Popular Science Monthly/Volume 21/October 1882/The Formation of Saline Mineral Waters
|THE FORMATION OF SALINE MINERAL WATERS.|
AS far back as we may go in the annals of mankind, we find mineral waters occupying a considerable place in the life of the peoples, and at last reach a point when they were the object of a veritable worship. Notwithstanding, however, the antiquity of the subject, and the importance that has always been attached to mineral waters, with the immense variety of labors of which they have been the object, the fundamental questions relating to them still remain enveloped in profound darkness. An examination of the most recent and most authoritative publications suffices to show us that, with regard to the most capital point, the origin and mode of formation of these waters, we, at the end of fifteen centuries, are not much further advanced than were the Romans. Modern science, it is true, has put the ancient nymph to flight, and driven from his sanctuary the little beneficent god that presided at each fountain; but it has not yet succeeded in raising the statue of the truth upon the vacant altar.
Mineral waters take up the substances which give them their peculiar composition, and acquire all their medical value, at greater or less distances within the globe. It is, then, for geology, the science which deals with the formation of the globe, to seek the solution of the questions of their origin.
The number of mineral springs is immense, and the variations among them seem infinite; but there exist among them certain groups which distinguish and separate themselves at the first glance. Among these we place in the first rank the saline waters, or those of which the sea is the type. This is the division which I propose to consider, and of which I shall endeavor to set forth the origin and method of formation.
It is a fact, which I may state as uncontested at the present time, that all spring-waters, whether mineralized or not, are of exterior origin—that is, are waters of infiltration derived from the atmosphere. When these waters return to the light without having met, in the strata they have traversed, either soluble minerals or gases other than those of the atmosphere, they constitute ordinary waters. If, on the contrary, they have met soluble substances or gases different from those of the atmosphere, they will return more or less charged with those substances, and will then be mineral waters. In studying them we need not, therefore, inquire about the origin of the water itself, for it comes from the atmosphere, but only about that of the saline substances which it has encountered in its course.
It has been known from the most remote antiquity that considerable masses of saline substances, generally composed of gypsum, more rarely of rock-salt, exist at numerous points of our globe. The two salts we have named are the ones that have hitherto attracted the attention of commercial and scientific men; but the saline beds are in reality of a more complex composition than is superficially indicated by the predominance of those compounds.
The hypotheses which have been proposed to account for the origin of these salts, though many, may be grouped around three principal heads: 1. Free sulphuric acid, coming up from the depths of the globe, has acted upon carbonate of lime already formed and produced gypsum. 2. Hydro-sulphuric acid, coming in like manner from the depths of the globe, has absorbed oxygen from the air, has become sulphuric acid, and, acting upon the already formed limestone, has produced gypsum, as in the previous theory. 3. Salts already formed in the interior of the globe have been brought up to the surface partly in solution, partly sublimed. These hypotheses are all wholly gratuitous; and, with regard to the first, it is impossible to admit that sulphuric acid, already formed, coming up from the depths of the globe, should have traversed, without saturating them, the enormous thickness of the underlying calcareous beds to make gypsum in the tertiary formation. The second hypothesis is more admissible, especially since M. Dumas has found gypsum that was evidently formed in that way; but while we may easily suppose a mass of limestone, of which one part has been transformed into gypsum by the action of free sulphuric acid, while the other part remains limestone, we can not admit such an intervention of the acid in a case where the gypsum is interpenetrated in all its parts with carbonate of lime. The third hypothesis, that the saline deposits have been brought up from the depths of the globe, is only a continued appeal to those mysterious actions which figure so prominently in the infancy of all the sciences; but, besides that it explains nothing, I believe that it is a real error. My studies of chemical geology have led me to the conclusion that the salts now held in solution in the waters of the seas, the salts existing in solid masses in the strata of our globe, and those which furnish the mineral constituents to saline waters, have a common origin, and that that origin is exterior to the first strata that were formed in the consolidation of the earth.
According to the conditions assumed in the nebular hypothesis of the origin of the earth, the rocks forming the first solid envelope of our globe solidified at a temperature of between 2,000° and 2,500° Cent. (3,600° and 4,500° Fahr.). Now, according to the law of dissociation, as discovered by Sainte-Claire Deville, chlorine, sulphur, and their constant compounds, with oxygen and hydrogen, were present in the atmosphere at the former temperature, and even below it, in a state of complete dissociation; and only at a much later stage than this was it possible for chlorine and sulphur to effect combinations so as to react upon the exterior crust and form sulphates and chlorides. The sulphates and chlorides, in their turn, could have been produced only at successive and extremely distant epochs. Thus, to mention only the two chlorides which constitute the largest part of the saline substances contained in marine waters, the chloride of sodium and the chloride of magnesium; the former has been formed at a high temperature, for it can support a high temperature without suffering decomposition. But the chloride of magnesium can not have been composed until a prodigiously more advanced epoch—that is, one nearer to our own time, when the temperature of the earth had descended to about the boiling-point of water; for the chloride of magnesium can not support such a temperature in the presence of water without being decomposed.
As the process of cooling continued, the atmosphere kept incessantly yielding by condensation the water which it held, and this water kept perpetually dissolving the soluble salts which it found already present, and also those that were continuously produced by the action of the acids it bore with it. As we have already seen, these salts were sulphates and chlorides. On the other hand, the metals competent to combine with the sulphur and the chlorine were necessarily those existing in the rocks that formed the first crust of consolidation, and these metals were, as we shall be able to show, lithium, potassium, sodium, magnesium, and calcium. Now, it is these five metals which, united with chlorine and sulphur, constitute nearly the total amount of the salts dissolved in the waters of the seas. Such, to my mind, is the origin of the salts which mineralize the seas; and it is an origin wholly exterior. Thus, from the most ancient aqueous period of our planet, from the time when its external temperature was not notably below the boiling-point of water, and from a time, consequently, before any trace of life was possible, the seas have had essentially the same composition they have to-day. Zoölogists have for a long time regarded this fact as necessary, because the animals whose remains have been found in the most ancient sedimentary beds did not differ as to their general plan from their congeners in modern seas, and could not, consequently, have lived in waters that differed sensibly in composition from the water of existing seas.
If the whole mass of chlorides and sulphates was originally dissolved in the sea-waters, then the only way we have of explaining the origin of the saline formations which exist in the sedimentary beds is to assume that they have been left by the spontaneous evaporation of parts of the ancient seas accidentally isolated from the oceans. I came to this conclusion long ago, not by a more or less intuitive suggestion, but drawn to it by the logic of facts and the ideas I am about to present. This conclusion once formulated, I have taken the consequences, as numerous as important, which it involves, and have submitted them to an experimental verification.
In approaching the experimental side of my investigations, I had first to study in its details what takes place when the waters of existing seas are left to spontaneous evaporation.
First, a very weak precipitation occurs of carbonate of lime with a trace of strontium, and of hydrated sesquioxide of iron, mingled with a slight proportion of manganese. The water then continues to evaporate, but remains perfectly limpid, without forming any other deposit than the one I have mentioned, till it has lost eighty per cent, of its original volume. It then begins to leave an abundant precipitate of perfectly crystallized sulphate of lime with two equivalents of water, or gypsum, identical in geometrical form and chemical composition with that of the gypsum-beds. This deposit continues till the water has lost eight per cent more of its original volume, then all precipitation ceases till two per cent more of the original quantity of water has evaporated away. Then a new deposition begins, not of gypsum, but of chloride of sodium, or sea-salt. The separation between the end of the deposition of gypsum and the beginning of that of chloride of sodium is so marked that it is utilized on a grand scale in the works of the salt-marshes. The salters allow all the gypsum to be deposited in the ordinary basins, and then run the water thus cleared of gypsum into special vessels, whereby they obtain a pure salt, which they can collect down to the very bottom of the basin after the last mother-waters have been drawn off. The deposition of pure or commercial salt continues till the volume of the water has been again reduced by one half, when a precipitation of sulphate of magnesia begins to take place with it. This continues, the two salts being deposited in equal quantities, till only three per cent of the original quantity of water is left. Finally, when the water has been concentrated to two per cent, carnallite, or the double chloride of potassium and magnesium, is deposited. Spontaneous evaporation can not go much further. The residual mother-water will not dry up at the ordinary temperature, even in the hottest regions of the globe; its chief constituent is chloride of magnesium. A body of sea-water, evaporated naturally, will then leave a series of deposits in which we will find as we dig down the following minerals in order:
Deliquescent salts, including chiefly chloride of magnesium.
Carnallite, or the double chloride of potassium and magnesium.
Mixed salts, including chloride of sodium and sulphate of magnesia.
Sea-salt, mixed with sulphate of magnesia.
Weak deposits of carbonate of lime, with sesquioxide of iron, etc.
The examination of this list and the facts that have been expounded draw with them a large number of consequences; I will only call attention to two of them. The first is, that the different groups of substances named in the list should become more and more rare as we ascend from the base to the summit; for each of them corresponds with a more advanced period of evaporation, and the chances for its production become less and less favorable as we rise. The second consequence—and I regard it as a capital one—is, that, when we meet one of the superior groups, we should expect to find, below it, all the other groups which were deposited previously to it in the order of evaporation. These consequences are constantly verified in the saline beds which exist so numerously on the globe. Thus, to bring up again the two most important salts, numerous beds of gypsum are known, without rock-salt, or any of the other deposits, above them—which verifies our first conclusion; but no bed of rock-salt is known without gypsum—which verifies the second conclusion. More than this, a vast saline bed is now known which corresponds with the complete period set forth in our list—that is, with the complete period of evaporation; it is the formation at Stassfurt, Prussia, the disposition of which corresponds exactly with that shown in the list. The study of the Stassfurt bed further reveals an entirely new fact—the presence, in a part quite above the mean of deliquescent salts, of an important deposit of boracic acid combined with magnesium. All the geologists and engineers who have ever given their attention to the Stassfurt deposit have been unanimous in calling for volcanic intervention, and looking to the depths of the globe for the explanation of the origin of the boracic acid, and the place it occupies in the upper part of the bed; and they have all also given a more or less preponderant part to the play of volcanic agencies in the formation of the whole. This conclusion was the more natural, because the ordinary laws of chemistry seemed to oppose the probability of boracic acid, even if it existed, being found in the last mother-waters. Borate of magnesia is almost insoluble, and should have been among the first and lowest of the substances deposited. Adhering to the principle that there are no saline substances in the interior of the globe below the sedimentary formations, I silenced my protestations as a chemist, and applied myself to inquire if, contrary to all previsions, boracic acid did not exist in the ultimate mother-waters of the salt-marshes of the south of France. The result justified my geological induction to a degree beyond all possible anticipation. Not only does boracic acid exist in the ultimate mother-waters, but it exists there in a quantity relatively so considerable that a single drop of the water is enough to show it, either by the hydrogen method or by that of spectrum analysis. Thus, the presence of this substance in the upper part of the Stassfurt bed ceases to be an objection to the theory that the bed is purely and simply the result of the evaporation of the ancient seawaters; and its presence here furthermore gives a confirmation, as striking as unforeseen, to that theory.
My chemical and experimental studies are very far from constituting the only foundation for my theory of the sedimentary origin of the saline formations of the globe. Existing nature offers us in abundance, and on even a vaster scale than was shown in the ancient ages, phenomena which it is sufficient to analyze to bring forth anew, even to the slightest details, the manifestations which accompanied and determined the precipitation of saline matters in the estuaries of the ancient seas. From among the numerous instances of this kind that present themselves to my thought, I select three—the mouths of the Rhône, the Caspian Sea, and the Dead Sea.
The map of the mouths of the Rhône shows that the lands for about twenty miles from the Mediterranean are cut up by numerous lakes, the principal of which, that of Valcares, exceeds sixty kilometres in superficial area. These lakes have been formed by the action of the alluvial contents of the river, which, being deposited on the bottom of the sea, have raised bars inclosing bodies of water. The water within the ponds, evaporating under the summer heat, is depressed in level. If the ponds were wholly isolated from each other, and had no communication with the sea, there would accumulate in each of them after a time, varying with its depth, a deposit of gypsum, and above this one of sea-salt, and so on through the series we have described; and, as the ponds are as a rule quite shallow, the saline deposits would all be thin. Things, however, do not go on thus. Most of the ponds communicate with each other and with the sea; consequently, when the water in them evaporates, the level is re-established with water that comes from the sea. In this way a pond, the bottom of which is only a few feet below the level of the sea, would become filled with saline substances if the canal of communication did not become choked. The Lake of Lavalduc, the surface of which is several yards below that of the Mediterranean, is depositing gypsum.
These facts show us saline deposits in process of formation under our very eyes, and it is not at all necessary, in order to explain their formation, to invoke changes of relief or any perturbations in the crust of the globe, but only to take an exact account of the manner in which a delta is formed, and of the circumstances that are a consequence of that mode of formation; and, although the delta of the Rhône is not one of the most extensive of the deltas of the modern period, it is one of the most remarkable and complete in respect to the spontaneous formation of saline deposits. If we should go into details, we should find a complete concordance between what is going on in the delta of the Rhône and what has been revealed in the study of the saliferous formations of past ages. Thus, saline deposits are forming in these more or less wholly isolated ponds: we have, then, a saliferous horizon in the delta of the Rhône, with the deposits generally separated; on the other hand, each of the deposits nearly always appears of a lenticular form, because the ponds of the estuaries necessarily, from the mode of their origin, assume that shape. The same principal characteristics are presented by most of the beds existing in the sedimentary deposits.
If, now, by any incident, a pond which has been closed and has deposited its gypsum is again brought into communication with the sea, life will reappear in it, and mollusks will leave their shells on top of the gypsum. If evaporation is resumed, life will disappear for a second time, and new strata of gypsum will be precipitated upon the marls containing the marine shells. This condition is exhibited in the delta of the Rhône, where the Lavalduc Lake, which has been isolated from the sea for centuries, has sunk to fifteen metres below its level. It has a stratum of gypsum on the bottom, and above it is a thick formation of mud, that has become a true marl where it has dried, at the base of which, and at a level corresponding with the epoch at which the water was nearly normal sea-water, marine shells are abundant. It may occasionally happen that a marine estuary, in which a deposit of gypsum has taken place, shall receive an accession of fresh instead of salt water, and then we shall have deposits containing freshwater fossils above the gypsum. We can see from this how valueless, as an argument against the theory that gypsums are products of the evaporation of sea-water, is the assertion, so often put forward in that guise, that gypsums are sometimes found covered with fresh-water deposits.
Similar phenomena and another illustration of our theory are exhibited on a colossal scale in the Caspian Sea. On the eastern side of that sea is the Gulf of Karabogaz, relatively small, but having a superficial area of at least twenty thousand square kilometres. It contains no living beings except some of the lower organisms, and its shores are marked by a complete sterility. Its only communication with the Caspian Sea is by a shallow channel which allows water to flow over from the sea, but lets none back. The excessive evaporation always going on in that hot and arid region causes a constant depression of the level of the water in the gulf, and this induces an incessant flow from the sea. In the absence of a counter-current from the Karabogaz to the Caspian, all of the salt that is brought in with the inflowing water remains in the Karabogaz; the amount of salt thus regularly added to the quantity already held there is not less than three hundred and fifty thousand tons every twenty-four hours. It is easy to prognosticate the future of this gulf. If the channel of communication is kept open, the water, now nearly saturated, will continue to deposit gypsum; but the constant accession of water from the sea will prevent its reaching for an extremely long time the degree of concentration that will permit the deposition of salt. There will in this case be produced in the Karabogaz a colossal deposit of gypsum, to which no parallel can be found in the ancient formations. If, on the other hand, the channel becomes obstructed, evaporation in the Karabogaz will go on more rapidly, for it receives no important affluent of fresh water. Then, at the end of a time which will not be prodigious, we shall have a saline deposit identical with that of Stassfurt, having large masses of gypsum at the bottom, and deliquescent salts with boracic acid at the top. The latter ending is more likely to be realized; for the level of the Caspian Sea itself is falling under the excess of evaporation over the supply of water brought by the rivers into it, and will at no very distant period sink below the level of the bottom of the channel that connects the Karabogaz with it.
The character of the Dead Sea is still in question. M. Lartet, of Toulouse, who was connected with the expedition of the Due de Luynes in 1866, fully recognizes the close analogy of its waters with the mother-waters that are left from the evaporation of the waters of normal seas; but he believes that its waters owe their quality to thermal springs, and that the sea never could have been in communication with the Mediterranean or the Red Sea. He supports his view by the assumed absence of certain substances common to sea-waters from its waters; but, as I have found these substances by analysis in Dead Sea waters, I consider the argument resting upon that ground invalid. M. Lortet, of Lyons, has recently made a discovery that indicates that this lake once formed part of a normal sea. He has found near the Lake of Tiberias a plateau covered with gravel and rounded pebbles, situated at the exact level of the Mediterranean Sea. If the body of water that washed these gravels and pebbles once occupied the valley of the Jordan and the Dead Sea, then there once existed here a gulf like that of Karabogaz. Separated from the oceans by some accident, it has dried up; its less soluble salts have been gradually deposited in the shallower parts of the basin and constitute the saline masses which are now found in the region—products of the sea instead of being the causes of its saltness; and the deliquescent salts have become concentrated, as in modern estuaries, into the still liquid part that constitutes a mother-water fully comparable in all respects to the mother-waters of the salt-marshes of the south of France.
I conclude with a summary of the principal facts to which I have directed attention.
At the moment when the first crust of consolidation began to form around our globe, chlorine and sulphur, now existing in combinations, were in the atmosphere. When the temperature had sufficiently diminished, these two bodies, reacting on the exterior crust of our globe, formed, at intervals otherwise extremely distant, combinations (sulphates and chlorides) by uniting with the metals that existed and still exist in the rocks constituting that first envelope. These metals, combined almost exclusively with sulphur and chlorine, are precisely the ones (lithium, potassium, sodium, magnesium, calcium) that still mineralize the waters of the seas. The salts thus formed, dissolved in the waters from the most ancient age of the aqueous period of our planet, have, then, a wholly exterior origin. Later, under the influence of causes often extremely insignificant in themselves, parts of these seas have been isolated from the oceans; they have evaporated, and, according to the degree of completeness in which concentration has been effected, they have deposited salt-beds sometimes of a complex enough nature, but which have uniformly presented the typical character that they begin with deposits of gypsum. Such is the origin of the saline masses that exist in the interior of our globe. Whenever waters of infiltration reach these saline deposits, they dissolve more or less considerable quantities of them, and, when they come out again into the light, they constitute what are called saline mineral waters.
- An objection may be raised to this idea, from the fact that a large number of chlorides and sulphurous products are developed in modern volcanic phenomena which we all agree came up from a very profound zone. I refer to it to state that I am prepared to meet it and have elements sufficient to show that the two orders of facts are perfectly reconcilable.