GEOLOGICAL FORMATION OF SALT.
Sir Isaac Newton, in his incomparable work upon Optics, likens a
particle of salt to a chaos, because of its “being dense, hard, dry,
earthy in the centre; and rare, soft and moist in the circumference.”
This ingenious definition is what one would expect from such an
observant and profound investigator; and I do not think that we shall
be able to find a better description of a salt-crystal than that which
this great philosopher has bequeathed us.
Regarding the original formation of rock-salt, there are many opinions,
theories and conjectures, and to the present day it is an undecided
question. We are, as I have previously stated, in complete ignorance
of the origin of the chloride of sodium; we must consider it as one of
those geological secrets upon which we shall never be able to enlighten
ourselves, if we cannot obtain stronger evidence than that which we
have at present. Science is at fault in this, as she is in many other
subjects which have perplexed and interested from time to time those
who study and seek to unravel the various obscure and complicated
phenomena of nature.
No satisfactory or elucidatory theory has, as yet, been advanced to
account for the occurrence of the formation of salt. Some geologists
have maintained that it was deposited from the ocean, but in what way
they do not explain; indeed, it is difficult to suppose how it could
have been so, for salt, or rather sea water, holds in solution many
ingredients which are not present in this rock. Besides, the several
strata above it contain organic remains, as do also those below, though
altogether of an entirely different kind; rock-salt itself contains
none whatever; from this fact some have inferred that the formation
took place during the epoch which elapsed between the destruction of
one creation and the calling of another into existence. Others suppose
that it is simply the result of volcanic action: this hypothesis is
correct to a certain extent, as far as isolated salt lakes like that
of Tsomoriri in Western Thibet, and that lake midway between Bombay
and Nagpur, are concerned; or those huge mountains consisting entirely
of fossil salt, like the one near Cardona, fifty miles from Barcelona,
in Spain, or those in Lahore, or in Peru; but it altogether fails as
regards non-isolated salt lakes and salt marshes, or such a large
inland sea as the Caspian. Some light may be thrown upon it by the
recent explorations in the North-Western Provinces of India, for Mr.
Wynne tells us that “the geological structure” (of the Indian Salt
Range) “of the trans-Indus extension of the Salt Range repeats in a
great measure that of the western portion of the Salt Range proper,
but with some considerable differences. The Palœozoic rocks, so far
as presented by the red-marl, rock-salt, and gypsum, are quite the
same, and so are the Carboniferous and Triassic groups, but others
of the sub-Carboniferous beds present themselves with a different
association from those of cis-Indus.” Mr. Wynne also informs us that
the mineral productions of the range are valuable, and consist of the
salt of Kalabagh and the Lun Nullah, the alum of Kalabagh and the
Chichali Pass, the coal or lignite from the Jurassic[41] beds of the
Kalabagh Hills: we also learn that gypsum is present with the salt,
as it is in Poland, Transylvania, and Hungary; for in these three
countries there is a layer of gypsum between the stratum of stone and
the bed of salt. This gypsiferous layer is of various colours; it is
crystallised, striated, and mixed with sea-shells: this admixture would
decidedly lead us to conclude that the salt was originally deposited in
bygone ages from the sea. On the contrary, the salt in Cheshire is not
accompanied by a bed of gypsum, there are no vestiges of marine exuviæ,
nor indeed any organic remains to be detected in any of the strata.
If the formation of salt (I am referring to mountains of rock-salt
such as we see near Cordova, in Spain,[42] and salt-mines as we see
in Galicia, and Cheshire, and also isolated salt lakes, like that
which exists in Western Thibet) is solely due to volcanic action, or
marine explosion, we may easily account for its irregular and unequal
distribution; also for its elevation into mountains, and as beds
beneath the surface of the earth, by reason of the greater or less
force which was employed for its upheaval; and also the thickness
or solidity of those strata of rocks through which it was propelled
in its upward course. If this were so, it is strange that it should
be entirely free from organic remains, whose absence therefore is a
formidable objection to this theory. Being accompanied by gypsum in
some districts and not in others, would decidedly point to the presumed
fact that salt has been the result of some volcanic agency; for were
it not so we should find, on the contrary, owing to gradual formation,
that gypsum would invariably be present with it, in the same way as we
find one stratum of rock either above or below the stratum of another
rock.
From the fact that deposits of salt are not confined to any particular
group of strata—for while the salt-mines of Galicia belong to the
tertiary formations, those in the State of New York are found in the
middle of the Silurian system—we may say that salt is not subject
to geological laws by reason of its somewhat erratic appearances in
different strata. As the chlorides of sodium and gypsum are frequently
sublimed from volcanic vents, an igneous origin has been ascribed to
many of the beds of salt and gypsum; and Mr. Bakewell threw out the
suggestion that the consolidation of both salt and gypsum must have
been effected by heat, because the great deposit of gypsum that occurs
with rock-salt at Bex, in Switzerland, was found by M. Carpentier to
be anhydrous when exposed to the atmosphere. If this hypothesis is
correct, and if salt and gypsum[43] were at some period in a state of
fusion, it is difficult to believe that when consolidated they are so
perfectly distinct and in two different strata, so that one contains
organic remains, whilst the other is altogether free from the slightest
vestige. It may have been possible that one was in a state of fusion
when the other was consolidated, and different degrees of heat might
have been necessary for the purpose.
We also may account for the absence of organic remains in rock-salt to
the following cause: the chloride of sodium, when in a state of fusion,
might have possessed the property of disintegrating, dissolving, and
absorbing within itself, however minute they might be, all particles of
organic matter with which it came into contact. Dr. Mantell writes: “It
cannot, however, be with certainty determined whether the absence or
paucity of fossils in a deposit is owing to the actual reduction of the
amount of life in the seas of a given area, or to the mineral character
of the strata not having been favourable to the preservation of organic
remains.”
A very serious difficulty presents itself in the great thickness of
many strata of salt; which, if regarded as the solid residuum of
sea-water, must have necessarily required a proportionate volume of
water, unless the seas of those distant periods contained a larger
amount of saline ingredients than they do at the present time: an
inference for which there are no reasonable grounds.
Wherever there are deposits of the chloride of sodium, they are almost
always accompanied with layers and intercalations of gypsum; and the
peculiar circumstance of two powerful acids, the sulphuric (in the
gypsum, or sulphate of lime), and the muriatic or hydro-chloric (in
the chloride of sodium), being so abundantly and uniformly present,
seems to point to a common origin; both are productions of volcanic
agency, though of the two I think salt frequently owes its origin more
to the subterranean activity than the gypsum, because we find there
are beds of salt where there is no gypsum, and isolated salt lakes
which might have been elevated into mountains had the process, during
their production, been of the same force as that used in the formation
of rock-salt, owing to an unexplainable interruption and premature
desinence.
The relation between the formation of gypsum and volcanic action seems
to be borne out by the fact that in North America, where the coal
measures are not associated with rocks resulting from volcanic agency,
there are no gypsum-beds; while on the contrary, there are large
deposits of gypsum, where igneous rocks are interpolated beneath the
stratum of coal, in Nova Scotia.[44]
Sir Charles Lyell, after a careful inquiry into the phenomena
exhibited by these strata of gypsum, gives his opinion that the
production of these gypsiferous beds in the carboniferous sea was
closely connected with volcanic agency, whether in the form of heated
vapours or stufas, or of hot mineral springs, or some other effects
resulting from submarine igneous irruptions.
Salt or brine springs occur in various parts of the United States in
the _old transition slate_ rocks. Sir Charles Lyell tells us that,
“in the middle of the horizontal Silurian rocks, in the State of New
York, there is a formation of red, green, and blueish-grey marls, with
beds of gypsum, and occasional salt-springs, the whole being from 800
to 1000 feet, and indistinguishable in mineral character from parts
of the Trias of Europe.” Salt-springs also occur in England in the
coal measures. The rock-salt of Cheshire and the brine-springs of
Worcestershire occur in what is called the _old red sandstone_ group.
The salt of Ischl, in the Austrian Alps, belongs to the _oolitic_, as
does also that found in the Lias of Switzerland. The immense mass or
bed of salt near Cordova occurs in the _cretaceous_ group; while the
salt deposit of Wieliczka belongs to the _supracretaceous_ group.[45]
The reader doubtless remembers, as I stated in the first chapter, that
the origin of salt is one of those enigmas of nature which, as yet, has
completely frustrated the most accomplished and scientific geologists,
and no suggestion has yet been made which will satisfactorily and
conclusively account for its formation; for whatever hypothesis
has been stated, there is sure to be an objection so difficult to
overcome, that the author has been fain to admit that it is thoroughly
impracticable, and therefore inadmissible. That it is decidedly not
amenable to the received laws of geology, is apparent, which all must
admit; therefore one cannot possibly apply them so as to determine the
place it occupies in relation to other strata, or practically fix that
period of time in which it was deposited; for it is erratic, and its
position is anomalous—erratic in the variety of appearances it assumes
in creation, and anomalous because it belongs to no particular strata,
and therefore no exact period of time can be assigned to it as to other
formations.
That salt is either due to volcanic agency, marine explosion, or to
overflow of sea-water and subsequent evaporation, or resilience, and
ultimate deposition, are the only three hypotheses which can with any
credibility be advanced to account for its formation.
That we have it presented to us in _six_ different conformations are
facts which when considered separately seem to point to one common
origin, but when taken as a whole indicate a separate inception.
Is it due to volcanic agency? In some respects it undoubtedly is,
otherwise how can we reasonably account for those gigantic mountains of
fossil or rock salt, which rise up isolated in the midst of a country
perfectly free for miles round of saline deposits, which present not
even the slightest trace of it? How can we account for it by any other
means when we find it in intimate relation with gypsum, which we know
is solely the production of subterranean activity? What reason can
we possibly assign for those salt lakes which are above or below the
sea-level and are perfectly solitary, and which have no communication
with the sea or with rivers, if they are not phenomena resulting from
volcanic agency? And how can we account for those masses of salt below
the earth’s surface which in some countries is of such adamantine
hardness that it requires to be blasted with gunpowder, if it is not
the production of volcanic force? If so, why is it that no remains
of organic matter are found imbedded in it? How comes it, if it is
the result of subterranean agency, that organic remains are found in
the gypsum and none in the salt, when both are caused by volcanic
explosion? Thus we see the theory of volcanic explosion is met with a
most formidable objection.
If marine explosion is the sole cause of the formation of salt, and
if the sea has through rents and crevices of the earth forced up its
superabundant saline constituents wherever there has been a vent for
their egress, and which has in the course of time become condensed
owing to the evaporation of the water or through its percolating into
the lower strata, another difficulty crops up quite as unanswerable
seemingly as that which stands in the way of the volcanic hypothesis:
there are no remains of marine organisms to be found, nor are there any
traces of vegetable matter.
The overflow and evaporation of sea-water and the subsequent deposition
of salt holds good in certain respects as regards salt lakes and salt
marshes when they are in close proximity or in the same locality; but
then those other inorganic constituents which are found as a general
rule in sea-water are not present in those open reservoirs, which is a
difficulty as formidable as the others, and admits of no evasion.
These are the three hypotheses with their obstacles; the hypotheses
feasible, the obstacles apparently unanswerable.
We have salt, or the chloride of sodium, presented to us in six
different conditions, viz.: _sea or salt water, salt or brine springs,
salt lakes, salt mines, mountain or fossil or rock salt, and salt
marshes_. The characteristics of salt are just the same fundamentally,
whether we extract it by evaporation from sea-water or salt lakes;
whether we obtain it from salt-springs; whether we dig it out of the
earth or by the excavation of salt mountains; or whether we acquire it
from salt marshes: there is no alteration in its ingredients, though it
may be impure from the admixture of arsenic or the sulphates of soda
and magnesia, or other impurities, or it may be discoloured red by
the oxide of iron derived from decomposed trap-rocks; still, for all
that, the chloride of sodium remains intact. The properties of salt are
not subject to the slightest change or modification: the acid is the
hydrochloric or muriatic, the base sodium, and the combination, the
chloride of sodium.
We find salt, or the chloride of sodium, in sea-water, the amount
averaging from 4 to 5·7 per cent., so that we see it is present in
no inconsiderable quantity; it is more or less impure from other
salts being held in solution in conjunction: where it comes from no
suggestion has yet been broached. We know that it is present, and we
also know that it can be obtained by adopting certain measures for
extracting it; and we are aware, from recent investigations, that the
colour and density of the sea is dependent on the quantity held in
solution. This is all we really know regarding the presence of the
chloride of sodium in the ocean.
The salt which we obtain from brine-springs contains the same
constituents as that which we extract from the sea, though in their
course upwards they collect on their way soluble salts, and therefore
the water goes through certain modifications, which the reader
doubtless recollects. For instance, the brine-springs of Lancashire and
Worcestershire rise up through strata of sandstone and red marl, which
contain large beds of rock-salt. The origin of the brine, therefore,
may be derived from beds of fossil-salt; but as the muriate of soda is
one of the products in volcanic regions, the original source of salt
may be as deep as that of lava.[46]
We have also seen that the base of all mineral waters is the chloride
of sodium, and that their ingredients are collected and dissolved as
they ascend to the surface; therefore they may probably both have the
same origin as the sea, as regards the chloride of sodium, which they
both hold in solution. We can account for their other characteristics
by the wide expansiveness of the sea, which is perpetually absorbing
and emitting vapours, and by the several strata through which the
mineral waters pass. There may be, though there is nothing that we can
advance as corroborative, a subterranean communication existing between
them, which would imply a common origin, the differences arising from
the physical surroundings, atmospheric influences, and the absorption
of soluble salts from the several strata.
What is the origin of salt lakes and salt marshes? This is, to a
certain extent, more easily explained. One theory as to the origin of
salt lakes (we naturally include inland seas, such as the Caspian and
the Aral) is the overflow and subsequent retirement of the sea-water,
their sites having been originally the bed of the ocean when it receded
to its present limits, leaving in its course depressions of land,
volumes of water of various depths, elevations, and extent of surface,
according to their deepness, altitude, or angles of declivity.[47]
Other ingenious hypotheses have been broached, which, I need hardly
say, are not worth considering, as they are entirely visionary. In
the case of isolated salt lakes, the above theory is not applicable;
and geologists tell us that they are doubtless the result of volcanic
agency, but at what period of time it is impossible to estimate, for
the density of the water found in them is not equable, and neither is
their specific gravity the same as that of sea-water, nor are there any
remains of marine organisms; and as their depth is variable, they are
not confined to any particular strata.
I have hinted previously that these isolated salt lakes are (if I may
venture to designate them as such) geological abortions. Had the power
which forced them into their present situation been accompanied by
that agency which has raised such huge masses as those near Cordova,
in Spain, and by the Dead Sea, and which probably brought about their
present crystalline form, others by reason of some unexplainable and
gradual transition, by chemical means, or decrease of temperature,
which naturally would occur the nearer it approached the earth’s
surface, these lakes might have developed into beds or mountains of
salt.
The salt which is dug out of the earth, and that which is excavated out
of isolated salt-mountains, are alike in every respect, and are much
more probably the result of volcanic explosion than of the deposition
of salt from sea-water, accruing from evaporation while pent up in
confined spaces. It may have been, though incalculable ages ago,
deposited from the sea, and then in course of time forced up while in a
state of fusion by some internal disruption.
We thus see that the six conditions under which we find the chloride
of sodium more or less indicate a common origin from sea-water,
notwithstanding the absence of marine organisms.
If we take salt as a whole, leaving out of the question altogether the
different conditions in which it is found, and with no reference at all
to its existing either in the earth, above the earth, in lakes, or in
the sea, but looking at it simply as it is, a mass of rock, or a volume
of water holding it in solution, it inclines one to the belief that it
possesses a dual inchoation, though the original source of both may
have been connate; but owing to extraneous causes which were brought
to bear, one branch became crystallised rock-salt, while the other,
through immaturity, remains in a state of solution. One is rock-salt,
which has been heaved up by volcanic power, and the other is what is
known as sea-water; the former has produced the mines, and the solitary
mountains, and the Indian Salt Range, and that salt which generates
mineral waters, and, it may be, those saline lakes like that which
exists between Bombay and Nagpur.
According to Sir Charles Lyell, sea-water has access to volcanic
foci. He says: “Although the theory which assumes that water plays a
principal part in volcanic operations does not necessarily imply the
proximity of volcanic vents to the ocean, it seems still to follow
naturally that the superficial outbursts of steam and lava will be
most prevalent where there is an incumbent body of salt water, or
any regions rather than in the interior of a continent, where the
quantity of rain-water is reduced to a minimum. The experiments of the
most eminent chemists have gradually removed, one after another, the
objections which were first offered to the doctrine that the salt water
of the sea plays a leading part in most volcanic eruptions. Sir Humphry
Davy observed that the fumes which escaped from Vesuvian lava deposited
common salt.”
All the principal volcanoes are situated close to the sea, and
therefore the hypothesis that a communication exists between them is
practically certain; their proximity to the sea, and the deposition
of salt from the fumes of lava, as Sir Humphry Davy noticed, are two
strong facts. But for all that, it does not prove satisfactorily that
salt is solely the result of volcanic agency, and indirectly from the
sea, because there is not the slightest trace of the remains of marine
organisms, unless they are totally destroyed and obliterated when it is
in a state of fusion; if so, it is more conclusive that salt such as we
find it is solely due to volcanic force. Salt may have been in times
past, as the observations of Sir Humphry Davy seem to corroborate, and
as confirmed by more recent chemists, deposited by volcanic agency in
the same way that salt is deposited by fumes of hydrochloric acid,
which are emitted with the lava during eruptions of such volcanoes as
Vesuvius and Etna, by reason of some communication with the sea.
As hydrochloric acid is found in the vapours which are disengaged from
red-hot lava, and as magnesia, which is not volatile, is left in the
lava itself, constituting one of its most important elements, it would
certainly lead one to surmise that there is a communication which,
though not always in existence, may be periodically caused by the
action of the volcano.
Both MM. St. Claire Deville and Fouqué have succeeded in demonstrating
the perfect accordance of the chemical composition of the products of
volcanic eruptions, both gaseous and solid, with the doctrine that salt
water has been largely present in volcanic foci. If so, why are there
no salts of magnesia in volcanic fumeroles? These salts are readily
decomposable by hot steam, and when water and heat are present they
produce hydrochloric acid and magnesia. M. Fouqué affirmed that he
witnessed an eruption of Mount Etna in 1865; the gaseous emanations
agreed in kind with those which we might have looked for if large
volumes of sea-water had gained access to reservoirs of subterranean
lava, and if they had been decomposed and expelled with the lava.[48]
We have obtained three facts, viz., that communications probably exist
between volcanic foci and sea-water; that fumes of hydrochloric acid
which accompany the lava deposit common salt; and that the salts of
magnesia are decomposed by heat; and what more probable than that all
living organisms which pass with the sea-water are utterly obliterated?
By the preceding observations, the reader will see that salt is not
subject to geological laws, by reason of its being confined to no
particular strata, and by the absence of organic remains; and that it
is not derived from sea-water, because there are no marine organisms to
be found in it.
That though it may have a pristine source, it has (though it may appear
paradoxical) a dual inchoation—by its being found as rock-salt, and by
its being present in sea-water, and, as I have stated, in a condition
of immaturity.
Rock-salt appears to be the result of volcanic agency, from its being
almost invariably (with but few exceptions) in juxtaposition with
gypsum, which is known to be of volcanic origin; by its being found
forced up independently of other formations, even through the crust
of the earth; by the presence of fumes of hydrochloric acid with lava
during volcanic eruptions.
It has undoubtedly an igneous origin, and the entire absence of organic
remains may be accounted for by the fact that while in a state of
fusion it may have disintegrated, absorbed into itself, or altogether
obliterated all remains of living organisms with which it may have
come into immediate contact. All other formations have preserved the
impress and structure of vegetable and animal life; salt is the sole
exception to the rule; and if while in a state of fusion it possessed
the property of destroying and obliterating all marks of animal and
vegetable remains, we can easily account for their absence.[49]
We have also seen that sea-water has access to volcanic foci, by reason
of fumes of hydrochloric acid, which deposit common salt, and by the
proximity of the volcanoes to the sea.
One question is naturally evolved out of this: does the sea obtain its
saline constituents from vast reservoirs, or beds of salt, through the
medium of communication with volcanic foci?
This question I leave unsolved, for were we to discuss it, we should
probably have to enter into other matters which would be somewhat
foreign to my subject. My opinion is that sea-water (if my hypothesis
that it is nothing else than salt in a state of immaturity is correct)
obtains its chloride of sodium in this way; and, if so, it accounts
at once for the absence of marine organisms, upon which phenomenon
geologists have always laid so much stress. Besides, if salt is
derived from evaporation of sea-water, and subsequent deposition of
salt, we should be able to obtain remains of marine organisms, if
not those of land animals. This one fact alone would tend to prove
that sea-water is the result of some subterranean communication with
reservoirs of salt, through the media of volcanic foci.
We have thus before us certain geological facts relative to salt, which
show that though it has not been discovered in the old stratified
rocks, it is nevertheless met with in nearly all the later formations,
and also that it is in process of formation, and notably so in the
Crimea. This undoubtedly is the case; but still we cannot apply any
of the laws of geology so as to make our conjectures confirmative by
certain facts which support one hypothesis and overthrow another.