Popular Science Monthly/Volume 32/March 1888/Underground Waters as Social Factors
|UNDERGROUND WATERS AS SOCIAL FACTORS.|
By Professor G. A. DAUBRÉE,
MEMBER OF THE FRENCH ACADEMY OF SCIENCES.
FROM the most remote times, the beneficent springs that jet from the interior of the earth have excited the gratitude and admiration of men. Like the sea and rivers, they have been deified by the peoples of the Indo-European family; and the worship that has been given to them, and the fables with which superstition has invested them, express the degree to which popular imagination has been struck by their mysterious origin, their inexhaustible flow, and their secret properties. The Greeks attributed to the fountain of Dodona, in Epirus, the faculty of discovering hidden truths and uttering oracles. The fountain of Egeria was supposed to possess the same power, and was intrusted to the guardianship of the Vestal Virgins. The fountains of Castalia, on the flank of Parnassus, and of Hippocrene, near Helicon, were believed to communicate the poetic spirit.
The Gauls had special veneration for the springs to which they went m search of health. The old romances of chivalry in their fancies of a fountain of youth, where spent forces and lost charms could be recovered, were only reproducing a myth of old Greece.
The perennial nature of springs, which was for a long time regarded as a sacred mystery, was also their most striking characteristic to those who sought to explain it without reference to religion and poetry. According to Aristotle's idea, which was adopted by Seneca and prevailed till the sixteenth century, "the interior of the earth contains deep cavities and much air, which must necessarily be cooled there. Motionless and stagnant, it is not long in being converted into water, by a metamorphosis like that which, in the atmosphere, produces rain-drops. That thick shadow, that eternal cold, that condensation which is disturbed by no movement, are the always subsisting and incessantly acting causes of the transmutation of air."
Simple and manifest as it appears to us now, the origin of springs was late in being recognized. Vitruvius suspected it, in his work on architecture; but it was Bernard Palissy who, after long studies on the constitution of the country in which he lived, overthrew the ancient fancies. According to his "Admirable Discourse on the Nature of Waters and Fountains, both Natural and Artificial," springs are generated by the infiltration of rain-waters or melted snow toward the interior of the earth, through cracks, till they reach "some place having a bottom of stone or contiguous rock." Palissy further sought means for establishing artificial fountains "in imitation of nature and as nearly approaching it as possible, by following the method of the sovereign Fountain-maker." He added the profound thought, which lies to-day at the foundation of experimental geology, "It is impossible to imitate Nature in anything, except we first contemplate her effects, and take her for our pattern and example." Hence we understand why springs are inexhaustible, because they are unceasingly renewed by the play of permanent forces; they result from a circulation which is in some respects symmetrical with the great aerial circulation of water.
Violent phenomena, like earthquakes, have certainly the prerogative of exciting the imagination. But other phenomena, though they take place slowly and in silence, are none the less worthy of interest; of this character are the mechanism and the fruitful action of the subterranean waters, of which springs are the exterior manifestation. Aside from their usefulness to man, the importance of the study of them is all the greater in that their work is not alone applicable to the present time. Since the crust of the earth has existed, and during all the periods of its development, the water circulating within it, sometimes at very high temperatures, has produced considerable and varied effects, which have in one way or another durably registered themselves, and the explanation of which is facilitated by recent experiments. It is, in fact, this incessant circulation which has engendered a large number of mineral species. The present functions of underground waters will first engage our attention, the examination of their part in the formation of minerals in ancient epochs being reserved for future studies.
As the course of rivers depends on the exterior contours of the soil, so is the régime of subterranean waters an immediate consequence of the nature and mode of arrangement of the masses through which they move.
Except for a very thin covering of vegetable soil, which is a kind of epidermis, the crust of the earth is composed of materials to which the name of rocks is applied, even when, like sand and clay, they are of little coherency. All of these masses have been formed successively, during periods of extremely long duration, and in the midst of conditions of which they bear in themselves the characteristic marks. They are veritable monuments, which delineate in their essential traits the successive revolutions of our globe.
The rocks constituting the greater part of the continents are called stratified, because they are divided into large parallel layers, to which are given the name of strata or beds. It is certain that the rocks of this category, whatever their composition, have been formed in the seas or lakes by sediments and organisms: a sure proof of this truth is furnished by their pebbles and sands, the origin of which can not be different from that of the present deposits of the ocean; the innumerable remains of fossilized marine animals are a still more eloquent testimony to it; and the disposition in beds completes the analogy with contemporaneous sediments. All of these formations may be traversed by mineral masses, disposed in more or less vertical irregular veins, which are usually contrasted in character to the incasing parts. Having risen from very deep regions, they are designated as eruptive rocks.
Some among these various materials are impervious to the passage of water. One of the most so among them is clay, a very abundant hydrated silicate of alumina, which, mixed with carbonate of lime, is also abundant as marl. Granite, and similar rocks, such as the schists, of which slate represents a well-known variety, have the same property, provided the fissures that traverse them are not too open. Thus, although the incessant invasion of water constitutes one of the chief obstacles to the miner's work, there are exploitations that keep quite dry in consequence of the impermeability of the incasing masses.
Other materials are easily permeable by water, as we may observe every day in sand and gravel. The same is the case with rocks which, not being themselves porous, are cut and cross-cut by crevices. Many compact limestones give instantaneous passage to water, which is drained away by their crevices as it would be by artificial conduits.
The regime of subterranean waters is exhibited in simple and clear characters in the deposits known as the ancient alluviums, the drift, and the quaternary deposits, which cover most of the continents as with a carpet. Their gravels and sands, usually associated with clays, greedily absorb water into interstices which represent a notable fraction (perhaps a third) of their total volume. Arrested in its descent by impermeable masses, it accumulates and forms a sheet or shallow body, from which it may be seen to exude through all the openings that may be made into it. This sheet has received several common names: as in France, nappe des puits (well-water) and nappe d'infiltration (infiltration-water); in Germany, Grundwasser; in England, ground-water; and in Italy, acqua di suolo, acqua di livello. A Greek term, which is cosmopolitan, is preferable, and it is found in the word phreatic. In a horizontal direction, the phreatic waters may occupy extensive surfaces, even whole countries, like the arenaceous deposits that serve as their receptacle.
An artificial excavation is not always necessary to make manifest the existence of ground-water. It appears in natural hollows of the soil, takes advantage of ravines of slight depth to issue in springs, which are sometimes impetuous and voluminous enough to constitute considerable streams at their source. The great sheet of the plain of Lombardy thus discharges itself into the beds of the rivers which plow the land in such a way that, after the streams have been drained by numerous irrigation-canals, they rise again spontaneously a little farther down, without apparently having received any new supply. The inexhaustible abundance of this interior sheet also receives here an agricultural application which is, perhaps, to the present time, unique. The water which is drawn from it by means of shallow wells called fontanelle is, in consequence of its nearly constant temperature, which is higher in winter than that of the ambient air, eminently suitable for irrigation. By forcing it to flow constantly in a thin sheet over the ground, the peasants are able, in a cold country, to cut their grass in January as in the summer-time. There are more than a thousand of these artificial subsoil springs, occupying a zone about two hundred kilometres long, extending from Ticino to Verona.
All rocks which are penetrable to water by means of fissures are also capable of containing phreatic water. The water in these sheets is not stagnant, but is animated by a slow and continuous motion. Among the facts that prove this we may cite the transportation in the subsoil of impurities like coal-tar, in the same direction, over several hundred metres, in a series of wells, the alignment of which marks the direction of the current. This movement is due to the general incline of the sheet.
In volcanic masses, the scoriaceous dejections and the lava-flows, with their cavities of various dimensions, offer no less facility for infiltrations. Rain-waters penetrate them and reappear lower down. Among the flows of fifty volcanoes in Auvergne, that which issues from the Puy de Gravenoire, near Clermont, gives rise to three springs: first at Fontanat; then at Royat, where they issue from a cave opened in the scoria surmounted by prismatic lava; and at the lower end of the flow the water is discharged under similar conditions to the advantage of the city of Clermont. In the same way, after having formed at Murois those scoriaceous caves to which George Sand has lent an infernal aspect, the long flow of the Tartaret gives out in its course a series of springs, around which several villages have grouped themselves. Thus fire is found to prepare the way for water by creating subterranean conduits for it.
The natural action of the waters which we have studied in the superficial deposits is indicated with equal clearness at a greater depth, in the midst of the stratified rocks. In these last, in fact, certain beds, very penetrable to water, alternate with others which arrest its passage. Whether the beds be horizontal or inclined, the relief of the soil is frequently so gashed that the impermeable support of the filtering and water-bearing stratum crops out and determines a flow by virtue of hydrostatic laws. These natural reservoirs thus produce springs which are permanent, provided successive rains furnish a sufficient supply of water, while they also sometimes simply give place to irregular oozings. These effusions occur not on the continents only, but also in sea-basins.
The sedimentary rocks, in their great thickness, inclose a succession of water-sheets or water-levels occupying distinct stages, and extending, with uniform characters, under whole countries, like the strata to which they are subordinated. It is proper to remark here that by the term water-sheet is not meant a real bed of water, lodged in a cavity, between solid masses that serve as walls to it, but water filling the minute interstices or the cracks of a rock. Continuous and regular in sand, these sheets are usually discontinuous and irregular in limestones and sandstones, in which the water only occupies more or less spacious fissures. When natural issues are wanting, human industry is able, by boring, to make openings down to the subterranean waters, which it causes to jet up to the surface, and sometimes to a considerable height above. The thought of undertaking such works is a very ancient one. The Egyptians had recourse to them forty centuries ago; and they were executed in France, in 1126, at Artois, whence the name of artesian wells has been given to them.
The water-levels of the cretaceous strata, from which the French artesian waters issue, are not always of advantage; but in the north of France and in Belgium they constitute the most formidable obstacle which miners have to encounter in reaching the coal-beds.
A striking confirmation of the theory of the source of supply of the artesian waters has been observed at Tours, where the water, spouting with great velocity from a well a hundred and ten metres in depth, brings up, together with fine sand, fresh-water shells and seeds, in such a state of preservation as to show that they could not have been more than three or four months on their voyage. Some of the wells of the wady Rir have also ejected fresh-water mollusks, fish, and crabs, still living, which must, therefore, have made a still more rapid transit. Caves, in limestone regions, play a part of the first order in the movements of the interior waters. Their presence is manifested at the surface by depressions of various shapes, such as are called "swallow-holes" in the north of England, and "sink-holes" in the United States. These cavities draw in the surface-waters and remove them from sight, to reappear at some other place, oftentimes in exceptionally voluminous fountains. They can be pointed out by the hundred in some parts of France, although only a small proportion of them are revealed by a visible discharge. The internal hollows are often aligned with dislocations of the ground, with which they are connected as effects of fractures, ultimately corroded and rounded off by water. The caves of Baume in the chain of the Jura correspond with a series of tunnel-holes and sinkings from the prolongation of which arises the river Seille. The Jurassic limestone of La Charente is marked by pits of various depths, with yawning mouths, into which the Tardouère and the Baudiat disappear near La Rochefoucauld, to gush out bubbling farther down and give rise to the Tourne. In the departments of the Var and the Maritime Alps, numerous sink-holes (scialets) feed, through secret channels, powerful springs that issue from the sea-bottom not far from the shore. The limestone around Mont Ventoux is riddled in a zone of seventy kilometres by natural wells and unfathomable pits, many of which bear names well known in the local legends. The waters which these rocks have stored up are poured out at their lowest point, and give rise, in a picturesque grotto, to the copious fountain of Vaucluse, which was formerly regarded as a beneficent divinity. Compared with the depth of the rains at different stations in the basin, the mean outflow of the fountain indicates a volume of infiltration equal to about six tenths of the quantity of rain-water. The limestone under the valley of the Loire, at Orleans, is plowed by interior currents from which the water-supply of the city is directly taken. The waters begin to be lost at a point some forty kilometres above the city, and return to the river about thirty kilometres below. The Iton, in the department of the Eure, fails to flow over the surface for several kilometres, and is called the Sec-Iton, or dry Iton; but its waters are reached in their subterranean course by excavations of twenty metres. Similar facts may be observed in all parts of the globe. By a similar kind of drainage the cavernous limestone of the Apennines gives rise to the Aqua Martia, which was brought to Rome b. c. 608 by the consul Quintus Marcius, and which still continues to be of prime importance to the city; "the most celebrated water in the universe," enthusiastically says Pliny, "a franchise of salubrity, one of the benefits granted to Rome by the favor of the gods."
A grain of truth sometimes lies at the bottom of the ancient fictions. Was not the observation of water-courses which are ingulfed and appear again the origin of the fable of the fountain of Arethusa, which the Greeks regarded as the reappearance of the river Alpheus? After a pursuit from Peloponnesus across the Ionian Sea, it was supposed to overtake the nymph personified in this fountain at the moment when it gushed out near Syracuse.
Besides moving through the interstices, fissures, and cavities of the crust of the globe, water exists everywhere in another state, in which, although quite invisible, it is of hardly less importance. All rocks, including the most compact ones, inclose water within their pores, however minute they may be, where it is held by capillary attraction, and is not apparent to our instruments of highest magnifying power. But it may be disengaged by desiccation, when the rock will be found to have lost a sensible fraction, some ten thousandths at least, of its weight. At the same time some of the qualities of the rock are modified; for workers in slate, sandstone, and other rocks find it a matter of great difference, in the facility of their tasks, whether these stones still hold their quarry-water or have been dried in the air. The Romans availed themselves of the porosity of the onyx to soak it in certain liquids which would enliven the color of the stones that they used in their cameos. Under this form of intimate latent impregnation, though relatively in extremely feeble proportions, water is incorporated in the deep masses of the terrestrial crust, in immense absolute quantities, •which are perhaps commensurable with the volume contained in the seas on the surface.
Various physical circumstances, such as the configuration of the soil and the vicinity of rivers or of the sea, have always had a great influence on the grouping and destinies of populations. The presence of particular minerals has had a similar determining influence. The useful metals, coal, and petroleum, have caused important cities to be created and to grow—as Virginia City, Leadville, Eureka, Oil City, and Petrolia, in the United States.
Underground water, a more commonplace substance, which has attracted much less attention, eminently deserves to be considered when we seek for the natural causes that have contributed to the formation of large agglomerations of men. Pliny the Elder remarked that mineral waters had peopled the earth with new cities and Olympus with new gods. Recent excavations in Gallic villages have brought to light vast piscinæ marble monuments, theatres, statues, mosaics, and other unmistakable vestiges of a vanished luxury, as at Nereis, Vichy, Plombières, Bagneres-de-Luchon, and Aix in Provence. Universal celebrity attaches to Baiæ, where every Roman was ambitious to have a country-house, and the ancient splendor of which is attested by ruined temples and palaces. The word "bath" and its equivalents in different languages form the roots of many place-names. Those who lived by the manufacture of salt have necessarily grouped themselves around the marine springs from which their towns have received names embodying the root-form of the word salt or its equivalents—Salins, Chateau-Salins, Salival, Marsal, Salies, Salat, Saleons, Saltz, Saltzbronn, Salzhausen, Salzungen, Salzburg, Hall, Reichenhall, etc.
So populations tend to group themselves around copious fountains of fresh, potable water, where the frequency of the villages is often in striking contrast with the sparsity of the settlements in more arid localities. These contrasts result from the constitution of the soil. The junction of the Jurassic formation with the impermeable clays of the lias on which it rests is marked by a line of frequent springs, around which habitations and villages stand thickly, as in the vicinity of Metz; while the absence of masses of population on the neighboring limestones, where water is reached only at a great depth, is matter of special remark. This abundant and regular water-supply is found under these conditions, and at the same geological level, in many parts of France, England, and Germany, where it always attracts thick populations.
While the cretaceous table-lands of Champagne lack springs, they flow out in abundance at the foot of the cliffs. Many of them bear the generic name of "Somme," because they are the origin or the top of a brook—as Somme-Suippe, Somme-Vesle, Somme-Tourbe, Somme-Bionne, and some fifty others. Around these springs, not far from arid and almost desert regions, are situated villages which gratefully borrow their names from the waters to which they owe their life; a kind of paternity which is not rare. In France numerous places, such as Fontainebleau, Fontanat, Fontanille, Fontvannes, Fontoy, Fontenoy, and Fontanay, derive their names from the Latin words fons and fontanetum, and some names are repeated many times. The same fact is apparent in Italy and Spain, where more than eight hundred names have the same origin; also in Germany, where the forms Brun, Bronn, and Born occur. The city of Paderborn is built upon forty springs which give rise to the Pader. Not far away is Lippspring, a word expressing the origin of the Lippe. This word "spring" in England and the United States, and "ain" in the north of Africa, convey the same idea. Eau, Aix, Aigues, Acqua, Aqua, and Waters, figure likewise in many words, with the signification of spring-water.
Nothing more clearly exemplifies the attractive force of subterranean waters than those collections of tilled-lands and habitations among the oases that are scattered over deserts. Strabo compared the Sahara to a panther's skin, the ground of which is the desert, while the black spots correspond with the somber verdure of the oases. These spots are aggregated in groups, like archipelagoes in the sea, in a zone of that desert which is confined between the thirtieth and thirty-seventh degrees of latitude. Algeria contains more than three hundred of them. Certain rainy regions, like Mount Atlas, send water by underground routes, which reaches them through sandy beds contained between impermeable strata of clay, and is thus protected against evaporation. Sometimes, when the water-sheet is not very deeply situated, it is utilized by digging holes where the roots of the palm-trees have grown down toward it. At many other points the water, impelled by the pressure upon it, opens a passage to the surface, and gives rise to springs or natural artesian wells. These appearances of water in the midst of arid and desert steppes constitute centers around which a life has developed itself under the protection from the sun and the simoom afforded by the palm and fruit trees. From a very remote epoch the natives have known how to imitate nature by opening issues for the interior water-sheet; but the perilous labor of digging was not inviting to workmen, and m-any of the ancient wells have become obstructed. The villages have become depopulated for want of water, the oases have shrunk, and gradually the desert has resumed the possession of the soil.
The first well bored after the French occupation of Algeria, at Tamema, spouted on the 19th of June, 1856, and was blessed by a marabout under the name of the Fountain of Peace. Numerous other borings revealed the existence of an underground river lying for a distance of a hundred and thirty kilometres beneath the wady Rir. At present one hundred and seventeen bored wells, together with five hundred native wells, give exit, from a mean depth of seventy metres, to a volume of water fully equal to that of the Seine, at Paris, in its lowest stage. Cultivated lands have been created, the native population has doubled, and the value of the oases has more than quintupled; a complete transformation of this part of the Sahara has been effected, by the agency of underground waters, within thirty years. Most of the manufacturing cities of the middle and north of England are situated upon the New Red Sandstone, where, besides excellent building-stone and proximity to the coal-fields, they enjoy the inestimable advantage of the presence of inexhaustible reservoirs of water purified by natural filtration, and easy of extraction. Belfast, in Ireland, is similarly situated. The water-bearing gravels are particularly worthy of attention from this point of view. With the inexhaustible and easily accessible provisions of water which they contain, they present to man an almost infinite expansion. This accounts for the existence upon these deposits, from most ancient times, of numerous important cities and capitals, like London, Paris, and Berlin. But in London the arenaceous and phreatic stratum has limitations which were opposed for several centuries to growth in particular directions. For a long time, according to Mr. Prestwich's observations, the population, by an instinct easy to understand, continued strictly concentrated on the principal water-sheet, and on a few isolated strips of gravel, as at Islington and Highbury. In the suburbs, likewise, the thick populations were collected on the larger gravel-beds rich in water, while in the same region, although the soil was everywhere cultivated and productive, the houses were very sparsely scattered. But the situation has greatly changed within the last seventy years, a supply of water having been brought from a distance, and the city has spread very rapidly over the clayey grounds.
Numerous populations still depend on wells for their drinking-water; Lombardy and Venice, with two million inhabitants; the extensive plains of Hungary; at least half of the German Empire; a part of the Russian Empire, seven times as large as France, and populated by about twelve million souls; and, according to the explorer Abbé David, the whole of the great northern plain of the Chinese Empire, containing more than a hundred million inhabitants. Besides these vast plains which represent more than a third of the continents, there are numberless valleys, with water-bearing subsoils, which have attracted to themselves aggregations of men. We can then affirm that a very important fraction of the human race depends for its principal drink wholly upon water which is furnished by the phreatic strata of ancient or modern alluviums. We never find such concentrations of inhabitants in countries where the soil is formed from granitic and schistose rocks, without being covered by disaggregated materials. These rocks permit water to descend to their interior only with the greatest difficulty. Springs are likewise weak among them, but very numerous; and the population is by force disseminated among them in isolated houses, and constitutes at most only little hamlets. The inhabitants, thus dispersed, differ in manners and character from those whom an indefinite abundance of underground water has drawn together and condensed into large groups.
Such are some of the social influences of subterranean waters, the importance of which has not always been fully appreciated.—Translated for the Popular Science Monthly from the Revue des Deux Mondes.