The Earth As Modified By Human Action Part 35

Especially destructive are they when, by any accident, a cavity is opened into them to a considerable depth, thereby giving the wind access to the interior, where the sand is thus first dried, and then scooped out and scattered far over the neighboring soil. The dune is now a magazine of sand, no longer a rampart against it, and mischief from this source seems more difficult to resist than from almost any other drift, because the supply of material at the command of the wind is more abundant and more concentrated than in its original thin and widespread deposits on the beach. The burrowing of conies in the dunes is, in this way, not unfreqnently a cause of their destruction and of great injury to the fields behind them. Drifts, and even inland sand-hills, sometimes result from breaking the surface of more level sand deposits, far within the range of the coast dunes. Thus we learn from Staring, that one of the highest inland dunes in Friesland owes its origin to the opening of the drift sand by the uprooting of a large Oak. [Footnote: De Bodem van Nederland, i. p. 425.]

Great as are the ravages produced by the encroachment of the sea upon the western sh.o.r.es of continental Europe, they have been in some degree compensated by spontaneous marine deposits at other points of the coast, and we have seen in a former chapter that the industry of man has reclaimed a large territory from the bosom of the ocean. These latter triumphs are not of recent origin, and the incipient victories which paved the way for them date back perhaps as far as ten centuries. In the meantime, the dunes had been left to the operation of the laws of nature, or rather freed, by human imprudence, from the fetters with which nature had bound them, and it is scarcely three generations since man first attempted to check their destructive movements. As they advanced, he unresistingly yielded and retreated before them, and they have buried under their sandy billows many hundreds of square miles of luxuriant cornfields and vineyards and forests.

On the west coast of France a belt of dunes, varying in width from a quarter of a mile to five miles, extends from the Adour to the estuary of the Gironde, and covers an area of nine hundred and seventy square kilometres, or two hundred and forty thousand acres. When not fixed by vegetable growths, these dunes advance eastwards at a mean rate of about one rod, or sixteen and a half feet, a year. Wo do not know historically when they began to drift, but if we suppose their motion to have been always the same as at present, they would have pa.s.sed over the s.p.a.ce between the sea coast and their present eastern border, and covered the large area above mentioned, in fourteen hundred years. We know, from written records, that they have buried extensive fields and forests and thriving villages, and changed the courses of rivers, and that the lighter particles carried from them by the winds, even where not transported in sufficient quant.i.ties to form sand-hills, have rendered sterile much land formerly fertile. [Footnote: The movement of the dunes has been hardly less destructive on the north side of the Gironde. See the valuable articles of Elisee Reclus in the Revue des Deux Mondes for December 1862, and several later numbers, ent.i.tled "Le Littoral de la France."] They have also injuriously obstructed the natural drainage of the maritime districts by choking up the beds of the streams, and forming lakes and pestilential swamps of no inconsiderable extent. In fact, so completely do they embank the coast, that between the Gironde and the village of Mimizan, a distance of one hundred miles, there are but two outlets for the discharge of all the waters which flow from the land to the sea; and the eastern front of the dunes is bordered by a succession of stagnant pools, some of which are more than six miles in length and breadth. [Footnote: Laval, Memoire sur les Dunes du Golfe de Gascongne, Annales des Ponte et Chaussees, 1847, p. 223. The author adds, as a curious and unexplained fact, that some of these pools, though evidently not original formations but mere acc.u.mulations of water dammed up by the dunes, have, along their western sh.o.r.e, near the base of the sand-hills, a depth of more than one hundred and thirty feet, and hence their bottoms are not less than eighty feet below the level of the lowest tides. Their western banks descend steeply, conforming nearly to the slope of the dunes, while on the north-east and south the inclination of their beds is very gradual. The greatest depth of these pools corresponds to that of the sea ten miles from the sh.o.r.e. Is it possible that the weight of the sands has pressed together the soil on which they rest, and thus occasioned a subsidence of the surface extending beyond their base?

A more probable explanation of the fact stated in the note is suggested by Elisee Reclus, in an article ent.i.tled Le Littoral de la France, in the Revue des Deux Mondes for September 1, 1864, pp. 193, 194. This able writer believes such pools to be the remains of ancient maritime bays, which have been cut off from the ocean by gradually acc.u.mulated sand banks raised by the waves and winds to the character of dunes.] A range of dunes extends along the whole western coast of Jutland and Schleswig-Holstein, and the movement of these sand-hills was formerly, and at some points still is, very destructive. The rate of eastward movement of the drifting dunes varies from three to twenty-four feet per annum. If we adopt the mean of thirteen feet and a half for the annual motion, these dunes have traversed the widest part of the belt in about twenty-five hundred years. Historical data are wanting as to the period of the formation of these dunes and of the commencement of their drifting; but there is recorded evidence that they have buried a vast extent of valuable land within three or four centuries, and further proof is found in the fact that the movement of the sands is constantly uncovering ruins of ancient buildings, and other evidences of human occupation, at points far within the present limits of the uninhabitable desert. Andresen estimates the average depth of the sand deposited over this area at thirty feet, which would give a cubic mile and a half for the total quant.i.ty. [Footnote: Andresen, On Klitformationen, pp. 56, 79, 82]

The drifting of the dunes on the coast of Prussia commenced not much more than a hundred years ago. The Frische Nehrung is separated from the mainland by the Frische Haff, and there is but a narrow strip of arable land along its eastern borders. Hence its rolling sands have covered a comparatively small extent of dry land, but fields and villages have been buried and valuable forests laid waste by them. The loose coast-row has drifted over the inland ranges, which, as was noticed in the description of these dunes on a former page, were protected by a surface of different composition, and the sand has thus been raised to a height which it could not have reached upon level ground. This elevation has enabled it to advance upon and overwhelm woods, which, upon a plain, would have checked its progress, and, in one instance, a forest of many hundred acres of tall pines was destroyed by the drifts between 1804 and 1827.

Control of Dunes by Man.

There are three princ.i.p.al modes in which the industry of man is brought to bear upon the dunes. First, the creation of them, at points where, from changes in the currents or other causes, new encroachments of the sea are threatened; second, the maintenance and protection of them where they have been naturally formed; and third, the removal of the inner rows where the belt is so broad that no danger is to be apprehended from the loss of them.

In describing the natural formation of dunes, it was said that they began with an acc.u.mulation of sand around some vegetable or other accidental obstruction to the drifting of the particles. A high, perpendicular cliff, which deadens the wind altogether, prevents all acc.u.mulation of sand; but, up to a certain point, the higher and broader the obstruction, the more sand will heap up in front of it, and the more will that which falls behind it be protected from drifting further. This familiar observation has taught the inhabitants of the coast that an artificial wall or dike will, in many situations, give rise to a broad belt of dunes. Thus a sand dike or wall, of three or four miles in length, thrown in 1610 across the Koegras, a tide-washed flat between the Zuiderzee and the North Sea, has occasioned the formation of rows of dunes a mile in breadth, and thus excluded the sea altogether from the Koegras. A similar dike, called the Zijperzeedijk, has produced another scarcely less extensive belt in the course of two centuries. A few years since, the sea was threatening to cut through the island of Ameland, and, by encroachment on the southern side and the blowing off of the sand from a low flat which connected the two higher parts of the island, it had made such progress, that in heavy storms the waves sometimes rolled quite across the isthmus. The construction of a breakwater and a sand dike have already checked the advance of the sea, and a large number of sand-hills has been formed, the rapid growth of which promises complete future security against both wind and wave. Similar effects have been produced by the erection of plank fences, and even of simple screens of wattling and reeds. [Footnote: Staring, De Bodem van Nederland, i., pp. 329-331. Id., Voormaals en Thans, p. 163. Andresen, Om Klitformationen, pp. 280, 295.

The creation of new dunes, by the processes mentioned in the text, seems to be much older in Europe than the adoption of measures for securing them by planting. Dr. Dwight mentions a case in Ma.s.sachusetts, where a beach was restored, and new dunes formed, by planting beach gra.s.s.

"Within the memory of my informant, the sea broke over the beach which connects Truro with Province Town, and swept the body of it away for some distance. The beach gra.s.s was immediately planted on the spot; in consequence of which the beach was again raised to a sufficient height, and in various places into hills."--Travels, iii., p. 93.]

The dunes of Holland are sometimes protected from the das.h.i.+ng of the waves by a revetement of stone, or by piles; and the lateral high-water currents, which wash away their base, are occasionally checked by transverse walls running from the foot of the dunes to low-water mark; but the great expense of such constructions has prevented their adoption on a large scale. [Footnote: Staring, i., pp. 310, 332.] The princ.i.p.al means relied on for the protection of the sand-hills are the planting of their surfaces and the exclusion of burrowing and grazing animals. There are gra.s.ses, creeping plants, and shrubs of spontaneous growth, which flourish in loose sand, and, if protected, spread over considerable tracts, and finally convert their face into a soil capable of cultivation, or, at least, of producing forest trees. Krause enumerates one hundred and seventy-one plants as native to the coast sands of Prussia, and the observations of Andresen in Jutland carry the number of these vegetables up to two hundred and thirty-four.

Some of these plants, especially the Arundo arenaria or arenosa, or Psamma or Psammophila arenaria--Klittetag, or Hjelme in Danish, helm in Dutch, Dunenhalm, Sandschilf, or Hugelrohr in German, gourbet in French, and marram in English--are exclusively confined to sandy soils, and thrive well only in a saline atmosphere. [Footnote: There is some confusion in the popular use of these names, and in the scientific designations of sand-plants, and they are possibly applied to different plants in different places. Some writers style the gourbet Calamagrostis arenaria, and distinguish it from the Danish Klittetag or Hjelme.] The arundo grows to the height of about twenty-four inches, but sends its strong roots with their many rootlets to a distance of forty or fifty feet. It has the peculiar property of flouris.h.i.+ng best in the loosest soil, and a sand-shower seems to refresh it as the rain revives the thirsty plants of the common earth. Its roots bind together the dunes, and its leaves protect their surface. When the sand ceases to drift, the arundo dies, its decaying roots fertilizing the sand, and the decomposition of its leaves forming a layer of vegetable earth over it.

Then follows a succession of other plants which gradually fit the sand-hills by growth and decay, for forest planting, for pasturage, and sometimes for ordinary agricultural use.

But the protection and gradual transformation of the dunes is not the only service rendered by this valuable plant. Its leaves are nutritious food for sheep and cattle, its seeds for poultry; [Footnote: Bread, not indeed very palatable, has been made of the seeds of the arundo, but the quant.i.ty which can be gathered is not sufficient to form an important economical resource.--Andresen, Om Klitformationen, p. 160.] cordage and netting twine are manufactured from its fibres, it makes a good material for thatching, and its dried roots furnish excellent fuel. These useful qualities, unfortunately, are too often prejudicial to its growth. The peasants feed it down with their cattle, cut it for rope-making, or dig if up for fuel, and it has been found necessary to resort to severe legislation to prevent them from bringing ruin upon themselves by thus improvidently sacrificing their most effectual safeguard against the drifting of the sands. [Footnote: Bergsoe, Reventlovs Virksomhed, ii., p. 4.]

In 1539 a decree of Christian III., king of Denmark, imposed a fine upon persons convicted of destroying certain species of sand-plants upon the west coast of Jutland. This ordinance was renewed and made more comprehensive in 1558, and in 1569 the inhabitants of several districts were required, by royal rescript, to do their best to check the sand-drifts, though the specific measures to be adopted for that purpose are not indicated. Various laws against stripping the dunes of their vegetation were enacted in the following century, but no active measures were taken for the subjugation of the sand-drifts until 1779, when a preliminary system of operation for that purpose was adopted. This consisted in little more than the planting of the Arundo arenaria, and other sand-plants, and the exclusion of animals destructive to those vegetables. [Footnote: Measures were taken for the protection of the dunes of Cape Cod, in Ma.s.sachusetts, during the colonial period, though I believe they are now substantially abandoned. A hundred years ago, before the valley of the Mississippi, or even the rich plains of Central and Western New York, were opened to the white settler, the value of land was relatively much greater in New England than it is at present, and consequently some rural improvements were then worth making, which would not now yield sufficient returns to tempt the investment of capital. The money and the time required to subdue and render productive twenty acres of sea-sand on Cape Cod, would buy a "section" and rear a family in Illinois. The son of the Pilgrim, therefore, abandons the sea-hills, and seeks a better fortune on the fertile prairies of the West. See Dwight, Travels, i., pp. 92, 93.] Ten years later, plantations of forest trees, which have since proved so valuable a means of fixing the dunes and rendering them productive, were commenced, and have been continued ever since. [Footnote: Andresen, Om Klitformationen, pp. 237, 240.] During this latter period, Bremontier, without any knowledge of what was doing in Denmark, experimented upon the cultivation of forest trees on the dunes of Gascony, and perfected a system, which, with some improvements in matters of detail, is still largely pursued on those sh.o.r.es.

The example of Denmark was soon followed in the neighboring kingdom of Prussia, and in the Netherlands; and, as we shall see hereafter, these improvements have been everywhere crowned with most flattering success.

Under the administration of Reventlov, a little before the close of the last century, the Danish Government organized a regular system of improvement in the economy of the dunes. They were planted with the arundo and other vegetables of similar habits, protected against trespa.s.sers, and at last partly covered with forest trees. By these means much waste soil has been converted into arable ground, a large growth of valuable timber obtained, and the further spread of the drifts, which threatened to lay waste the whole peninsula of Jutland, to a considerable extent arrested.

In France, the operations for fixing and reclaiming the dunes--which began under the direction of Bremontier about the same time as in Denmark, and which are, in principle and in many of their details, similar to those employed in the latter kingdom--have been conducted on a far larger scale, and with greater success, than in any other country.

This is partly owing to a climate more favorable to the growth of suitable forest trees than that of Northern Europe, and partly to the liberality of the Government, which, having more important landed interests to protect, has put larger means at the disposal of the engineers than Denmark and Prussia have found it convenient to appropriate to that purpose. The area of the dunes already secured from drifting, and planted by the processes invented by Bremontier and perfected by his successors, is about 100,000 acres. [Footnote: "These plantations, perseveringly continued from the time of Bremontier, now cover more than 40,000 hectares, and compose forests which are not only the salvation of the department, but const.i.tute its wealth." --Clave, Etudes Forestieres, p. 254. Other authors have stated the plantations of the French dunes to be much more extensive.] This amount of productive soil, then, has been added to the resources of France, and a still greater quant.i.ty of valuable land has been thereby rescued from the otherwise certain destruction with which it was threatened by the advance of the rolling sand-hills.

The improvements of the dunes on the coast of West Prussia began in 1795, under Soren Bjorn, a native of Denmark, and, with the exception of the ten years between 1807 and 1817, they have been prosecuted ever since. The methods do not differ essentially from those employed in Denmark and France, though they are modified by local circ.u.mstances, and, with respect to the trees selected for planting, by climate. In 1850, between the mouth of the Vistula and Kahlberg, 6,300 acres, including about 1,900 acres planted with pines and birches, had been secured from drifting; between Kahlberg and the eastern boundary of West Prussia, 8,000 acres; and important preliminary operations had been carried on for subduing the dunes on the west coast. [Footnote: Kruse, Dunenbau, pp. 34, 38, 40.]

The tree which has been fonnd to thrive best upon the sand-hills of the French coast, and at the same time to confine the sand most firmly and yield the largest pecuniary returns, is the maritime pine, Pinus maritima, a species valuable both for its timber and for its resinous products. It is always grown from seed, and the young shoots require to be protected for several seasons, by the branches of other trees, planted in rows, or spread over the surface and staked down, by the growth of the Arundo arenaria and other small sand-plants, or by wattled hedges. The beach, from which the sand is derived, has been generally planted with the arundo, because the pine does not thrive well so near the sea; but it is thought that a species of tamarisk is likely to succeed in that lat.i.tude even better than the arundo. The shade and the protection offered by the branching top of this pine are favorable to the growth of deciduous trees, and, while still young, of shrubs and smaller plants, which contribute more rapidly to the formation of vegetable mould, and thus, when the pine has once taken root, the redemption of the waste is considered as effectually secured.

In France, the maritime pine is planted on the sands of the interior as well as on the dunes of the seacoast, and with equal advantage. This tree resembles the pitch pine of the Southern American States in its habits, and is applied to the same uses. The extraction of turpentine from it begins at the age of about twenty years, or when it has attained a diameter of from nine to twelve inches. Incisions are made up and down the trunk, to the depth of about half an inch in the wood, and it is insisted that if not more than two such slits are cut, the tree is not sensibly injured by the process. The growth, indeed, is somewhat checked, but the wood becomes superior to that of trees from which the turpentine is not extracted. Thus treated, the pine continues to flourish to the age of one hundred or one hundred and twenty years, and up to this age the trees on an acre yield annually 300 pounds of essence of turpentine, and 250 pounds of resin, worth together not far from ten dollars. The expense of extraction and distillation is calculated at about four dollars, and a clear profit of more than five dollars per acre is left. [Footnote: These processes are substantially similar to those employed in the pineries of the Carolinas, but they are better systematized and more economically conducted in France. In the latter country, all the products of the pine, even to the cones, find a remunerating market, while, in America, the price of resin is so low, that in the fierce steamboat races on the great rivers, large quant.i.ties of it are thrown into the furnaces to increase the intensity of the fires. In a carefully prepared article on the Southern pineries published in an American magazine--I think Harper's--a few years ago, it was stated that the resin from the turpentine distilleries was sometimes allowed to run to waste; and the writer, in one instance, observed a ma.s.s, thus rejected as rubbish, which was estimated to amount to two thousand barrels. Olmsted saw, near a distillery which had been in operation but a single year, a pool of resin estimated to contain three thousand barrels, which had been allowed to run off as waste.--A Journey in the seaboard Slave States, 1863, p. 345.] This is exclusive of the value of the timber, when finally cut, which, of course, amounts to a very considerable sum. In Denmark, where the climate is much colder, hardier conifers, as well as the birch and other northern trees, are found to answer a better purpose than the maritime pine, and it is doubtful whether this tree would be able to resist the winter on the dunes of Ma.s.sachusetts. Probably the pitch-pine of the Northern States, in conjunction with some of the American oaks, birches, and poplars, and especially the robinia or locust, would prove very suitable to be employed on the sand-hills of Cape Cod and Long Island. The ailanthus, now coming into notice as a sand-loving tree, some species of tamarisk, and perhaps the Aspressus macrocarpa, already found useful on the dunes in California, may prove valuable auxiliaries in resisting the encroachment of drifting sands, whether in America or in Europe, and the intermixing of different species would doubtless be attended with as valuable results in this as in other branches of forest economy. It cannot, indeed, be affirmed that human power is able to arrest altogether the incursions of the waves on sandy coasts, by planting the beach, and clothing the dunes with wood. On the contrary, both in Holland and on the French coast, it has been found necessary to protect the dunes themselves by piling and by piers and sea-walls of heavy masonry. But experience has amply shown that the processes referred to are entirely successful in preventing the movement of the dunes, and the drifting of their sands over cultivated lands behind them; and that, at the same time, the plantations very much r.e.t.a.r.d the landward progress of the waters. [Footnote: See a very interesting article ent.i.tled "Le Littoral de la France," by Elisee Reclus, in the Revue des Deux Mondes for December, 1862, pp. 901, 936.]

Besides the special office of dune plantations already noticed, these forests have the same general uses as other woods, and they have sometimes formed by their droppings so thick a layer of vegetable mould that the sand beneath has become sufficiently secured to allow the wood to be felled, and the surface to be ploughed and cultivated with ordinary field crops.

In some cases it has been found possible to confine and cultivate coast sand-hills, even without preliminary forestal plantation. Thus, in the vicinity of Cap Breton in France, a peculiar process is successfully employed, both for preventing the drifting of dunes, and for rendering the sands themselves immediately productive; but this method is applicable only in exceptional cases of favorable climate and exposure.

It consists in planting vineyards upon the dunes, and protecting them by hedges of broom, Erica scoparia, so disposed as to form rectangles about thirty feet by forty. The vines planted in these enclosures thrive admirably, and the grapes produced by them are among the best grown in France. The dunes are so far from being an unfavorable soil for the vine, that fresh sea-sand is regularly employed as a fertilizer for it, alternating every other season with ordinary manure. The quant.i.ty of sand thus applied every second year, raises the surface of the vineyard about four or five inches. The vines are cut down every year to three or four shoots, and the raising of the soil rapidly covers the old stocks.

As fast as buried, they send out new roots near the surface, and thus the vineyard is constantly renewed, and has always a youthful appearance, though it may have been already planted a couple of generations. This practice is ascertained to have been followed for two centuries, and is among the oldest well-authenticated attempts of man to resist and vanquish the dunes. [Footnote: Boitel, Mise en valeur des Terres pauvres, pp. 212, 218.]

The artificial removal of dunes, no longer necessary as a protection, does not appear to have been practiced upon a large scale except in the Netherlands, where the numerous ca.n.a.ls furnish an easy and economical means of transporting the sand, and where the construction and maintenance of sea and river dikes, and of causeways and other embankments and fillings, create a great demand for that material. Sand is also employed in Holland, in large quant.i.ties, for improving the consistence of the tough clay bordering upon or underlying diluvial deposits, and for forming an artificial soil for the growth of certain garden and ornamental vegetables. When the dunes are removed, the ground they covered is restored to the domain of industry; and the quant.i.ty of land recovered in the Netherlands by the removal of the barren sands which enc.u.mbered it, amounts to hundreds and perhaps thousands of acres.

Inland Dunes.

Vast deposits of sand, both in the form of dunes and of plains, are found far in the interior of continents, in the Old World and in the New. The deserts of Gobi, of Arabia, and of Africa have been rendered familiar by the narratives of travellers, but the sandy wilderness of America, and even of Europe, have not yet been generally recognized as important elements in the geography of the regions where they occur.

There are immense wastes of drifting sands in Poland and other interior parts of Europe, in Peru, and in the less known regions of our own Western territory, where their extent is greater than that of all the coast dunes together which have hitherto been described by European and American geographers. [Footnote: On the Niobrara river alone, the dunes cover a surface of twenty thousand square miles.--Hayden, Report on Geological Survey of Wyoming, 1870, p. 108.] The inland sand-hills of both hemispheres are composed of substantially the same material and aggregated by the action of the same natural forces as the dunes of the coast. There is, therefore, a general resemblance between them, but they appear, nevertheless, to be distinguished by certain differences which a more attentive study may perhaps enable geologists to recognize in the sandstone formed by them. The sand of which they are composed comes in both princ.i.p.ally from the bed of the sea being brought to the surface in one case by the action of the wind and the waves, in the other by geological upheaval. [Footnote: American observers do not agree in their descriptions of the form and character of the sand-grains which compose the interior dunes of the North American desert. C. C. Parry, geologist to the Mexican Boundary Commission, in describing the dunes near the station at a spring thirty-two miles west from the Rio Grande at El Paso, says: "The separate grains of the sand composing the sand-hills are seen under a lens to be angular, and not rounded, as would be the case in regular beach deposits."--U. S. Mexican Boundary Survey, Report of, vol i., Geological Report of C. C. Parry, p. 10.

In the general description of the country traversed, same volume, p. 47, Colonel Emory says that on an "examination of the sand with a microscope of sufficient power," the grains are seen to be angular, not rounded by rolling in water.

On the other hand, Blake, in Geological Report, Pacific Railroad Rep., vol. v., p. 119, observes that the grains of the dune sand, consisting of quartz, chalcedony, carnelian, agate, rose quartz, and probably chrysolite, were much rounded; and on page 241, he says that many of the sand grains of the Colorado desert are perfect spheres. On page 20 of a report in vol. ii. of the Pacific Railroad Report, by the same observer, it is said that an examination of dunes brought from the Llano Estacado by Captain Pope, showed the grains to be "much rounded by attrition."

The sands described by Mr. Parry and Colonel Emory are not from the same localities as those examined by Mr. Blake, and the difference in their character may be due to a difference of origin or of age. In New Mexico, sixty miles south of Fort Stanton, there are island dunes composed of finely granulated gypsum.--American Naturalist, Jan. 1871, p. 695.] The sand of the coast dunes is rendered, to a certain extent, cohesive by moisture and by the saline and other binding ingredients of sea-water, while long exposure to meteoric influences has in a great measure deprived the inland sands of these const.i.tuents, though there are not wanting examples of large acc.u.mulations of sand far from the sea, and yet agglutinated by saline material. Hence, as might be expected, inland dunes, when not confined by a fixed nucleus, are generally more movable than those of the coast, and the form of such dunes is more or less modified by their want of consistence. Thus, the crescent or falciform shape is described by all observers as more constant and conspicuous in these sandhills than in those of littoral origin; they tend less to unite in continuous ridges, and they rarely attain the height or other dimensions of the dunes of the seash.o.r.e. Meyer describes the sand-hills of the Peruvian desert as perfectly falciform in shape and from seven to fifteen feet high, the chord of their arc measuring from twenty to seventy paces. The slope of the convex face is described as very small, that of the concave as high as 70 degrees or 80 degrees, and their surfaces were rippled. No smaller dunes were observed, nor any in the process of formation. The concave side uniformly faced the north-west, except towards the centre of the desert, where, for a distance of one or two hundred paces, they gradually opened to the west, and then again gradually resumed the former position. Tschudi observed, in the same desert, two species of dunes, fixed and movable, and he ascribes a falciform shape to the movable, a conical to the fixed dunes, or medanos. "The medanos," he observes, "are hillock-like elevations of sand, some having a firm, others a loose base. The former [latter], which are always crescent-shaped, are from ten to twenty feet high, and have an acute crest. The inner side is perpendicular, and the outer or bow side forms an angle with a steep inclination downwards. [Footnote: The dunes of the plains between Bokhara and the Oxus are all horse-shoe shaped, convex towards the north, from which the prevailing wind blows.

On this side they are sloping, inside precipitous, and from fifteen to twenty feet high.--Burnes, Journal in Bokhara, ii., pp. 1, 2.] When driven by violent winds, the medanos pa.s.s rapidly over the plains. The smaller and lighter ones move quickly forward, before the larger; but the latter soon overtake and crush them, whilst they are themselves s.h.i.+vered by the collision. These medanos a.s.sume all sorts of extraordinary figures, and sometimes move along the plain in rows forming most intricate labyrinths.... A plain often appears to be covered with a row of medanos, and some days afterwards it is again restored to its level and uniform aspect.... "The medanos with immovable bases are formed on the blocks of rocks which are scattered about the plain. The sand is driven against them by the wind, and as soon as it reaches the top point, it descends on the other side until that is likewise covered; thus gradually arises a conical-formed hill. [Footnote: The sand-hills observed by Desor in the Algerian desert were fixed, changing their form only on the surface as sand was blown to and from them.--Sahara und Atlas, 1865, p. 21.] Entire hillock chains with acute crests are formed in a similar manner.... On their southern declivities are found vast ma.s.ses of sand, drifted thither by the mid-day gales. The northern declivity, though not steeper than the southern, is only sparingly covered with sand. If a hillock chain somewhat distant from the sea extends in a line parallel with the Andes, namely, from S. S. E.

to N. N. W., the western declivity is almost entirely free of sand, as it is driven to the plain below by the south-east wind, which constantly alternates with the wind from the south." [Footnote: Travels in Peru, New York, 1848, chap. ix.] It is difficult to reconcile this description with that of Meyen, but if confidence is to be reposed in the accuracy of either observer, the formation of the sand-hills in question must be governed by very different laws from those which determine the structure of coast dunes. Captain Gilliss, of the American navy, found the sand-hills of the Peruvian desert to be in general crescent-shaped, as described by Meyen, and a similar structure is said to characterize the inland dunes of the Llano Estacado and other plateaus of the North American desert, though those latter are of greater height and other dimensions than those described by Meyen. There is no very obvious explanation of this difference in form between maritime and inland sand-hills, and the subject merits investigation. It is, however, probable that the great mobility of the flying dunes of the Peruvian desert is an effect of their dryness, no rain falling in that desert, and of the want of salt or other binding material to hold their particles together.

Inland Sand Plains.

The inland sand plains of Europe are either derived from the drifting of dunes or other beach sands, or consist of diluvial deposits, or are ancient sea-beds uplifted by geological upheaval. As we have seen, when once the interior of a dune is laid open to the wind, its contents ars soon scattered far and wide over the adjacent country, and the beach sands, no longer checked by the rampart which nature had constrained them to build against their own encroachments, are also carried to considerable distances from the coast. Few regions have suffered so much from this cause, in proportion to their extent, as the peninsula of Jutland. So long as the woods, with which nature had planted the Danish dunes, were spared, they seem to have been stationary, and we have no historical evidence, of an earlier date than the sixteenth century, that they had become in any way injurious. From that period there are frequent notices of the invasions of cultivated grounds by the sands; and excavations are constantly bringing to light proof of human habitation and of agricultural industry, in former ages, on soils now buried beneath deep drifts from the dunes and beaches of the seacoast.

[Footnote: For details, consult Andresen, Om Klitformationen, pp. 223, 236.]

Extensive tracts of valuable plain land in the Netherlands and in France have been covered in the same way with a layer of sand deep enough to render them infertile, and they can be restored to cultivation only by processes a.n.a.logous to those employed for fixing and improving the dunes. [Footnote: When the deposit is not very deep, and the adjacent land lying to the leeward of the prevailing winds is covered with water, or otherwise worthless, the surface is sometimes freed from the drifts by repeated harrowings, which loosen the sand, so that the wind takes it up and transports it to grounds where acc.u.mulations of it are less injurious.] Diluvial sand plains, also, have been reclaimed by these methods in the Duchy of Austria, between Vienna and the Semmering ridge, in Jutland, and in the great champaign country of Northern Germany, especially the Mark Brandenburg, where artificial forests can be propagated with great ease, and where, consequently, this branch of industry has been pursued on a great scale, and with highly beneficial results, both as respects the supply of forest products and the preparation of the soil for agricultural use.

As has been already observed, inland sands are generally looser, dryer, and more inclined to drift, than those of the seacoast, where the moist and saline atmosphere of the ocean keeps them always more or less humid and cohesive. The sands of the valley of the Lower Euphrates--themselves probably of submarine origin, and not derived from dunes are advancing to the north-west with a rapidity which seems fabulous when compared with the slow movement of the sand-hills of Gascony and the Low German coasts. Loftus, speaking of Niliyya, an old Arab town a few miles east of the ruins of Babylon, says that, "in 1848, the sand began to acc.u.mulate around it, and in six years, the desert, within a radius of six miles, was covered with little, undulating domes, while the ruins of the city were so buried that it is now impossible to trace their original form or extent." [Footnote: Travels and Researchs in Chaldaea, chap. ix.

Dwight mentions (Travels, vol. iii, p. 101) an instance of great mischief from the depasturing of the beach gra.s.s which had been planted on a sand plain in Cape Cod: "Here, about one thousand acres were entirely blown away to the depth, in many places, of ten feet.... Not a green thing was visible except the whortleberries, which tufted a few lonely hillocks rising to the height of the original surface and prevented by this defence from being blown away also. These, although they varied the prospect, added to the gloom by their strongly picturesque appearance, by marking exactly the original level of the plain, and by showing us in this manner the immensity of the ma.s.s which had been thus carried away by the wind. The beach gra.s.s had been planted here, and the ground had been formerly enclosed; but the gates had been left open, and the cattle had destroyed this invaluable plant."] Loftus considers this sand-flood as the "vanguard of those vast drifts which advancing from the south-east, threaten eventually to overwhelm Babylon and Baghdad." An observation of Layard, cited by Loftus, appears to me to furnish a possible explanation of this irruption. He "pa.s.sed two or three places where the sand, issuing from the earth like water, is called 'Aioun-er-rummal,' sand springs." These "springs" are very probably merely the drifting of sand from the ancient subsoil, where the protecting crust of aquatic deposit and vegetable earth has been broken through, as in the case of the drift which arose from the upturning of an oak mentioned on a former page. When the valley of the Euphrates was regularly irrigated and cultivated, the underlying sands were bound by moisture, alluvial slime, and vegetation; but now, that all improvement is neglected, and the surface, no longer watered, has become parched, powdery, and naked, a mere accidental fissure in the superficial stratum may soon be enlarged to a wide opening, that will let loose sand enongh to overwhelm a province. The Landes of Gascony. The most remarkable sand plain of France lies at the south-western extremity of the empire, and is generally known as the Landes, or heaths, of Gascony. Clave thus describes it: "Composed of pure sand, resting on an impermeable stratum called alios, the soil of the Landes was, for centuries, considered incapable of cultivation. [Footnote: The alios, which from its color and consistence was supposed to be a ferruginous formation, appears from recent observations to contain little iron and to owe most of its peculiar properties to vegetable elements carried down into the soil by the percolation of rain-water. See Revue des Eaux et Forets for 1870, p.

801.] Parched in summer, drowned in winter, it produced only ferns, rushes, and heath, and scarcely furnished pasturage for a few half-starved flocks. To crown its miseries, this plain was continually threatened by the encroachments of the dunes. Vast ridges of sand, thrown up by the waves, for a distance of more than fifty leagues along the coast, and continually renewed, were driven inland by the west wind, and, as they rolled over the plain, they buried the soil and the hamlets, overcame all resistance, and advanced with fearful regularity.

The whole province seemed devoted to certain destruction, when Bremontier invented his method of fixing the dunes by plantations of the maritime pine." [Footnote: Etudes Forestieres, p. 250. See, also, Reclus, La Terre, i., 105, 106.] Although the Landes had been almost abandoned for ages, they show numerous traces of ancient cultivation and prosperity, and it is princ.i.p.ally by means of the encroachments of the sands that they have become reduced to their present desolate condition.

The destruction of the coast towns and harbors, which furnished markets for the products of the plains, the damming up of the rivers, and the obstruction of the smaller channels of natural drainage by the advance of the dunes, were no doubt very influential causes; and if we add the drifting of the sea-sand over the soil, we have at least a partial explanation of the decayed agriculture and diminished population of this great waste. When the dunes were once arrested, and the soil to the east of them was felt to be secure against invasion by them, experiments, in the way of agricultural improvement, by drainage and plantation, were commenced, and they have been attended with such signal success, that the complete recovery of one of the dreariest and most extensive wastes in Europe may be considered as both a probable and a near event. [Footnote: Lavergne, Economie Rurale de la France, p. 300, estimates the area of the Landes of Gascony at 700,000 hectares, or about 1,700,000 acres. The same author states (p. 301), that when the Moors were driven from Spain by the blind cupidity and brutal intolerance of the age, they demanded permission to establish themselves in this desert; but political and religious prejudices prevented the granting of this liberty. At this period the Moors were a far more cultivated people than their Christian persecutors, and they had carried many arts, that of agriculttire especially, to a higher pitch than any other European nation. But France was not wise enough to accept what Spain had cast out, and the Landes remained a waste for three centuries longer.

For a brilliant account of the improvement of the Landes, see Edmond About, Le Progres, chap. vii.

The forest of Fontainebleau, which contains above 40,000 acres, is not a plain, but its soil is composed almost wholly of sand, interspersed with ledges of rock. The sand forms not less than ninety-eight per cent of the earth, and, as it is almost without water, it would be a drifting desert but for the artificial propagation of forest trees upon it. The Landes of Sologne and of Brenne are less known than those of Gascony, because they are not upon the old great lines of communication. They once compoaed a forest of 1,200,000 acres, but by clearing the woods have relapsed into their primitive condition of a barren sand waste.

Active efforts are now in progress to reclaim them.]

In the northern part of Belgium, and extending across the confines of Holland, is another very similar heath plain, called the Campine. This is a vast sand flat, interspersed with marshes and inland dunes, and, until recently, considered almost wholly incapable of cultivation.

Enormous sums had been expended in reclaiming it by draining and other familiar agricultural processes, but without results at all proportional to the capital invested. In 1849, the unimproved portion of the Campine was estimated at little less than three hundred and fifty thousand acres. The example of France prompted experiments in the planting of trees, especially the maritime pine, upon this barren waste, and the results have now been such as to show that its sands may both be fixed and made productive, not only without loss, but with positive pecuniary advantage. [Footnote: Economie Rurale de la Belgique, par Emile De Laveleye, Revue des Deux Mondes, Juin, 1861, pp. 6l7-644. The quant.i.ty of land annually reclaimed on the Campine is stated at about 4,000 acres. Ca.n.a.ls for navigation and irrigation have been constructed through the Campine, and it is said that its barren sands, improved at an expense of one hundred dollars per acre, yield, from the second year, a return of twenty-five dollars to the acre.]

There are still unsubdued sand wastes in many parts of interior Europe not familiarly known to tourists or even geographers. "Olkuez and Schiewier in Poland," says Naumann, "lie in true sand deserts, and a boundless plain of sand stretches around Ozenstockau, on which there grows neither tree nor shrub. In heavy winds, this plain resembles a rolling sea, and the sand-hills rise and disappear like the waves of the ocean. The heaps of waste from the Olkuez mines are covered with sand to the depth of four fathoms." [Footnote: Geognosie, ii., p. 1173.] No attempts have yet been made to subdue the sands of Poland, but when peace and prosperity shall be restored to that unhappy country, there is no reasonable doubt that the measures, which have proved so successful on similar formations in Germany and near Odessa, may be employed with advantage in the Polish deserts. [Footnote: "Sixteen years ago," says an Odessa landholder, "I attempted to fix the sand of the steppes, which covers the rocky ground to the depth of a foot, and forms moving hillocks with every change of wind. I tried acacias and pines in vain; nothing would grow in such a soil. At length I planted the varnish tree, or ailanthus, which succeeded completely in binding the sand." This result encouraged the proprietor to extend his plantations over both dunes and sand steppes, and in the course ot sixteen years this rapidly growing tree had formed real forests. Other landholders have imitated his example with great advantage.--Rentsch, Der Wald, pp. 44, 45.]

CHAPTER VI.

GREAT PROJECTS OF PHYSICAL CHANGE ACCOMPLISHED OR PROPOSED BY MAN.

Cutting of Isthmuses--Ca.n.a.l of Suez--Maritime Ca.n.a.ls in Greece--Ca.n.a.ls to Dead Sea--Ca.n.a.ls to Libyan Desert--Maritime Ca.n.a.ls in Europe--Cape Cod Ca.n.a.l--Changes in Caspian--Diversion of the Nile--Diversion of the Rhine--Improvements in North American Hydrography--Soil below Rock--Covering Rock with Earth--Desert Valleys--Effects of Mining--Duponchel's Plans of Improvement--Action of Man on the Weather--Resistance to Great Natural Forces--Incidental Effects of Human Action--Nothing small in Nature.

In a former chapter I spoke of the influence of human action on the surface of the globe as immensely superior in degree to that exerted by brute animals, if not essentially different from it in kind. The eminent Italian geologist, Stoppani, goes further than I had ventured to do, and treats the action of man as a new physical clement altogether sui generis. According to him, the existence of man const.i.tutes a geological period which he designates as the ANTHROPOZOIC ERA. "The creation of man," says he, "was the introduction of a new element into nature, of a force wholly unknown to earlier periods." "It is a new telluric force which in power and universality may be compared to the greater forces of the earth." [Foonote: Corso Di Geologia, Milano, 1873, vol ii., cap.

x.x.xi., section 1327.] It has already been abundantly shown that, though the undesigned and unforeseen results of man's action on the geographical conditions of the earth have perhaps been hitherto greater and more revolutionary than the effects specially aimed at by him, yet there is scarcely any a.s.signable limit to his present and prospective voluntary controlling power over terrestrial nature.