Theory of the Earth, Volume 1 (of 4)
by
James Hutton
Part 6 out of 6
corps souterrains, une circulation qui fait continuellement changer de
place aux elemens de la matiere, jusqu'a ce que reunis par la force des
affinites, les corpuscules similaires prennent la forme que la nature
leur a assignee."
Those nodular bodies or figured parts which are here inclosed in the
rock, are evidently what may be called calcedony agates. M. Patrin is
persuaded, from the examination of them, that they had not been formed
in the manner of German agates, which he supposes is by mean of
infiltration; and he has endeavoured to conceive another manner of
operating, still however by means of water, which I suppose, according
to this hypothesis, is to dissolve substances in one part, and deposits
them in another, There must certainly be some great _desideratum_ in
that mineral philosophy which is obliged to have recourse to such
violent suppositions. First, water is not an universal solvent, as it
would require to be, upon this supposition; secondly, were water allowed
to be an universal menstruum, here is to be established a circulation
that does not naturally arise from the mixture of water and earth; and,
lastly, were this circulation to be allowed, it would not explain the
variety which is found in the consolidation and concretion of mineral
bodies.
So long, therefore, as we are to explain natural appearances by
reasoning from known principles, and not by ascribing those effects to
preternatural causes, we cannot allow of this regular operation which
M. Patrin alleges to be acting in the interior parts of the most solid
bodies. This is indeed evident, that there has been a cause operating
in the internal parts of the most solid bodies, a cause by which the
elements, or constituent parts of those solid bodies, have been moved
and regularly disposed, as this author very well observes must have been
the case in our agates or eyed stones; but to ascribe to water this
effect, or to employ either an ineffectual or an unknown cause, is not
to reason philosophically with regard to the history of nature; it is to
reason phantastically, and to imagine fable.
M. Monnet has imagined a petrifying power in water very different
from any that has hitherto been conceived, I believe, by natural
philosophers, and I also believe, altogether inconsistent with
experience or matter of fact; but as it is not without good reason that
this naturalist has been induced to look out for a petrifying cause
different from any hitherto supposed, and as he has endeavoured very
properly to refute the systems of petrification hitherto received,
I would beg leave to transcribe his reasoning upon the subject in
corroboration of the present theory of consolidation by the means of
fusion.
It is upon occasion of describing one of the species of alpine stone
or schistus which contains quartzy particles. _Nouveau voyage
mineralogique, etc._ Journal de Physique Aoust 1784.
"Il y a loin de cette pierre, que je regarde comme une variete de roches
ardoisees, aux veritable ardoises. La composition de toutes ces pierres
est due aux terres quartzeuses et argileuses, et a la terre talqueuse,
que je demontrerai un jour etre une espece particuliere et distincte des
autres, qui constitue les bonnes ardoises, et fait, ainsi que le quartz,
qu'elles resistent aux injures de l'air, sans s'effleurir, comme je
ferai voir que cette terre, qu'on designera sous la denomination de
terre talqueuse, si l'on veut, resiste au grand feu sans se fondre. Les
differences de toutes ces pierres, quoique composees des memes matieres,
mais dans des proportions differentes, sont frappantes, et pourroient
faire croire qu'elles n'appartiennent pas a ce genre. Mais qui ne voit
ici que toutes ces differences, ou ces varietes, ne sont dues qu'aux
modifications de la matiere premiere, qu'elle a eprouvees, soit en se
melant avec des matieres heterogenes, prevenantes du debris des etres
qui ont existe, comme l'argile, par exemple, qui, de l'aveu de presque
tous les naturalistes, est le produit de l'organization des plantes, ou
soit en se melant avec de la matiere deja solidifiee depuis long-temps?
Or nous ne craignons pas de dire, ce que nous avons dit plusieurs fois
quand l'occasion s'en est presentee, que cette matiere unique, que
se modifie selon les occasions et les circonstances, et qui prend
un caractere analogue au matieres qu'elle rencontre, est l'eau, que
beaucoup de naturalistes cherchent vainement ailleurs. Ils ne peuvent
comprendre, malgre les exemples frappans qui pourroient les porter a
adopter cette opinion, que ce fluide general soit l'element des corps
solides du regne mineral, comme il est de ceux du regne vegetal et du
regne animal. L'on cherche serieusement, par des experiences chimiques,
a decouvrir si l'eau est susceptible de se convertir en terre comme
si la nature n'avoit pas d'autre moyen que nous de la faire passer de
l'etat fluide a l'etat solide. Voyez le spath calcaire et le quartz
transparens; est il a presumer qu'ils ne sont que le resultat du depot
des matieres terreuses fait par les eaux? Mais, dans ce ca-la encore,
il faut supposer que l'eau qui est restee entre ces partie s'est
solidifiee; car, qu'est-elle donc devenue, et quel est donc le lien qui
a uni ces parties et leur a fait prendre une forme reguliere? Il est
vrai qu'on nous parle d'un suc lapidifique; mais c'est-la un etre de
raison, dont il seroit bien plus difficile d'etablir l'existence, que de
croire a la solidification de l'eau. On nous donne cependant comme un
principe certain que l'eau charie d'un lieu a un autre les matieres
qu'il a dissoutes, et qu'elle les depose a la maniere des sels. Mais
c'est supposer une chose dementie par l'experience; savoir, que l'eau
ait la propriete de dissoudre les matieres terreuses, telles que la
quartzeuse. A la verite, M. Auchard de Berlin y joint de l'air fixe;
mais cet air fixe ne sauroit tenir en dissolution un atome de quartz
dans l'eau; et quelle qu'ait ete l'exactitude de ceux qui ont repete
les experiences de M. Auchard, on n'a pu reussir a imiter la nature,
c'est-a-dire, a former des cristaux quartzeux, comme il a annonce. Que
l'eau ait la faculte de tenir en dissolution quelques petites parties
de terre calcaire, au moyen de cet air fixe, il n'en faut pas conclure
qu'elle puisse former de cette maniere tous les cristaux calcaires, sans
que l'eau elle-meme y concoure pour sa part; car ce seroit conclure
quelque fois que la partie seroit egale au tout. Voyez ces geodes
calcaire et argileuses, qui renferment des cristaux nombreux de quartz
ou de spath calcaire; ne sont ils que le resultat du depot de l'eau
qui y a ete renfermee, ou que la cristallization pure et simple des
molecules que vous supposez avoir ete tenues en dissolution par cette
eau? Il naitroit de cette opinion une foule d'objections qu'il seroit
impossible de resoudre. Cependant M. Guettard, dans la mineralogie du
Dauphine, qui vient de paroitre, ouvrage tres-estimable a beaucoup
d'egards, explique, selon cette maniere de penser, la formation de
cristallizations quartzeuses qu'on trouve dans certaines geodes de
cette province, et celle des mines de cristal des hautes montagnes. En
supposant meme comme vraie l'explication qu'il en donne, on trouveroit
en cela un des plus grands probleme, et des plus difficiles a resoudre
qu'il y ait en mineralogie; car d'abord il faudroit expliquer comment un
si petite quantite d'eau que celle qui a ete renfermee dans les geodes,
et celle qui est parvenue dans les fentes des rochers, ont pu fournir
un si grande quantite de matiere que celle qui constitue ces
cristallisations, et ce qui n'est pas le moins difficile a concevoir,
comment l'eau a pu charrier cette matiere a travers tant de matieres
differentes, et la conserver precisement pour cette destination;
comment, par exemple, l'eau est venue deposer de la terre quartzeuse
dans les masses enormes de pierres calcaires, qui forment la cote qui
domine le village de Champigny, a quatre lieues de Paris, au dela de
Saint-maur; car s'il nous faut citer un exemple frappant de cette
singularite, et a portee d'etre vue des naturalistes qui sont dans la
capitale, je ne puis mieux faire que de citer cette cote, une des plus
curieuses de la France, et que je me propose de fair connoitre en detail
dans la troisieme partie de la mineralogie de la France. On verra,
dis-je, dans cette bonne pierre a chaux, et une de plus pure des
environs de Paris, de tres-abondantes cristallisations de quartz
transparent, et quelque fois de belle eau, que les ouvriers sont forces
de separer de la partie calcaire, a laquelle elles adherent fortement.
Mais c'est trop nous arreter a combattre une opinion qui doit son
origine aux premieres idees qu'ont eues les premiers observateurs en
mineralogie, qui se detruira d'elle meme comme tant d'autres dont il
nous reste a peine le souvenir."
We find here an accurate naturalist, and a diligent observer, who, in
conformity with what my sentiments are upon the subject, thinks it
impossible that the crystallizations in close cavities, and concretions
of different solid substances within each other, which so frequently
occur in the mineral regions, could have been produced, by means of
solution and crystallization, from a fluid vehicle. But what has he
now substituted in place of this solution, in order to explain
appearances?--a mere supposition, viz. that nature may have the power
of converting water, in those secret places, into some other thing; or
rather that the substance of water is here converted into every other
thing; for, though he has only mentioned quartz and calcareous spar,
what mineral substance is there that may not be found in those close
cavities? They are actually almost all, not even excepting gold; for,
small grains of gold are inclosed within the cavities of a porous stone,
in the Siberian mine. Now, for what purpose should nature, (to the power
of which we are not to set a limit) have such an object in view as
to convert water into every thing, unless it were to confound human
understanding? For, so far as human experience has been as yet able to
reach, there would appear to be certain elementary substances; and among
these is water, or the principles of that fluid[43]. But because water
is so generally found in bodies, and so necessarily in most of the
operations of this world, why convert it into every other thing? Surely,
for no better reason than that there has not occurred to this mineralist
any other way of explaining certain natural appearances which aqueous
solution could not produce. Here is no dispute about a matter of fact;
it is on all hands allowed, that in certain cavities, inaccessible to
any thing but heat and cold, we find mineral concretions, which contain
no water, and which, according to the known operations of nature, water
could not have produced; must we therefore have recourse to water acting
according to no known principle, that is to say, are we to explain
nature by a preternatural cause?
[Note 43: Water is now considered by men of science, as a compound
substance; this doctrine, which seems to follow so necessarily from the
experiments of the French philosophers, must be tried by the growing
light of chemical science. In the oxygenating operation of inflammable
and combustible bodies when burning, those ingenious chemists overlooked
the operation of _phlogistic matter_, which has no weight, and
which escapes on that occasion, as I have had occasion to show in a
dissertation upon phlogiston, and in the Philosophy of Light, Heat,
and Fire. How far this view, which I have given of those interesting
experiments, may lead to the explanation of other collateral phenomena,
such as that of the water produced, I will not pretend to conjecture.
One thing is evident, that if the weight of the water, procured in
burning inflammable and vital air, be equal to that of those two gasses,
we would then have reason to conclude, either that water were a compound
substance, or that vital air, and inflammable vapour were compounds of
water and the matter of light, or solar substance.]
I dare say that this is not the view that M. Monnet takes of the
subject, when he thinks to explain to himself the concretion of
those different substances by means of water; but, according to my
apprehension of the matter, his theory, when sifted to the bottom, will
bear no other construction; and, unless he shall consider water like the
matter of heat, as capable of producing the fluidity of fusion, and of
being also again abstracted from the fluid, by pervading the most solid
body, which would then be a substance different from water, he must
employ this aqueous substance as a menstruum or solvent for solid
bodies, in the same manner as has been done by those naturalists whom
he he justly censure, and conform to those erroneous ideas which first
observations, or inaccurate knowledge of minerals, may have suggested to
former naturalists.
It is the dissolution and concretion of siliceous substance, no
doubt, that gives such difficulty to our naturalists in explaining
petrifaction: they have, however, something apparently in their favour,
which it may be proper now to mention.
In the _first_ place, although siliceous substance is not soluble, so
far as we know, by simple water, it is soluble by means of alkaline
substance; consequently, it is possible that it may be dissolved in the
earth.
_Secondly_, The water of Giezer in Iceland, actually petrifies bodies
which are alternately imbibed with that hot water and exposed to the
air. This water, therefore, not only contains siliceous substance in a
dissolved state, but deposits this again, either by means of cooling,
or being aerated, or of evaporating. Consequently, without knowing the
principle upon which it proceeds, we here perceive a natural operation
by which siliceous petrifaction may be performed.
_Lastly_, We have another principle for the dissolution of siliceous
substance. This is the fluor acid which volatilises the siliceous
substance. This, however, requires certain conditions, which cannot be
found as a general cause in the mineral regions.
Thus we would seem to have every thing necessary for explaining the
concretion and crystallization of siliceous bodies, provided we could
find the proper conditions requisite for that operation; for whether it
shall be by means of acid or alkaline substances that siliceous matter
is to be dissolved, volatilised, and transported from one place to
another, it is necessary that those dissolving substances should be
present upon those occasions. Nor is it sufficient only to dissolve the
siliceous substance which is to be transported; the necessary conditions
for the concretion again of the dissolved substances, whatever these may
be, are also absolutely required for this operation. Now, though those
requisite conditions may be, upon many occasions, allowed in the earth,
it is not according to the theory of our modern naturalists, who explain
petrifaction upon the principles of simple infiltration of water,
that any advantage can be taken of those conditions; nor are natural
appearances to be explained without employing more complicated chemical
agents in the mineral regions.
To this subject of the petrifactions of Giezier, I may now add the
information which we have received in consequence of a new voyage from
this country to Iceland.
When Sir Joseph Banks returned from his expedition to Iceland, he landed
at this place; and, having brought specimens of the petrifications of
Giezer, Dr Black and I first discovered that these were of a siliceous
substance. I have always conjectured that the water of Giezer must be
impregnated with flinty matter by means of an alkaline substance, and
so expressed my opinion in the Theory of the Earth published in the
Transactions of the Edinburgh Royal Society. We have therefore been very
desirous of procuring some of that water, in order to have it analysed.
An opportunity favourable to our views has occurred this summer. Mr
Stanley set out from this place with the same purpose of examining
Iceland. He was so good as to ask of Dr Black and I what inquiries we
would incline that he should make. We have now, by the favour of this
gentleman, obtained specimens of the petrifactions of Giezer; and, what
is still more interesting, we have procured some of the water of those
petrifying boiling springs.
It appears from these specimens, that the boiling water which is ejected
from those aqueous volcanoes, if we may use the expression, is endued
with the quality of forming two different species of petrifaction or
incrustation; for, besides the siliceous bodies, of which we had before
received specimens, the same stream of water incrustates its channel
with a calcareous substance. All the specimens which I have seen consist
of incrustation, some purely siliceous, some calcareous, and others
mixed of those two, more or less.
Dr Black has been analysing the water; and he finds in it siliceous
matter dissolved by an alkaline substance, in the manner of liquor
silicum[44]. My conjecture has thus been verified.
[Note 44: See Trans. of the Edin. Royal Society.]
It must not be alleged that nature may operate in the mineral regions,
as she does here upon the surface in the case of Giezer. Such an
argument as this, however sound it may be in general, will not apply to
the subject of which we treat at present. There is no question about
the limiting the powers of nature; we are only considering nature as
operating in a certain determined manner, viz. by water acting simply
upon the loose materials of the land deposited at the bottom of the
sea, and accumulated in regular strata, one upon another, to the most
enormous depth or thickness. This is the situation and condition of
things in which nature is to operate; and we are to find the means of
consolidating those strata, and concreting every species of substance
in almost every possible composition, according to some known physical
principle. Here is an operation which is limited; for, we must reason
strictly, according to the laws of nature, in the case which we have
under consideration; and we cannot suppose nature as ever transgressing
those laws.
It is acknowledged, that, by means sometimes of an aeriform, sometimes
of an alkaline, perhaps also of an acid substance, calcareous matter is
dissolved in the earth, and certain metallic substances, such as lead
and iron. This solution also, upon particular occasions, (where the
proper conditions for separating the solvent from the dissolved
substance exist), forms certain concretions; these are sometimes a mere
incrustation, as in the case of the siliceous incrustation of Giezer,
sometimes again in a crystallised or sparry form, as in the case of
stalactical concretions. But here is no question of those cases where
the proper conditions may be found; first, of dissolving the substance
which is afterwards to be concreted; secondly, of separating the
menstruum from the dissolved substance; and, lastly, of removing the
fluid deprived of its solution, and of supplying a new solution in its
room; the question is, how far those concretions are formed where those
conditions do not take place. Now, this last case is that of almost all
mineral concretions.
It must not be here alleged that certain concretions have been found in
mines posterior to these having been worked by man; consequently, that
those concretions have been formed by nothing but the infiltration of
water. In those cases, where such concretions are truly found, I am
persuaded that all the conditions proper to that operation will also
be found; and it is only, I believe, in those cases where such proper
conditions may be found, that this aqueous concretion ever appears. Now,
if we shall except calcareous stalactite, and the bog ore of iron, How
seldom is it that any appearance of those aqueous mineral concretion
ever is found? Those very few cases in which they are found, afford the
strongest proof against these being operations general to the globe, or
proper mineral concretions; because it is only where all the necessary
conditions conspire in each contributing its part, that the effect is
accomplished; and this is a thing which cannot possibly take place in
the aquiform strata below the surface of the sea. But, without attending
to this clear distinction of things perfectly different, naturalists are
apt to see false analogies, and thus in generalising to form the most
erroneous theories.
I shall now give an example of this fallaceous manner of reasoning; it
is in the case of certain mineral appearances which are erroneously
considered as stalactical concretions.
The only true stalactical bodies are of a calcareous substance; they are
formed by water containing this substance in a dissolved state; and the
principles upon which this particular concretion is formed are well
known. It is therefore easy to compare other concretions, which may have
some superficial resemblance to these stalactical bodies, in order to
see if they have proceeded upon the same principle of concretion from
a dissolved state, or by water depositing its dissolved substance in a
similar manner.
There are two different mineral substances which give appearances of
this sort. These are certain concretions of calcedony, and also of
iron-ore, which are thought to have such resemblance to stalactical
concretions as, by some superficial observers, to be reckoned of the
same kind. It is now proposed to show that those conclusions are not
well founded; and that, in this case of calcedony and iron-ore, it could
not be upon the principle of stalactical concretion that the bodies now
in question had their forms.
The principle upon which calcareous substance is dissolved in water, and
made to concrete by the evaporation of the acid substance, or fixed
air by which it had been dissolved, is too well known to require any
explanation in this place; we are only to consider the sensible effects
of those operations of which we know so well the proper conditions.
There are just two distinct views under which we may consider all
stalactical concretions formed; these are the incrustation of
the calcareous substance concreting upon a foreign body, and the
incrustation of the same substance upon itself. By the first any manner
of shape may be formed, provided there be a solid body, upon the surface
of which the calcareous solution is made to pass. By the second, again,
we have various forms; but we know the principles upon which they had
been made. These are the shape and motions of the fluid which gives the
calcareous concretion. Now, these principles are always to be perceived,
more or less, in all the bizarre or fantastical, as well as regular
shapes which are produced by stalactical concretions. At present, we
shall confine our views to one particular shape, which is simple,
regular, and perfectly understood wherever it is formed.
Drops of water falling from a roof, and forming stalactite, produce
first tubular bodies, and then gradually consolidate and increase those
pendulous bodies by incrustation. These appearances are thought to be
observed in the calcedony and ferruginous concretions, which has led
some mineralists to conclude, that those concretions had been formed
in the same manner, by means of water. We are now to show that these
mineral appearances are not analogous to stalactites in their formation,
and that they have evidently been formed in a different manner.
It must be evident, that, in the formation of those pendulous bodies,
each distinct stalactite must be formed by a separate drop of water;
consequently, that no more stalactites can be formed in a given space,
than there could have subsisted separate drops of water. Now, a drop of
water is a very determined thing; and thus we have a principle by which
to judge of those mistaken appearances.
Let us suppose the gut of water to be but one eighth of an inch,
although it is a great deal more, we should have no stalactites formed
nearer to each other than that measure of space. But those mineral
concretions, which are supposed to be stalactical, are contained in half
that space, or are nearer to each other than the tenth or twentieth of
an inch. I have them like needles, and in every degree of proximity or
contiguity, at the same time that they are perfectly solid. Therefore,
it is plainly impossible that they could have been formed upon this
principle of calcareous stalactite. But, it is only by this false
resemblance, that any argument can be formed for the concretion of those
bodies from an aqueous solution; in every other respect they are true
mineral concretions; and, that these have had a very different origin,
has been already the subject of investigation, and will be more
particularly examined in the course of this work.
The term _infiltration_, which has been much employed for explaining
mineral appearances, is too vague, imperfect, or unexplicit, for
science, whether as the means of knowing nature, or the subject of
confutation. This is not the case with that of stalactite; here is
a term that implies a certain natural operation, or a most distinct
process for attaining a certain end; and we know the principles upon
which it proceeds, as well as the several steps that may be traced in
the general result. It is an operation which has not only been analysed
to its principles; it is also a process which is performed by man,
proceeding on his acquired knowledge. Now, were this operation common to
the mineral regions, as it is proper to the surface of this earth; we
could not remain in any degree of suspense with regard to the origin of
those mineral bodies; for, having the true clue of knowledge, we should
be able to unravel the most intricate and mysterious appearance. But, so
far from this being the case, the more we come to inquire into nature,
and employ this principle, the less we find it applicable, and the more
involved in darkness is our science.
The places where these false appearances of stalactite are found, are
precisely those in which, from the nature of things, all possibility for
such an operation is excluded. For, How can this take place within a
closs cavity in the mineral regions? The term _vegetation_ may as well
be employed for the explanation of those appearances: But what would
now be said of such an explication? It is high time that science were
properly applied to the natural history of this earth, and mineralists
not allowed to impose upon themselves with false reasoning, or to please
themselves with the vain attempt of explaining visible effects by
unknown causes.
Such various inconsistent opinions, respecting petrifaction or mineral
concretion, as I have now exposed, opinions that are not founded on any
sound physical principle, authorise me to conclude that they are all
erroneous. If this be admitted, it will follow that we have no proof
of any proper mineral concretion except that which had proceeded by
congelation from the fluid state of fusion. This has been the doctrine
which I have held out in my Theory of the Earth; and this will be more
and more confirmed as we come to examine particular mineral appearances.
CHAP. VIII.
The Nature of Mineral Coal, and the Formation of Bituminous Strata,
investigated.
SECT. I.--Purpose of this Inquiry.
In the first chapter, I have given a perfect mark by which to judge,
of every consolidated stratum, how far that had been the operation or
effect of water alone, or if it had been that of heat and fusion. This
is the particular veins or divisions of the consolidated stratum,
arising from the contraction of the mass, distended by heat, and
contracted in cooling. It is not an argument of greater or lesser
probability; it is a physical demonstration; but, so far as I see, it
would appear to be for most mineralists an unintelligible proposition.
Time, however, will open the eyes of men; science will some day find
admittance into the cabinet of the curious. I will therefore now give
another proof,--not of the consolidation of mineral bodies by means
of fusion, for there is no mineral body in which that proof is not
found,--but of the inconsistency of aqueous infiltration with the
appearances of bodies, where not only fusion had been employed for the
consolidation, but where the application of heat is necessary, and along
with it the circumstances proper for _distillation_.
Short-sighted naturalists see springs of water issuing from the earth,
one forming calcareous incrustations, the other depositing bituminous
substances. Here is enough for them to make the theory of a world; on
the one hand, solid marble is explained, on the other, solid coal.
Ignorance suspects not error; their first step is to reason upon a false
principle;--no matter, were they only to reason far enough, they would
soon find their error by the absurdity into which it lands them. The
misfortune is, they reason no farther; they have explained mineralogy
by infiltration; and they content themselves with viewing the beautiful
specimens in their cabinet, the supposed product of solution and
crystalization. How shall we inform such observators; How reason with
those who attend not to an argument!
As naturalists have explained all mineral concretions from aqueous or
other solution, and attributed to infiltration the formation of those
stony bodies in which there are marks of their original composition,
so have they explained to themselves, I suppose, the origin of those
bituminous bodies which are found among the strata of the earth. In the
case of stony substances, I have shown how unfounded all their theories
are for the production of those concretions, crystallizations, and
consolidated bodies. I am here to examine the subject of inflammable and
combustible bodies, which I believe have been little considered by those
theorists who suppose mineral bodies consolidated by infiltration. It is
here that we shall find an infinite difference between the aqueous and
igneous theories; for, we shall find it impossible to explain by the one
certain operations which must have necessarily required the great agent
generally employed in the other.
The subject of this chapter is a touch-stone for every theory of the
earth. In every quarter of this globe, perhaps in every extensive
country, bituminous strata are to be found; they are alternated with
those which are called aquiform, or which had been evidently formed by
subsidence of certain moved materials at the bottom of the sea; so far,
therefore, all those strata have had the same origin. In this point
I think I may assert, that all the different theories at present are
agreed; and it is only concerning certain transformations of those
strata, since their original collection, that have been ascribed to
different causes.
Of these transformations, which the strata must have undergone, there
are two kinds; one in relation to change of place and position; the
other in relation to solidity or consistence. It is only the last of
those two changes which is here to be the subject of consideration;
because, with regard to the first, there is nothing peculiar in these
bituminous strata to throw any light, in that respect, upon the others.
This is not the case with regard to the transformation in their chemical
character and consistence; bituminous bodies may not be affected by
chemical agents, such as fire and water, in the same manner as the
argillaceous, siliceous, micaceous, and such other strata that are
alternated with the bituminous; and thus we may find the means for
investigating the nature of that agent by which those strata in general
have been transformed in their substance; or we may find means for the
detecting of false theories which may have been formed with regard
to those operations in which the original deposits of water had been
changed.
We have had but two theories, with regard to the transformation of
those bodies which have had a known origin, or to the change of their
substance and consistence; the one of these which I have given is that
of heat or fusion; the other, which I wish to be compared with mine,
is that of water and infiltration. It is by this last that all authors
hitherto, in one shape or another, have endeavoured to explain the
changes that those strata must have undergone since the time of their
first formation at the bottom of the sea. They indiscriminately apply
the doctrine of infiltration to those strata of mineral coal as to any
other; they say that bituminous matter is infiltrated with the water,
impregnates certain strata of earth with bituminous matter, and thus
converts them into mineral coal, and bituminous strata. This is not
reasoning physically, or by the inductive method of proceeding upon
matter of fact; it is reasoning fantastically, or by making gratuitous
supposition founded merely on imagination. It was thus that natural
philosophers reasoned before the age of science; the wonder now is,
how men of science, in the present enlightened age, should suffer such
language of ignorance and credulity to pass uncensured.
The subject which I am now to treat of consists of peculiar strata of
the earth, bodies which we may investigate through all the stages of
their change, which is extreme; for, from vegetable bodies produced upon
the habitable earth, they are now become a mineral body, and the most
perfect coal,--a thing extremely different from what it had been, and
a thing which cannot be supposed to have been accomplished by the
operation of water alone, or any other agent in nature with which we
are acquainted, except the action of fire or heat. It is therefore
impossible for a philosopher, reasoning upon actual physical principles,
not to acknowledge in this a complete proof of the theory which has been
given, and a complete refutation of that aqueous operation which has
been so inconsiderately supposed as consolidating the strata of the
earth, and forming the various mineral concretions which are found in
that great body.
To see this, it will be sufficient to trace the progress of vegetable
and animal substances, (bodies which had certainly lived by means of a
former earth), to this changed state in which they have become perfect
mineral bodies, and constitute a part of the present earth. For, as
these changes are perfectly explained by the one theory, and absolutely
inconsistent with the other, there arises from this a conviction that
must be irresistible to a person who can give proper attention to a
chain of reasoning from effect to cause.
But if we thus succeed to illustrate the theory of the earth by the
natural history of those particular strata, we have but one step farther
to make in order to bring all the other parts of the earth, whether
stratified or not, into the most perfect consistence with the theory;
now this step, it will be most easy to make; and I shall now mention
it, that so the reader may keep it in his view: Pyrites is a
sulphureo-metallic substance, which cannot be produced by means of
water, a substance which the influences of the atmosphere decomposes or
separates into its elements, and which even our imperfect art may
be considered as able to produce, by means of fusion in our fires.
Therefore, the finding of this creature of fire intimately connected
with those consolidated strata of mineral coal, adds the greatest
confirmation, were it necessary, to the doctrine of those mineral bodies
having been consolidated by fusion. This confirmation, however, is
not necessary, and it is not the only thing which I am at present to
illustrate in that doctrine. What I have now in view is, to homologate
the origin of those coal strata, with the production of every other
mineral substance, by heat or fusion; and this is what the intimate
connection of pyrites with those strata will certainly accomplish. This
will be done in the following manner:
Pyrites is not only found in great masses along with the coal strata; it
is contained in the veins which traverse those strata, and in the minute
ramifications of those veins, which are occasioned by the contraction of
the mass, and generally divide it into small cubical pieces; but besides
that extrinsic connection, (as it may be called,) with the stratum of
coal, pyrites is found intimately connected with that solid body, in
being mixed with its substance. If, therefore, it were proved, that
either the one or other of those two substances had been consolidated by
fusion, the other must be acknowledged as having had the same origin;
but now I am to prove, from the natural history of mineral coal, that
pyrites had been there formed by fusion; and then, by means of the
known origin of that sulphureo-metallic substance, we shall extend our
knowledge to the origin of every other mineral body.
The process of this argument is as follows: Every mineral body, I
believe, without exception, will be found so intimately connected with
pyrites, that these two things must be concluded as having been together
in a fluid state, and that, whatever may have been the cause of fluidity
in the one, this must have also caused the fluidity in the other;
consequently, whatever shall be proved with regard to the mineral
operations of pyrites, must be considered as proved of every other
mineral substance. But, from the connection of pyrites with mineral
coal, it is to be proved that the origin of this metallic body had been
fusion; and then it will appear, that all other mineral bodies must have
been more or less in fusion, or that they must have been consolidated
by means of heat, and not by any manner of solution or aqueous
infiltration. I therefore now proceed to take a view of the natural
history of coal strata,--a subject which mineralogists seem not inclined
to engage with, although the most ample data are to be found for that
investigation.
SECT. II.--Natural History of Coal Strata, and Theory of this
Geological Operation.
Fossil coal is the species of stratum best understood with regard to its
accidents, as being much sought after; at least, this is the case in
many parts of Britain, where it supplies the place of wood for burning.
This fossil body has the most distinguished character; for, being
inflammable or combustible in its nature, there is no other species of
stratum that may be confounded with it.
But, though coal be thus the most distinguishable mineral, and that
which is best understood in the science of mining, it is perhaps the
most difficult to be treated of in the science of mineralogy; for,
not having properly any distinguishable parts, we have nothing in the
natural constitution of this body, as we have in most other strata, to
lead us to the knowledge of its original state or first formation.
The varieties of coal are distinguished by their different manner of
burning; but, from appearances of this kind, no perfect judgement can be
formed with regard to the specific manner in which those strata had been
made; although, from chemical principles, some conclusion may be drawn
concerning certain changes which they have undergone since they had been
formed.
Thus we have one species of coal which is extremely fusible, abounds
with oil, and consequently is inflammable; we have another species again
which is perfectly fixed and infusible in the fire; therefore, we may
conclude upon principle, that, however, both those coals must have
undergone the operation of heat and fusion, in bringing them to their
present state, it is only the last that has become so much evaporated as
to become perfectly fixed, or so perfectly distilled, as to have been
reduced to a caput mortuum.
The argument here employed is founded upon this fact; that, from the
fusible species of coal, a caput mortuum may be formed by distillation,
and that this chemical production has every essential quality, or every
peculiar property, of the fixed and infusible species; although, from
the circumstances of our operation, this caput mortuum may not have
precisely the exterior appearance of the natural coal. But, we have
reason to believe, it is not in the nature of things to change the
infusible species, so as to make it fusible or oily. Now, that this body
was not formed originally in its present state, must appear from this,
that the stratum here considered is perfectly solid; but, without
fusion, this could not have been attained; and the coal is now supposed
to be infusible. Consequently, this fixed substance, which is now,
properly speaking, a perfect coal, had been originally an oily
bituminous or fusible substance. It is now a fixed substance, and an
infusible coal; therefore, it must have been by means of heat and
distillation that it had been changed, from the original state in which
this stratum had been formed.
We have thus, in the examination of coal strata upon chemical
principles, received a certain lesson in geology, although this does not
form a proper distinction by which to specify those strata in general,
or explain the variety of that mineral. For, in this manner, we
could only distinguish properly two species of those strata; the one
bituminous or inflammable; the other proper coal, burning without smoke
or flame. Thus it will appear that, as this quality of being perfectly
charred is not originally in the constitution of the stratum, but an
accident to which some strata of every species may have been subjected,
we could not class them by this property without confounding together
strata which had differences in their composition or formation.
Therefore, we are led to inquire after some other distinction, which may
be general to strata of fossil coal, independent of those changes which
this substance may have undergone after it had been formed in a stratum.
Perfect mineral coal being a body of undistinguishable parts, it is only
in its resolution that we may analyse it, and this is done by burning.
Thus, in analysing coal by burning, we have, in the ashes alone, that by
which one species of coal may be distinguished from another; and, if we
should consider pure coal as having no ashes of itself, we should then,
in the weight of its ashes, have a measure of the purity of the coal,
this being inversely as the quantity of the ashes. Now, though this be
not accurately true, as the purest coal must have some ashes proper to
itself, yet, as this is a small matter compared with the quantity of
earthy matter that may be left in burning some species of coal, this
method of analysis may be considered as not far removed from the truth.
But, in distinguishing fossil coal by this species of chemical analysis,
not only is there to be found a perfect or indefinite gradation from a
body which is perfectly combustible to one that is hardly combustible in
any sensible degree, we should also fall into an inconveniency similar
to that already mentioned, of confounding two things extremely different
in their nature, a bituminous body, and a perfect charcoal. Thus, if we
shall found our distinction upon the fusibility and different degree of
having been charred, we shall confound fossil coals of very different
degrees of value in burning, or of very different compositions as
strata; if, again, we found it upon the purity of composition, in
judging from the ashes, we shall confound fossil bodies of very
different qualities, the one burning with much smoke and flame, the
other without any; the one fusible almost like wax, the other fixed and
infusible as charcoal.
It will now appear, that what cannot be done in either the one or other
of those two methods, may in a great degree, or with considerable
propriety, be performed in employing both.
Thus, whether for the economical purposes of life, or the natural
history of fossil coal, those strata should be considered both with
regard to the purity of their composition as inflammable matter
deposited at the bottom of the sea, and to the changes which they
have afterwards undergone by the operation of subterranean heat and
distillation.
We have now considered the original matter of which coal strata are
composed to be of two kinds; the one pure bitumen or coal, as being
perfectly inflammable or combustible; the other an earthy matter,
with which proper coal may be variously mixed in its composition, or
intimately connected, in subsiding from that suspended state by which it
had been carried in the ocean. It is a matter of great importance, in
the physiology of this globe, to know that the proper substance of coal
may be thus mixed with heterogeneous bodies; for, supposing that this
earthy matter, which has subsided in the water along with coal, be no
farther connected with the combustible substance of those strata, than
that it had floated in the waters of the ocean, and subsided _pari
passu_ with the proper materials of the coal, we hence learn a great
deal with regard to the state in which the inflammable matter must have
been at the time of its formation into strata. This will appear by
considering, that we find schistus mixed with coal in the most equal or
uniform manner, and in almost every conceivable degree, from the purest
coal to the most perfect schistus. Hence we have reason to conclude,
that, at the formation of those strata, the bituminous matter, highly
subtilised, had been uniformly mixed with the earth subsiding in the
water.
Not only is the bituminous matter of coal found mixed in every different
proportion with the earthy or uninflammable materials of strata, but the
coaly or bituminous composition is found with perhaps every different
species of substance belonging to strata. This is certain, that we have
the coaly matter intimately mixed with argillaceous and with calcareous
strata.
Thus it will appear, that it is no proper explanation of the formation
of coal strata, to say that vegetable matter is the basis of those
strata; for though, in vegetation, a substance proper for the formation
of bituminous matter is produced, it remains to know by what means, from
a vegetable body, this bituminous matter is produced, and how it comes
to be diffused in that subtile state by which it may be uniformly mixed
with the most impalpable earth in water. Could we once resolve this
question, every other appearance might be easily explained. Let us
therefore now endeavour to discover a principle for the resolving of
this problem.
There are two ways in which vegetable bodies may be, in part at least,
resolved into that subtilised state of bituminous matter after which we
inquire; the one of these is by means of fire, the other by water. We
shall now consider these severally as the means of forming bituminous
strata, although they may be both employed by nature in this work.
When vegetable bodies are made to burn, there is always more or less of
a fuliginous substance formed; but this fuliginous substance is no
other than a bituminous body in that subtilised state in which it is
indefinitely divided, and may be mixed uniformly with any mass of matter
equally subtilised with itself. But this is precisely what we want, in
order to compose the strata of coal in question. If, therefore, there
were to be found in the ocean such a fund of this fuliginous substance
as might suffice for the formation of bituminous strata, no difficulty
would be left in explaining the original of fossil coal. But tho'
sufficient quantity of this fuliginous matter might not be found for the
explanation of natural appearances, yet there cannot be a doubt that
more or less of this matter must be produced in the mineral operations
of the globe, and be found precisely in that place where it is required
for the forming of those strata of coal.
In order to conceive this, we are to consider, that there are actually
great quantities of coal strata in a charred state, which indicates
that all their more volatile oleaginous or fuliginous matter had been
separated by force of subterranean heat; and, we are to suppose that
this had been transacted at the bottom of the ocean: Consequently, a
subtile oleaginous, bituminous, or fuliginous substance, must have been
diffused in that ocean; and this bituminous matter would be employed in
forming other strata, which were then deposited at the bottom of the
waters.
But besides this quantity of bituminous matter which is necessarily
formed in the mineral operations of the earth, and with regard to the
quantity of which we can never form a proper estimate, there must enter
into this same calculation all the fuliginous matter that is formed in
burning bodies upon the surface of this earth. This bituminous matter of
smoke is first delivered into the atmosphere, but ultimately it must
be settled at the bottom of the sea. Hence though, compared with the
quantity that we think required, each revolution of the globe produces
but a little in our estimation, yet the progress of time, in reforming
worlds, may produce all that is necessary in the formation of our
strata.
There now remains to explain the other way in which bituminous matter
may be obtained from vegetable bodies, that is, by means of water. For
this purpose we must begin with a part of natural history that will
throw some light upon the subject.
All the rivers in Scotland run into the sea tinged with a brown
substance; this is most evident in some of them after a flood, and while
yet the river is swelled; but, in travelling to the north of Scotland
in the summer season, without any rain, I saw all the rivers, without
exception, of a brown colour, compared with a river of more clear water.
This colour proceeds from the moss water, as it is called, which runs
into the rivers, or the infusion of that vegetable substance which
forms combustible turf, called peat. Now, this moss water leaves, upon
evaporation, a bituminous substance, which very much resembles fossil
coal. Therefore, in order to employ this vegetable infusion, delivered
into the ocean for the purpose of forming bituminous strata at its
bottom, it is only required to make this bituminous matter separate and
subside.
If now we consider the immense quantity of inflammable vegetable
substance, dissolved in water, that is carried into the sea by all the
rivers of the earth, and the indefinite space of time during which those
rivers have been pouring in that oily matter into the sea; and if we
consider, that the continual action of the sun and atmosphere upon this
oily substance tends, by inspissation, to make it more and more dense or
bituminous, we cannot hesitate in supposing a continual separation
of this bituminous matter or inspissated oil from the water, and
a precipitation of it to the bottom of the sea. This argument is
corroborated by considering, that, if it were otherwise, the water of
the sea must have, during the immense time that rivers are proved
to have run, be strongly impregnated with that oily or bituminous
substance; but this does not appear; therefore we are to conclude, that
there must be the means of separating that substance from the water in
which it had been dissolved.
If there is thus, from the continual perishing of animal and vegetable
bodies upon the surface of this earth and in the sea, a certain supply
of oily or bituminous matter given to the ocean, then, however small a
portion of this shall be supposed the whole oily or inflammable matter
produced upon the surface of the earth, or however long time it may
require for thus producing a stratum or considerable body of coal,
we must still see in this a source of the materials proper for the
production of that species of strata in the bottom of the sea.
We have now considered the proper materials of which pure fossil coal
is chiefly formed; we have at present to consider what should be the
appearances of such a substance as this collected at the bottom of the
sea, and condensed or consolidated by compression and by heat. We should
thus have a body of a most uniform structure, black, breaking with a
polished surface, and more or less fusible in the fire, or burning with
more or less smoke and flame, in proportion as it should be distilled
or inspissated, less or more, by subterranean heat. But this is the
description of our purest fossil coals, which burn in giving the
greatest quantity of heat, and leave the smallest quantity of ashes.
In order to form another regular species of coal, let us suppose that,
along with the bituminous substance now considered, there shall be
floating in the water of the ocean a subtile earthy substance, and that
these two different substances shall subside together in an uniform
manner, to produce a stratum which shall be covered with immense weight,
compressed, condensed, and consolidated as before, we should thus have
produced a most homogeneous or uniform body to appearance, but not so
in reality. The mixture of heterogeneous matter, in this case, is too
minute to be discovered simply by inspection; it must require deep
reflection upon the subject, with the help of chemical analysis,
to understand the constitution of this body, and judge of all the
circumstances or particulars in which it differs from the former. It is
worth while to examine this subject with some attention, as it will give
the most instructive view of the composition of bituminous strata, both
those which are not considered as coal, and also the different species
of that mineral body.
In the first place then, if the mixture of those two different
substances had been sufficiently perfect, and the precipitation uniform,
the solid body of coal resulting from this mixture, would not only
appear homogeneous, but might break equally or regularly in all
directions; but the fracture of this coal must visibly differ from the
former, so far as the fracture of this heterogeneous coal cannot have
the polished surface of the pure bituminous body; for, the earthy matter
that is interposed among the bituminous particles must affect the
fracture in preventing its surface from being perfectly smooth. This
imperfect plane of the fracture may be improved by polishing; in which
case the body might be sufficiently smooth to have an agreeable polish;
but it cannot have a perfect polish like a homogeneous body, or appear
with that glassy surface which is naturally in the fracture of the pure
bituminous coal.
But this is also a perfect description of that species of coal which is
called in England Kennel coal, and in Scotland Parrot coal. It is so
uniform in its substance that it is capable of being formed on the
turning loom; and it receives a certain degree of polish, resembling
bodies of jet.
Thus, we have a species of coal in which we shall find but a small
degree of fusibility, although it may not be charred in any degree.
Such an infusible coal may therefore contain a great deal of aqueous
substance, and volatile oily matter; consequently may burn with smoke
and flame. But this same species of coal may also occasionally be
charred more or less by the operation of subterranean heat; and, in
that case, we should have a variety of coal which could only be
distinguished, from a similar state of pure bituminous coal, by the
ashes which they leave in burning. At least, this must be the case, when
both species are, by sufficient distillation, reduced to the state of
what may be properly termed a chemical coal.
But in the natural state of its composition, we find those strata of
kennel or parrot coal, possessing a peculiar property, which deserves to
be considered, as still throwing more light upon the subject.
We have been representing these strata of coal as homogeneous to
appearance, and as breaking indifferently in all directions; this last,
perhaps, is not so accurate; for they would seem to break chiefly into
two directions, that is, either parallel or perpendicular to the bed.
Thus we have this coal commonly in rectangular pieces, in which it
is extremely difficult to distinguish the direction of the bed, or
stratification of the mass. By an expert eye, however, this may be in
general, or at least sometimes, distinguished, and then, by knowing
the habit of the coal in burning, a person perfectly ignorant of the
philosophy of the matter may exhibit a wonderful sagacity, or even of
power over future events, in applying this body to fire; for, at his
pleasure, and unknown to those who are not in the secret; he may
apparently, in equal circumstances, make this coal either kindle
quietly, or with violent cracking and explosions, throwing its splinters
at a distance.
The explanation lies in this, that, though the rectangular mass of coal
appears extremely uniform in its structure, it is truly a stratified
mass; it is therefore affected, by the sudden approach of fire in a very
different manner, according as the edge of the stratum, which is seen in
four of the sides of this supposed cube, shall be applied to the fire,
or the other two sides, which are in the line of the stratum, or
parallel to the bed of coal. The reason of this phenomenon now remains
to be considered.
When the edge of the coal is exposed to the fire, the stratification of
the coal is opened gradually by the heat and expanding vapours, as a
piece of wood, of a similar shape, would be by means of wedges placed in
the end way of the timber. The coal then kindles quietly, and quickly
flames, while the mass of this bituminous schistus is opening like the
leaves of a book, and thus exhibits an appearance in burning extremely
like wood. But let the fire be applied to the middle of the bed, instead
of the edge of the leaves, and we shall see a very different appearance;
for here the expanded aqueous vapours, confined between the _laminae_,
form explosions, in throwing off splinters from the kindling mass; and
this mass of coal takes fire with much noise and disturbance.
The ashes of this coal may be determined as to quality, being in general
a subtile white earth; but, as to quantity, the measure of that earth
produces an indefinite variety in this species of coal; for, from the
kennel or parrot coal, which is valuable for its burning with much
flame, to that black schistus which our masons use in drawing upon
stone, and which, though combustible in some degree, is not thought to
be a coal, there is a perfect gradation, in which coal may be found with
every proportion of this earthy alloy.
Among the lowest species of this combustible schistus are those
argillaceous strata in Yorkshire from whence they procure alum in
burning great heaps of this stone, which also contains sulphur, to
impregnate the aluminous earth with its acid. We have also, in this
country, strata which differ from those aluminous schisti only in the
nature of the earth, with which the bituminous sediment is mixed. In
the strata now considered, the earth, precipitated with the bituminous
matter, being calcareous, has produced a limestone, which, after burning
especially, is perfectly fissile.
Therefore, with regard to the composition of mineral coal, the theory
is this. That inflammable, vegetable, and animal substances, in a
subtilised state, had subsided in the sea, being mixed more or less with
argillaceous, calcareous, and other earthy substances in an impalpable
state. Now, the chemical analysis of fossil coal justifies that theory;
for, in the distillation of the inflammable or oily coal, we procure
volatile alkali, as might be naturally expected.
Thus we have considered fossil coal as various, both in its state and
composition; we have described coal which is of the purest composition,
as well as that which is most impure or earthy; and we have shown that
there is a gradation, from the most bituminous state in which those
strata had been formed in being deposited at the bottom of the sea,
to the most perfect state of a chemical coal, to which they have been
brought by the operation of subterranean fire or heat.
We have been hitherto considering fossil coal as formed of the
impalpable parts of inflammable bodies, united together by pressure, and
made to approach in various degrees to the nature of a chemical coal,
by means of subterranean heat; because, from the examination of those
strata, many of them have evidently been formed in this manner.
But vegetable bodies macerated in water, and then consolidated by
compression, form a substance of the same kind, almost undistinguishable
from some species of fossil coal. We have an example of this in our turf
pits or peat mosses; when this vegetable substance has been compressed
under a great load of earth, which sometimes happens, it is much
consolidated, and hardens, by drying, into a black body, not afterwards
dilutable or penetrated by water, and almost undistinguishable in
burning from mineralised bodies of the same kind.
Also, when fossil wood has been condensed by compression and changed by
the operation of heat, as it is frequently found in argillaceous strata,
particularly in the aluminous rock upon the coast of Yorkshire, it
becomes a jet almost undistinguishable from some species of fossil coal.
There cannot therefore be a doubt, that if this vegetable substance,
which is formed by the collection of wood and plants in water upon the
surface of the earth, were to be found in the place of fossil coal, and
to undergo the mineral operations of the globe, it must at least augment
the quantity of those strata, though it should not form distinct strata
by itself.
It may perhaps be thought that vegetable bodies and their impalpable
parts are things too far distant in the scale of magnitude to be
supposed as subsiding together in the ocean; and this would certainly be
a just observation with regard to any other species of bodies: But the
nature of vegetable bodies is to be floatant in water; so that we may
suppose them carried at any distance from the shore; consequently, the
size of the body here makes no difference with regard to the place or
order in which these are to be deposited.
The examination of fossil coal fully confirms those reasonable
suppositions. For, _first_, The strata that attend coal, whether the
sandstone or the argillaceous strata, commonly, almost universally,
abound with the most distinct evidence of vegetable substances; this
is the impressions of plants which are found in their composition.
_Secondly_, There is much fossil coal, particularly that termed in
England clod coal, and employed in the iron foundry, that shows
abundance of vegetable bodies in its composition. The strata of this
coal have many horizontal interstices, at which the more solid shining
coal is easily separated; here the fibrous structure of the compressed
vegetable bodies is extremely visible; and thus no manner of doubt
remains, that at least a part of this coal had been composed of the
vegetable bodies themselves, whatever may have been the origin of the
more compact parts where nothing is to be distinguished.
The state in which we often find fossil wood in strata gives reason to
conclude that this body of vegetable production, in its condensed state,
is in appearance undistinguishable from fossil coal, and may be also in
great quantity; as, for example, the Bovey coal in Devonshire.
Thus the strata of fossil coal would appear to be formed by the
subsidence of inflammable matter of every species at the bottom of the
sea, in places distant from the shore, or where there had been much
repose, and where the lightest and most floatant bodies have been
deposited together. This is confirmed in examining those bodies of
fossil coal; for, though there are often found beds of sand-stone
immediately above and below the stratum of the coal, we do not find any
sand mixed in the strata of the coal itself.
Having found the composition of coal to be various, but all included
within certain rules which have been investigated, we may perceive in
this an explanation of that diversity which is often observed among the
various strata of one bed of coal. Even the most opposite species of
composition may be found in the thickness of one bed, although of very
little depth, that is to say, the purest bituminous coal may, in the
same bed, be conjoined with that which is most earthy.
Fossil coal is commonly alternated with regular sand-stone and
argillaceous strata; but these are very different bodies; therefore,
it may perhaps be inquired how such different substances came to be
deposited in the same place of the ocean. The answer to this is easy; we
do not pretend to trace things from their original to the place in which
they had been ultimately deposited at the bottom of the sea. It is
enough that we find the substance of which we treat delivered into
the sea, and regularly deposited at the bottom, after having been
transported by the currents of the ocean. Now the currents of the ocean,
however regular they may be for a certain period of time, and however
long this period may be protracted, naturally change; and then the
currents, which had given birth to one species of stratum in one place,
will carry it to another; and the sediment which the moment before
had formed a coal stratum, or a bed of that bituminous matter, may
be succeeded either with the sediment of an argillaceous stratum, or
covered over with a bed of sand, brought by the changed current of the
sea.
We have now considered all the appearances of coal strata, so far as
these depend upon the materials, and their original collection. But,
as those bituminous strata have been changed in their substance by the
operation of subterranean heat and inspissation, we are now to look for
the necessary consequences of this change in the body of the stratum;
and also for other mineral operations common to fossil coal with
consolidated strata of whatever species.
If coal, like other mineral strata, have been inspissated and
consolidated by subterranean heat, we should find them traversed with
veins and fissures; and, if the matter found in those veins and fissures
corresponds to that found in similar places of other strata, every
confirmation will be hence given to the theory that can be expected from
the consideration of those bituminous strata. But this is the case; we
find those fissures filled both with calcareous, gypseous, and pyritous
substances. Therefore, we have reason to conclude, that the strata of
fossil coal, like every other indurated or consolidated body in the
earth, has been produced, _first_, by means of water preparing and
collecting materials proper for the construction of land; and,
_secondly_, by the operation of internal fire or subterranean heat
melting and thus consolidating every known substance of the globe.
Not only are those sparry and pyritous substances, which are more
natural to coal strata, found forming veins traversing those strata in
various directions, but also every other mineral vein may occasionally
be found pervading coal mines, or traversing bituminous strata. Gold,
silver, copper, lead, calamine, have all, in this manner, been found in
coal.
There remains now only to consider those bituminous strata of fossil
coal in relation to that change of situation which has happened more
or less to every stratum which we examine; but which is so much better
known in those of coal, by having, from their great utility in the arts
of life, become a subject for mining, and thus been traced in the earth
at great expense, and for a long extent.
Coal strata, which had been originally in a horizontal position, are now
found sometimes standing in an erect posture, even almost perpendicular
to the plane in which they had been formed. Miners therefore distinguish
coal strata according as they deem them to approach to the one or other
of those two extremes, in terming them either flat or edge seams or
veins. Thus, it will appear, that every possible change from the
original position of those strata may have happened, and are daily found
from our experience in those mines.
But besides the changed position of those strata, in departing from the
horizontal line or flat position in which they had been formed, there is
another remarkable change, termed by miners a _trouble_ in the coal. The
consideration of this change will further illustrate the operations of
nature in placing that which had been at the bottom of the sea above its
surface.
Strata, that are in one place regularly inclined, may be found bended,
or irregularly inclined, in following their course. Here then is a
source of irregularity which often materially effects the estimates
of miners, judging from what they see, of those parts which are to be
explored; and this is an accident which they frequently experience.
But, without any change in the general direction of the stratum, miners
often find their coal broke off abruptly, those two parts being placed
upon a higher and lower situation in respect to each other, if flat
beds, or separated laterally if they are edge seams. This is by miners
termed a _slip, hitch_, or _dyke_.
These irregularities may either be attended with an injected body of
subterraneous lava or basaltes, here termed whin-stone, or they may not
be attended, at least apparently, _i.e._ immediately, with any such
accident. But experienced miners know, that, in approaching to any of
those injected masses of stone, which are so frequent in this country,
their coal is more and more subject to be troubled.
As there is, in this country of Scotland, two different species of
mountains or hills, one composed both in matter and manner exactly
similar to the Alps of Switzerland, the other of whin-stone, basaltic
rock, or subterraneous lava; and as the fossil coal, argillaceous and
sand-stone strata, are found variously connected with those hills,
nothing can tend more to give a proper understanding, with regard to the
construction of the land in general, of the globe than a view of those
different bodies, which are here found much mixed together in a little
space of country, thus exhibiting, as it were in miniature, what may be
found in other parts of the world, upon a larger scale, but not upon any
other principle. I will therefore endeavour to give a short description
of the mineral state of this country with regard to coal, so far as my
experience and memory will serve.
This country might very properly be considered as consisting of primary
and secondary mountains; not as supposing the primary mountains original
and inexplicable in their formation, any more than those of the latest
production, but as considering the one to be later in point of time, or
posterior in the progress of things. The first are those which commonly
form the alpine countries, consisting of various schisti, of quartzy
stone, and granites. The second, again, are the whinstone or basaltic
hills scattered up and down the low country, and evidently posterior to
the strata of that country, which they break, elevate, and displace.
Thus there are in this country, as well as every where else, three
things to be distinguished; first, the alpine or elevated country;
secondly, the flat or low country; and, thirdly, that which has been of
posterior formation to the strata which it traverses, in whatever shape
or quality; whether as a mountain, or only as a vein; whether as a
basaltes, a porphyry, or a granite, or only as a metal, a siliceous
substance, or a spar.
Those three things which are here distinguished do not differ with
regard to the chemical character of their substances; for, in each of
these, every different substance is to be found, more or less; and it
is not in being composed of materials peculiar to itself, that makes an
alpine country be distinguished from a flat country; it is chiefly in
the changes which the strata of the alpine country have been made to
undergo, posterior to their original collection, that the rocks of the
alpine country differ from those of the flat country.
But the observation that is most to the purpose of the present subject
of bituminous strata, is this; it is chiefly in the strata of the flat
country that fossil coal are found; there are none that I know of in
all the alpine countries of Scotland; and it is always among the strata
peculiar to the flat country that fossil coal is found. Now, this
appearance cannot be explained by saying that the materials of mineral
coal had not existed in the world while those primary strata were formed
in the sea. I have already shown, (chap. 4.) that there had been the
same system of a world, producing plants, and thus maintaining animals,
while the primary strata were formed in the sea; I have even adduced an
example of coal strata among those primary schisti, although this be an
extremely rare occurrence: Consequently, we are under the necessity of
looking out for some other cause.
If the changes which have been evidently superinduced in the strata of
alpine countries arise from the repeated operations of subterranean
fire, or to the extreme degree in which those strata have been affected
by this consolidating and elevating cause, it will be natural to suppose
that the bituminous or combustible part among those stratifications, may
have been mostly consumed upon some occasion during those various and
long continued operations; whereas, in the flat beds of the low country,
although there is the most perfect evidence for the exertion of heat in
the consolidation of those strata, the general quantity of this has been
a little thing, compared with the universal manifestation of this cause
in the operations of the alpine countries, the strata of which have been
so much displaced in their situations and positions.
To illustrate this, strata of sand-stone are found in both the alpine
and flat countries of Scotland. About Leadhills, for example, there are
abundance of those strata; but, in the flat country, the generality of
the sand-stone is so little changed as to appear to every enlightened
naturalist aquiform strata; whereas the most enlightened of those
philosophers will not perhaps attribute the same original to a similar
composition in the alpine country, which is so much changed from its
original state. It is not because there had been wanting a sufficient
degree of heat to consolidate the sand-stone in the coal country; for I
can show specimens of sand-stone almost contiguous with coal, that have
been extremely much consolidated in this manner. But this is only a
particular stratum; and the general appearance of the sand-stone, as
well as other strata in the coal countries, is that of having been
little affected by those subterranean operations of heat by which those
bodies in the alpine country have been changed in their structure,
shape, and position.
If we shall thus allow the principle of consolidation, consequently also
of induration, to have been much exerted upon the strata of the alpine
country, and but moderately or little upon those of the low country of
Scotland, we shall evidently see one reason, perhaps the only one, for
the lesser elevation of the one country above the level of the sea, than
the other. This is because the one resists the powers which have been
employed in leveling what has been raised from the bottom of the sea,
more than the other; consequently, we find more of the one remaining
above the level of the sea than of the other.
Let us now take the map of Scotland, in order to observe the mixture of
those two different species of countries, whereof the one is generally
low and flat, the other high and mountainous; the one more or less
provided with fossil coal, the other not.
From St Abb's Head, on the east of Scotland, to the Mull of Galloway,
on the west, there runs a ridge of mountains of granite, quartz, and
schistus strata, which contain not coal. On each side of this ridge we
find coal countries; Northumberland, on the one side, and, on the other,
the shires of Ayr, Lanark, and the Lothians; the one is a mountainous
country, the others are comparatively low or flat countries. Let us now
draw another alpine line from Buchan and Caithness, upon the east, to
the island of Jura, on the west; this traverses a mountainous country
destitute of coal, and, so far as I know, of any marks of marine bodies.
But, on each side of this great alpine ridge, we find the hard country
skirted with one which is lower, flatter, or of a softer nature,
in which coal is found, upon the one side, in the shires of Fife,
Clackmannan, and Stirling; and, on the other, in that hollow which runs
from the Murray Frith south-west, in a straight line, directed upon the
end of Mull, and composed, for the most part, of water very little above
the level of the sea. Here, to be sure, the coal is scarce, or not so
evident; but there is coal upon the sea coast in several places of this
great Bay betwixt Buchan and Caithness; and the lowness of the country,
across this part of the island, is almost sufficient testimony that it
had been composed of softer materials.
Thus the coal country of Scotland may be considered as in one band
across the island, and included in the counties of Ayr, Lanark, and all
those which border upon the Frith of Forth. Now, in all this tract of
coal and tender strata, we do not find ridges of alpine stone or primary
mountains, but we find many hills of solid rock, little mountains, from
500 to 1000 feet high; such as that beautiful conical hill North Berwick
Law, Torpender Law, Arthur's Seat, the Lowmands, and others of inferior
note. That is to say, the whole of this included space, both sea and
land, has been invaded from below with melted masses of whin-stone,
breaking up through the natural strata of the country, and variously
embossing the surface of the earth at present, when all the softer
materials, with which those subterranean lavas had been covered, are
washed away or removed from those summits of the country. Hence there is
scarcely a considerable tubercle, with which this country also abounds,
that may not be found containing a mass of whin-stone as a nucleus.
But besides those insulated masses of whinstone that form a gradation
from a mountain to a single rock, such, for example, as that on which
the Castle of Edinburgh is built, we find immense quantities of the same
basaltic rock interjected among the natural strata, always breaking and
disordering them, but often apparently following their directions for a
considerable space with some regularity. We also find dykes of the same
substance bisecting the strata like perpendicular veins of rock; and, in
some places, we see the connection of these rocks of the same substance,
which thus appear to be placed in such a different form in relation to
the strata.
It will thus appear, that the regular form, and horizontal direction of
strata throughout this country of coal, now under contemplation, has
been broken and disordered by the eruption and interjection of those
masses of basaltic stone or subterraneous lava; and thus may be
explained not only the disorders and irregularities of coal strata, but
also the different qualities of this bituminous substance from its
more natural state to that of a perfect coal or fixed infusible and
combustible substance burning without smoke. This happens sometimes to a
part of a coal stratum which approaches the whin-stone.
Having thus stated the case of combustible or bituminous strata, I would
ask those naturalists, who adhere to the theory of infiltration and the
operation of water alone, how they are to conceive those strata formed
and consolidated. They must consider, that here are immense bodies of
those combustible strata, under hundreds, perhaps thousands, of fathoms
of sand-stone, iron-stone, argillaceous and calcareous strata. If they
are to suppose bituminous bodies collected at the bottom of the sea,
they must say from whence that bitumen had come; for, with regard to the
strata below those bituminous bodies, above them, and between them,
we see perfectly from whence had come the materials of which they are
formed. They cannot say that it is from a collection of earthy matter
which had been afterwards bituminized by infiltration; for, although
we find many of those earthy strata variously impregnated with the
bituminous and coaly matter, I have shown that the earthy and the
bituminous matter had subsided together; besides, there are many of
those coaly and bituminous strata in which there is no more than two or
three _per cent._ of earthy matter or ashes after burning; therefore
the strata must have been formed of bituminous matter, and not simply
impregnated with it.
To avoid this difficulty, we shall allow them to form their strata,
which certainly is the case in great part, by the collection of
vegetable bodies; then, I desire them to say, in what manner they are
to consolidate those bodies. If they shall allege that it is by simple
pressure, How shall we conceive the numerous veins of spar and pyrites,
which traverse those strata in all directions, to be formed in
those bodies consolidated by the compression of the superincumbent
masses?--Here is a manifest inconsistency, which proves that it could
not be. But, even were we to suppose all those difficulties to be over
come, there is still an impossibility in the way of that inconsiderate
theory, and this will appear more fully in the following chapter.
SECT. III.--The Mineralogical Operations of the Earth illustrated from
the Theory of Fossil Coal.
There is not perhaps a greater difference among the various qualities of
bodies than that which may be observed to subsist between the burning of
those two substances, that is, the inflammable bodies on the one hand,
and those that are combustible on the other. I have treated of that
distinction in Dissertations upon subjects of Natural Philosophy, part
3d. where I have considered the different effects of those two kinds
of bodies upon the incident light; and, in a Dissertation upon the
Philosophy of Fire, etc. I have distinguished those two kinds of
substances in relation to their emitting, in burning, the fixed light
which had constituted a part of those inflammable and combustible
bodies.
All animals and vegetable bodies contain both those different chemical
substances united; and this phlogistic composition is an essential part
in every animal and vegetable substance. There are to be found in those
bodies particular substances, which abound more or less with one of
those species of phlogistic matter, but never is the one species of
those burning substances to be found naturally, in animal and vegetable
bodies, without being associated with the other; and it is all that the
chemical art can do to separate them in a great degree upon occasion.
Pure ardent spirit may perhaps be considered as containing the one, and
the most perfect coal the other; the chemical principle of the one
is proper carbonic matter; and of the other it is the hydrogeneous
principle, or that of inflammable air.
Thus we so far understand the composition of animal and vegetable
substances which burn or maintain our fires; we also understand the
chemical analysis of those bodies, in separating the inflammable from
the combustible substance, or the volatile from the fixed matter, the
oil from what is the proper coal. It is by distillation or evaporation,
the effect of heat, that this separatory operation is performed; and we
know no other means by which this may be done. Therefore, wherever we
find peculiar effects of that separatory operation, we have a right to
infer the proper cause.
The subject, which we are to consider in this section, is not the
composition of strata in those of mineral coal, but the transformation
of those, which had been originally inflammable bodies, into bodies
which are only combustible, an end which is to be attained by the
separation of their volatile or inflammable substances. In the last
section, I have shown of what materials the strata of mineral coal had
been originally formed; these are substances containing abundance of
inflammable oil or bitumen, as well as carbonic matter which is properly
combustible; and this is confirmed by the generality of those strata,
which, though perfectly consolidated by fusion, retain still their
inflammable and fusible qualities. But now the object of investigation
is that mineral operation by which some of those strata, or some parts
of a fusible and inflammable stratum, have been so changed as to become
infusible and only combustible.
We have now examined those strata which may be considered as either
proper mineral coal, or as only a bituminous schistus; we are now to
class along with these another species of this kind of matter, which has
had a similar origin, although it may assume a different character.
According to the common observations of mankind, the eminent quality by
which coal is to be distinguished, is the burning of that substance,
or its capacity for making a fire. Therefore, however similar in other
respects, a substance which had not that eminent quality of coal could
hardly be considered as being allied to it; far less could it be
supposed, as being in every other respect the same. We are however
to endeavour to show, that there are truly substances of this kind,
substances which to common observation, having none of the properties of
coal with respect to fire, consequently, no utility for the purpose of
burning, might be considered as another species of mineral, while at the
same time they are truly at bottom a composition very little different
from those which we have considered as the most perfect coal.
It must be recollected that we have distinguished coal in general as
of two different species, one perfect or proper coal, containing no
perceptible quantity of either oil or phlegm; the other as burning with
smoke and flame, consequently containing both aqueous and oleaginous
substances which it emits in distillation. It is the first of these
which we are now to consider more particularly, in order to see the
varieties which may be found in this species of mineral substance.
When that bituminous fossil, which is the common coal of this country,
is submitted to heat it is subject to melt more or less, and emits smoke
which is composed of water and oil. If it be thus completely distilled,
it becomes a perfect coal of a porous or spongy texture. Such a
substance as this is extremely rare among minerals; I have however found
it. It is in the harbour of Ayr, where a whinstone dyke traverses the
coal strata, and includes some of that substance in the state of coals
or cinder. I pointed this out many years ago to Dr Black; and lately I
showed it to Professor Playfair.
But the culm of South Wales, the Kilkenny coal of Ireland, and the blind
coal of Scotland, notwithstanding that these are a perfect coal, or
charred to a coal, have nothing of the porous construction of the
specimen which I have just now mentioned; they are perfectly solid, and
break with a smooth shining surface like those which emit smoke and
flame.
Here is therefore a mineral operation in the preparation of those coals
which we cannot imitate; and here is the clearest evidence of the
operation of mineral fire or heat, although we are ignorant of the
reason why some coal strata are charred, while others are not, and why,
in some particular cases, the charred coal may be porous or spongy like
our coals, while in general those blind coals (as they are called) are
perfectly solid in their structure.
But to what I would call more particularly the attention of mineral
philosophers is this, that it is inconceivable to have this effect
produced by means of water; we might as well say that heat were to be
the cause of ice. The production of coal from vegetable bodies, in which
that phlogistic substance is originally produced, or from animal bodies
which have it from that source, is made by heat, and by no other means,
so far as we know. But, even heat alone is not sufficient to effect that
end, or make a perfect coal; the phlogistic body, which is naturally
compound, consisting of both inflammable and combustible substances,
must be separated chemically, and this must be the operation of heat
under the proper circumstances for distillation or evaporation.
Here is the impossibility which in the last chapter I have alleged the
aqueous theory has to struggle against; and here is one of the absolute
proofs of the igneous theory. Not only must the aqueous part of those
natural phlogistic bodies be evaporated, in order to their becoming
coal, but the oily parts must also, by a still increased degree of heat,
be evaporated, or separated by distillation from the combustible part.
Here, therefore, is evidently the operation of heat, not simply that
of fusion in contradiction to the fluidity of aqueous solution, but
in opposition to any effect of water, as requiring the absence or
separation of that aqueous substance.
But those natural appearances go still farther to confirm our theory,
which, upon all occasions, considers the compression upon the bodies
that are submitted to the operation of heat, in the mineral regions, as
having the greatest efficacy in modifying that operation. Coal strata,
which are in the neighbourhood of each other, being of those two
opposite species, the one fusible and inflammable, the other infusible
and combustible, afford the clearest proof of the efficacy of
compression; for, it is evident, that the coal, which was once
bituminous or fusible, cannot be charred without the distillation of
that substance; therefore, prevent the distillation by compression and
the charring operation cannot proceed, whatever should be the intensity
of the heat; and then, fusion alone must be the effect upon the
bituminous body. But now, as we have both those species of coal in the
vicinity of each other, and even the same strata of coal part charred,
while the rest is not, this natural appearance, so far from being a
stumbling block, as it must be to the opposite theory, is most clearly
explained by the partial escape of vapours from the mineral regions, and
thus confirms the theory with regard to the efficacy of compression.
It is owing to the solidity of those natural charred coals, and the want
of oil, that they are so very difficult of kindling; but, when once
kindled in sufficient quantity, they make a fire which is very durable.
There are even some of them which, to common observation, seem to be
altogether incombustible. I have of this kind a specimen from a stratum
at Stair, which shall be afterwards mentioned.
M. Struve, in the Journal de Physique for January 1790, describes a
mineral which he calls _plombagine charbonneuse ou hexaedre_; and gives
for reason, _parce qu'elle ressemble extremement au charbon de pierre
schisteux, ou d'hexaedre_. He says farther, "Il est tres commun, dans
une roche qui forme un passage entre les granits et les breches, qu'on
n'a trouvee jusqu'a present qu'on masses roulees dans le pays de Vaud."
He concludes his paper thus: "Ce fossile singulier ne paroit pas
appartenir a la Suisse seule. J'ai dans ce moment devant les yeux une
substance parfaitement semblable, si on excepte la couleur qui tient le
milieu entre le gris de fer et le rouge modere; elle vient du pays de
Gotha de la Friedrischs-grube, proche d'Umneau. On le regarde comme un
eisenrahm uni a du charbon de pierre."
The specimen which I have from Stair upon the water of Ayr, so far as I
can understand, perfectly resembles this _plombagine_ of M. Struve. It
consumes very slowly in the fire, and deflagrates like plumbago with
nitre. Now this comes from a regular coal stratum; and what is more
remarkable, in this stratum is contained a true plumbago, Farther up the
country, the Earl of Dumfries has also a mine containing plumbago along
with other coal strata; and though the plumbago of these two mines have
not all the softness and beauty of the mineral of the same species from
Cumberland, they are nevertheless perfect plumbago.
I have a specimen of steatetical whinstone or basaltes from some part of
Cumberland, in which is contained many nodules of the most perfect and
beautiful plumbago. It is dispersed through this stone in rounded masses
of all sizes from a nut to a pin's head; and many of these are mixed
with pyrites. There is therefore reason to believe that this plumbago
had been in fusion.
Now, if we consider that every species of coal and every species of
plumbago are equally, that is, perfectly combustible, and yield, in
burning, the same volatile principles, differing only perhaps a little
in the small quantity of fixed matter which remains, we shall be
inclined to believe, that they have all the same origin in a vegetable
substance; and that they are diversified by some very small composition
of other matter. This being allowed, one thing is certain, that it is by
the operation of mineral fire or heat that those combustible substances,
however composed, have been brought to their present state of coal,
although we are ignorant of the circumstances by which their differences
and their peculiar chemical and mechanical qualities have been produced.
Let us resume in a few words. There is not perhaps one substance in the
mineral kingdom by which the operation of subterraneous heat is, to
common understanding, better exemplified than that of mineral coal.
Those strata are evidently a deposit of inflammable substances which all
come originally from vegetable bodies. In this state of their formation,
those coal strata must all be oleagenous or bituminous. In many of them,
however, these volatile parts are found wanting; and, the stratum is
found in the state of the most perfect coal or caput mortuum. There, is,
I presume, no other means to be found by which this eminent effect could
be produced, except by distillation; and, this distillation perhaps
proceeded under the restraining force of an immense compression.
To this theory it must not be objected, that all the strata of coal,
which are found in the same place or neighbourhood, are not reduced to
that caput mortuum or perfect coaly state. The change from a bituminous
to a coaly substance can only take place in proportion as the
distillation of the volatile parts is permitted. Now this distillation
must be permitted, if any passage can be procured from the inflammable
body submitted to the operation of subterraneous heat; and, one stratum
of coal may find vent for the passage of those vapours, through some
crevice which is not open to another. In this way, doubtless, some of
those bodies have been inspissated or reduced to a fixed coal, while
others, at a little distance, have retained most of their volatile
parts.
We cannot doubt of this distilling operation in the mineral regions,
when we consider that in most places of the earth we find the evident
effects of such distillation of oily substances in the naphta and
petroleum that are constantly emitted along with water in certain
springs. These oily substances are no other than such as may be
procured, in a similar manner, from the fusible or inflammable coal
strata; we have therefore every proof of this mineral operation that the
nature of things admit of. We have also sufficient evidence that those
fusible and inflammable coals, which have not been distilled to a caput
mortuum, had been subjected to the operation of subterraneous heat,
because we find those fusible coals subject to be injected with pyrites,
as well as the more perfect coal.
If we now consider those various appearances of mineral bodies which
are thus explained by the theory of mineral fire, or exertion of
subterraneous heat, appearances which it is impossible to reconcile by
any supposition of aqueous solution, or that unintelligible language
of mineral infiltration which has of late prevailed, we shall be fully
satisfied, that there is a uniform system in nature of providing a power
in the mineral regions, for consolidating the loose materials deposited
at the bottom of the sea, and for erecting those masses of mineralized
substances into the place of land; we shall thus be led to admire the
wisdom of nature, providing for the continuation of this living world,
and employing those very means by which, in a more partial view of
things, this beautiful structure of an inhabited earth seems to be
necessarily going into destruction.
END OF VOLUME FIRST.
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