the scoriaceous and glassy materials
which abound in the neighbourhood of the active volcanic vents. Where,
it is asked, do we find among the older rocks of the globe the heaps of
lapilli, dust, and scoriæ, with the glassy and pumiceous rocks that now
occur so abundantly in all volcanic districts?
In reply to this objection, we may point out that these accumulations
of loose materials are of such a nature as to be capable of easy
removal by denuding agents, and that as they are formed upon the land
they will, if not already washed away by the action of rain, floods,
rivers, &c., run great risk of having their materials distributed,
when the land sinks beneath the waters of the ocean and the surface
is covered by new deposits. With respect to the glassy rocks it must
be remembered that the action of water, containing carbonic acid and
other substances, in percolating through such masses has a tendency
to set up crystalline action, and these glassy rocks easily undergo
'devitrification'; it would therefore be illogical for us to expect
glassy rock-masses to retain their vitreous character through long
geological periods, during which they have been subjected to the action
of water and acid gases.
But careful observation has shown that the scoriaceous and vitreous
rocks are by no means absent among the igneous materials ejected during
earlier periods of the earth's history. Their comparative infrequency
is easily accounted for when we remember, in the first place, the ease
with which such materials would be removed by denuding forces, and in
the second place, the tendency of the action of percolating water to
destroy their characteristic features, by filling up their vesicles
with crystalline products and by effecting devitrification in their
mass.
[Sidenote: SIMILARITY OF ANCIENT AND RECENT LAVAS.]
If we go back to the very oldest known rock-masses of the globe, those
which are found underlying the fossiliferous Cambrian strata, we find
abundant evidence that volcanic action took place during the period
in which these materials were being accumulated. Thus, in the Wrekin,
as Mr. Allport has so well shown, we find clear proofs that before
the long-distant period of the Cambrian, there existed volcanoes
which ejected scoriæ, lapilli, and volcanic dust, and also gave rise
to streams of lava exhibiting the characteristic structures found in
glassy rocks. In these rocks, which have undergone a curious alteration
or devitrification, we still find all those peculiar structures--the
sphærulitic, the perlitic, and the banded--so common in the rhyolites
of Hungary, with which rocks the Wrekin lavas, in their chemical
composition, precisely agree. Prof. Bonney, too, has shown that the
rocks of Charnwood Forest, which are also probably of pre-Cambrian age,
contain great quantities of altered volcanic agglomerates, tuffs, and
ashes. I have found the sphærulitic, perlitic, and banded structures
exhibited by British lavas of the Cambrian, Silurian, Devonian and
Carboniferous periods, as well as in those of Tertiary age; and in
connection with these different lavas we find vast accumulations,
sometimes thousands of feet in thickness, of volcanic agglomerates and
tuffs which have undergone great alteration.
All these facts point to one conclusion--namely, that during all past
geological periods, materials similar to those which are now being
extruded from volcanic vents were poured out on the earth's surface
by analogous agencies. If we could trace the lava-streams of the
present day down to the great subterranean reservoirs from which their
materials have been derived, we should doubtless find that at gradually
increasing depths, where the pressure would be greater and the escape
of heat from the mass slower, the rocky materials would by degrees
assume more and more crystalline characters. We should thus find
obsidian or rhyolite insensibly passing into quartz-felsite and finally
into granite; trachyte passing into orthoclase-porphyry and syenite;
and basalt passing into dolerite, augite-porphyry, and gabbro.
On the other hand, if we could replace the great masses of stratified
rocks which must once have overlain the granites, syenites, diorites,
and gabbros, we should find that, as we approached the original
surface, these igneous materials would gradually lose their crystalline
characters, and when they were poured out at the surface would take the
forms of rhyolite, trachyte, andesite, and basalt--all of which might
occasionally assume the glassy forms known as obsidian or tachylyte.
[Sidenote: ALTERED FORMS OF ANCIENT LAVAS.]
But while we insist on the essential points of similarity between
the lavas poured out upon the surface of the earth during earlier
geological periods and those which are being extruded at the present
day, we must not forget that by the action of percolating water and
acid gases, the mineral constitution, the structure, and sometimes
even the chemical composition of these ancient lavas may undergo
a vast amount of change. In not a few cases these changes in the
characters of a lava may be carried so far that the altered rock bears
but little resemblance to the lava from which it was formed, and it may
be found desirable to give it a new name. Among the rocks of aqueous
origin we find similar differences in the materials deposited at
different geological periods. Clay, shale and clay-slate have the same
composition, and the two latter are evidently only altered forms of the
first mentioned, yet so great is the difference in their characters
that it is not only allowable, but desirable, to give them distinctive
names.
In the same way, among the deposits of the earlier geological periods
we find rocks which were doubtless originally basalts, but in which
great alterations have been produced by the percolation of water
through the mass. The original rock has consisted of crystals of
felspar, augite, olivine, and magnetite distributed through a glassy
base. But the chemical action of water and carbonic acid may have
affected all the ingredients of the rock. The outward form of the
felspar crystals may be retained while their substance is changed to
kaolinite, various zeolites, and other minerals; the olivine maybe
altered to serpentine and other analogous minerals; the magnetite
changed to hydrous peroxide of iron; the augite may be changed to
uralite or hornblende; and the surrounding glassy mass more or
less devitrified and decomposed. The hard, dense, and black rock
known as basalt has under these circumstances become a much softer,
earthy-looking mass of a reddish-brown tint, and its difference from
basalt is so marked that geologists have agreed to call it by another
name, that of 'melaphyre.' Even in their ultimate chemical compositions
the 'melaphyres' differ to some extent from the basalts, for some of
the materials of the latter may have been removed in solution, and
water, oxygen, and carbonic acid have been introduced to combine with
the remaining ingredients.
But if we carefully study, by the aid of the microscope, a large series
of basalts and melaphyres, we shall find that many rocks of the former
class show the first incipient traces of those changes which would
reduce them to the latter class. Indeed, it is quite easy to form a
perfect series from quite unaltered basalts to the most completely
changed melaphyres. Hence we are justified in concluding that all the
melaphyres were originally basalts, just as we infer that all oaks were
once acorns.
Now changes, similar to those which we have seen to take place in
the case of basaltic lavas, are exhibited by the lavas of every
other class, which have been exposed to the influence of the same
agencies,--namely, the passage of water and acid gases. But inasmuch as
the minerals composing the basic lavas are for the most part much more
easily affected by such agencies than are the minerals of acid lavas,
the ancient basic rocks are usually found in a much more highly altered
condition than are the acid rocks of equivalent age.
[Sidenote: NAMES GIVEN TO ALTERED LAVAS.]
We thus see that each of the classes of modern lavas has its
representative in earlier geological periods, in the form of rocks
which have evidently been derived from these lavas, through alterations
effected by the agency of water and acid-gases that have permeated
their mass. Thus, while the basalts are represented among the ancient
geological formation by the melaphyres, the andesites are represented
by the porphyrites, and the trachytes and rhyolites by different
varieties of felstones. And, as we can form perfect series illustrating
the gradual change from basalt to melaphyre, so we can arrange other
series demonstrating the passage of andesites into porphyrites, and of
trachytes and rhyolites into felsites.
It must be remembered, however, that these changes do not take place
in anything like determinate periods of time. Occasionally we may
find lavas of ancient date which have undergone surprisingly little
alteration, and in other cases there occur lavas belonging to a
comparatively recent period which exhibit very marked signs of change.
The alteration of the lavas and other igneous rocks does not, however,
stop with the production of the melaphyres, porphyrites, and felstones.
By the further action of the water and carbonic acid of the atmosphere,
the basic lavas are reduced to the soft earthy mass known as 'wacke,'
and the intermediate and acid lavas to the similar material known as
'claystone.' As the passage of water and carbonic acid gas through
these rock-masses goes on, they are eventually resolved into two
portions, one of which is insoluble in water and the other is soluble.
The insoluble portion consists principally of quartz, the crystals
of which are almost unattacked by water and carbonic acid, and the
hydrated silicate of alumina. All the sands and clays, which together
make up more than nine-tenths of the stratified rocks of the globe,
are doubtless derived, either directly or indirectly, from these
insoluble materials separated during the decomposition of volcanic and
plutonic rocks. The soluble materials, which consist of the carbonates,
sulphates and chlorides of lime, magnesia, soda, potash, and iron,
give rise to the formation of the limestones, gypsum, rock-salt,
ironstones, and other stratified masses of the earth's crust. We thus
see how the igneous materials of the globe, by their decomposition,
famish the materials for the stratified rock-masses. The relations of
the different plutonic and volcanic rocks to one another and to the
materials which are derived from them are illustrated in the following
table.
[Sidenote: RELATIONS OF ALTERED TO UNALTERED LAVAS.]
Unaltered Altered Decomposed
Plutonic Rocks lavas lavas Rocks
Granite {Quartz-felsite } Rhyolite and }
{ ('quartz-porphyry')} Obsidian } Felstone }
} }
Syenite {Orthoclase-porphyry } Trachyte } } Claystones
}
Diorite {Hornblende-porphyry } Andesite Porphyrite }
Miascite {Liebnerite porphyry } Phonolite ? --
Gabbro {Augite-porphyry and } Basalt Melaphyre Wacke
{Dolerite }
Some petrographers, indeed, have maintained the principle that rocks
belonging to widely separated geological periods, even when they
exhibit no essential points of difference, should nevertheless be
called by distinct names. But such a system of classification is
calculated rather to hinder than to advance the cause of science. If
the palæontologist were to adopt the same principle and give distinct
names to the same fossil, when it was found to occur in two different
geological formations, we can easily understand what confusion would
be occasioned, and how the comparison of the fauna and flora of the
different formations would be thereby rendered impossible. But the
naturalist, in his diagnosis of a species, wisely confines himself
to the structure and affinities of the organism before him; and in
the same way the petrographer, in giving a name to a rock, ought
to be guided only by his studies of its chemical composition, its
mineralogical constitution, and its structure, putting altogether
out of view its geographical distribution and geological age. Only
by strict attention to this principle can we hope to arrive at such
comparisons of the rocks of different areas and different periods, as
may serve as the basis for safe inductions.
Before leaving this question of the relation which exists between the
igneous rocks of different ages, it may be well to notice several facts
that have been relied upon, as proving that the several geological
periods are distinguished by characteristic igneous products.
It has frequently been asserted that the acid igneous rocks are present
in much greater quantities in connection with the older geological
formations than axe the basic; while, on the other hand, the basic
igneous rocks are said to have been extruded in greater abundance in
the more recent geological periods. But in considering this question it
must not be forgotten that, as a general rule, the basic rocks undergo
decomposition and disintegration far more rapidly than do the acid
rocks. In consequence of this circumstance the chance of our finding
their recognisable representatives among the older formations, is much
less in the case of the former class of rocks than in the latter. As a
matter of fact, however, we do find great masses of gabbro, diabase,
and melaphyre associated even with the oldest geological formations,
while trachytes and rhyolites abound in many volcanic districts where
active vents exist at the present day. Upon a general review of the
subject, it may well be doubted whether the supposed preponderance of
acid igneous materials in the earlier periods of the earth's history,
and of basic igneous materials during the later periods, rests on any
substantial basis of observation.
[Sidenote: AUGITIC AND HORNBLENDIC ROCKS.]
Another difference which has frequently been relied upon, as
distinguishing the older igneous rocks from those of more recent date,
is the supposed fact that the former are characterised by the presence
of hornblende, the latter by the presence of augite. It may be admitted
that this distinction is a real one, but its significance and value are
greatly diminished when we remember the relations which exist between
the two minerals in question. Hornblende and augite are interesting
examples of a dimorphous substance; in chemical composition they are
identical, or rather they are liable to variation between the same
limits, but in their crystalline forms and optical characters they
differ from one another. It has been proved that hornblende is the
stable, and augite the unstable condition of the substance in question.
If hornblende be fused and allowed to cool, it crystallises in the form
of augite. On the other hand, augite-crystals in rocks of ancient date
are found undergoing gradual change and passing into hornblende. The
mineral uralite has the outward form of augite, but the cleavage and
optical properties of hornblende; and there are not wanting many facts
pointing to the conclusion that rocks which now contain hornblende were
originally augitic masses, in which the unstable mineral in their
midst has been gradually converted into the stable one.
There are, however, two minerals which up to the present time have
been found in association only with the older and newer rock-masses
respectively. These are _muscovite_, or the white form of mica, which
occurs in so many granites, but has not yet been discovered in any
modern representative of that rock; and leucite, which is not yet known
in rocks of older date than the Tertiary.
When we remember that muscovite would appear to be a product of
deep-seated igneous action, and is only found in rock-masses that have
been formed under such conditions, we shall be the less surprised at
its non-occurrence in rocks of recent date, especially if we bear in
mind the fact that very few of the younger granitic rocks have as yet
been exposed at the surface by denudation.
With respect to leucite, on the other hand, it must be remembered that
it is a very unstable mineral which appears to be easily changed into
felspar. It is by no means improbable, therefore, that some ancient
igneous rocks which now contain felspar were originally leucitic rocks.
To the view that the action of volcanic forces upon the globe during
past geological times was similar in kind to that which we now observe
going on around us, still another objection has been raised. It has
been asserted that some of the deposits of igneous rock associated
with the older geological formations are of such a nature that they
could not possibly have been accumulated around volcanic vents of the
kind which we see in operation around us.
[Sidenote: VOLCANIC ORIGIN OF ANCIENT IGNEOUS ROCKS.]
Mr. Mallet has declared that the igneous products of the Palæozoic
period differ fundamentally in character from those materials formed
by volcanic action during the later Secondary and the Tertiary
periods. Upon what observations these generalisations are based he
has given us no information, and the enormous mass of facts which
have been collected in recent years concerning the structure of the
lavas and fragmental volcanic deposits of the pre-Cambrian, Cambrian,
Silurian, Devonian and Carboniferous periods, all point to a directly
opposite conclusion. The more carefully we carry on our investigations
concerning these ancient lavas, by the aid of chemical analysis and
microscopic study, the more are we convinced of the essential identity
of the ancient and modern volcanic rocks, both in their composition
and their minute structure. Of great masses of dust produced by
crushing, such as Mr. Mallet has supposed to have been formed during
the earlier geological periods, there is not the smallest evidence;
but we everywhere find proofs, when the rocks are minutely examined,
of the vesicular structure so characteristic of materials produced by
explosive volcanic action.
It has frequently been asserted that in the great districts covered by
basaltic lavas which we find in the Rocky Mountains of North America,
in the Deccan of India, in Abyssinia, and even in the Western Isles of
Scotland, we have proofs of the occurrence, during earlier geological
periods, of volcanic action very different in character from that
which at present takes place on our globe. It has been asserted that
the phenomena observed in these districts can only be accounted for by
supposing that great fissures have opened in their midst, from which
lavas have issued in enormous floods unaccompanied by the ordinary
explosive phenomena of volcanoes.
It must be remembered, however, that none of the districts in question
have been subjected to careful and systematic examination with a view
to the discovery of the vents from which these masses of lava have
issued, with the exception of that which occurs in our own islands. In
this case, in which superficial observers have spoken of the district
as being covered with horizontal lava-sheets piled upon one another to
the depth of 3,000 feet, careful study of the rock-masses has shown
that the accumulations of basalt really consist of a great number of
lava-currents which have issued at successive epochs covering enormous
periods of time. During the intervals between the emission of these
successive lava-currents the surfaces of the older ones have been
decomposed, and formed soils upon which forests have grown up; they
have been eroded by streams, the valleys so formed being filled with
gravels; and lakes have been originated on their surfaces in which
various accumulations have taken place.
[Sidenote: TRULY-VOLCANIC ORIGIN OF LAVA PLATEAUX.]
It has been demonstrated, moreover, that the basal-wrecks of no less
than five volcanic mountains, each of which must have rivalled Etna in
its proportions, existed within this area, and the connection of the
lava-currents, which have deluged the surrounding tracts, with these
great volcanoes has been clearly proved. It is probable that when
more careful and systematic researches are carried on in the other
districts, in which widely-spread sheets of basaltic rocks exist,
similar volcanic vents will be discovered. It must also be remembered
that if such a country as Iceland were subjected to long-continued
denudation, the mountain peaks and cones of loose materials would be
worn away, the whole island being thus reduced to a series of plateaux
composed of lava-sheets, the connection of which with the crystalline
materials filling the great volcanic vents, a superficial observer
might altogether fail to recognise.
But even where we cannot trace the former existence of great volcanic
mountains, like those which once rose in the Hebrides, it would
nevertheless be very rash to conclude that the vast plateaux of
lava-rock must have been formed as gigantic floods unaccompanied by
ordinary volcanic action. Mr. Darwin has pointed out that in crossing
districts covered by lava, he was frequently only able to determine
the limits of the different currents of which it was made up, by an
examination of the age of the trees and the nature of the vegetation
which had sprung up on them. And everyone who has travelled much in
volcanic districts can confirm this observation; what appears at first
sight to be a great continuous sheet of lava proves upon more careful
observation to be composed of a great number of distinctly different
lava-currents, which have succeeded one another at longer or shorter
intervals.
We must remember, too, how various in kind are the volcanic
manifestations which present themselves under different circumstances.
Sometimes the amount of explosive action at a volcanic vent is very
great, and only fragmental ejections take place, composed of the frothy
scum of the lava produced by the escape of gases and vapours from its
midst. But in other cases the amount of explosive action may be small,
and great volumes of igneous materials may issue as lava-streams. In
such cases, only small scoriæ-cones would be formed around the vents,
and one half of such cones is commonly swept away by the efflux of the
lava-currents, while the remainder may be easily removed by denuding
action or be buried under the lava-currents issuing from other vents
in the neighbourhood. Thus it may easily come to pass that what a
superficial observer takes for an enormous mass of basaltic lava poured
out from a great fissure at a single effort, may prove upon careful
observation to be made up of innumerable lava-currents, each of which
is of moderate dimensions; and it may further be found that these
lava-currents, instead of being the product of a single paroxysmal
effort from one great fissure, have been accumulated by numerous small
outbursts taking place at wide intervals, from a great number of minor
orifices.
[Sidenote: SHIFTING OF VOLCANIC ACTION IN DIFFERENT AREAS.]
Having then considered the arguments which have been adduced in support
of the view that the volcanic phenomena of former geological periods
differ from those which are still occurring upon the globe, we may
proceed to state the general conclusions which have been drawn from the
study of the volcanic rocks of the different geological periods.
From a survey of the volcanic rocks of different ages, we are led to
the interesting and important conclusion that the scene of volcanic
action has been continually shifting to fresh areas at different
periods of the earth's history. We find repeated proofs that the
volcanic energy has made its appearance at a certain part of the
earth's crust, has gradually increased in intensity to a maximum, and
then as slowly declined. But as these manifestations have died away
at one part of the earth's surface, they have gradually made their
appearance at another. In every district which has been examined, we
find abundant proofs that volcanic energy has been developed at certain
periods, has disappeared during longer or shorter periods, and then
reappeared in the same area. And on the other hand, we find that there
is no past geological period in which we have not abundant evidence
that volcanic outbursts took place at some portion of the earth's
surface.
To take the case of our own islands for example. We know that during
the pre-Cambrian periods volcanic outbursts occurred, traces of which
are found both in North and South Wales, in the Wrekin Chain in
Shropshire, in Charnwood Forest, and in parts of Scotland and Ireland.
In Cambro-Silurian times we have abundant proofs, both in North Wales
and the Lake district, that volcanic action on the very grandest
scale was taking place during the Arenig and the older portion of
the Llandeilo periods, and again during the deposition of the Bala
or Caradoc beds. The lavas, tuffs, and volcanic agglomerates ejected
during these two periods have built up masses of rock many thousands of
feet in thickness. Snowdon and Cader Idris among the Welsh mountains,
and some of the higher summits of the Lake district, have been carved
by denudation from the vast piles of volcanic materials ejected during
these periods.
In Devonian or Old-Red-Sandstone times, volcanic activity was renewed
with fresh violence upon that part of the earth's surface now occupied
by the British Islands. Along the line which now forms the Grampians
there rose a series of volcanoes of the very grandest dimensions. Ben
Nevis, and many others among the higher Scotch mountains, have been
carved by denudation from the hard masses of granite, quartz-felsite,
and other plutonic rocks which formed the central cores of these
ancient volcanic piles. The remains of the great lava-sheets, and of
the masses of volcanic agglomerate ejected from these grand Devonian
volcanoes, make up hill-ranges of no mean altitude, like the Sidlaws,
the Ochils, and the Pentlands.
[Sidenote: ANCIENT BRITISH VOLCANOES.]
The volcanic action of the Devonian period was prolonged into
Carboniferous times, but was then evidently diminishing gradually in
violence. Instead of great central volcanoes, such as existed in the
earlier period, we find innumerable small vents which threw out tuffs,
agglomerates and lavas, and were scattered over the districts lying
around the bases of the now extinct Devonian volcanoes. In the central
valley of Scotland and in many parts of England, we find abundant
proofs of the existence of these small and scattered volcanic vents
during Carboniferous times. The well-known hill of Arthur's Seat, which
overlooks the city of Edinburgh, and many castle-crowned crags of the
Forth and Clyde valleys, are the worn and denuded relics of these small
volcanoes. There are some indications which point to the conclusion
that the volcanic action of the Newer Palæozoic epoch had not entirely
died out in Permian times, but the evidence upon this point is not
altogether clear and satisfactory.
During nearly the whole of the Secondary or Mesozoic periods the
volcanic forces remained dormant in the area of the British Isles.
Some small volcanic outbursts, however, appear to have occurred in
Triassic times in Devonshire. But in other areas, such as the Tyrol,
South-eastern Europe and Western America, the Triassic, Jurassic, and
Cretaceous periods were marked by grand manifestations of volcanic
activity.
The volcanic forces which had during the long Mesozoic periods deserted
our part of the earth's surface, appear to have returned to it in
full rigour in the Tertiary epoch. In the Newer-Palæozoic periods
the direction of the great volcanic band which traversed our islands
appears to have been from north-east to south-west; but in Tertiary
times a new set of fissures were opened running from north to south.
There is evidence that during the Eocene or Nummulitic period, the
first indications of the subterranean forces having gathered strength
below the district were afforded by the issue of calcareous and
siliceous springs, and soon fissures were opened which emitted scoriæ,
tuffs, and lavas. The intensity of the volcanic action gradually
increased till it attained its maximum in the Miocene period, when
a great chain of volcanic mountains stretched north and south along
the line of the Inner Hebrides, the north-east of Ireland, and the
sea which separates Great Britain from Ireland. The basal-wrecks of a
number of these volcanoes can be traced in the islands of Skye, Mull,
Rum, and parts of the adjoining mainland. We have already seen that
along this great band of volcanic action, which traverses the Atlantic
Ocean from north to south, a number of active vents still exist, though
their energy is now far less intense than was the case in former times.
The only vestiges of the action of these now declining volcanic forces,
at present found in our islands, are the hot springs of Bath and a few
other warm and mineral springs; but in connection with this subject it
must be remembered that our country occasionally participates in great
earthquake-vibrations, like that which destroyed Lisbon in the year
1759.
[Sidenote: ANCIENT VOLCANOES IN OTHER DISTRICTS.]
If we were to study any other part of the earth's surface, we should
arrive at precisely the same conclusion as those to which we have
been conducted by our examination of the British Islands--namely,
that during past geological times the subterranean forces had made
themselves felt in the area, had gradually attained a maximum, and then
as gradually declined, passing through all those varied cycles which we
have described in a former chapter. And we should also find that these
periods of volcanic activity alternated with other periods of complete
quiescence which were of longer or shorter duration. But on comparing
two different districts, we should discover that what was a period of
volcanic activity in the one was a period of repose in the other, and
_vice versâ_.
From these facts geologists have been led to the conclusion which we
have already enunciated--namely, that the subterranean forces are in a
state of continual flux over the surface of the globe. At one point of
the earth's crust these forces gradually gather such energy as to rend
asunder the superincumbent rock-masses and make themselves manifest
at the surface in the series of phenomena characteristic of volcanic
action. But after a longer or shorter interval of time--an interval
which must probably be measured by millions of years--the volcanic
forces die out in that area to make their appearance in another.
Hence, although we may not be able to prove the fact by any
mathematical demonstration, a strong presumption is raised in favour
of the view that the subterranean energy in the earth's crust is a
constant quantity, and that the only variations which take place are in
the locality of its manifestation.
Upon this question whether the amount of this subterranean energy
within the earth's crust is at the present time increasing, stationary,
or declining, we are not altogether destitute of evidence. There are
some considerations connected with certain astronomical hypotheses, to
which we shall hereafter have to refer, that might lead us to entertain
the view that the subterranean activity was once far greater than it
is at present, and that during the long periods of the earth's past
history it has been slowly and gradually declining. And those who
examine the vast masses of igneous materials which have been poured
out from volcanic vents during the earlier periods of the earth's
history may be inclined, at first sight, to point to them as affording
conclusive proof of this gradual decline.
[Sidenote: SUPPOSED DECLINE OF VOLCANIC ACTION.]
But a more careful study of the rocks in question will probably cause
a geologist to pause before jumping to such a conclusion. If we look
at the vast masses of volcanic materials erupted in Miocene times in
our own island and in Ireland, for example, we might be led to imagine
that we have the indications of a veritable 'Reign of Fire,' and that
the evidence points to a condition of things very different indeed
from that which prevails at the present day. But in arriving at such
a conclusion we should be neglecting a most important consideration,
the disregard of which has been the fertile parent of many geological
errors. Many independent lines of evidence all point to the inference
that these volcanic ejections are not the result of one violent effort,
but are the product of numerous small outbreaks which have been
scattered over enormous periods of time.
When we examine with due care the lavas, tuffs, and other volcanic
ejections which constitute such mountain-masses as those of the
Hebrides, of the Auvergne, and of Hungary, we find clear proofs
that the ancient Miocene volcanoes of these districts were clothed
with luxuriant forests, through which wild animals roamed in the
greatest abundance. The intervals between the ejections of successive
lava-streams were often so great, that soils were formed on the
mountain-slope, and streams cut deep ravines and valleys in them.
The island of Java is situated near the very heart of what is at the
present day the most active volcanic centre on the face of the globe,
yet vegetable and animal life flourish luxuriantly there, and the
island is one of the richest and most fertile spots upon the face of
the globe. Not all the terrors of occasional volcanic outbreaks will
ever drive the Neapolitan vine-dressers from the fertile slopes of
Vesuvius, for its periods of repose are long, and its eruptions are of
short duration.
These considerations lead the geologist to conclude that the evidence
afforded by the ancient volcanic rocks is clear and positive in support
of the view that the manifestations of the subterranean forces in the
past agree precisely in their nature and in their products with those
taking place around us at the present time. On the question of great
secular changes having occurred in the amount of volcanic energy in
past geological periods, the evidence must be pronounced negative, or
at the best doubtful.
But even if the geologist confesses himself unable to establish the
fact of any decline in the subterranean energies during the vast
periods of which he takes cognisance, it must be remembered that such
decline may really be going on; for vast as was the duration of the
geological epochs, they probably constitute but a fraction of those far
grander periods which are required by the speculations of the physical
astronomer.