is the heat corresponding to the work expended in the crushing,
expressed in British units of heat. The following were the rocks
experimented upon: Caen stone, Portland (both oolites), magnesian
limestone, sandstones of various sorts, carboniferous limestones
(marbles), the older slates (Cambrian and Silurian), basalts, various
granites and porphyries, thus ranging from the newest and least
resistant to the oldest and most resistant rocks. The results have been
tabulated, and are given in detail in my Paper, now in possession of the
Royal Society. The minimum obtained is 331 and the maximum 7,867 British
units of heat developed, by transformation of the work of crushing one
cubic foot of rock. If we apply the results to a thickness of solid
crust of 100 miles (British), of which the upper twenty-one miles
consist of neozoic, newer palæozoic, older palæozoic and azoic rocks in
nearly equal proportion as to thickness, and the remaining eighty miles
of crystalloid rocks (acid and basic magmas of Durocher) of physical
properties which we may assume not very different from those of our
known granites and porphyries--and which, in so far as they may differ,
would give a still _higher_ co-efficient of work transformed into heat
than I have attributed to them by ranging them as only equal to the
granites, etc.--then we obtain a mean co-efficient for the entire
thickness of crust of 100 miles of 6,472 British units of heat,
developable from each cubic foot of its material, if crushed to powder.
It results from this that each cubic mile of the mean material of such
a crust, when crushed to powder, developes sufficient heat to melt 0·876
cubic miles of ice into water at 32°, or to raise 7·600 cubic miles of
water from 32° to 212° Fahr., or to boil off 1·124 cubic miles of water
at 32° into steam of one atmosphere, or, taking the average melting
point of rocky mixtures at 2,000° Fahr., to melt nearly three and a-half
cubic miles of such rock, if of the same specific heat.
Of the heat annually lost by our globe and dissipated into space,
represented by 777 cubic miles of ice melted, as before stated, the
chief part is derived from the actual hypogeal source of a hotter though
not necessarily fused nucleus, and nearly, if not wholly, is quite
independent of the heat of Vulcanicity, which is developed as a
consequence of its loss or dissipation. But were we to take the extreme
case, and suppose it possible that all the heat the globe loses annually
resulted from the transformation of the work of internal crushing of its
shell, we shall find that the total volume of rock needed to be crushed
in order to produce the required amount of lost heat is perfectly
insignificant as compared with the volume of the globe itself, or that
of its shell. For, as 1·270 cubic miles of crushed rock developes heat
equivalent to that required to melt one cubic mile of ice to water at
32°, and if we assume the volume of our globe's _solid_ crust to equal
one-fourth of the total volume of the entire globe, 987 cubic miles of
rock crushed annually would supply the whole of the heat dissipated in
that time. But that is less than the _one sixty-five millionth_ of the
volume of the crust only.
But a very small portion of the total heat annually lost by our globe
is sufficient to account for the whole of the volcanic energy of every
sort, including thermal waters, manifested annually upon our earth. In
the absence of complete data, we can only approximately calculate what
is the annual amount of present volcanic energy of our planet. This
energy shows itself to us in three ways: 1. The heating or fusing of the
ejected solid matters at volcanic vents. 2. The evolution of steam and
other heated elastic fluids by which these are carried. 3. The work of
raising through a certain height all the materials ejected. To which we
must add a large allowance for waste, or thermal mechanical and chemical
energy ineffectually dissipated in and above the vents. All these are
measurable into units of heat.
I have applied this method of calculation to test the adequacy of the
source I have assigned for volcanic heat, in two ways, viz.: 1. To the
phenomena presented during the last two thousand years by Vesuvius, the
best known Volcano in the world; and 2. To the whole of the four hundred
and odd volcanic cones observed so far upon our globe, of which not more
than one-half have ever been known in activity.
It is impossible here to refer to the details of the method or steps of
these calculations. The result however is, that making large allowances
for presumably defective data, _less than one-fourth_ of the total
telluric heat annually dissipated (as already stated in amount) is
sufficient to account for the annual volcanic energy at present expended
by our globe.
It is thus represented by the transformation into heat of the work of
crushing about 247 cubic miles of (mean) rock, a quantity so perfectly
insignificant, as compared with the volume of the globe itself, as to be
absolutely inappreciable in any way but by calculation; and as its
mechanical result is only the vertical transposition transitorily of
material within or upon our globe, the proportion of the mass of which
to the whole is equally insignificant, so not likely in any way to
produce changes recognisable by the astronomer.
Space here forbids my entering at all upon that branch of my
investigation which is based upon the experimental results, above
mentioned, of the total contraction of fused rocks: for these, the
original Paper can, I hope, be hereafter referred to. I am enabled,
however, to prove thus how enormously more than needful has been the
store of energy dissipated since our globe was wholly a melted mass, for
the production, through the contraction of its volume, of all the
phenomena of elevation and of Vulcanicity which its surface presents.
And how very small is the amount of that energy in a unit of time as now
operative, when compared with the same at very remote epochs in our
planet's history.
I have said that if we can find a true cause in Nature for the
origination of volcanic _heat_, all the other known phenomena, at and
about volcanic vents, become simple. Lavas and all other solid ejecta of
Volcanoes, from all parts of the earth's surface, as well as basalts,
present in chemical and physical constitution close resemblance, and may
be all referred to the melting of more or less fusible mixtures of
siliceous crystalloid rocks with aluminous (slates, etc.) and calcareous
rocks. Their general chemical composition, and the higher or lower
temperatures of fusion resulting therefrom, together with the higher or
lower temperatures to which they have been submitted at the different
volcanic foci, determine their difference of flow (under like surface
conditions) and of mineral character after ejection and cooling.
St. Clair de Ville and Fouqué have shown that the gaseous ejections, of
which steam forms probably 99 per cent., are such as arise from water
admitted to a _pre-existent focus of high temperature_.
Whether sea or fresh water is not material, when we bear in mind that
the chemical constituents found in sea water and in natural fresh waters
that have penetrated the soil are, on the whole, alike in kind and only
differ in proportions. But I must pass almost without notice all the
varied and instructive phenomena which are presented by volcanic vents,
for to treat of these at all would be to more than double the size of
this sketch.
In the source that has been pointed out as that from which volcanic heat
itself is derived, viz., the secular cooling of our globe, and the
effects of that upon its solid shell, we are enabled to point to that
which is the surest test of the truth of any theory--that it not only
enables us to account for all the phenomena, near or remote, but to
predict them. We see here linked together as parts of one grand play of
forces, those of contraction by cooling, producing by _direct_
mechanical action the elevation of mountain chains, and by their
_indirect_ action, by transformation of mechanical work into heat, the
production of Volcanoes; and both by direct and by indirect action, of
Earthquakes, never previously shown to have thus the physical connection
of one common cause, but merely supposed, more or less, to be connected
by their distribution upon our earth's surface.
We now discern thus the physical cause _why_ Volcanoes are distributed,
viewed largely, linearly, and follow the lines of elevation; we see
equally why their action is uncertain, non-periodic, fluctuating in
intensity, with longer or shorter periods of repose, shifting in
position, becoming extinct here, appearing in new activity or for the
first time there. We have an adequate solution of the before
inexplicable fact of their propinquity, and yet want of connection. We
have an adequate cause for the fusion of rock at local points without
resorting to the baseless hypothesis of perennial lakes of lava, etc.
For the first time, too, we discern a true physical cause for earthquake
movement, where volcanic energy does not show itself. The crushing of
the world's solid shell, whether thick or thin, goes on _per saltum_ and
at ever-shifting places, however steadily the tangential pressures
producing it may act. Hence crushing _alone_ may be shown to develope
amply sufficient impulse to produce the most violent Earthquakes,
whether they be or be not at a given place or time connected with
volcanic outburst or possible injection, or with tangential pressures,
enough still, in some cases, to produce partial permanent elevation.
When subterraneous crushing takes place, and the circumstances of the
site do not permit the access of water, there may be Earthquake, but can
be no Volcano; where water is admitted, there may be both.
And thus we discern why there are comparatively few submarine
Volcanoes, the floor of the ocean being, on the whole,
water-tight--"puddled," as an engineer would say, by the huge deposit of
incoherent mud, etc., that covers most of it, and probably having a
thicker crust beneath it than beneath the land.
We see, moreover, that the geological doctrine of absolute uniformity
cannot be true as to Vulcanicity, any more than it can for any other
energy in play in our world. Its development was greatest at its
earliest stages, when the great masses of the mountain chains were
elevated. It is even now--though as compared to men's experience, and
even to all historic time, apparently uniform and always the same--a
decaying energy.
The regimen of our planet as part of the Cosmos, which seems to some
absolute (and presented to Playfair no trace of a beginning nor
indication of an end), is not absolute, and only seems to us to be so
because we see so little of it, and of its long perspective in time.
This the now established doctrine of the conservation of energy renders
certain.
With this source for volcanic heat, too, in our possession, we can look
from our own world to others, and predict within certain limits, which
must widen as our knowledge of the facts of their substance and surface
becomes greater, what have been and what are the developments of
Vulcanicity which have taken place or are occurring in or upon them.
Looking to our own satellite, we see for the first time a sufficient
physical cause for the enormous display of volcanic energy there which
the telescope divulges to us; one which is not to be explained alone by
the commonly made statement of the small density of the moon, but by
the fact that as the rate of her cooling from a given temperature, as
compared with that of our earth (apart from questions of the chemical
nature of the two bodies, or of their specific heats, etc.), has been
inversely as their respective masses, and directly as their surfaces, so
has the rate of cooling of the moon been vastly greater than that of the
earth, and the energy due to contraction by cooling more intense and
rapidly developed in our satellite than upon our globe.
We have thus traced, in meagre and broken outline only--because space
admitted no more--the progress of Science to its existing state as
respects Vulcanicity, in its two branches of Vulcanology and of
Seismology, and pointed out their more intimate relations and points of
connection, and been at length able to refer them, on the sure basis of
physical laws, to one common cause, and that one derived from no
hypothesis, but simply from the postulate of our world as a terr-aqueous
globe cooling in space.
What I have here advanced with reference to volcanic energy, which
appertains to my own researches, I do not conceal from myself, nor from
the reader, has yet to await the reception generally and the award of
the true men of science of the world.
That, like every new line of thought which has attempted or succeeded in
supplanting the old, it will meet with opposition, I make no doubt.
My belief, however, is that in the end it will be found to have added a
fragment to the edifice of true knowledge.
The interpretation which I have given of the nature and origin of
volcanic activity points at once to the function in the Cosmos which it
is its destiny to fulfil. It is the instrument provided for the purpose
of continually preserving the earth's solid shell in a state to follow
down after the descending nucleus. It does this by an apparatus or play
of mechanism whereby the material of the solid shell, locally or along
certain lines, is not only crushed, but the crushed material is blown
out as dust, or expelled as liquid rock from between the walls of the
shell, which are thus enabled to approach each other; and thus, by
relief of the tangential thrusts, to permit the shell to descend, which
it is obvious that crushing alone, unless it extended to the whole mass
of the shell, could not accomplish.
It is a wonderful example of Nature's mechanism thus to see how simple
are the means by which this end is accomplished. The same inevitable
crush that dislocates the solid shell along certain lines, produces the
heat necessary to expel to the surface the material crushed.
When attempted to be made the basis for philosophic discovery, "final
causes" are no doubt barren, as Bacon has said; but when we have
independently and by strict methods arrived at a result, we may justly
appeal, as a test of its truth, to its showing itself as plainly
fulfilling a needful end, and, by a distinctly discernible mechanism,
preserving that harmony and conservation which are the obvious law of
the universe.
As has been said, if I mistake not by Daubeny, John Phillips, by
Herschell, and by myself, the function of the Earthquake and the
Volcano is not destructive but, preservative. But we now see that: that
the preservative scope of this function, as respects our earth, is far
wider than what has been previously attributed to it. The Volcano does
not merely throw up new fertile soil, and tend, in some small degree, to
restore to the dry land the waste for ever going on by rain and sea; it
fulfils a far weightier and more imperative task; it--by a mechanism the
power of which is exactly balanced to the variable calls demanded of it,
and which working almost imperceptibly, although in a manner however
terrible its surface-action may at times appear to us little
men[G]--prevents at longer intervals such sudden and unlooked-for
paroxysms in the mass of our subsiding earth's shell as would be
attended with wide-spread destruction to all that it inhabit.
To the popular mind, Volcanoes and Earthquakes are only isolated items
of curiosity amongst "the wonders of the world:" few geologists even
appear to realise how great and important are the relations of
Vulcanicity to their science, viewed as a whole. Yet of Vulcanicity it
is not too much to say, that in proportion as its nature and doctrines
come to be known and understood as parts of the Cosmos, the nearer will
it be seen to lie at the basis of all Physical Geology.
[A] For a fuller account of the literature and history of advancement of
human knowledge as to Earthquakes, here merely glanced at, I must refer
to my First Report on the Facts of Earthquakes, "Reports, British
Association, 1850," and to the works of Daubeny, Lyell, Phillips and
others, its _complete_ history remaining yet to be written.
[B] Yet how indistinctly formed were Young's ideas, and indistinct in
the same direction as those of Humboldt, becomes evident by a single
sentence: "When the agitation produced by an Earthquake extends further
than there is any reason to suspect a subterraneous communication, it is
probably propagated through the earth nearly in the same manner as a
noise is conveyed through the air."--_Lectures, Nat. Phil._, Vol. I.
[C] The Right Rev. Charles Graves, F.R.S., etc., then Fellow of Trinity
College, Pres. R. I. Acad., and now Bishop of Limerick, on presentation
of the Academy's Cunningham Medal.
[D] In this Report, though I have never before referred to it, and do so
now with reluctance, I have always felt that the Author did me some
injustice. The only reference made to my labours, published the
preceding year only, is in the following words: "Many persons have
regarded these phenomena (viz., Earthquakes) as due in a great measure
to vibrations ... and the subject has lately been brought under our
notice, in a Memoir by Mr. Mallet, 'On the Dynamics of Earthquakes,' in
which he has treated it in a more determinate manner, and in more
detail, than any preceding writer" (p. 74). If that Paper of mine be
collated with this Report, it will be, I believe, found that, as
respects the earthquake part, the latter tint parades, in a mathematical
dress, some portions of the general theory of earthquake movements,
previously published by me as above stated. So, also, in the chapter (p.
90) referring to Seismometry, and the important uses to Geology that
might be (and since have been, to some extent) made of it, no mention is
made of those instruments previously proposed by me, nor of my
anticipation of their important uses. This is but too mortifyingly
suggestive of the--
"Pereant qui mea ante mihi dixerunt."
Having left this unnoticed for so many years, and during which the
Author has preceded me to that bourne where our errors to each other
must be forgotten, I should certainly not have now trespassed on the
good rule, _De mortuis nil nisi bonum_, had I not observed very recently
one amongst other results probably attributable to it. In Professor
Phillips's "Vesuvius," if any one will refer to the passage beginning
"The mechanism of earthquake movement has been investigated by competent
hands. The late eminent mathematician, Mr. Hopkins, explained these
tremors in the solid earth by the general theory of vibratory motion,"
etc. (pages 257-259)--I think he must, in the absence of collateral
information, conclude that, not I, but Mr. Hopkins, was the discoverer
of the Theory of Earthquakes as explained by the general theory of
vibratory motion.
Probably my friend, Professor Phillips, had not recently referred to
those Memoirs and Reports of twenty-four years back, and I am thoroughly
convinced that, if he has here perpetuated an injustice, he has done so
unintentionally and unwittingly.