Popular Science Monthly/Volume 34/February 1889/Underground Waters in Rock Transformations
By Prof. G. A. DAUBRÉE.
EVIDENTLY different actions from those which engendered the metalliferous deposits have propagated themselves through considerable masses, and have impressed a peculiar stamp upon them. The rocks that have been marked by such actions exhibit at once the characteristics of the sedimentary rocks and some of those of the eruptive rocks. While retaining the stratified disposition which they owe to their sedimentary origin, they are often studded with crystalline and anhydrous silicates, which they would not have contained if they had continued in their normal state. These rocks, of a somewhat mixed nature, are called metamorphic, a term given in allusion to the changes which they have undergone since they were deposited, and to which they owe their present appearance. Stratified rocks have sometimes acquired these characters in the vicinity of eruptive rocks. In several localities of the Tyrol, the Triassic limestone in contact with melaphyre has been transformed into white marble for a thickness of more than five hundred metres, while pyroxene, spinel, tourmaline, and other crystalline minerals have been developed at the same time.
Clay schists have suffered mineralogical transformations in proximity with granitic eruptions. Even half a century ago, De Boblaye pointed out the presence, in Brittany, of fossil shells among the schistose rocks, which also contained, in testimony of the heat to which they had been subjected, large crystals of silicious minerals, as of andalusite or made, and staurotide. The groupings of the latter species in the form of a cross have been long remarked, and have caused the name of croisette to be given to it. These remarkable modifications of the schists, which constitute a sort of radiation around the granitic flows, extend to distances varying from a few hundred metres to three kilometres. The heat to which the strata have been subjected by the intrusion of the eruptive mass is undoubtedly one of the causes of it; but the watery emanations which accompanied the eruption of the granite, and which are revealed to us by inclusions in the mass, attest that water has played a no less important part in it.
There is, however, something still more remarkable than this in the phenomena of metamorphism. Sedimentary rocks, occupying whole regions, bear evidence of profound modifications, without its being possible to discover the slightest eruptive cropping out. One of the most common examples of this phenomenon is that in which clay rocks have become phyllads. The rocks of that name, although they consist essentially, like the clays, of silicates of alumina, differ from them in their cohesion. They refuse to mix with water. The strata of the Ardennes, the Taunus, and other regions of western Europe, in which this mineralogical condition was first verified, belong to the most ancient geological epochs; and from that fact this crystalline texture was for a long time regarded as exclusively appertaining to sedimentary deposits of a very remote age. Hence the name of transition beds which was given them. It was thought that in the sea in which these matters were deposited, following the primitive or crystalline beds, there continued to operate a chemical precipitation of silicates which were mingled with arenaceous and calcareous deposits. It was subsequently recognized that this half-crystalline condition resulted from a transformation posterior to sedimentation.
The opinion that the mineralogical condition of these beds is not a necessary consequence of their antiquity, receives confirmation from the fact that formations in other countries, also belonging to the most ancient systems, do not participate in these crystalline characters, but in their argillaceous rocks are similar to those which are found in recent formations. This is the case in Sweden, Russia, the United States, and Canada. But it has been observed in those places that the strata have not been strongly dislocated as in the regions we have just been speaking of, but have retained their original horizontality. To this circumstance they doubtless owe their preservation. The mineralogical contrast between formations of the same age corresponds, therefore, with an essential difference in their bearing.
There are countries where formations of less antiquity have also suffered profound transformations. The Alps afford fundamental data on this subject. In the face of the rocks of different ages—Carboniferous, Triassic, and Tertiary—which enter into their composition, one is surprised at the special physiognomy which each one of them presents as compared with what we observe in beds of the same age in other regions, where they have remained horizontal. A general influence has, therefore, acted upon a part of the vast region of the Alps. It has affected rocks of every epoch, even those of the Lower Tertiary—that is, a series of beds many thousand metres thick—and that, although eruptive rocks are very rare in it.
With the mineralogical changes which we have just noticed is associated a modification of texture that depends on the same cause. It is well known in the slates as the schistose or laminated structure. The fissile rocks which it characterizes have the property of detaching themselves in thin plates—that is, of cleaving in certain directions. Observations made in various countries have demonstrated the important fact that the planes of cleavage are quite distinct from the planes of stratification. Instead of being parallel to the layers, they are frequently oblique, and—what is still more conclusive—while the planes of stratification have been bent and exhibit a variety of inclinations, the planes of cleavage pursue a regular direction, regardless of the most pronounced inflections, and remain constantly parallel to one another. This independence shows, besides, that the planes of cleavage were produced, not only after the beds in which they are manifest were deposited, but also after they had lost their primary horizontality. The schistose disposition, very frequent in the most ancient fossiliferous rocks, sometimes persists in more recent formations, when they have been subjected to energetic dislocations. In many localities of the Alps slates are quarried into the Tertiary formation.
An important characteristic of the schistose rocks is the considerable deformations which the fossils in them have received, as is seen in the trilobites of the Angers slates. Not less frequently, the molluscan fossils called belemnites have been broken up and had their segments more or less scattered.
Since the schistose structure has been found to be independent of the stratification, the cause of a geometrical disposition so remarkable and so general has become the subject of various hypotheses. It has been successively attributed to electric effects, to terrestrial magnetism, to the heat of the globe, and to a beginning of crystallization. Exact observations, however, teach us that the cleavage of stratified beds is related on the one hand to the actions which have deformed the fossils in the same strata, and on the other hand to the axes of the warping and the great lines of dislocation. The phenomenon should most probably be attributed to mechanical action.
The demonstration has been confirmed by some very simple experiments. Clay under compression assumes a leafy texture; but, for this, it must have a certain degree of plasticity. If too dry, it crumbles; if too wet, the laminæ, while they are formed, are not separable. I have got more decisive results from forcing clay to flow, in a jet, under hydraulic pressure. In this case, very well defined leaflets are produced, and that upon bands of several metres, in the direction of the pressure and the movement. All of these artificially laminated pastes resemble natural schistose rocks in their fracture. In these various flows of the plastic mass, the neighboring particles do not advance uniformly. The differences in the velocities which they acquire cause them to slide upon one another; and the schistose texture, the direct consequence of this sliding, is, we may readily conceive, necessarily determined in reference to the direction of the flow. The deformations of fossils and the drawing out of belemnites have been reproduced in this way, and thus experimentally explained.
We shall now consider how the fundamental facts of metamorphism imply the necessary action of subterranean waters. The mineralogical modifications peculiar to the phenomena have incontestably taken place at a higher temperature than now prevails on the surface of the globe. We base this conclusion upon the analogies of these beds with the eruptive rocks, and especially upon the presence of numerous anhydrous silicates, which form one of their most remarkable features. The proper heat of the globe decreasing from the deeper parts toward the surface, the sediments deposited in the ocean, at the relatively low temperature that reigns there, should, when they have been covered by other strata, acquire a higher temperature by reason of their greater distance from the radiating surface. The superposition of masses as heavy as are those of some of the stratified beds has often been enough to determine, after their deposition, a considerable heating up of the lower masses, especially at periods when the increase of heat downward may have been at a more rapid rate than now. Thus the regular propagation of the heat of the globe has been competent to act upon entire formations.
There is, however, another source of heat, at once more immediate and more energetic, for the transformations with which we are occupied, although it has been long misunderstood. Heat is engendered by the mechanical actions that have left their marks at numerous spots on the crust of the globe. Instead of preserving the horizontal position which they assumed when they were deposited, these beds have often been thrown up, folded, and contorted in various ways; and the resultant dislocations are observable through several thousand metres of thickness. At every step, in the Alps, for example, in the face of escarpments where the rock shows itself to the quick, the least observing eye is attracted by the boldness of the inflections, and the mind pauses stupefied before the grandeur of the forces that have produced such effects. Not all the labor put in play in these colossal upthrows has been employed in actions purely mechanical. A part of it has been transformed into heat, and it is the effects of this heat that we have been studying.
Experience has come to confirm the last induction also. Clay has been forced to flow either between cylinders like those of iron-rolling mills, or under trituration in malaxating tubs, such as are used in some brick-yards. In either case the rock is considerably heated up after a very short time, without subjecting it to any material pressure. In these operations the heating is greater in proportion as the clayey part is harder and more resistant. We have then reason to believe that in nature, when rocks more coherent and less plastic than ordinary clay have been submitted to mechanical actions powerful enough to determine an interior movement, even if it be of little amplitude, they will be found in conditions still more favorable to their being heated. It has, therefore, been enough for argillaceous masses to undergo a lamination under the effect of dislocations in the crust of the earth for their temperature to be notably raised.
But heat alone, however intense it may be, can not explain the most characteristic effects of metamorphism, nor the uniformity with which they have been produced over considerable spaces; for the conductivity of the rocks is extremely weak. Then, contrary to what would be the case were the action simply a calorific one, the effects have not always been most energetic in the parts in contact with the eruptive rocks. The water included in all the rocks, whether in their pores or in combination, has of necessity intervened as an auxiliary to the heat. The nature of the minerals produced, of the hydrated silicates, like chlorite, for example, no less than the uniformity of their disposition in vast masses, denote the intervention of this interior water. Thus, in this order of geological phenomena, when we might have believed that heat, accompanied by certain chemical actions, was the sole agent, subterranean water has also had its part to play.
This conclusion regarding the fundamental cause of metamorphism, although it had been justified by observation, still needed an experimental sanction. For that, the investigator should put himself in circumstances as nearly as possible like those in which Nature seems to have acted, and obtain the reproduction of characteristic minerals. I have tried to realize this. The principal difficulty in operating under the enormous pressure acquired by the vapor of water when the temperature approaches the point of dull redness is to find walls capable of resisting it. Water having been placed in a glass tube, which was then sealed by a lamp, this tube was introduced into a second tube of iron, with very thick walls, which was also closed, but not without difficulty, at the forge. In order to counterbalance the tension of the vapor in the interior of the glass tube, which might cause it to burst, care was taken to pour water outside of this tube, between its walls and those of the iron tube. The apparatus was set upon the dome of the furnace of a gas-factory in contact with masonry at a dull-red heat, in a thick bed of sand, where it remained for several weeks. Under these conditions, explosions of extreme violence took place. The most strongly resistant tubes were thrown into the air, bursting with a noise comparable to that of a cannon-shot. It was not possible to multiply the proofs to the extent that was desirable; but those that were made were sufficient to reveal facts quite different from those which we had deduced in laboratories under ordinary conditions.
The water acted very energetically upon the glass, which underwent a complete transformation, in composition and appearance. It was replaced by a white mass, quite opaque, resembling porcelain, with swellings and blisters, the results of softening. There had been developed, at the expense of a part of the substance, numbers of minute crystals, colorless and limpid like rock-crystal, with which they are identical, even to small details in the forms. These artificial crystals appeared, now isolated, now grouped into geodes which it was impossible to distinguish, except for the difference in dimensions, from those of nature. Another product of the same experiments deserves no less attention. It is pyroxene, which appears in little green, brilliant, and transparent crystals, exactly like those of the Alps. For the first time an anhydrous silicate had been seen to be produced by the action of water.
The acquisition of another kind of power by water under such conditions was exemplified by the conversion of pine-wood into a bright and hard black substance resembling anthracite, and consisting simply of carbon associated with small quantities of volatile substances. It was shown, by its granulation in small globules, to have passed, in the water, through a kind of fusion. The reactions from which these products resulted are all the more interesting because they were obtained with a very small quantity of water, hardly equal to a third of the weight of the metamorphosed glass. Furthermore, the new products crystallized at a temperature considerably lower than their point of fusion. It is thus proved that water highly superheated acquires an energy that was unknown to belong to it. It destroys combinations that were reputed to be stable, and in the presence of which it was regarded as inert; and it composes others, among which are the anhydrous silicates. The production of these silicates in the crust of the earth escapes our observation, because it requires a temperature greatly superior to that of boiling water. But it must be going on in the depths of the rocks, where there is no lack of imprisoned water, nor of temperatures and pressures incomparably higher than those of our most potent experiments. It is hardly necessary to say more concerning the application of these synthetic results to questions concerning the metamorphic transformation of entire regions.
Other facts in nature are explained in these experiments. First, they teach us the origin of quartz in the crust of the earth, where it appears everywhere and in the most diverse bearings. Have not the veinlets of this mineral, for example, which traverse quartzites and phyllads in every direction, probably separated themselves at the expense of the incasing rock, and in the presence of water and heat, just as the quartz was extracted from glass? An action of the same kind is recognizable in the metalliferous veins. Sometimes the temperature there is high enough for the silicates also to be generated. The veins in which the green emeralds of Peru are found associated with crystallized quartz, calcite, and pyrites are evidently of aqueous formation.
Thus, by going back to ancient periods, we have seen how numerous species of minerals are produced concerning whose origin the observation of facts occurring to-day can not inform us. These numerous minerals, whether metalliferous or stony, occurring in various formations, are the final result of the work of water, which is found in some way stereotyped in them. We have thus succeeded in discovering the intimate operations of that liquid in laboratories which it abandoned long ago, in fissures of greater or less magnitude, and in blisters or the simple pores of the rocks. We are instructed concerning the manner in which it circulated, by vestiges of different kinds which permit us to reconstitute the various circumstances of its course.
The external features of an organized being make its constitution known only in an incomplete manner. An adequate anatomical study must penetrate to its interior organs and tissues. Thus, existing thermal springs, even if we take care to scrutinize in the most careful manner the constitution of the country and the conditions where they issue, do not suffice to reveal their economy with precision. Their constantly flowing columns of water, even when they are not accompanied with irrespirable gases, prevent our reaching their channels of ascent. In the very exceptional cases in which it is possible to penetrate below their orifices of emergence, as at Bourbonne and Plombières, the curious facts which we observe cause us to regret that we can not descend lower. Nature seems to have desired to withdraw from our sight the actual workings of subterranean waters, especially when they are engendering minerals. Water is not more rare than heat in the masses of the interior of the globe. Even when it does not circulate in natural channels, it is at least present, held imbibed in the most compact rocks. In clays, although combined, it is not less susceptible of acting chemically than in the free condition. Thus, what we have obtained only with many difficulties in our experiments, the action of superheated water, is found vigorously exemplified everywhere in the interior of the rocks, where the effective resistance to enormous pressures permits the realization of more complete results than are possible with the fragile apparatus of our laboratories.
The circumstance that heat stored in masses of so little conducting power as stony substances is preserved for a very long time, is eminently favorable to chemical combinations and to crystallization. Nature possesses another superior advantage over man in having extremely long lapses of time at her disposal. The importance of this advantage, in the application in which we are now regarding it, appears plainly from what has occurred in the Roman masonry of Plombières. Besides this, reactions which go on slowly do not require so high a temperature as those that are of shorter duration.
The study of waters in their course and effects in ancient epochs thus seems to complete the history and broaden the view of their subterranean works. Here, then, a real exchange of light takes place. The past illuminates the present as much as the present illuminates the past. There is nothing, moreover, to prove that phenomena of this character do not continue down to our own days. We have a right to believe that similar actions are still going on, but in interior regions beyond the reach of our powers of observation. Superheated water, which betrays its existence through. thermal springs and volcanic exhalations, to all appearance is slowly and silently engendering considerable and permanent effects in the interior of the globe, and is giving birth to various minerals as it did in former days.
In the same way as in our organism all the parts of the body owe their development to the support which they receive from the circulation of the blood, so in the crust of the earth, water, by its incessant subterranean circulation and its predominantly chemical work, accomplishes a kind of vital action which is perpetuated through ages. May we not justly apply to these mineralogical and geological results, so worthy of our curiosity and derived from a single cause, Leibnitz's favorite epigraph, In varietate unitas?—Translated for the Popular Science Monthly from the Revue des Deux Mondes.