Popular Science Monthly/Volume 33/September 1888/Underground Waters and Mineral Veins
|UNDERGROUND WATERS AND MINERAL VEINS.|
By Prof. G. A. DAUBRÉE.
BEFORE occupying himself with the great masses that constitute the crust of the earth, and yielding to cupidity rather than to scientific curiosity, man attempted to discover the genesis of certain minerals. Have the middle ages not seen more than one alchemist, in his passionate search for the philosopher's stone, trying to discover the secret of Nature, and reproduce the processes by which she has created in the rocks gold, the most noble, as they said in those days, of the metals, and certainly the most precious?
According to the system of Thales, adopted by Aristotle, water was the universal principle of things. "If the elements are born of one another," wrote Seneca, "why may not the earth be produced from water? Like the human body, the earth includes a number of humors, some of which, hardened when they came to maturity; whence the metallic earths, and stony substances, which are nothing but petrified liquids."
The hypotheses relative to the nature of mineral substances which were current down to the last century are related to this doctrine. Bernard Palissy, one of the most penetrative minds of his time, wrote: "All mineral matters that you call dead bodies were also created as well as vegetable substances, and have been in travail to produce seeds from which others could be engendered. Crystal is not so dead but that it is given to it to know how to separate itself from other waters, and from itself, with its angles and diamond-points, in the midst of them. Thus, mineral matters are not so inert but they bring forth and gradually produce most excellent things. These mineral matters are intermingled and unrecognized among the waters, in the matrix of the earth, so that every human and brute creature is engendered under a kind of water. The matters of metals are concealed in such a way that it is impossible for man to distinguish them before they are congealed, just as no one can tell that water in which salt is dissolved is saline without tasting it with his tongue." Then, replying to the alchemists who had recourse in their experiments to the highest furnace-temperatures then known, Palissy added, "When you have tried everything by fire, you will find that my saying is true, that water is the beginning and origin of all natural things." One could not reason more ingeniously respecting an idea wholly of the imagination, but which could hardly have been sounder, at a time when, chemistry not having yet put on a scientific character, the nature of the substances whose origin it was sought to ascertain was still almost unknown. Struck by the admirable regularity of the motions of the stars, a number of minds were led, by a mystical generalization, to draw from them consequences applicable to the phenomena of our planet. According to a doctrine that goes back to the Chaldeans, and is also found among the Egyptians, sidereal influences contribute to the maturing (that is, to the subterranean transformation) of mineral substances. Mysterious relations were supposed between the celestial bodies of our solar system and metals, the luster of which has some resemblance to the color of the stars. Conformably to the principle of likes, gold corresponded with the sun, silver with the moon, iron with Mars, copper with Venus, lead with Saturn, and tin with Jupiter. Strange as it may seem, this fancy had not been abandoned two centuries ago. An old German practical miner's manual, the "Bergbüchlein," the earliest known edition of which is dated in 1505, contains figures in which metalliferous veins may be seen running into the interior of the earth, and in the sky the planets which correspond respectively with the various metals, and from which the generative effluviæ are flowing. "With the birth and growth of a metallic mineral," it is said in the book, "are involved, on the one side an agent, and on the other a subordinate substance or matter, which is capable of being set into activity, like something in fermentation. The general agent is the sky, with its movements, the revolution of its planets, and its luminous radiation. This is why each metallic mineral is subject to a special influence from its particular planet." Thus, in a little work of only a few pages, and which was simply intended to give the most necessary knowledge to the practical miner, was placed for the same consideration of utility as determines the employment of the compass, the notion of this pretended affinity between the metals and the planets.
The mode of formation, or, as Buff on said, the genesis of minerals is one of the most interesting questions of their history. But the problem could not be approached until geologists had furnished precise data on the conditions of their bearing. Satisfactory solutions have recently been obtained in the case of a certain number of mineral species. Synthetic experiment, placing itself in the circumstances that seem to have presided at their formation, has succeeded in reproducing them, with their crystalline forms, and all their essential characteristics, and has thus completed the demonstration of their origin. By means of this method of demonstration, we have been able to ascertain that many minerals are due to the action of subterranean waters. From the most ancient epochs, these waters have circulated through the crust of the earth, where they have left, at a multitude of points, signs revealing the part they have played, and the course they have taken, even more clearly than contemporary phenomena •have done.
The sedimentary beds, formed like the deposits which the sea spreads every day in the bottom of its basin, are often distinguishable from one another, even at first sight, by certain exterior characters. The differences are, for the most part, produced by the action of subterranean waters, as is demonstrated by the animal and vegetable fossils, which were for a long time designated as petrifactions, or, rather, by the chemical changes which these fossilized bodies have evidently undergone.
Here, shells and corals, showing forms perfectly preserved down to their slightest details, are no longer composed of carbonate of lime, as they certainly were during the life of the animal to which they belonged, but are essentially different substances, quartz having entirely taken the place of the calcium carbonate. There are also other minerals, such as pyrites and sulphate of baryta, which have penetrated and crystallized within the cavities which the bodies of these invertebrates occupied.
The silicified woods, which are very frequently met, assert still more clearly the intervention of a liquid. Not only can the least trained eye recognize their external shape, but the ligneous texture also is still maintained, even to the cells and other inmost parts, as distinctly as in the living wood. It is not, then, a simple molding of silica, performed in the vacant spots that have been left by the disappearance of the vegetable substance, but the effect of a molecular substitution, gradual and slow, which has preserved to us the most delicate organs of various plants. A liquid, such as water, has been able of itself to produce these substitutions of one body for another, by depositing the substances which it held dissolved.
Changes due in like manner to an aqueous influence have induced the formation of the rounded bodies called nodules, which have been sometimes confounded with organic productions, although they are wholly mineral. Flint, which is a variety of quartz, often appears under a tubercular form. Nodules of it are found, in a parallel alignment to the structure of the chalk, in the chalk-beds of England and France. They have been produced subsequently to the deposition of the strata, and have often imbedded fossils upon which they have molded themselves. There are also calcareous nodules that have been produced in a similar manner. The most recent quaternary deposits, like the diluvian clay, or loess, present a large number of them. The same form appears very frequently in the carbonate of iron, which is particularly abundant in the clays of the coal-beds, and is mined in several counties of Great Britain. These balls may be recognized by their metallic luster and brassy color, and their surface spiked with crystalline points. They are formed of pyrites or bisulphuret of iron, and abound in the chalk, the plastic clay, and the carboniferous rocks. When, as the result of denudations, they appear isolated on the surface of rocks of an entirely different nature, people have sometimes been led to suppose them fallen from the sky; and so they have been given, in some parts of France, the vulgar names of thunder-stones or aërolithes. The substance which has been formed into these concretionary forms appears to have been subjected to the influence of a liquid vehicle, like quarry-water, or the water which has been imbibed by rocks. The tendency of dissolved matter to agglomerate, under the influence of attraction, into a spherical shape, has been opposed by the unequal resistance of the mass from which it isolates itself. Hence, the tubercular forms.
In the case of the blackish coatings called dendrites, the forms of which bear a deceptive resemblance to those of mosses, the deposit is wholly inorganic; water, branching out by capillary attraction through extremely minute cracks, has deposited oxide of manganese in them.
The marbles called veined give evidence of another mode of action of subterranean waters. Their varied aspect is due to little veins of white crystalline carbonate of lime winding around in a mass of dark color and amorphous character, but of the same chemical composition. Fissures, intersecting in every direction, are first produced in the rock, under the influence of mechanical actions; the cavities thus opened have served as channels for waters which, on their passage, have dissolved a part of the substance to deposit it afterward purified by crystallization—a fact very much like what we habitually observe in our laboratories. This mode of veined structure is most frequent in the limestones of regions that have been dislocated. The Alps furnish many examples of it along escarpments of a considerable extent.
Modifications have also been imposed upon the eruptive rocks, under the influence of the waters that have traversed them; but they are of a different character from those that we have been considering, not only on account of the heat that has prevailed among them, but also on account of the composition of the rocks. Various mineral species, grouped under the name of zeoliths, may be observed among the eruptive rocks, as crystals adorning innumerable cavities, such as we may see produced in existing volcanic lavas, by the disengagement of the vapors which these lavas exhale down to the moment when they are completely solidified. It is easily seen that these zeoliths were not formed at the same time as the mother-rock, but after it had become consolidated and turgid. They always assume the same disposition, whatever the age of the rocks. Sometimes agate is associated with them, as at Oberstein, in the Palatinate, where that stone was mined in antiquity, and in Uruguay, where it is extracted at this time. Its concentric zones, laid one upon another in successive moldings, testify clearly to a gradual deposit, evidently of an aqueous nature. Waters of incrustation are producing under our eyes deposits of carbonate of lime of identical structure. The varied colorations of the successive zones of agate which are utilized in the making of cameos correspond with very slight variations in the nature of the precipitant liquid. The limpid crystals of Iceland-spar, to which physics is indebted, since Huygens, for most important discoveries in double refraction and the polarization of light, are associated with zeoliths in the cavities of ancient lavas, and originated at the same epoch.
We do not any longer have to resort to erroneous or vague conjectures to explain the origin of these minerals, for we have a demonstration, that might be called experimental, which throws the clearest light on all their details of it. The excavations made at Plombières in 1851, to increase the flow of the springs, brought to light in the deep trenches of the subsoil a part of the old Roman underground conduits that had escaped the ravages of the barbarians. They also disclosed a masonry-work of bêton and bricks carefully built around the thermal springs, so as to isolate them from the neighboring river and the gravel, in which they were in danger of being scattered and cooled. Every spring imprisoned in this masonry, as it rose from its source, could only escape, to flow on to the fish-ponds, through a vertical chimney of cut stone. Carefully examining the bricks which had been immersed for centuries in the mineral water, I discovered that they had undergone a transformation of the most interesting character. New combinations, silicates of the family of the zeolites, had been developed in the cavities with which the bricks were riddled; chabesite in striated crystals, grouped exactly like those in Nature, and with the same angles; and christianite, the crystals of which, intersecting one another in the form of a cross, are identical with those of the volcanic rocks. There was, besides, a product of opal, translucent and colored like drops of dew. The tissue of the bricks contained little fibrous and radiated globules, which optical characteristics showed to be chalcedony. The same species had been formed even in the minutest pores of the brick. These minerals, of contemporaneous production, were found later in the Roman masonry-works of Luxeuil and Bourbonne-les-Bains. With the aid of time, thermal water had there acted chemically on the bricks and on the limestone, and had engendered gradually and by a curious collaboration the substances we have named, without calling in the high temperature that we were ready to suppose was necessary, nor water very strongly mineralized. A very slow but incessant action was sufficient. Does not this demonstration, even to the minutest particulars, account for the formation in ancient epochs of zeolite, agate, and the substances that usually accompany them? In view of their complete similarity with those of which Plombières has revealed the history, can we not say that all these minerals were reproduced in rocks still incompletely cooled, by the chemical action of hot or lukewarm waters which infiltrated themselves into their easily permeable texture, and of which the parasitic zeolites accurately trace the ancient history?
On account of the multiplicity and extent of the works of exploitation that traverse the metalliferous beds in numerous countries, and the mathematical exactness with which all the details of their forms and composition are revealed every day, these beds bring to bear very precious data on the functions as mineralizers of the ancient springs. The veins of the most usual type have the form of plaques rarely exceeding a few metres in thickness. Horizontally, they are prolonged to considerable superficies, sometimes to ten or fifteen kilometres. This measurement is given by the extent to which workings have been promoted, and can be visibly exemplified on the surface of the ground, when the quartzose parts of the veins, persistent against denuding agencies, appear as steep ridges, of imposing height, which the eye can follow in the distance. Sometimes they stretch out like a gigantic wall irregularly notched, sometimes they rise in needles. In depth, the veins are prolonged indefinitely, and mining excavations, however deeply they may be carried, fail to reach their lower limit.
At first sight, metalliferous veins contrast in their mineral composition with the incasing rocks, to whatever category they may belong, even when they are welded to them. They are formed of very different minerals; and it is necessary to distinguish in them the useful substances or minerals, and the stony matter or gangue. The last very often occur in a decidedly predominating proportion, and much that is unforeseen in the returns of a mining operation results from their variations in quantity and richness. The various substances constituting veins sometimes assume a symmetrical disposition in respect to the two walls, showing that they result from deposits made successively one upon another, as happens in a crystallizing pan or in fountain pipes that are incrusted with stony substance.
Metalliferous veins are rarely isolated, but usually form systems or groups, connected by a bond of parallelism and by similarity of composition. They occur exclusively in regions that have suffered dislocations, of which they appear as if they were a consequence. The constitution of the soil of France well brings out this correlation. While veins are wanting in the districts in which the beds have nearly preserved their original horizontality, they are found by thousands, although of inferior richness, in the central plateau, the Vosges, the Pyrenees, and the Breton peninsula. They often border upon the eruptive rocks, with which they are visibly connected as if by a bond of relationship. Many countries famous for their metallic riches, like Cornwall, Hungary, and the State of Nevada, furnish striking examples of this last alliance.
Metalliferous veins betray their origin by their forms and the independent manner in which they cut rocks of every kind. Their formation is due to large vertical breaks, called faults, which have given an outlet to the substances and have ultimately been filled with them. The concomitance of the veins and of grand dislocations sufficiently testifies that metallic matters and their gangues have been brought from down up, that is, from the deeper regions of the globe toward the surface. From this fact, it was at first inferred that the ascension of the vein-filling minerals was accomplished by sublimation or at least by fusion, but this has been shown by many circumstances to be inexact. The specimens of the collections teach of themselves alone that their various minerals have been precipitated one after another, distributing themselves in an order quite different from their degree of fusibility and volatility. It should also be observed that the greater part of them are found outside of the veins, and in circumstances in which they can not have been deposited except by the intermediary action of water.
These views, which the direct observation of Nature had suggested to Elie de Beaumont, have been experimentally verified by De Senarmont. Working with close tubes under pressure, and at a temperature very much higher than that of boiling water, this eminent observer succeeded in reproducing the minerals of the veins from the most common substances—quartz, sulphate of baryta, fluor-spar, iron and copper pyrites, blende, sulphuret of antimony, glance, spathic iron, and carbonate of zinc; all of which laboratory-minerals, in a crystallized condition, quite resembled the analogous natural minerals. The fact of the contemporaneous formation of many of these, as exemplified in the basins of existing springs, as at Bourbonne-les-Bains, come later to confirm and complete this demonstration. Deep fractures or faults, which so numerously furrow the crust of the earth, have therefore endured various destinies in the series of the ages. Some have remained empty, or have been filled only with fragments detached from their walls. Others have furnished a way of exit for fluid eruptive rocks, basalts and porphyries, for example; and, finally, there are those with which we are now concerned, which have served, by the intervention of water, as channels for metalliferous emanations.
These emanations have not been borne exclusively into faults. Sometimes they have filled interstices of irregular and various forms, thus constituting ore-bearing masses, now adjoining eruptive rocks, as if they had followed them, now incased in stratified beds. Whatever their forms, these masses are often in relation with faults which have served as vents for emanations, partly watery, from the interior of the earth.
Among the metalliferous deposits of the last category, some, still better than the veins, demonstrate the intervention of mineral or thermal waters. The masses of hydrated peroxide of iron, frequent at Berry, where the Romans mined for them, and in Périgord, Lorraine, Franche-Comté, and other districts, have been attributed, with much probability, to the presence of gaseous springs, in which iron was dissolved as a bicarbonate. The form of globules with concentric laminations, or pitholiths, which they affect, strikingly resembles the little spheroids of carbonate of lime that are deposited every day in the basin from which the thermal springs of Carlsbad gush and whirl. At times we may recognize clearly that solutions of peroxide of iron have acted upon the limestone which they bathed, for they have gradually corroded it. This chemical action has also been exercised on animal and vegetable matters. At many places in Alsace the mineral contains minute fibrous fragments consisting of woody remnants, or the wood, without losing its texture, has been completely replaced by peroxide of iron and quartz.
Nothing is clearer than the intervention of subterranean waters in the origin of many masses of calamine, in which zinc occurs in the condition of a carbonate and a hydrated silicate, at Veielle-Montagne, for example, not far from Aix-la-Chapelle. The mining works have enabled us to recognize and follow the channels of the generating springs in all their details. The calcareous walls between which they made their way have been attacked, and, as we have just seen to have been done with peroxide of iron, zinc mineral has been gradually substituted for carbonate of lime. The springs that held the mineral in solution issued from faults, and insinuated themselves into the permeable strata, flowing upon the surface of the impermeable beds. Vestiges of fossil shells, sometimes including the minerals of zinc and lead, in Westphalia, for example, likewise attest the substitution of metallic combinations for limestone. The lead and silver mines of Laurium, one of the principal sources of Athenian wealth, which figured in the budget of that state from the year 520 b. c., have revealed, perhaps still more evidently, in their vast excavations, the same processes of Nature.
Similar instances occur in many other countries. We cite in France the various calamine beds on the circumference of the central plateau; and, in the United States, in the Rocky Mountains, the important beds that have given rise to the towns of Eureka and Leadville. Notwithstanding local differences, all these masses of calamine present striking analogies, quite independent of the age of the beds in which they are spread. The metallic sheets are always in the same relation of situation with respect to the permeability and chemical nature of the rocks as they would be to-day, if the metalliferous waters were continuing to flow. It is thus made possible to determine exactly all the ruling conditions of these ancient zinc-bearing streams.
Phosphorus is most usually found in the crust of the earth in the state of phosphate of lime or phosphorite. It is extracted for the wants of agriculture from certain layers of the stratified beds and in the cretaceous formation, particularly in the beds called the gault, the same as those from which flows the water of the well of Grenelle. This mineral has been worked very actively since 1855 in several of the departments of France, in England, Bavaria, North Germany, Russia, Spain, and Poland. It also exists in remarkable quantities in the Jurassic. It often, in these beds, contains animal forms, as of bones, indicating that it has passed through life. But when it appears in eruptive rocks and metalliferous veins, its origin is wholly independent of the action of organized beings. Like the metals, the phosphorus now contained in the sedimentary beds originated chiefly from the interior reservoirs of the globe, whence it has also been brought by the agency of thermal springs. This is illustrated in the important beds of Estremadura, where phosphorite associated with quartz constitutes numerous vertical veins, which have been filled up from below. Incidentally, the substance has penetrated the calcareous strata, and has assumed the forms of fossils within them, thus bringing a new proof of aqueous precipitation.
Still more frequently than any other substance, is quartz distributed in large veins. The granitic plateau of France, Brittany, the Vosges, and the Pyrenees presents numerous examples of it. These veins can often be perceived from a considerable distance. Besides crystallized quartz, they frequently contain parcels of metalliferous minerals, and thus represent transitions toward metalliferous veins proper. The ribboned texture of chalcedony and agate, which abound in these veins, and the manner in which they are related to deposits of precisely the same nature, inclosed in adjoining strata, confirm the view of their aqueous origin and permit their age to be determined. This is exemplified in the department of the Loire, where these veins are thus associated with porphyry, and at the northern point of Morvan; and in the remarkable quartz-veins of the Sierra Nevada, in California, which are auriferous at some points. Comprised within a zone some nine or ten miles wide, they extend from north to south along the chain for nearly two hundred miles. One of the most considerable of these, "the great quartz-vein," can be followed over more than thirty-five miles.
In fact, all these outflowings of quartz and connected minerals, whatever may be the diversity of their forms, in veins, masses, or strata, attest, not less authentically than the metalliferous beds, the intervention and generative power of subterranean waters which have been long since exhausted. We shall next see that waters sufficiently superheated deposit as quartz-crystal the silica which they hold in solution. It is thus explained how this mineral has in a certain way become the binder-up of the fractures of the terrestrial crust.—Translated for the Popular Science Monthly from the Revue des Deux Mondes.