These three may combine in several cases, and on the part of the
observer must combine with measurements, angular and linear, and with
geodetic operations to be conducted in the shaken country.
The methods of application in detail are described fully, as well as
their actual application and results, in my work published in 1862 (2
vols.), entitled "The First Principles of Observational Seismology, as
developed in the Report to the Royal Society of London of the Expedition
made by Command of the Society into the Interior of the Kingdom of
Naples, to investigate the Circumstances of the Great Earthquake of
December, 1857," to the many illustrations of which the pecuniary grant,
in aid, of £300 was most liberally made to the publishers (Messrs.
Chapman and Hall) by the Society.
It is not my intention here, nor would space allow, of my going into the
details of observation, nor of the deductions and conclusions I have
recorded in those volumes. I have referred to their contents as marking
the advent of a new method. I have ventured to call it a new _organon_
in the investigation of Earthquakes, and, through them, of the deep
interior of our earth; and will only add that the method, on this its
very first trial, proved fertile and successful. The depth of focus for
this shock of December, 1857, was about seven to eight geographical
miles below sea level, roughly stated. It gives me great pleasure to add
that my friend, Dr. Oldham, Director-General of the Geological Survey of
India, has since applied these same methods to the phenomena of the
great Cachar Earthquake of the 10th January, 1869, and with success. The
pressure of official duties has, he informs me, as yet prevented his
fully working out his results, but they appear so far to indicate, as we
should expect, a depth of focus or origin considerably greater than in
the European case of 1857. Some account of Dr. Oldham's results were
this year communicated to the Geological Society of London through
myself, they are of great interest and importance.
Such, briefly and imperfectly sketched, is the existing state of
Seismology. As a branch of exact science it is, as it were, an affair of
yesterday. It is with reluctance that I have been compelled, in this
review, to refer to my own work so prominently. The harvest has been and
still is plenteous, but in this field of intellectual work the labourers
are few. This must continue to be so as long as Geology shall continue
to be viewed in public estimation (in England at least) as a fashionable
toy, that everyone who has been to school is supposed capable of
handling; and until all who profess to be geologists shall have learnt
that, to make sound progress, they must first become mathematicians,
physicists and chemists.
It is to the general imperfect knowledge of these sciences amongst
geologists that speculative errors show such vitality, and that Geology
makes such poor progress towards becoming the interpretation of the
world as a machine (_Erdkunde_).
It is for the same reason that Seismology and Vulcanology make little
progress; the first cannot be pursued beyond its present boundaries, nor
can even its present position be understood or explained by anyone
unfamiliar with the laws of wave motion, of all classes of waves; and it
would be easy to show, by quoting from various British or foreign
text-books on Geology, how extremely imperfect is the grasp of some of
the authors upon the subject of earthquake-wave motion, even such as
they admit and endeavour to explain and apply: in fact, many geologists
appear never to have framed to themselves any clear idea of what _is_ a
wave of any sort, liquid or elastic. The general silence as to seismic
theory of French geological writers is remarkable, to whatever cause
attributable. It has been said that French philosophers show themselves
little disposed to acknowledge or to follow the lead of their foreign
compeers in any branch of science. If this be true, or in so far as it
may be so, it is unworthy of French science, which has such boundless
claims upon our homage. I am disposed to attribute the fact in this case
to other circumstances; and, amongst these, to the small extent to which
our language is known amongst French scientific men.
Germany has shown more desire to cultivate this branch of science.
Although, as yet, the distinct enunciation of its fundamental principles
has but sparsely found its way into her text-books, several able
monographs, such as those of Schmidt and of Höttinger, prove how
completely some of her philosophers have mastered and how well applied
them. The men of science of Northern Italy, amongst whom so many
glorious names are to be found on the roll of discovery, have shown
themselves quite alive to the importance of Seismology; and I know of no
more clear, exact and popular exposition of its principles and
application, and of its cosmical relations, than is to be found in a
small volume by Professor Gerolamo Boccardo, published at Genoa in 1869,
entitled _Sismopirologia Terremoti, Vulcani e lente oscillazione del
suolo, saggio di una teoria di Geographia Fisica_.
My object, so far, has been to mark the progress of ascertained
theoretic notions as to Seismology. I have, therefore, passed without
notice many speculative monographs, and the treatment upon Earthquakes,
whether speculative or historical, and however able, that constitutes a
prominent feature of nearly all systematic works on Geology.
That which may be at present viewed as achieved and certainly
ascertained in theoretic Seismology is the clear conception of the
nature of earthquake motion; the relations to it of great sea or other
water wave commotions; the relations to it of sound waves--as to which,
however, more remains to be known; and the relations of all these to
secondary effects, tending in various ways to modify more or less the
topographic and other conditions of the land or sea bottom. And in
descriptive Seismology the present distribution of the earthquake bands
or regions of greatest seismic prevalence and activity are tolerably
ascertained, and their connection with volcanic lines and those of
elevation rendered more evident. Viewed alone, nothing can yet be said
to be absolutely ascertained as to the immediately antecedent cause or
causes of the impulse. The function of Earthquake, as part of the
cosmical machine, has become more clear, as the distinctive boundaries
between Earthquake and permanent elevation of the earth have been made
evident; and it has been seen that Earthquake, however contemporaneous
occasionally with permanent elevation, is not the cause, though it may
be one of the consequences of the same forces which produce elevation;
and thus, that an infinite number of Earthquakes, however violent, and
acting through however prolonged a time, can never act as an agent of
permanent elevation, unless, indeed, on that minute scale in which
surface elevation may arise from secondary effects, like that of the
Ullah Bund.
Much remains to be done, and much may be expected even from the
continuation, if done in a systematic and organised manner, of the
statistic record of Earthquakes in connection with those other branches
of cosmical statistics, Climatology, Meteorology, Terrestrial Magnetism,
etc., the observation of which is already, to a certain extent,
organised over a large portion of the globe.
And now let us look back for a moment to ask, How, by what mental path
of discovery, have we arrived at what we have passed in review?
The facts of Earthquakes have been before men for unknown ages "open
secrets," as Nature's facts have been well called; "but eyes had they
and saw not." Facts viewed through the haze of superstition, or of
foregone notions of what Nature _ought_ to do, cease to be facts. When,
after the great Calabrian Earthquake of 1783, the Royal Academy of
Naples sent forth its commission of its learned members to examine into
the effects, they had spread around them in sad profusion all that was
necessary to have enabled them to arrive at a true notion of the nature
of the shock, and thence a sound explanation of the varied and great
secondary effects they witnessed, and of which they have left us the
records in their Report, and the engravings illustrative of it. But we
look in vain for any light; the things seen, often with distortion or
exaggeration, are heaped together as in the phantasmagoria of a wild
and terrible dream, from which neither order nor conclusion follow.
Why was this? Why were these eminent _savants_ no more successful in
explaining what they saw than the ignorant peasants they found in the
Calabrian mountains?
Because physical science itself was not sufficiently advanced, no doubt;
but also because they had no notion of applying such science as they
had, to the very central point itself of the main problem before them,
freed from all possible adventitious conditions, and so, as it were,
attacking it in the rear. How different might have been the result of
their labours, had they begun by asking themselves, What is an
earthquake? Can we not try to find out what it _is_ by observing and
_measuring_ what it has done? We see the converse mode of dealing with
Nature in Torricelli. "Nature abhors a vacuum," was told him, as the
wisdom of his day. Possibly: but her abhorrence is limited, for I find
it is _measured_ by the pressure of a column of water of thirty-four
feet in height. We need not pursue the story with Pascal, up to the top
of the Puy de Dôme.
This lesson is instructive generally to all investigators, and
particularly here; for Vulcanology, to which we are about now to turn,
has occupied until almost to-day much the same position that Seismology
did in those of the Neapolitan Commissioners.
Whole libraries have been written with respect to it dealing with
_quality_, but _measure_ and _quantity_ remain to be applied to it.
To a very preponderant class in the civilised world no knowledge is of
much interest or value that does not point to what is called a
"practical result," one measurable into utility or coin. I do not stop
to remark as to the bad or as to certain good results of this tendency
of mind; but I may venture to point out to all, that the exact knowledge
of the nature of earthquake motion, even during the short time that it
has become known, has not been barren in results absolutely practical
and utilitarian. The minute investigation of the destruction of
buildings, etc., and the deductions that have been made as to the
relations between the form, height, materials, methods of building,
combination of timber and of masonry, and many other architectural or
constructive conditions, have made it certain now that earthquake-proof
houses and other edifices can be constructed with facility, and at no
great increase, if any at all, of cost. I can affirm that there is no
physical necessity why in frequently and violently shaken countries,
such as Southern Italy or the Oriental end generally of the
Mediterranean, victims should hereafter continue by thousands to be
sacrificed by the fall of their ill-designed and badly built houses.
Were a "Building Act" properly framed, put in force by the Italian
Government in the Basilicatas and Capitanata, etc., so that new houses
or existing ones, when rebuilt, should be so in accordance with certain
simple rules, a not very distant time can be foreseen when Earthquakes,
passing through these rich and fertile but now frequently sorely
afflicted regions, should come and go, having left but little trace of
ruin or death behind. Some disasters there must always be, for we
cannot make the flanks of mountains, nor the beds of torrents, etc.,
always secure; but the main mortality of all Earthquakes is in the
houses or other inhabited buildings. Make these proof, and the wholesale
slaughter is at an end.
The principles we have established have been thus practically applied in
another direction. The Japanese Government, with the keen and rapid
perception of the powers inherent in European science which
characterises now that wonderful people, has commenced to illuminate its
coasts by lighthouses constructed after the best European models. But
Japan is greatly convulsed by earthquakes, and lighthouses, as being
lofty buildings, are peculiarly liable to be destroyed by them.
The engineer of the Japanese Government for these lights, Mr. Thomas
Stevenson, C.E. (one of the engineers to the Commissioners of Northern
Lights), was instructed to have regard, in the design of those
lighthouses, to their exposure to shock. I was consulted by Mr.
Stevenson as to the general principles to be observed; and those
edifices have been constructed so that they are presumedly proof against
the most violent shocks likely to visit Japan; not, perhaps, upon the
best possible plan, but upon such as is truly based upon the principles
I have developed. Mr. Stevenson has published some account of their
construction.
The earthquake regions of South America might with incalculable benefit
apply those ideas; and, indeed, they have been, to some extent, already
applied by my friend, Mr. William Lloyd, Member of the Institution of
Civil Engineers, to the New Custom Houses constructed from his designs
at Valparaiso.
As one of these utilitarian views, and an important one, it will occur
to many to ask--Can the moment of the occurrence or the degree of
intensity of earthquake shock be predicted, or is it probable that at a
future day we may be able to predict them? At present, any prediction,
either of the one or the other, is impossible; and those few who have
professed themselves in possession of sufficient grounds for such
prediction are deceivers or deceived. Nor is it likely that, for very
many years to come, if ever, science shall have advanced so as to render
any such prediction possible; but it is neither impossible nor
improbable that the time shall arrive when, within certain, perhaps
wide, limits as to space, previous time, and instant of occurrence, such
forewarnings may be obtainable.
Earthquakes, like storms and tempests, and nearly all changes of
weather, are not periodic phenomena, nor yet absolutely uncertain or, so
to say, accidental as to recurrence.
They are quasi-periodic, that is to say, some of their conditions as to
causation rest upon a really periodic basis, as, for example, the
recurrence of storms upon the periodic march of the earth, and sun and
moon, etc., and the recurrence of Earthquakes upon the secular cooling
of our earth; but the conditions in both are so numerous and complicated
with particulars, that we cannot fully analyse them--hence, cannot
reduce the phenomena to law, and so cannot predict recurrence. Yet
storms and tempests--which were, along with pestilences and Earthquakes,
amongst the natural phenomena which Bishop Butler deemed in his own day
impossible of human prediction--have already, through the persistent and
systematised efforts of meteorological observers, become to a certain
extent foreseeable; and medical science assures us that it has rendered
that, though to a much less degree of probability, true of pestilences.
We may, therefore, give the utilitarian some hope, that if he will help
us along--who value our accessions of knowledge primarily upon a
different standard to his--in our talk of discovery, our posterity, in a
century or two hence, may not improbably possess the advantage of being
able, in some degree, to predict their Earthquakes. I fear the
inducement will go but a small way with the utilitarian generation,
whose bent tends much towards asking, "What has posterity ever done for
them?"
But though we cannot as yet predict the time when an Earthquake may take
place in any locality, we can, on mixed statistic and dynamic grounds,
in many cases state the limits of probable violence of the next that may
recur. For example, the three shafts of marble columns of the Temple of
Serapis, at Pozzuoli, each of about 41-1/2 feet in height, and 4 feet 10
inches in diameter at the base, remain standing alone, since they were
uncovered, in the year 1750.
Now, as we can calculate exactly what velocity of earthquake-wave motion
would be required to overset these, we are certain that, during the last
one hundred and twenty-two years, the site of the Temple, and we may
say Naples and the Phlegræan fields generally, have never experienced a
shock as great as the very moderate one that would overset these
columns. A shock whose wave particle had a horizontal velocity of only
about 3-1/2 feet (British) per second would overturn these columns;
which is only about one-fourth the velocity (within the meizoseismic
area) of the great shock of 1857, that produced wide-spread destruction
in the Basilicatas, and not enough to throw down any reasonably
well-built house of moderate height.
Naples, so far as Earthquake is concerned, whether coming from the
throes of Vesuvius or elsewhere, has a pretty good chance of safety. She
may possibly (though not probably) be some day smothered in ashes; but
is in little danger of being shaken to the earth. During this time there
have been taking place, larger eruptions of Vesuvius and earthquake
shocks from other centres, together probably about the same number of
times as the numbers of those years, when those columns have been more
or less shaken.
We may therefore affirm that the probability (on the basis of this
experience _only_) is, say 120 to 1, that the next shock, whether
derived from Vesuvius, or elsewhere, that may shake Pozzuoli, will be
one less in power than would be needed to overturn the shafts of the
Temple of Serapis there.
* * * * *
Let us now turn to the second branch of our subject--viz.,
Vulcanology--upon which, as yet, we have secured less firm standing
ground than we have seen we possess in Seismology, for which reason we
took that first into consideration.
It is the part of Vulcanology to co-ordinate and explain all the
phenomena of past or present times visible on our globe which are
evidences of the existence and action, whether local or general, of
temperatures within our globe greatly in excess of those of the surface,
and which reach the fusing points of various mineral compounds as found
arriving, heated or fused, at the surface.
The stratigraphic geologist sees that such heated or fused masses have
come up from beneath, throughout every epoch that he can trace; but he
cannot fail to discern more or less a change in the order or character
of those outcomings, as he traces them from the lowest and oldest
formations to those of the present day. He sees immense outpourings of
granitoid or porphyrytic rocks that have welled up and overflowed the
oldest strata--huge dykes filling miles of fissures that had been
previously opened for the reception of the molten matter that has filled
them, and often passing through those masses of previously outpoured
rock; later he sees huge tables of basaltic rock poured forth over all.
One grand characteristic common to all these--commonly called plutonic
products--being that, whether they were poured forth over the surface or
injected into cavities in other rocks, the movements of the fused
material were, on the whole, hydrostatic and _not explosive_.
At the present day, whatever other evidences we have of high temperature
below our globe's surface, that which primarily fixes the eye of the
geologist is the Volcano, whose characteristic, as we see it in
activity, _is explosive_. But though there is this great characteristic
difference between the plutonic and the volcanic actions and their
products, the two, when looked at largely, are seen so to inosculate,
that it is impossible not to refer them to an agency common to both,
however changed the modes of its action have been between the earliest
epochs of which traces are presented to us and the present day.
To us little men, who, as Herschell has well said, in referring to the
methods of measuring the size of our globe, "can never see it all at
once, but must creep like mites about its surface," the Volcano, in the
stupendous grandeur of its effects, tends to fix itself in our minds in
exaggerated proportions to its true place in the cosmic machine; and, in
fact, nearly all who have sought to expound its nature and mode of
origination have occupied themselves far too exclusively with describing
and theorising upon the strange and varied phenomena which the volcanic
cone itself and its eruptions present, and too often, in the splendour
and variety of these, have very much lost sight of what ought to be the
centre-point of all such studies, namely, to arrive at some sound
knowledge of what is the _primum mobile_ of all these wonderful efforts.
Nor has the distinction been very clearly seen between the main
phenomena presented at and about volcanic active mouths, which can be
employed to elucidate the nature of the causation at work far below, and
those most varied and curious, and in other respects most pregnant and
instructive phenomena, mechanical and chemical, which are called into
action in and by the ejected matter of the volcanic cone after its
ejection. It can help us but little or very indirectly, in getting at a
true conception of the nature and source of the heat itself of the
Volcano, to examine, for example, all the curious circumstances that are
seen in the movements and changes in the lava that has already flowed
from its mouth; but it would be of great importance if we can ascertain,
by any form of observation around the cone, from what depth it has come,
or at what depth the igneous origin lies.
The physician, endeavouring to ascertain the real nature of small-pox or
measles, will scarcely make much progress who, however curiously or
minutely, confines his attention to the pustules that he sees upon the
skin.
Yet the Volcano, or rather all volcanic activity as now operative upon
our globe, is, as it were, an experiment of Nature's own perpetually
going on before us, the results of which, if well chosen--that is, as
Bacon says, by keeping to the main and neglecting the accidents--can,
when colligated and correctly reasoned upon, in relation to our planet
as a whole, give us the key to the enigma of terrestrial Vulcanicity in
its most general sense, and at every epoch of our world's geognostic
history, and show us its true place and use in the cosmical machine. Let
us glance at the history of past speculation on this subject, from which
so little real knowledge is to be derived, and then at the salient facts
of Vulcanology as now seen upon our earth, and finally see if we can
connect these with other great cosmical conditions, so as to arrive at
a consistent explanation in harmony with all.
We gain nothing absolutely from the knowledge of the so-called
"ancients" as to Volcanoes in Europe at least, where alone historic
records likely to refer to them exist. The Volcanoes of Europe are few
and widely scattered. The Greeks saw but little of them, and the Romans
were all and at all times most singularly unobservant of natural
phenomena.
Cæsar never mentions the existence in France of the Volcanoes of
Auvergne, so much like those he must have seen in Italy and Sicily; and
Roman writers pass in silence that great volcanic region, though
inhabited by them, and their language impressed upon the places, as
Volvic (_volcano-vicus_) seems with others to indicate; and though there
is some reason to believe that one or other of the Puys was in activity
within the first five hundred years of our epoch, the notices which
Humboldt and others have collected as from Plato, Pausanius, Pliny,
Ovid, etc., teach nothing.
Whatever of mere speculation there may have been, volcanic theory, or
what has passed for such, there was none before 1700, when Léméry
brought forward a trivial experiment, the acceptance of which, even for
a moment, as a sufficient cause for volcanic heat (and it retarded other
or truer views for years), we can now only wonder at. Breislak's origin,
in the burning of subterranean petroleum or like combustibles, was
scarcely less absurd than Léméry's sulphur and iron filings.
Davy, in the plenitude of his fame, and full of the intense chemical
activities of the metals of the alkalies which he had just isolated,
threw a new but transient verisimilitude upon the so-called chemical
theory of Volcanoes, by ascribing the source of heat to the oxidation of
those metals assumed to exist in vast, unproved and unindicated masses
in the interior of the earth. But Davy had too clear an intellect not to
see the baseless nature of his own hypothesis, which in his last work,
the "Consolations in Travel," he formally recanted; and it only survived
him in the long-continued though unconvincing advocacy of Dr. Daubeny.
So far, the origin of the heat had been sought always, in the crude
notion of some sort of _fuel consumed_, whether that were petroleum or
potassium and sodium; but as no fuel was to be found, nor any indicated
by the products, so far as known, of the volcanic heat, so what has been
called the mechanical theory, in a variety of shapes, took its place.
This, in whatever form, takes its lava and other heated products of the
volcano ready made from a universal ocean of liquid material, which it
supposes constitutes the interior or nucleus of our globe, and which is
only skinned over by a thin, solid crust of cooled and consolidated
rock, which was variably estimated at from fourteen to perhaps fifty
miles in thickness. Here was a boundless supply of more than heat, of
hot lava ready made, the existence of which at these moderate depths the
then state of knowledge of hypogeal temperature, which was supposed to
go on increasing with depth at the rate of about 1° Fahrenheit, for
every thirty or forty feet, seemed quite to sustain.
The difficulty remained, how was this fiery ocean brought to the
surface or far above it? To account for this two main notions prevailed,
and, indeed, have not ceased to prevail. Some unknown elastic gases or
vapour forced it up through fissures or rents pre-existent, or produced
by the tension of the elastic and liquid pressure below.
The form in which this view took most consistency, and approaching most
nearly to truth, finds the elastic vapour in steam generated from water
passed down through fissures from the sea or from the land surface. But
to this the difficulty was started, that fissures that could let down
water would pass up steam. The objection, when all the conditions are
adequately considered, has really no weight; and it has been completely
disposed of, since within a few years it has been proved that capillary
infiltration goes on in all porous rocks to enormous depths, and that
the capillary passages in such media, though giving free vent to
water--and the more as the water is warmer--are, when once filled with
liquid, proof against the return through them of gases or vapours. So
that the deeply seated walls of the ducts leading to the crater, if of
such material, may be red hot and yet continue to pass water from every
pore (like the walls of a well in chalk), which is flushed off into
steam that cannot return by the way the water came down, and must reach
the surface again, if at all, by the duct and crater, overcoming in its
way whatever obstructions they may be filled with.
And this remarkable property of capillarity sufficiently shows how the
lava--fused below or even at or above the level of infiltration--may
become interpenetrated throughout its mass by steam bubbles, as it
usually but not invariably is found to be.
Nor is it difficult to see such a mechanism between volcanic ducts and
fissures conveying down water, as large and open pipes, for a large part
of their depth, as shall bring down water to foci of volcanic heat,
without the power of the water flowing back except as steam and through
the crater.
Indeed, the facts known as to geysers, and those of half-drowned-out
Volcanoes such as Stromboli--whose action is intermittent just as much
as that of a geyser--show that this is not merely probable. There is,
therefore, no need for the hypothesis of those who have supposed all the
huge volumes of steam blown off from Volcanoes in eruption to come from
vesicular water pre-existent in the minute cavities of crystalline or
other rocks before their fusion into lava: a fact not proved for many
classes of rock, and for none in sufficient quantity to account for the
vast volume of steam required and for the irregularity of its issue.
It is rather to anticipate, but I may state at once that, so far as the
admission of superficial waters to the interior, and to any depth to
which fissures or dislocation can extend, I believe no valid physical or
mechanical difficulties exist, taking into account _all_ the conditions
that may come into play together.
Another set of views has been suggested and supported by various
writers, which proposes to account for the rise of lava on purely
hydrostatic principles. The solid crust, fractured into isolated
fragments by tensions due to its own contraction, is supposed to sink
into the sea of lava on which it floats; and much ingenuity has been
expended in imagining the mechanism by which, in places, the liquid
matter is supposed to rise _above_ the surface of the crust.
I have no space for discussing these views further than to assert that,
in the existing state of our globe, and even admitting a solid crust of
only 60,000 metres thick, dislocation of the crust by _tension_ is not
possible. The solid crust of our globe, as I hope we shall see further
on, is not in a state of tension, and has not been so since it was
extremely thin, a mere pellicle as compared with the liquid nucleus, but
is, on the contrary, in a state of _tangential compression_.
However tenable, in other respects, may be the volcanic theory which
rests upon the assumption of a very _thin_ crust and a universal ocean
of fused rock beneath, it fails wholly to explain many of the most
important circumstances observable as to the distribution and movements
of existing Volcanoes on our globe.
It affords no adequate explanation of the configuration of the lines of
Volcanoes, nor of their occurrence in the ocean bed, nor of their
existence in high latitudes, near the Poles, where, no matter how or at
what rate our globe cooled from liquidity, the crust must be thickest;
nor of the independence of eruptive action of closely adjacent volcanic
vents; nor of the non-periodicity, the sudden awakening-up to activity,
the as sudden exhaustion, the long repose, the gradual decay of action
at particular vents, and of much more that might be stated and
sustained as difficulties left by that theory unexplained, or that are
of a nature even opposed to it.
The researches of the last few years have, however, as it appears to me,
rendered any theory that demands as its postulates a _very thin crust_,
and a universal liquid nucleus beneath it, absolutely untenable.
Without attaching any importance to the arguments of Mr. Hopkins, based
upon precession and nutation, it appears to me, on various other
grounds, some of which have been urged by Sir William Thompson, that the
earth's solid crust is not a thin one, at least not thin enough to
render it conceivable that water can ever gain admission to a fluid
nucleus, if any such still exist, situated at so great a depth; and
without such access we can have no Volcano. It is not necessary to go to
the extent of a crust of 800 or 1,000 miles thick: with one of half the
minor thickness, I believe it may be proved, on various grounds,
hydraulic amongst others, that neither water could reach the nucleus,
nor the liquid matter of the nucleus reach the surface. Mr. Hopkins
having proved to his own satisfaction an enormous thickness for the
crust, and seeing clearly the difficulties that this involved to the
generally accepted volcanic theory, and having no other to substitute
for it, fell back upon that most vague and weak notion of the existence
of isolated lakes of liquid rock, existing at comparatively small depths
beneath the earth's surface within the solid and relatively cold crust,
each supplying its own Volcano, or more than one, with ready-made lava.
What is to produce these lakes of fused matter in the midst of similar
solidified matter? what is perpetually to maintain their fluidity in the
midst of solid matter continually cooling? what has given them their
local position? why near or less near the surface? what should have
arranged them in directions stretching in some cases nearly from Pole to
Pole?
Surely this creation of imaginary lakes, merely because it happens to
fit the vacant chink that seems needed to wedge up a falling theory, is
an instance of that abuse of hypothesis against which Newton so
vehemently declaims--"_Hypotheses non fingo._"
Hypothesis, to be a philosophic scaffolding to knowledge, must, as
Whewell has said, "be close to the facts, and not merely connected with
them by arbitrary and untried facts." Yet this appears accepted by Lyell
(10th edition, Vol. II., p. 227, and elsewhere); by Phillips
("Vesuvius," pp. 331, 332); by Scrope, if, as I hope, I mistake him not
("Volcanoes," pp. 265, 307-8); though none of these excellent
authorities seem either quite clear or quite satisfied with the notion;
and in the very passage referred to, Lyell _may_ have possibly a much
more philosophic notion in view, where he says: "It is only necessary,
in order to explain the action of Volcanoes, to _discover some cause
which is capable of bringing about such a concentration of heat as may
melt one after the other certain portions of the solid crust_, so as to
form seas, lakes or oceans of subterraneous lava." (Vol. II, pp. 226,
227). If by this is meant, that all that is needed to complete a true
theory of volcanic action is to discover _an adequate cosmical cause for
the heat_--that is to say, a prime mover to which all its phenomena may
be traced back, which shall be at once reconcilable with the conditions
of our planet as a cooling mass in space and with facts of Vulcanology
as they are now seen upon it--then I entirely agree with it.
It has been my own object to endeavour to discover and develope that
adequate cause in a Paper "On Volcanic Energy, an Attempt to develope
its True Nature and Cosmical Relations," read (in abstract) before the
Royal Society of London ("Proceedings, Royal Society," Vol. XX., May,
1872), and now (October, 1872) under consideration of Council with a
view to publication.
I propose concluding this review of the progress of Vulcanology (in
which I have had to limit myself to reviewing merely the chief stages of
advance towards knowledge of the nature and origin of volcanic heat
itself, and have had to pass without notice the vast and important mass
of facts and reasonings collected by so many labourers as to its visible
phenomena and products, and the still greater mass of speculation, good
and bad, on every branch of the subject), by giving a necessarily very
brief and imperfect sketch of my own views as in that Paper in part
developed. It will first be necessary to retrace our steps a little, in
order to gain such a point as shall afford us a fuller view of the whole
problem before us.
It is not necessary to dilate, even did space allow, upon the many
points which bind together Earthquakes and Volcanoes as belonging to the
play of like forces. These are generally admitted; and in various ways,
more or less obscure, geologists generally have supposed some relations
between these and the forces of elevation, which have raised up mountain
chains, etc.
No one, however, that I am aware of, prior to myself, in the Paper just
alluded to, has attempted to show, still less to prove upon an
experimental basis, that all the phenomena of elevation, of volcanic
action, and of Earthquakes, are explicable as parts of one simple
machinery--namely, the play of forces resulting from the secular cooling
of our globe. We have seen that, on the whole, both Earthquakes and
Volcanoes follow along the great lines of elevation of our surface. Any
true solution of the play of forces which has produced any one of those
three classes of phenomena must connect itself with them all, and be
adequate to account for all. And this would have earlier been seen, had
geologists generally framed for themselves any correct notions of the
mechanism of elevation itself, and seen its real relation with the
secular cooling of our planet. But the play of forces resulting from
this secular cooling has never, until very recently, been adequately or
truly stated. The arbitrary assumption and neglect of several essential
conditions by La Place, in his celebrated Paper "On the Cooling of the
Earth," in the fifth volume of the "Mécanique Céleste," and the
arbitrary and unsustainable hypothesis of Poisson upon the same subject,
have tended to retard the progress of physical Geology as to the nature
of elevation: the first, by leaving the geologist in doubt as to whether
our globe were cooling at all; the second, by suggesting distorted
notions as to the mode of its cooling and consolidation. On the other
hand, neither geologists nor mathematicians generally have framed for
themselves any clear notions of the mechanism of elevation. Had a true
conception been formed of the forces and interior movements brought
necessarily into operation by the secular cooling of the globe,
geologists could scarcely have failed to see that their notion as to the
way and direction in which the forces producing elevation have actually
acted could not, if arising from refrigeration, be those which they have
almost universally supposed, namely--some force acting vertically
upwards, _i.e._, radially from the centre of the sphere. Had geologists
only looked at Nature with open eye, they must have seen that mountain
ranges, and elevations generally (exclusive of volcanic cones),
presented circumstances absolutely incompatible with their having been
thrust up by any force _primarily_ acting in the direction of a radius
to the spheroid.
Yet this is the erroneous notion of the mechanism of elevation which to
the present hour prevails amongst geologists, so far as they in general
have framed to themselves any distinct idea of such mechanism at all.
Thus, only to cite two examples from recent authors of justly high
reputation. Lyell says of the probable subterranean sources, whether of
upward or downward movement, when permanently uplifting a country, and
in reference to the crumpling of strata on mountain flanks by lateral
pressure, it would be rash to assume these able to resist a power of
such stupendous energy, "_if its direction, instead of being vertical_,
happened to be oblique or horizontal." This is somewhat vague--and I
trust I do not mistake or misrepresent the illustrious author--yet it
is the most explicit expression I can find in the "Principles of
Geology" as to his notion of the primary direction of elevatory force
(Edit. 10, Vol. I., p. 133). That Mr. Scrope's idea is that only of
primary radial or vertical direction of such forces, is apparent on
inspecting his Diagram No. 64 ("Volcanoes," p. 285), and in the use of
the words, "an axial wedge of granite," which, on the next page, we find
is "liquefied granite;" and if we read on to page 294, and refer also to
pages 50 and 51, I believe there can be no doubt that _vertical_ or
_direct up-thrust_ is the author's notion of the primary direction of
all forces of elevation. The true nature of these forces was, however,
clearly seen and most justly stated by Constant Prevost ("Compt. Rend.,"
Tome XXXI., 1850, and "Bulletin de la Société Géolog. de France," Tome
II., 1840) as consisting, not in forces of some unknown origin acting
primarily in the vertical, but in _tangential pressures acting
horizontally, and resolved by mutual pressures at certain points into
vertical resultants_. These Prevost rightly attributed to the
contraction of the earth's solid crust. The same idea has been adopted
by Elie de Beaumont as the true mechanism of the elevation of mountain
ranges; and although De Beaumont's views as to the thinness he assigns
to the solid and contracting crust, and his strange deduction as to the
parallelism of contemporaneous mountain chains uplifted by its spasmodic
action along certain lines, may be untenable, his notion generally as to
the play of forces producing mountain elevation is much more nearly
correct.
Mr. Hopkins's notion is simply that of the geologists. Anyone who reads
his well-known papers on elevation and the formation of fissures, etc.,
must see that he views all elevatory forces as of liquids or
quasi-liquids forced up and acting primarily _vertically_ upon the
strata above them, and that these strata are not under tangential
compression, but under tension. Hence the mathematical deductions
contained in those papers as to the directions in which elevatory forces
act, and in which fissures are formed by them, are not in any way a
setting forth of such facts as occur in Nature, and, much attention as
they have attracted, can only now be viewed as exercises of mathematical
skill misapplied, because based upon data not to be found in Nature. In
fact, those papers do but misrepresent Nature, and, like many other
mathematical investigations based on untrue or insufficient data, have
tended to retard knowledge.
The views which I have put forward in the Paper I have referred to, read
to the Royal Society, recapitulated in skeleton, so to say, are as
follows. Omitting those portions which treat of our globe from the
period of the first liquefaction out of a nebulous condition, and of the
earliest stages of the cooling by radiation into space, when the crust
was extremely thin, and of the deformation of the spheroid as one of the
first effects of its contraction, and through that the general shaping
out of continents and ocean beds; I have endeavoured to show that the
rate of contraction of the crust, while very thin, exceeded that of the
large fluid nucleus supporting it, and so gave rise to _tangential
tensions_ in the crust, and fracturing it into segments; next, that as
the crust thickened, these _tensions_ were gradually converted into
_tangential pressures_, the contraction of the nucleus now beginning to
exceed (for equal losses of heat) that of the crust through which it
cooled. At this stage these tangential pressures gave rise to the
_chief_ elevations of mountain chains--not by liquid matter by any
process being injected from beneath vertically, but by such pressures,
mutually reacting along certain lines, being resolved into the vertical,
and forcing upwards more or less of the crust itself. The great outlines
of the mountain ranges and the greater elevation of the land were
designated and formed during the long periods that elapsed in which the
continually increasing thickness of crust remained such that it was
still, as a whole, flexible enough, or opposed sufficiently little
resistance to crushing, to admit of this uprise of mountain chains by
resolved tangential pressures. I have shown that the simple mechanism of
such tangential pressures is competent to account for all the complex
phenomena both of the elevations and of the _depressions_ that we now
see on the earth's surface (other than continents and ocean beds),
including the production of gaping fissures (in directions generally
orthogonal to those of tangential pressure). And as our earth is still a
cooling body, and the crust, however now thicker and more rigid, is
still incapable of sustaining the tangential pressures to which it is
now exposed, so I by no means infer that slow and small (relatively)
movements of elevation and depression may not be still and now going on
upon the earth's surface; in fact all the phenomena of elevation and
depression, rending, etc., which at a much remoter epoch acted upon a
much grander and more effective scale. So that, for aught my views say
to the contrary, all the mountain chains in the world may be possibly
increasing in stature year by year, or at times; but in any case at a
rate almost infinitesimally small in its totality over the whole earth
to that with which their ridges were originally upreared.
But the thickness of the earth's crust--thus constantly added to, by
accretion of solidifying matter from the still liquid or pasty nucleus,
as the whole mass has cooled--has now assumed such a thickness as to be
able to offer a too considerable resistance to the tangential pressures,
to admit of its giving way to any large extent by resolution upwards;
yet the cooling of the whole mass is going on, and contraction, though
unequal, both of thick crust and of hotter nucleus beneath also, whether
the latter be _now_ liquid or not. Were the contraction, lineal or
cubical, for equal decrements or losses of heat, or in equal
times--equal both in the material of the solidified crust and in that of
the hotter nucleus--there could be no such tangential pressures as are
here referred to, at any epoch of the earth's cooling. But in accordance
with the facts of experimental physics, we know that the co-efficient of
contraction for all bodies is greater as their actual temperature is
higher, and this both in their solid and liquid states.
Hence for equal decrements of heat, or by the cooling in equal times,
the hotter nucleus contracts more than does its envelope of solid
matter.
The result is now, as at all periods since the signs changed of the
tangential forces thus brought into play--_i.e._, since they became
tangential _pressures_--that the nucleus tends to shrink away as it
were from beneath the crust, and to leave the latter, unsupported or but
partially supported, as a spheroidal dome above it.
Now what happens? If the hollow spheroidal shell were strong enough to
sustain, as a spheric dome, the tangential thrust of its own weight and
the attraction of the nucleus, the shell would be left behind altogether
by the nucleus, and the latter might be conceived as an independent
globe revolving, centrally or excentrically, within a shell outside of
it. This, however, is not what happens.
The question then arises, Can the solid shell support the tangential
thrust to which it would be thus exposed? By the application to this
problem of an elegant theorem of Lagrange, I have proved that it cannot
possibly do so, no matter what may be its thickness nor what its
material, even were we to assume the latter not merely of the hardest
and most resistant rocks we know anything of, but even were it of
tempered cast-steel, the most resistant substance (unless possibly
iridio-osmium exceed it) that we know anything about. Lagrange has shown
that if P be the normal pressure upon any flexible plate curved in both
directions, the radii of these principal curvatures being r' and r'',
and T the tangential thrust at the point of application and due to the
force P, then: