A like example is observed in the Anapus near Syracuse, which, below the

junction of its two branches, is narrower, though swifter than either of

them, and such cases are by no means unfrequent. The immediate effect of

the confluence of two rivers upon the current below depends upon local

circumstances, and especially upon the angle of incidence. If the two

nearly coincide in direction, so as to include a small angle, the join

current will have a greater velocity than the slower confluent, perhaps

even than either of them. If the two rivers run in transverse, still

more if they flow in more or less opposite, directions, the velocity of

the principal branch will be retarded both above and below the junction,

and at high water it may even set back the current of the affluent.

On the other hand, the diversion of a considerable branch from a river

retards its velocity below the point of separation, and here a deposit

of earth in its channel immediately begins, which has a tendency to turn

the whole stream into the new bed. "Theory and the authority of all

hydrographical writers combine to show that the channels of rivers

undergo an elevation of bed below a canal of diversion."--Letter of

Fossombroni, in Salvagnoli, Raccolta di Documenti, p. 32. See the early

authorities and discussions on the principle stated in the text, in

Frisi, Del modo di regolare i Fiumi e i Torrenti, libro iii., capit. i.,

and Mongotti, Idraulica, ii., pp. 88 et seqq., and see p. 498, note,

ante.

In my account of these improvements I have chiefly followed Fossombroni,

under whose direction they were principally executed. Many of

Fossombroni's statements and opinions have been controverted, and in

comparatively unimportant particulars they have been shown to be

erroneous.--See Lombardini, Guida allo studio dell' Idrologia, cap.

xviii., and same author, Esame degli Studi sul Tevere, Section 33.]

Between the beginning of the fifteenth century and the year 1761,

thirty-one destructive floods of the Arno are recorded; between 1761,

when the principal streams of the Val di Chiana were diverted into that

river, and 1835, not one. [Footnote: Fossombroni, Memorie

Idraulico-storiche, Introduzione, p. xvi. Between the years 1700 and

1799 the chroniclers record seventeen floods of the Arno, and twenty

between 1800 and 1870, but none of these were of a properly destructive

character except those in 1844, 1864, and 1870, and the ravages of this

latter were chiefly confined to Pisa, and were occasioned by the

bursting of a dike or wall. They are all three generally ascribed to

extraordinary, if not unprecedented, rains and snows, but many inquirers

attribute them to the felling of the woods in the valleys of the upper

tributaries of the Arno since 1835. See a paper by Griffini, in the

Italia Nuova, 18 Marzo, 1871.]

Results of Operations.

It is now a hundred years since the commencement of the improvements in

the Val di Chiana, and those of the Maremma have been in more or less

continued operation for above a generation. They have, as we have seen,

produced important geographical changes in the surface of the earth and

in the flow of considerable rivers, and their effects have been not less

conspicuous in preventing other changes, of a more or less deleterious

character, which would infallibly have taken place if they had not been

arrested by the improvements in question.

The sediment washed into the marshes of the Maremma is not less than

12,000,000 cubic yards per annum. The escape of this quantity into the

sea, which, is now almost wholly prevented, would be sufficient to

advance the coast-line fourteen yards per year, for a distance of forty

miles, computing the mean depth of the sea near the shore at twelve

yards. It is true that in this case, as well as in that of other rivers,

the sedimentary matter would not be distributed equally along the shore,

and much of it would be carried out into deep water, or perhaps

transported by the currents to distant coasts. The immediate effects of

the deposit in the sea, therefore, would not be so palpable as they

appear in this numerical form, but they would be equally certain, and

would infallibly manifest themselves, first, perhaps, at some remote

point, and afterwards more energetically at or near the outlets of the

rivers which produced them. The elevation of the bottom of the sea would

diminish the inclination of the beds of the rivers discharging

themselves into it on that coast, and of course their tendency to

overflow their banks and extend still further the domain of the marshes

which border them would be increased in proportion.

It has been already stated that, in order to prevent the overflow of the

valley of the Tiber by freely draining the Val di Chiana into it, the

Papal authorities, long before the commencement of the Tuscan works,

constructed strong barriers near the southern end of the valley, which

detained the waters of the wet season until they could be gradually

drawn off into the Paglia. They consequently deposited most of their

sediment in the Val di Chiana and carried down comparatively little

earth to the Tiber. The lateral streams contributing the largest

quantities of sedimentary matter to the Val di Chiana originally flowed

into that valley near its northern end; and the change of their channels

and outlets in a southern direction, so as to raise that part of the

valley by their deposits and thereby reverse its drainage, was one of

the principal steps in the process of improvement.

We have seen that the north end of the Val di Chiana near the Arno had

been raised by spontaneous deposit of sediment to such a height as to

interpose a sufficient obstacle to all flow in that direction. If, then,

the Roman dam had not been erected, or the works of the Tuscan

Government undertaken, the whole of the earth, which has been arrested

by those works and employed to raise the bed and reverse the declivity

of the valley, would have been carried down to the Tiber and thence into

the sea. The deposit thus created would, of course, have contributed to

increase the advance of the shore at the mouth of that river, which has

long been going on at the rate of three metres and nine-tenths (twelve

feet and nine inches) per annum. [Footnote: See the careful estimates of

Rozet, Moyens de forcer les Torrents, etc., pp. 42, 44.] It is evident

that a quantity of earth, sufficient to effect the immense changes I

have described in a wide valley more than thirty miles long, if

deposited at the outlet of the Tiber, would have very considerably

modified the outline of the coast, and have exerted no unimportant

influence on the flow of that river, by raising its point of discharge

and lengthening its channel.

The Coast of the Netherlands. It has been shown in a former section that

the dikes of the Netherlands and the adjacent states have protected a

considerable extent of coast from the encroachments of the sea, an have

won a large tract of cultivable land from the dominion of the ocean

waters. The immense results obtained from the operations of the Tuscan

engineers in the Val di Chiana, and the Maremma have suggested the

question, whether a different method of accomplishing these objects

might not have been adopted with advantage. It has been argued, as in

the case of the Po, that a system of transverse inland dikes and canals,

upon the principle of those which have been so successfully employed in

the Val di Chiana and in Egypt, might have elevated the low grounds

above the ocean tides, by spreading over them the sediment brought down

by the Rhine, the Maes, and the Scheld. If this process had been

introduced in the Middle Ages, and constantly pursued to our times, the

superficial and coast geography, as well as the hydrography of the

countries in question, would undoubtedly have presented an aspect very

different from their present condition; and by combining the process

with a system of maritime dikes, which would have been necessary, both

to resist the advance of the sea and to retain the slime deposited by

river overflows, it is, indeed, possible that the territory of those

states would have been as extensive as it now is, and, at the same time,

somewhat elevated above its natural level.

The argument in favor of that method rests on the assumption that all

the sea-washed earth, which the tides have let fall upon the shallow

coast of the Netherlands, has been brought down by the rivers which

empty upon those shores, and could have been secured by allowing those

rivers to spread over the flats and deposit their sediment in

still-water pools formed by cross-dikes like those of Egypt.

But we are ignorant of the proportions in which the marine deposits that

form the soil of the polders have been derived from materials brought

down by these rivers, or from other more remote sources. Much of the

river slime has, no doubt, been transported by marine currents quite

beyond the reach of returning streams, and it is uncertain how far this

loss has been balanced by earth washed by the sea from distant shores

and let fall on the coasts of the Netherlands and other neighboring

countries.

We know little or nothing of the quantity of solid matter brought down

by the rivers of Western Europe in early ages, but, as the banks of

those rivers are now generally better secured against wash and abrasion

than in former centuries, the sediment transported by them must be less

than at periods nearer the removal of the primitive forests of their

valleys, though certainly greater than it was before those forests were

felled. Kladen informs us that the sedimentary matter transported to the

sea by the Rhine would amount to a cubic geographical mile in five

thousand years. [Footnote: Erdhunde, vol. i, p. 384. The Mississippi--a

river "undercharged with sediment"--with a mean discharge of about ten

times that of the Rhine, deposits a cubic geographical mile in

thirty-three years.] The proportion of this suspended matter which, with

our present means, could be arrested and precipitated upon the ground,

is almost infinitesimal, for only the surface-water, which carries much

less sediment than that at the bottom of the channel, would flow over

the banks, and as the movement of this water, if not checked altogether,

would be greatly retarded by the proposed cross-dikes, the quantity of

solid matter which would be conveyed to a given portion of land during a

single inundation would be extremely small. Inundations of the Rhine

occur but once or twice a year, and high water continues but a few days,

or even hours; the flood-tide of the sea happens seven hundred times in

a year, and at the turn of the tide the water is brought to almost

absolute rest. Hence, small as is the proportion of suspended matter in

the tide-water, the deposit probably amounts to far more in a year than

would be let fall upon the same area by the Rhine.

This argument, except as to the comparison between river and tide water,

applies to the Mississippi, the Po, and most other great rivers. Hence,

until that distant day when man shall devise means of extracting from

rivers at flood, the whole volume of their suspended material and of

depositing it at the same time on their banks, the system of cross-dikes

and COLMATAGE must be limited to torrential streams transporting large

proportions of sediment, and to the rivers of hot countries, like the

Nile, where the saturation of the soil with water, and the securing of a

supply for irrigation afterwards, are the main objects, while raising

the level of the banks is a secondary consideration.

CHAPTER V.

THE SANDS.

Origin of Sand--Sand now Carried to the Sea--Beach Sands of Northern

Africa--Sands of Egypt--Sand Dunes and Sand Plains--Coast Dunes--Sand

Banks--Character of Dune Sand--Interior Structure of Dunes--Geological

Importance of Dunes--Dunes on American Coasts--Dunes of Western

Europe--Age, Character, and Permanence of Dunes--Dunes as a Barrier

against the Sea--Encroachments of the Sea--Liimfjord--Coasts of

Schleswig-Holstein, Netherlands, and France--Movement of Dunes--Control

of Dunes by Man--Inland Dunes--Inland Sand Plains.

Origin of Sand.

Sand, which is found in beds or strata at the bottom of the sea or in

the channels of rivers, as well as in extensive deposits upon or beneath

the surface of the dry land, appears to consist essentially of the

detritus of rocks. It is not always by any means clear through what

agency the solid rock has been reduced to a granular condition; for

there are beds of quartzose sand, where the sharp, angular shape of the

particles renders it highly improbable that they have been formed by

gradual abrasion and attrition, and where the supposition of a crushing

mechanical force seems equally inadmissible. In common sand, the quartz

grains are the most numerous; but this is not a proof that the rocks

from which these particles were derived were wholly, or even chiefly,

quartzose in character; for, in many composite rocks, as, for example,

in the granitic group, the mica, feldspar, and hornblende are more

easily decomposed by chemical action, or disintegrated, comminuted, and

reduced to an impalpable state by mechanical force, than the quartz. In

the destruction of such rocks, therefore, the quartz would survive the

other ingredients, and remain unmixed, when they had been decomposed and

recomposed into new mineralogical or chemical combinations, or been

ground to slime and washed away by water currents.

The greater or less specific gravity of the different constituents of

rock doubtless aids in separating them into distinct masses when once

disintegrated, though there are veined and stratified beds of sand where

the difference between the upper and lower layers, in this respect, is

too slight to be supposed capable of effecting a complete

separation. [Footnote: In the curiously variegated sandstone of Arabia

Petraea--which is certainly a reaggregation of loose sand derived from

disaggregation of older rocks--the continuous veins frequently differ

very widely in color, but not sensibly in specific gravity or in

texture; and the singular way in which they are now alternated, now

confusedly intermixed, must be explained otherwise than by the weight of

the respective grains which compose them. They seem, in fact, to have

been let fall by water in violent ebullition or tumultuous mechanical

agitation, or deposited by a succession of sudden aquatic or aerial

currents flowing in different directions and charged with differently

colored matter.] In cases where rock has been reduced to sandy fragments

by heat, or by obscure chemical and other molecular forces, the

sand-beds may remain undisturbed, and represent, in the series of

geological strata, the solid formations from which they were derived.

The large masses of sand not found in place have been transported and

accumulated by water or by wind, the former being generally considered

the most important of these agencies; for the extensive deposits of the

Sahara, of the Arabian peninsulas, of the Llano Estacado and other North

and South American deserts, of the deserts of Persia, and of that of

Gobi, are supposed to have been swept together or distributed by marine

currents, and to have been elevated above the ocean by the same means as

other upheaved strata. Meteoric and mechanical influences are still

active in the reduction of rocks to a fragmentary state; [Footnote: A

good account of the agencies now operative in the reduction of rock to

sand will be found in Winkler, Zand en Duinen, Dockarm, 1865, pp. 4-20.

I take this occasion to acknowledge my obligations to this author for

assuming the responsibility of many of the errors I may have committed

in this chapter, by translating a large part of it from a former edition

of the present work and publishing it as his own.] but the quantity of

sand now transported to the sea seems to be comparatively

inconsiderable, because--not to speak of the absence of diluvial

action--the number of torrents emptying directly into the sea is much

less than it was at earlier periods. The formation of alluvial plains in

maritime bays, by the sedimentary matter brought down from the

mountains, has lengthened the flow of such streams and converted them

very generally into rivers, or rather affluents of rivers of later

geographical origin than themselves. The filling up of the estuaries has

so reduced the slope of all large and many small rivers, and,

consequently, so checked the current of what the Germans call their

Unterlauf, or lower course, that they are much less able to transport

heavy material than at earlier epochs. The slime deposited by rivers at

their junction with the sea, is usually found to be composed of material

too finely ground and too light to be denominated sand, and it can be

abundantly shown that the sand-banks at the outlet of most large streams

are of tidal, not of fluviatile, accumulation, or, in lakes and tideless

seas, a result of the concurrent action of waves and of wind. Large

deposits of sand, therefore, must in general be considered as of

ancient, not of recent formation, and many eminent geologists ascribe

them to diluvial action. Staring has discussed this question very fully,

with special reference to the sands of the North Sea, the Zuiderzee, and

the bays and channels of the Dutch coast. [Footnote: De Bodem van

Nederland, i., pp. 243, 246-377, et seqq. See also the arguments of

Bremontier as to the origin of the dune-sands of Gascony, Annales des

Ponts et Chaussees, 1833, 1er semestre, pp. 158, 161. Bremontier

estimates the sand anually thrown up on that coast at five cubic toises

and two feet to the running toise (ubi supra, p. 162), or rather more

than two hundred and twenty cubic feet to the running foot. Laval, upon

observations continued through seven years, found the quantity to be

twenty-five metres per running metre, which is equal to two hundred and

sixty-eight cubic feet to the running foot.--Annales des Ponts et

Chaussees, 1842, 2me semestre, p. 229. These computations make the

proportion of sand deposited on the coast of Gascony three or four times

as great as that observed by Andresen on the shores of Jutland. Laval

estimates the total quantity of sand annually thrown up on the coast of

Gascony at 6,000,000 cubic metres, or more than 7,800,000 cubic yards.]

His general conclusion is, that the rivers of the Netherlands "move sand

only by a very slow displacement of sand-banks, and do not carry it with

them as a suspended or floating material." The sands of the German Ocean

he holds to be a product of the "great North German drift," deposited

where they now lie before the commencement of the present geological

period, and he maintains similar opinions with regard to the sands

thrown up by the Mediterranean at the mouths of the Nile and on the

Barbary coast. [Footnote: De Bodem van Nederland, i., p. 339.]

Sand now carried to the Sea.

There are, however, cases where mountain streams still bear to the sea

perhaps relatively small, but certainly absolutely large, amounts of

disintegrated rock. [Footnote: The conditions favorable to the

production of sand from disintegrated rock, by causes now in action, are

perhaps nowhere more perfectly realized than in the Sinaitic Peninsula.

The mountains are steep and lofty, unprotected by vegetation or even by

a coating of earth, and the rocks which compose them are in a shattered

and fragmentary condition. They are furrowed by deep and precipitous

ravines, with beds sufficiently inclined for the rapid flow of water,

and generally without basins in which the larger blocks of stone rolled

by the torrents can be dropped and left in repose; there are severe

frosts and much snow on the higher summits and ridges, and the winter

rains are abundant and heavy. The mountains are principally of igneous

formation, but many of the less elevated peaks are capped with

sandstone, and on the eastern slope of the peninsula you may sometimes

see, at a single glance, several lofty pyramids of granite, separated by

considerable intervals, and all surmounted by horizontally stratified

deposits of sandstone often only a few yards square, which correspond to

each other in height, are evidently contemporaneous in origin, and were

once connected in continuous beds. The degradation of the rock on which

this formation rests is constantly bringing down masses of it, and

mingling them with the basaltic, porphyritic, granitic, and calcareous

fragments which the torrents carry down to the valleys, and, through

them, in a state of greater or less disintegration, to the sea. The

quantity of sand annually washed into the Red Sea by the larger torrents

of the Lesser Peninsula, is probably at least equal to that contributed

to the ocean by any streams draining basins of no greater extent.

Absolutely considered, then, the mass may be said to be large, but it is

apparently very small as compared with the sand thrown up by the German

Ocean and the Atlantic on the coasts of Denmark and of France. There

are, indeed, in Arabia Petraea, many torrents with very short courses,

for the sea-waves in many parts of the peninsular coast wash the base of

the mountains. In these cases, the debris of the rocks do not reach the

sea in a sufficiently comminuted condition to be entitled to the

appellation of sand, or even in the form of well-rounded pebbles. The

fragments retain their annular shape, and, at some points on the coast,

they become cemented together by lime or other binding substances held

in solution or mechanical suspension in the sea-water, and are so

rapidly converted into a singularly heterogeneous conglomerate, that one

deposit seems to be consolidated into a breccia before the next winter's

torrents cover it with another.

In the northern part of the peninsula there are extensive deposits of

sand intermingled with agate pebbles and petrified wood, but these are

evidently neither derived from the Sinaitic group, nor products of local

causes known to be now in action.

I may here notice the often repeated but mistaken assertion, that the

petrified wood of the Western Arabian desert consists wholly of the

stems of palms, or at least of endogenous vegetables. This is an error.

I have myself picked up in that desert, within the space of a very few

square yards, fragments apparently of fossil palms, and of at least two

petrified trees distinctly marked as of exogenous growth both by annular

structure and by knots. In ligneous character, one of these almost

precisely resembles the grain of the extant beech, and this specimen was

worm-eaten before it was converted into silex.]

The quantity of sand and gravel carried into the Mediterranean by the

torrents of the Maritime Alps, the Ligurian Apennines, the islands of

Corsica, Sardinia, and Sicily, and the mountains of Calabria, is

apparently great. In mere mass, it is possible, if not probable, that as

much rocky material, more or less comminuted, is contributed to the

basin of the Mediterranean by Europe, even excluding the shores of the

Adriatic and the Euxine, as is washed up from it upon the coasts of

Northern Africa and Syria. A great part of this material is thrown out

again by the waves on the European shores of that sea. The harbors of

Luni, Albenga, San Remo, and Savona west of Genoa, and of Porto Fino on

the other side, are filling up, and the coast near Carrara and Massa is

said to have advanced upon the sea to a distance of 475 feet in

thirty-three years. [Footnote: Bottger, Das Mittelmeer, p. 128.] Besides

this, we have no evidence of the existence of deep-water currents in the

Mediterranean, extensive enough and strong enough to transport quartzose

sand across the sea. It may be added that much of the rock from which

the torrent sands of Southern Europe are derived contains little quartz,

and hence the general character of these sands is such that they must be

decomposed or ground down to an impalpable slime, long before they could

be swept over to the African shore.

Sands of Northern Africa.

The torrents of Europe, then, do not at present furnish the material

which composes the beach sands of Northern Africa, and it is equally

certain that those sands are not brought down by the rivers of the

latter continent. They belong to a remote geological period, and have

been accumulated by causes which we cannot at present assign. The wind

does not stir water to great depths with sufficient force to disturb the

bottom, [Footnote: The testimony of divers and of other observers on

this point is conflicting, as might be expected from the infinite

variety of conditions by which the movement of water is affected. It is

generally believed that the action of the wind upon the water is not

perceptible at greater depths than from fifteen feet in ordinary to

eighty or ninety in extreme cases; but these estimates are probably very

considerably below the truth. Andresen quotes Bremontier as stating that

the movement of the waves sometimes extends to the depth of five hundred

feet, and he adds that others think it may reach to six or even seven

hundred feet below the surface.--Andresen, Om Klitformationen, p. 20.

Many physicists now suppose that the undulations of great bodies of

water reach even deeper. But a movement of undulation is not necessarily

a movement of translation, and besides, there is very frequently an

undertow, which tends to carry suspended bodies out to sea as powerfully

as the superficial waves to throw them on shore. Sand-banks sometimes

recede from the coast, instead of rolling towards it. Reclus informs us

that the Mauvaise, a sand-bank near the Point de Grave, on the Atlantic

coast of France, has moved five miles to the west in less than a

century.--Revue des Deux Mondes for December, 1862, p. 905.

The action of currents may, in some cases, have been confounded with

that of the waves. Sea-currents, strong enough, possibly, to transport

sand for some distance, flow far below the surface in parts of the open

ocean, and in narrow straits they have great force and velocity. The

divers employed at Constantinople in 1853 found in the Bosphorus, at the

depth of twenty-five fathoms and at a point much exposed to the wash

from Galata and Pera, a number of bronze guns supposed to have belonged

to a ship-of-war blown up about a hundred and fifty years before. These

guns were not covered by sand or slime, though a crust of earthy matter,

an inch in thickness, adhered to their upper surfaces, and the bottom of

the strait appeared to be wholly free from sediment. The current was so

powerful at this depth that the divers were hardly able to stand, and a

keg of nails, purposely dropped into the water, in order that its

movements might serve as a guide in the search for a bag of coin

accidentally lost overboard from a ship in the harbor, was rolled by the

stream several hundred yards before it stopped.] and the sand thrown

upon the coast in question must be derived from a narrow belt of sea. It

must hence, in time, become exhausted, and the formation of new

sand-banks and dunes upon the southern shores of the Mediterranean will

cease at last for want of material. [Footnote: Few seas have thrown up

so much sand as the shallow German Ocean; but there is some reason to

think that the amount of this material now cast upon its northern shores

is less than at some former periods, though no extensive series of

observations on this subject has been recorded. On the Spit of Agger, at

the present outlet of the Liimfjord, Andresen found the quantity during

ten years, on a beach about five hundred and seventy feet broad, equal

to an annual deposit of an inch and a half over the whole surface.--Om

Klitformationen, p. 56. This gives seventy-one and a quarter cubic feet

to the running foot--a quantity certainly much smaller than that cast up

by the same sea on the shores of the Dano-German duchies and of Holland,

and, as we have seen, scarcely one-fourth of that deposited by the

Atlantic on the coast of Gascony.]

But even in the cases where the accumulations of sand in extensive

deserts appear to be of marine formation, or rather aggregation, and to

have been brought to their present position by upheaval, they are not

wholly composed of material collected or distributed by the currents of

the sea; for, in all such regions, they continue to receive some small

contributions from the disintegration of the rocks which underlie, or

crop out through, the superficial deposits. [Footnote: See, on this

subject, an article in Aus der Natur, vol. xxx., p. 590.

The Florentine Frescobaldi, who visited the Sinaitic peninsula five

hundred years ago, observed the powerful action of the solar heat in the

disintegration of the desert rocks. "This place," says he, "was a ridge

of rocks burnt to powder by the sun, and this powder is blown away from

the rock by the wind and is the sand of the desert; and there be many

hills which are pure bare rock, and when the sun parcheth them, the wind

carries off the dust, and other sand is there none in that

land,"--Viaggio, pp. 69, 70. In Arabia Petraea, when a wind, powerful

enough to scour down below the ordinary surface of the desert and lay

bare a fresh bed of stones, is followed by a sudden burst of sunshine,

the dark agate pebbles are often cracked and broken by the heat; and

this is the true explanation of the occurrence of the fragments in

situations where the action of fire is not probable. If the fragments

are small enough to be rolled by the winds, they are in time ground down

to sand and contribute to the stock of that material which covers the

face of the desert, though the sand thus formed is but an infinitesimal

proportion of the whole.] In some instances, too, as in Northern Africa,

additions are constantly made to the mass by the prevalence of

sea-winds, which transport, or, to speak more precisely, roll the finer

beach-sand to considerable distances into the interior. But this is a

very slow process, and the exaggerations of travellers have diffused a

vast deal of popular error on the subject.

Sands of Egypt.

In the narrow valley of the Nile--which, above its bifurcation near

Cairo, is, throughout Egypt and Nubia, generally bounded by precipitous

cliffs--wherever a ravine or other considerable depression occurs in the

wall of rock, one sees what seems a stream of desert sand pouring down,

and common observers have hence concluded that the whole valley is in

danger of being buried under a stratum of infertile soil. The ancient

Egyptians apprehended this, and erected walls, often of unburnt brick,

across the outlet of gorges and lateral valleys, to check the flow of

the sand-streams. In later ages, these walls have mostly fallen into

decay, and no preventive measures against such encroachments are now

resorted to. But the extent of the mischief to the soil of Egypt, and

the future danger from this source, have been much overrated. The sand

on the borders of the Nile is neither elevated so high by the wind, nor

transported by that agency in so great masses, as is popularly supposed;

and of that which is actually lifted or rolled and finally deposited by

air-currents, a considerable proportion is either calcareous, and,

therefore, readily decomposable, or in the state of a very fine dust,

and so, in neither case, injurious to the soil. There are, indeed, both

in Africa and in Arabia, considerable tracts of fine, silicious sand,

which may be carried far by high winds, but these are exceptional cases,

and in general the progress of the desert sand is by a rolling motion

along the surface. [Footnote: Sand heaps, three and even six hundred

feet high, are indeed formed by the wind, but this is effected by

driving the particles up an inclined plane, not by lifting them.

Bremontier, speaking of the sand-hills on the western coast of France,

says: "The particles of sand composing them are not large enough to

resist wind of a certain force, nor small enough to be taken up by it,

like dust; they only roll along the surface from which they are

detached, and, though moving with great velocity, they rarely rise to a

greater height than three or four inches."--Memoirs sur les Dunes,

Annales des Ponts et Chaussecs, 1833, ler semestre, p, 148.

Andresen says that a wind, having a velocity of forty feet per second,

is strong enough to raise particles of sand as high as the face and eyes

of a man, but that, in general, it rolls along the ground, and is

scarcely ever thrown more than to the height of a couple of yards from

the surface. Even in these cases, it is carried forward by a hopping,

not a continuous, motion; for a very narrow sheet or channel of water

stops the drift entirely, all the sand dropping into it until it is

filled up.

Blake observes, Pacific Railroad Report, vol. v., p. 242, that the sand

of the Colorado desert does not rise high in the air, but bounds along

on the surface or only a few inches above it.

The character of the motion of sand drifts is well illustrated by an

interesting fact not much noticed hitherto by travellers in the East. In

situations where the sand is driven through depressions in rock-beds, or

over deposits of silicious pebbles, the surface of the stone is worn and

smoothed much more effectually than it could be by running water, and I

have picked up, in such localities, rounded, irregularly broken

fragments of agate, which had received from the attrition of the sand as

fine a polish as could be given them by the wheel of the lapidary.

Very interesting observations, by Blake, on the polishing of hard stones

by drifting sand will be found in the Pacific Railroad Report, vol. v.,

pp. 92, 230, 231. The grinding and polishing power of sand has lately

received a new and most ingenious application in America. Jets of sand,

and even of small particles of softer substances, thrown with a certain

force, are found capable of cutting the hardest minerals and metals. A

block of corundum, some inches thick, has been bored through in a few

minutes by this process, and it promises to be highly useful in

glass-cutting and other similar operations.] So little is it lifted, and

so inconsiderable is the quantity yet remaining on the borders of Egypt,

that a wall four or five feet high suffices for centuries to check its

encroachments. This is obvious to the eye of every observer who prefers

the true to the marvellous; but the old-world fable of the overwhelming

of caravans by the fearful simoom--which even the Arabs no longer

repeat, if indeed they are the authors of it--is so thoroughly rooted in

the imagination of Christendom that most desert travellers, of the

tourist class, think they shall disappoint the readers of their journals

if they do not recount the particulars of their escape from being buried

alive by a sand-storm, and the popular demand for a "sensation" must be

gratified accordingly. [Footnote: Wilkinson says that, in much

experience in the most sandy parts of the Libyan desert, and much

inquiry of the best native sources, he never saw or heard of any

instance of danger to man or beast from the mere accumulation of sand

transported by the wind. Chesney's observations in Arabia, and the

testimony of the Bedouins he consulted, are to the same purpose. The

dangers of the simoom are of a different character, though they are

certainly aggravated by the blinding effects of the light particles of

dust and sand borne along by it, and by that of the inhalation of them

upon the respiration. ]

Another circumstance is necessary to be considered in estimating the

danger to which the arable lands of Egypt are exposed. The prevailing

wind in the valley of the Nile and its borders is from the north, and it

may be said without exaggeration that the north wind blows for

three-quarters of the year. [Footnote: In the narrow valley of the Nile,

bounded as it is, above the Delta, by high cliffs, all air-currents from

the northern quarter become north winds, though of course varying in

partial direction, in conformity with the sinuosities of the valley.

Upon the desert plateau they incline westwards, and have already borne

into the valley the sands of the eastern banks, and driven those of the

western quite out of the Egyptian portion of the Nile basin.] The effect

of winds blowing up the valley is to drive the sands of the desert

plateau which border it, in a direction parallel with the axis of the

valley, not transversely to it; and if it ran in a straight line, the

north wind would carry no desert sand into it. There are, however, both

curves and angles in its course, and hence, wherever its direction

deviates from that of the wind, it might receive sand-drifts from the

desert plain through which it runs. But, in the course of ages, the

winds have, in a great measure, bared the projecting points of their

ancient deposits, and no great accumulations remain in situations from

which either a north or a south wind would carry them into the valley.

[Footnote: These considerations apply, with equal force, to the supposed

danger of the obstruction of the Suez Canal by the drifting of the

desert sands. The winds across the isthmus are almost uniformly from the

north, and they swept it comparatively clean of flying sands long ages

since. The traces of the ancient canal between the Red Sea and the Nile

are easily followed for a considerable distance from Suez. Had the

drifts upon the isthmus been as formidable as some have feared and

others have hoped, those traces would have been obliterated, and Lake

Timsah and the Bitter Lakes filled up, many centuries ago. The few

particles driven by the rare east and west winds towards the line of the

canal, will easily be arrested by plantations or other simple methods,

or removed by dredging. The real dangers and difficulties of this

magnificent enterprise--and they have been great--consisted in the

nature of the soil to be removed in order to form the line, and

especially in the constantly increasing accumulation of sea-sand at the

southern terminus by the tides of the Red Sea, and of sand and Nile

slime at the northern, by the action of the winds and currents. Both

seas are shallow for miles from the shore, and the excavation and

maintenance of deep channels, and of capacious harbors with easy and

secure entrances, in such localities, is doubtless one of the hardest

problems offered to modern engineers for practical solution. See post,

Geological Importance of Dunes, note.]

The sand let fall in Egypt by the north wind is derived, not from the

desert, but from a very different source--the sea. Considerable

quantities of sand are thrown up by the Mediterranean, at and between

the mouths of the Nile, and indeed along almost the whole southern coast

of that sea, and drifted into the interior to distances varying

according to the force of the wind and the abundance and quality of the

material. The sand so transported contributes to the gradual elevation

of the Delta, and of the banks and bed of the river itself. But just in

proportion as the bed of the stream is elevated, the height of the water

in the annual inundations is increased also, and as the inclination of

the channel is diminished, the rapidity of the current is checked, and

the deposition of the slime it holds in suspension consequently

promoted. Thus the winds and the water, moving in contrary directions,

join in producing a common effect.

The sand, blown over the Delta and the cultivated land higher up the

stream during the inundation, is covered or mixed with the fertile earth

brought down by the river, and no serious injury is sustained from it.

That spread over the same ground after the water has subsided, and

during the short period when the soil is not stirred by cultivation or

covered by the flood, forms a thin pellicle over the surface as far as

it extends, and serves to divide and distinguish the successive layers

of slime deposited by the annual inundations. The particles taken up by

the wind on the sea-beach are borne onward, by a hopping motion, or

rolled along the surface, until they are arrested by the temporary

cessation of the wind, by vegetation, or by some other obstruction, and

they may, in process of time, accumulate in large masses, under the lee

of

rocky projections, buildings, or other barriers which break the force of

the wind.

In these facts we find an important element in the explanation of the

sand drifts, which have half buried the Sphinx and so many other ancient

monuments in that part of Egypt. These drifts, as I have said, are not

wholly from the desert, but in largo proportion from the sea; and, as

might be supposed from the distance they have travelled, they have been

long in gathering. While Egypt was a great and flourishing kingdom,

measures were taken to protect its territory against the encroachment of

sand, whether from the desert or from the Mediterranean; but the foreign

conquerors, who destroyed so many of its religious monuments, did not

spare its public works, and the process of physical degradation

undoubtedly began as early as the Persian invasion. The urgent

necessity, which has compelled all the successive tyrannies of Egypt to

keep up some of the canals and other arrangements for irrigation, was

not felt with respect to the advancement of the sands; for their

progress was so slow as hardly to be perceptible in the course of a

single reign, and long experience has shown that, from the natural

effect of the inundations, the cultivable soil of the valley is, on the

whole, trenching upon the domain of the desert, not retreating before

it.

The oases of the Libyan, as well as of many Asiatic deserts, have no

such safeguards. The sands are fast encroaching upon them, and threaten

soon to engulf them, unless man shall resort to artesian wells and

plantations, or to some other efficient means of checking the advance of

this formidable enemy, in time to save these islands of the waste from

final destruction.

Accumulations of sand are, in certain cases, beneficial as a protection

against the ravages of the sea; but, in general, the vicinity, and

especially the shifting of bodies of this material, are destructive to

human industry, and hence, in civilized countries, measures are taken to

prevent its spread. This, however, can be done only where the population

is large and enlightened, and the value of the soil, or of the

artificial erections and improvements upon it, is considerable. Hence in

the deserts of Africa and of Asia, and thee inhabited lands which border

on them, no pains are usually taken to check the drifts, and when once

the fields, the houses, the springs, or the canals of irrigation are

covered or choked, the district is abandoned without a struggle, and

surrendered to perpetual desolation. [Footnote: In parts of the Algerian

desert, some efforts are made to retard the advance of sand dunes which

threaten to overwhelm villages. "At Debila," says Laurent, "the lower

parts of the lofty dunes are planted with palms, ... but they are

constantly menaced with burial by the sands. The only remedy employed by

the natives consists in little dry walls of crystallized gypsum, built

on the crests of the dunes, together with hedges of dead palm-leaves.

These defensive measures are aided by incessant labor; for every day the

people take up in baskets the sand blown over to them the night before

and carry it back to the other side of the dune."--Memoires sur le

Sahara, p. 14.]

Sand Dunes and Sand Plains.

Two forms of sand deposit are specially important in European and

American geography. The one is that of dune or shifting hillock upon the

coast, the other that of barren plain in the interior. The coast-dunes

are composed of sand washed up from the depths of the sea by the waves,

and heaped in more or less rounded knolls and undulating ridges by the

winds. The sand with which many plains are covered appears sometimes to

have been deposited upon them while they were yet submerged beneath the

sea, sometimes to have been drifted from the seacoast, and scattered

over them by wind-currents, sometimes to have been washed upon them by

running water. In these latter cases, the deposit, though in itself

considerable, is comparatively narrow in extent and irregular in

distribution, while, in the former, it is often evenly spread over a

very wide surface. In all great bodies of either sort, the silicious

grains are the principal constituent, though, when not resulting from

the disintegration of silicious rock and still remaining in place, they

are generally accompanied with a greater or less admixture of other

mineral particles, and of animal and vegetable remains, [Footnote:

Organic constituents, such as comminuted shells, and silicious and

calcareous exuviae of infusorial animals and plants, are sometimes found

mingled in considerable quantities with mineral sands. These are usually

the remains of aquatic vegetables or animals, but not uniformly so, for

the microscopic organisms, whose flinty cases enter so largely into the

sand-beds of the Mark of Brandenburg, are still living and prolific in

the dry earth. See Wittwer, Physikalische Geographic, p. 142. The desert

on both sides of the Nile is inhabited by a land-snail--of which I have

counted eighty, in estimation, on a single shrub barely a foot high--and

thousands of its shells are swept along and finally buried in the drifts

by every wind. Every handful of the sand contains fragments of them.

Forchhammer, in Leonhard und Bronn s Jahrbuch, 1841, p. 8, says of the

sand-hills of the Danish coast: "It is not rare to find, high in the

knolls, marine shells, and especially those of the oyster. They are due

to the oyster-eater [Haemalopus ostralegus], which carries his prey to

the top of the dunes to devour it." See also Staring, De Bodem van

Nederland, i., p. 821.] and they are also, usually somewhat changed in

consistence by the ever-varying conditions of temperature and moisture

to which they have been exposed since their deposit. Unless the

proportion of these latter ingredients is so large as to create a

considerable adhesiveness in the mass--in which case it can no longer

properly be called sand--it is infertile, and, if not charged with

water, partially agglutinated by iron, lime, or other cement, or

confined by alluvion resting upon it, it is much inclined to drift,

whenever, by any chance, the vegetable network which, in most cases,

thinly clothes and at the same time confines it, is broken. Human

industry has not only fixed the flying dunes by plantations, but, by

mixing clay and other tenacious earths with the superficial stratum of

extensive sand plains, and by the application of fertilizing substances,

it has made them abundantly productive of vegetable life. These latter

processes belong to agriculture and not to geography, and, therefore,

are not embraced within the scope of the present subject. But the

preliminary steps, whereby wastes of loose, drifting barren sands are

transformed into wooded knolls and plains, and finally, through the

accummulation of vegetable mould, into arable ground, constitute a

conquest over nature which precedes agriculture--a geographical

revolution--and, therefore, an account of the means by which the change

has been effected belongs properly to the history of man's influence on

the great features of terrestrial surface. I proceed, then, to examine

the structure of dunes, and to describe the warfare man wages with the

sand-hills, striving on the one hand to maintain and even extend them,

as a natural barrier against encroachments of the sea, and, on the

other, to check their moving and wandering propensities, and prevent

them from trespassing upon the fields he has planted and the habitations

in which he dwells.

COAST DUNES.

Coast dunes are oblong ridges or round hillocks, formed by the action of

the wind upon sands thrown up by the waves on the low beaches of seas,

and sometimes of fresh-water lakes. On most coasts, the supply of sand

for the formation of dunes is derived from tidal waves. The flow of the

tide is more rapid, and consequently its transporting power greater,

than that of the ebb; the momentum, acquired by the heavy particles in

rolling in with the water, tends to carry them even beyond the flow of

the waves; and at the turn of the tide, the water is in a state of

repose long enough to allow it to let fall much of the solid matter it

holds in suspension. Hence, on all low, tide-washed coasts of seas with

sandy bottoms, there exist several conditions favorable to the formation

of sand deposits along high-water mark. [Footnote: There are various

reasons why the formation of dunes is confined to low shores, and this

law is so universal, that when bluffs are surmounted by them, there is

always cause to suspect upheaval, or the removal of a sloping beach in

front of the bluff, after the dunes were formed. Bold shores are usually

without a sufficient beach for the accumulation of large deposits; they

are commonly washed by a sea too deep to bring up sand from its bottom;

their abrupt elevation, even if moderate in amount, would still be too

great to allow ordinary winds to lift the sand above them; and their

influence in deadening the wind which blows towards them would even more

effectually prevent the raising of sand from the beach at their foot.

Forchhammer, describing the coast of Jutland, says that, in high winds,

"one can hardly stand upon the dunes, except when they are near the

water line and have been cut down perpendicularly by the waves. Then the

wind is little or not at all felt--a fact of experience very common on

our coasts, observed on all the steep shore bluffs of 200 feet height,

and, in the Faroe Islands, on precipices 2,000 feet high. In heavy gales

in those islands, the cattle fly to the very edge of the cliffs for

shelter, and frequently fall over. The wind, impinging against the

vertical wall, creates an ascending current which shoots somewhat past

the crest of the rock, and thus the observer or the animal is protected

against the tempest by a barrier of air."-Leonhard und Bronn, Jahrbuch,

1841, p. 3. The calming, or rather diversion, of the wind by cliffs

extends to a considerable distance in front of them, and no wind would

have sufficient force to raise the sand vertically, parallel to the face

of a bluff, even to the height of twenty feet.] If the land-winds are of

greater frequency, duration, or strength than the sea-winds, the sands

left by the retreating wave will be constantly blown back into the

water; but if the prevailing air-currents are in the opposite direction,

the sands will soon be carried out of the reach of the highest waves,

and transported continually farther and farther into the interior of the

land, unless obstructed by high grounds, vegetation, or other obstacles.

The laws which govern the formation of dunes are substantially these. We

have seen that, under certain conditions, sand is accumulated above

high-water mark on low sea and lake shores. So long as the sand is kept

wet by the spray or by capillary attraction, it is not disturbed by

air-currents, but as soon as the waves retire sufficiently to allow it

to dry, it becomes the sport of the wind, and is driven up the gently

sloping beach until it is arrested by stones, vegetables, or other

obstructions, and thus an accumulation is formed which constitutes the

foundation of a dune. However slight the elevation thus created, it

serves to stop or retard the progress of the sand-grains which are

driven against its shoreward face, and to protect from the further

influence of the wind the particles which are borne beyond it, or rolled

over its crest, and fall down behind it. If the shore above the beach

line were perfectly level and straight, the grass or bushes upon it of

equal height, the sand thrown up by the waves uniform in size and weight

of particles as well as in distribution, and if the action of the wind

were steady and regular, a continuous bank would be formed, everywhere

alike in height and cross section. But no such constant conditions

anywhere exist. The banks are curved, broken, unequal in elevation; they

are sometimes bare, sometimes clothed with vegetables of different

structure and dimensions; the sand thrown up is variable in quantity and

character; and the winds are shifting, gusty, vertical, and often

blowing in very narrow currents. From all these causes, instead of

uniform hills, there rise irregular rows of sand-heaps, and these, as

would naturally be expected, are of a pyramidal, or rather conical

shape, and connected at bottom by more or less continuous ridges of the

same material.

Elisee Reclus, in describing the coast dunes of Gascony, observes that

when, as sometimes happens, the sands are not heaped in a continuous,

irregular bulwark, but deposited in isolated hillocks, they have a

tendency to assume a crescent shape, the convexity being turned

seawards, or towards the direction from which the prevailing winds

proceed. This fact, the geological bearing of which is obvious, is not

noticed by previous French writers or even by Andresen, though a

semi-lunar outline has been long generally ascribed to inland dunes. It

is, however evident that such a form would naturally be produced by the

action of a wind blowing long in a given direction upon a mass of loose

sand with a fixed centre--such as is constituted by the shrub or stone

around which the sand is first deposited--and free extremities. On a

receding coast, dunes will not attain so great a height as on more

secure shores, because they are undermined and carried off before they

have time to reach their greatest dimensions. Hence, while at sheltered

points in South-western France, there are dunes three hundred feet or

more in height, those on the Frisic Islands and the exposed parts of the

coast of Schleswig-Holstein range only from twenty to one hundred feet.

On the western shores of Africa, it is said that they sometimes attain

an elevation of six hundred feet. This is one of the very few points

known to geographers where desert sands are advancing seawards,

[Footnote: "On the west coast of Africa the dunes are drifting seawards,

and always receiving new accessions from the Sahara. They are constantly

advancing out into the sea."--Naumann, Geognosie, ii., p.1172.] and here

they rise to the greatest altitude to which sand-grains can be carried

by the wind. The hillocks, once deposited, are held together and kept in

shape, partly by mere gravity, and partly by the slight cohesion of the

lime, clay, and organic matter mixed with the sand; and it is observed

that, from capillary attraction, evaporation from lower strata, and

retention of rain-water, they are always moist a little below the

surface. [Footnote: "Dunes are always full of water, from the action of

capillary attraction. Upon the summits, one seldom needs to dig more

than a foot to find the sand moist, and in the depressions, fresh water

is met with near the surface."--Forchhammer, in Leonhard and Bronx, for

1841, p.5, note. On the other hand, Andresen, who has very carefully

investigated this as well as all other dune phenomena, maintains that

the humidity of the sand ridges cannot be derived from capillary

attraction. He found by experiment that a heap of drift-sand was not

moistened to a greater height than eight and a half inches, after

standing with its base a whole night in water. He states the minimum of

water contained by the sand of the dunes, one foot below the surface,

after a long drought, at two per cent, the maximum, after a rainy month,

at four per cent. At greater depths the quantity is larger. The

hygroscopicity of the sand of the coast of Jutland he found to be

thirty-three per cent, by measure, or 21.5 by weight. The annual

precipitation on that coast is twenty-seven inches, and as the

evaporation is about the same, he argues that rain-water does not

penetrate far beneath the surface of the dunes, and concludes that their

humidity can be explained only by evaporation from below.--Om

Klitformationen, pp. 106-110. In the dunes of Algeria, water in so

abundant that wells are constantly dug in them at high points on their

surface. They are sunk to the depth of three or four inches only, and

the water rises to the height of a metre in them.--Laurent, Memoire sur

le Sahara, pp. 11, 12, 13. The same writer observes (p. 14) that the

'hollows in the dunes are planted with palms which find moisture enough

a little below the surface. It would hence seem that proposal to fix the

dunes which are supposed to threaten the Suez Canal, by planting the

maratime pine and other trees upon them, is not altogether so absurd as

it has been thought to be by some of those disinterested philanthropists

of other nations who were distressed with fears that French capitalists

would lose the money they had invested in that great undertaking. Ponds

of water are often found in the depression between the sand-hills of the

dune chains in the North American desert.]

By successive accumulations, they gradually rise to the height of

thirty, fifty, sixty, or a hundred feet, and sometimes even much higher.

Strong winds, instead of adding to their elevation, sweep off loose

particles from their surface, and these, with others blown over or

between them, build up a second row of dunes, and so on according to the

character of the wind, the supply and consistence of the sand, and the

face of the country. In this way is formed a belt of sand-dunes,

irregularly dispersed and varying much in height and dimensions, and

sometimes many miles in breadth. On the Island of Sylt, in the German

Sea, where there are several rows, the width of the belt is from half a

mile to a mile. There are similar ranges on the coast of Holland,

exceeding two miles in breadth, while at the mouths of the Nile they

form a zone not less than ten miles wide. The base of some of the dunes

in the Delta of the Nile is reached by the river during the annual

inundation, and the infiltration of the water, which contains lime, has

converted the lower strata into a silicious limestone, or rather a

calcarous sandstone, and thus afforded an opportunity of studying the

structure of that rock in a locality where its origin and mode of

aggregation and solidification are known.

The tide, though a usual, is by no means a necessary condition for the

accumulations of sand out of which dunes are formed. The Baltic and the

Mediterranean are almost tideless seas, but there are vast ranges of

dunes on the Russian and Prussian coasts of the Baltic and at the mouths

of the Nile and many other points on the shores of the Mediterranean.

The vast shoals in the latter sea, known to the ancients as the Greater

and Lesser Syrtis, are of marine origin. They are still filling up with

sand, washed up from greater depths, or sometimes drifted from the coast

in small quantities, and will probably be converted, at some future

period, into dry land covered with sand-hills. There are also extensive

ranges of dunes upon the eastern shores of the Caspian, and at the

southern, or rather south-eastern, extremity of Lake Michigan.

[Footnote: The careful observations of Colonel J. D. Graham, of the

United States Army, show a tide of about three inches in Lake Michigan.

See "A Lunar Tidal Wave in the North American Lakes," demonstrated by

Lieut.-Colonel J. D. Graham, in the fourteenth volume of the Proceedings

of the American Association for the Advancement of Science.] There is no

doubt that this latter lake formerly extended much farther in that

direction, but its southern portion has gradually shoaled and at last

been converted into solid land, in consequence of the prevalence of the

north-west winds. These blow over the lake a large part of the year, and

create a southwardly set of the currents, which wash up sand from the

bed of the lake and throw it on shore. Sand is taken up from the beach

at Michigan City by every wind from that quarter, and, after a heavy

blow of some hours' duration, sand ridges may be observed on the north

side of the fences, like the snow wreaths deposited by a drifting wind

in winter. Some of the particles are carried back by contrary winds, but

most of them lodge on or behind the dunes, or in the moist soil near the

lake, or are entangled by vegetables, and tend permanently to elevate

the level. Like effects are produced by constant sea-winds, and dunes

will generally be formed on all low coasts where such prevail, whether

in tideless or in tidal waters.

Jobard thus describes the modus operandi, under ordinary circumstances,

at the mouths of the Nile, where a tide can scarcely be detected: "When

a wave breaks, it deposits an almost imperceptible line of fine sand.

The next wave brings also its contribution, and shoves the preceding

line a little higher. As soon as the particles are fairly out of the

reach of the water they are dried by the heat of the burning sun, and

immediately seized by the wind and rolled or borne farther inland. The

gravel is not thrown out by the waves, but rolls backwards and forwards

until it is worn down to the state of fine sand, when it, in its turn,

is cast upon the land and taken up by the wind." [Footnote: Staring, De

Bodun van Nederland, i., p. 327, note.] This description applies only to

the common every-day action of wind and water; but just in proportion to

the increasing force of the wind and the waves, there is an increase in

the quantity of sand, and in the magnitude of the particles carried off

from the beach by it, and, of course, every storm in a landward

direction adds sensibly to the accumulation upon the shore.

Sand Banks.

Although dunes, properly so called, are found only on dry land and above

ordinary high-water mark, and owe their elevation and structure to the

action of the wind, yet, upon many shelving coasts, accumulations of

sand much resembling dunes are formed under water at some distance from

the shore by the oscillations of the waves, and are well known by the

name of sand banks. They are usually rather ridges than banks, of

moderate inclination, and with the steepest slope seawards, [Footnote:

Kohl, Inseln und Marschen Schleswig Holsteins, ii., p. 33. From a

drawing in Andresen, Om Klitformationen, p. 24, it would appear that on

the Schleswig coast the surf-formed banks have the steepest slope

landwards, those farther from the shore, as stated in the text.] and

their form differs little from that of dunes except in this last

particular and in being lower and more continuous. Upon the western

coast of the island of Amrum, for example, there are three rows of such

banks, the summits of which are at a distance of perhaps a couple of

miles from each other; so that, including the width of the banks

themselves, the spaces between them, and the breadth of the zone of

dunes upon the land, the belt of moving sands on that coast is probably

not less than eight miles wide.

Under ordinary circumstances, sand banks are always rolling, landwards,

and they compose the magazine from which the material for the dunes is

derived. [Footnote: Sand banks sometimes connect themselves with the

coast at both ends, and thus cut off a portion of the sea. In this case,

as well as when salt water is enclosed by sea-dikes, the water thus

separated from the ocean gradually becomes fresh, or at least brackish.

The Haffs, or large expanses of fresh water in Eastern Prussia--which

are divided from the Baltic by narrow sand banks called Nehrungen, or,

at sheltered points of the coast, by fluviatile deposits called

Werders--all have one or more open passages, through which the water of

the rivers that supply them at last finds its way to the sea.] The

dunes, in fact, are but aquatic sand banks transferred to dry land. The

laws of their formation are closely analogous, because the action of the

two fluids, by which they are respectively accumulated and built up, is

very similar when brought to bear upon loose particles of solid matter.

It would, indeed, seem that the slow and comparatively regular movements

of the heavy, unelastic water ought to affect such particles very

differently from the sudden and fitful impulses of the light and elastic

air. But the velocity of the wind currents gives them a mechanical force

approximating to that of the slower waves, and, however difficult it may

be to explain all the phenomena that characterize the structure of the

dunes, observation has proved that it is nearly identical with that of

submerged sand banks. [Footnote: Forchhammer ascribes the resemblance

between the furrowing of the dune sands and the beach ripples, not to

the similarity of the effect of wind and water upon sand, but wholly to

the action of the wind; in the first instance, directly, in the latter,

through the water. "The wind-ripples on the surface of the dunes

precisely resemble the water-ripples of sand flats occasionally

overflowed by the sea; and with the closest scrutiny, I have never been

able to detect the slightest difference between them. This is easily

explained by the fact, that the water-ripples are produced by the action

of light wind on the water which only transmits the air-waves to the

sand."--Leonhard und Bronn, 1841, pp. 7, 8.] The differences of form are

generally ascribable to the greater number and variety of surface

accidents of the ground on which the sand hills of the land are built

up, and to the more frequent changes, and wider variety of direction, in

the courses of the wind.

CHARACTER OF DUNE SAND.

"Dune sand," says Staring, "consists of well-rounded grains of quartz,

more or less colored by iron, and often mingled with fragments of

shells, small indeed, but still visible to the naked eye. [Footnote:

According to the French authorities, the dunes of France are not always

composed of quartzose sand. "The dune sands" of different characters,

says Bremontier, "partake of the nature of the different materials which

compose them. At certain points on the coast of Normandy they are found

to be purely calcareous; they are of mixed composition on the shores of

Brittany and Saintonge, and generally quartzose between the mouth of the

Gironde and that of the Adour."--Memoire sur les Dunes, Annales des

Ponts et Chaussees, t. vii., 1833, 1er semestre, p. 146.

In the dunes of Long Island and of Jutland, there are considerable veins

composed almost wholly of garnet. For a very full examination of the

mechanical and chemical composition of the dune sands of Jutland, see

Andresen, Om Klitformationen, p. 110. Fraas informs us, Aus dem Orient,

pp. 176, 177, that the dune sands of the Egyptian coast arise from the

disintegration of the calcareous sandstone of the same region. This

sandstone, composed in a large proportion of detritus of both land and

sea shells mingled with quartz sand, appears to have been consolidated

under water during an ancient period of subsidence. A later upheaval

brought it to or near the surface, when it was more or less

disintegrated by the action of the waves and by meteoric influences--a

process still going on--and it is now again subsiding with the coast it

rests on.

The calcareous sand arising from the comminution of corals forms dunes

on some of the West India Islands.--Agassiz, Bulletin of the Museum of

Comparative Zoology, vol. i.] These fragments are not constant

constituents of dune sand. They are sometimes found at the very summits

of the hillocks, as at Overveen; in the King's Dune, near Egmond, they

form a coarse, calcareous gravel very largely distributed through the

sand, while the interior dunes between Haarlem and Warmond exhibit no

trace of them. It is yet undecided whether the presence or absence of

these fragments is determined by the period of the formation of the

dunes, or whether it depends on a difference in the process by which

different dunes have been accumulated. Land shells, such as snails, for

example, are found on the surface of the dunes in abundance, and many of

the shelly fragments in the interior of the hillocks may be derived from

the same source." [Footnote: De Bodem van Nederland, i., p. 323.]

Sand concretions form within the dunes and especially in the depressions

between them. These are sometimes so extensive and impervious as to

retain a sufficient supply of water to feed perennial springs, and to

form small permanent ponds, and they are a great impediment to the

penetration of roots, and consequently to the growth of trees planted,

or germinating from self-sown seeds, upon the dunes. [Footnote: Staring,

De Bodem van Nederland, i., p.317. See also Bergsoe, Reventlov's

Virksomhed, ii., p. 11.

"In the sand-hill ponds mentioned in the text, there is a vigourous

growth of bog plants accompanied with the formation of peat, which goes

on regularly as long as the dune sand does not drift. But if the surface

of the dunes is broken, the sand blows into the ponds, covers the peat,

and puts and end to its formation. When, in the course of time, marine

currents cut away the coast, the dunes move landwards and fill up the

ponds and thus are formed the remarkable strata of fossile peat called

Martorv, which appears to be unknown to the geologists of other parts of

Europe." -- Forchhammer, in Leonhard und Bronn, 1841, p. 18. Martorv has

a specific gravity thrice as great as that of ordinary peat in

consequence of the pressure of the sand.--Asbjornsen, Torv og Torvdrift,

p.26.]

Interior Structure of Dunes.

The interior structure of the dunes, the arrangement of their particles,

is not, as might be expected, that of an unorganized, confused heap, but

they show a strong tendency to stratification. This is a point of much

geological interest, because it indicates that sandstone may owe its

stratified character to the action of other forces as well as of water.

The origin and peculiar character of these layers are due to a variety

of causes.

For example, a south-west wind and current may deposit upon a dune a

stratum of a given color and mineral composition, and this may be

succeeded by a north-west wind and current, bringing with them particles

of a different hue, constitution, and origin. Again, if we suppose a

violent tempest to strew the beach with sand-grains very different in

magnitude and specific gravity, and, after the sand is dry, to be

succeeded by a gentle breeze, it is evident that only the lighter

particles will be taken up and carried to the dunes. If, after some

time, the wind freshens, heavier grains will be transported and

deposited on the former, and a still stronger succeeding gale will roll

up yet larger kernels. Each of these deposits will form a stratum. If we

suppose the tempest to be followed, after the sand is dry, not by a

gentle breeze, but by a wind powerful enough to lift at the same time

particles of very various magnitudes and weights, the heaviest will

often lodge on the dune while the lighter will be carried farther. This

would produce a stratum of coarse sand, and the same effect might result

from the blowing away of light particles out of a mixed layer, while the

heavier remained undisturbed. [Footnote: The lower strata must be older

than the superficial layers, and the particles which compose them may in

time become more disintegrated, and therefore finer than those deposited

later and above them.

Hull ingeniously suggests that, besides other changes, fine sand

intermixed with or deposited above a coarser stratum, as well as the

minute particles resulting from the disintegration of the grains of the

latter, may be carried by rain in the case of dunes, or by the ordinary

action or sea-water in that of sand-banks, down through the interstices

in the coarser layer, and thus the relative position of sand and gravel

may be changed.--Oorsprong der Hollandsche Duinen, p. 103.]

Still another cause of apparent stratification may be found in the

occasional interposition of a thin layer of leaves or other vegetable

remains between successive deposits, and this I imagine to be more

frequent than has been generally supposed. Some geologists have thought

that the sand strata of dunes are of annual formation; [Footnote:

Schomann, Geologische Wanderungen durch die Preussischen Ost-See

Provinzen, 1869, p. 81.] but the autumnal deposit of foliage from

neighboring trees and shrubs furnishes a more probable explanation of

the division of the sand-heaps into regular layers.

A late distinguished American admiral communicated to me an interesting

observation made by him at San Francisco, which has an important bearing

on the arrangement of the particles of sand in dunes and other irregular

accumulations of that substance. In laying out a navy-yard at that port,

a large quantity of earthy material was removed from the dunes and other

hillocks and carted to a low piece of ground which required filling up.

Sand of various characters, fine and coarse gravel, and common earth

were dropped promiscuously by the carts as accident or convenience

dictated, and of course they were all confusedly intermixed. Some time

after, when the new ground was consolidated, various excavations were

made in it, and the different materials of which the filling was

composed were found to be stratified with considerable regularity,

according to their specific gravity.

Two explanations of this remarkable fact suggest themselves to me,

which, however, do not perhaps exclude others. San Francisco is subject

to earthquakes, and though violent or even sensible shocks are not very

frequent, it is highly probable that, as is shown to be the case in many

other countries, by late seismological observations, there are, in the

course of the year, a great number of slight shocks which escape

unscientific observation. A frequent repetition of slight tremblings of

the earth would, like any other moderate mechanical agitation, probably

produce the separation of a miscellaneous mass, like that described,

into distinct layers. Again, the Pacific coast, like all others upon an

open sea, is exposed to incessant concussion from the shock of the

waves, which is repeated many thousand times a day. This concussion is

often sensibly felt by the observer, and it seems not in the least

improbable that the agitation may have tended to produce a stratified

arrangement in the case at San Francisco, as well as in all coast dunes

and other accumulations of loose mineral material in similar situations.

Kohl observes that the shore on the landward side of the files of dunes

often trembles from the shock of the waves on the beach, [Footnote:

Inseln und Marschen, etc., ii., p. 34.] and Villeneuve established by

careful experiment that at Dunkerque the ground is sensibly agitated by

the same cause, in stormy weather, to a distance of more than a mile

from the sea.

The eddies of strong winds between the hillocks must also occasion

disturbances and re-arrangements of the sand layers, and it seems

possible that the irregular thickness and the strange contortions of the

strata of the sandstone at Petra may be due to some such cause. A

curious observation of Professor Forchhammer suggests an explanation of

another peculiarity in the structure of the sandstone of Mount Seir. He

describes dunes in Jutland, composed of yellow quartzose sand intermixed

with black titanian iron. When the wind blows over the surface of the

dunes, it furrows the sand with alternate ridges and depressions,

ripples, in short, like those of water. The swells, the dividing ridges

of the system of sand ripples, are composed of the light grains of

quartz, while the heavier iron rolls into the depressions between, and

thus the whole surface of the dune appears as if covered with a fine

black network.

The sea side of dunes, being more exposed to the caprices of the wind,

is more irregular in form than the lee or land side, where the

arrangement of the particles is affected by fewer disturbing and

conflicting influences. Hence, the stratification of the windward slope

is somewhat confused, while the sand on the lee side is found to be

disposed in more regular beds, inclining landwards, and with the largest

particles lowest, where their greater weight would naturally carry them.

The lee side of the dunes, being thus formed of sand deposited according

to the laws of gravity, is very uniform in its slope, which, according

to Forchhammer, varies little from an angle of 30 degrees with the

horizon, while the more exposed and irregular weather side lies at an

inclination of from 5 degrees to 10 degrees. When, however, the outer

tier of dunes is formed so near the waterline as to be exposed to the

immediate action of the waves, it is undermined, and the face of the

hill is very steep and sometimes nearly perpendicular.

Geological Importance of Dunes.

These observations, and other facts which a more attentive study on the

spot would detect, might furnish the means of determining interesting

and important questions concerning geological formations in localities

very unlike those where dunes are now thrown up. For example, Studer

supposes that the drifting sand-hills of the African desert were

originally coast dunes, and that they have been transported to their

present position far in the interior, by the rolling and shifting

leeward movement to which all dunes not covered with vegetation are

subject. The present general drift of the sands of that desert appears

to be to the south-west and west, the prevailing winds blowing from the

north-east and east; but it has been doubted whether the shoals of the

western coast of Northern Africa, and the sands upon that shore, are

derived from the bottom of the Atlantic, in the usual manner, or, by an

inverse process, from those of the Sahara. The latter, as has been

before remarked, is probably the truth, though observations are wanting

to decide the question. [Footnote: "The North African desert falls into

two divisions: the Sahel, or western, and the Sahar, or eastern. The

sands of the Sahar were, at a remote period, drifted to the west. In the

Sahel, the prevailing east winds drive the sand-ocean with a progressive

westward motion. The eastern half of the desert is swept

clean."--Naumann, Geognosie, ii., p. 1173.] There would be nothing

violently improbable in the a priori supposition that they may have been

in part first thrown up by the Mediterranean on its Libyan coast, and

thence blown south and west over the vast space they now cover. But

inasmuch as it is now geologically certain that the Sahara is an

uplifted bed of an ancient sea, we may suppose that, while submerged, it

was, like other sea-bottoms, strewn with sand, and that its present

supply of that material was, in great proportion, brought up with it.

Laurent observed, some years ago, that marine shells of still extinct

species were found in the Sahara, far from the sea, and even at

considerable depths below the surface. [Footnote: Memoires sur le Sahara

Oriental p. 62] These observations have been confirmed past all question

by Desor, Martins, and others, and the facts and the obvious conclusion

they suggest are at present not disputed. But whatever has been the

source and movement of these sands, they can hardly fail to have left on

their route some sandstone monuments to mark their progress, such, for

example, as we have seen are formed from the dune sand at the mouth of

the Nile; and it is conceivable that the character of the drifting sands

themselves, and of the conglomerates and sandstones to whose formation

they have contributed, might furnish satisfactory evidence as to their

origin, their starting-point, and the course by which they have wandered

so far from the sea. [Footnote: Forchhammer, after pointing out the

coincidence between the inclined stratification of dunes and the

structure of ancient tilted rocks, says: "But I am not able to point out

a sandstone formation corresponding to the dunes. Probably most ancient

dunes have been destroyed by submersion before the loose sand became

cemented to solid stone, but we may suppose that circumstances have

existed somewhere which have preserved the characteristics of this

formation."--Leonhard und Bronn, 1841, p. 8, 9. Such formations,

however, certainly exist. Laurent (Memoire sur le Sahara, etc., p. 12)

tells us that in the Algerian desert there are "sandstone formation" not

only "corresponding to the dunes," but, actually consolidated within

them. "A place called El-Mouia-Tadjer presents a repetition of what we

saw at El-Baya; one of the funnels formed in the middle of the dunes

contains wells from two metres to two and a half in depth, dug in a sand

which pressure, and probably the presence of certain salts, have

cemented so as to form true sandstone, soft indeed, but which does not

yield except to the pickaxe. These sandstones exhibit an inclination

which seems to be the effect of wind; for they conform to the direction

of the sands which roll down a scarp occasioned by the primitive

obstacle."

"At New Quay the dune sands are converted to stone by an oxide of iron

held in solution by the water which pervades them. This stone, which is

formed, so to speak, under our eye, has been found solid enough to be

employed for building."-Esquiros, L'Angleterre, etc., in Revue des Deux

Mondes, 1864, pp. 44, 45.

The dunes near the mouth of the Nile, the lower sands of which have been

cemented together by the infiltration of Nile water, would probably show

a similar stratification in the sandstone which now forms their base.

Dana describes a laminated rook often formed by the infiltration of

water into the sand dunes on the Hawaian islands.--Corals and Coral

Islands, 1872, p.155.]

If the sand of coast dunes is, as Staring describes it, composed chiefly

of well-rounded, quartzose grains, fragments of shells, and other

constant ingredients, it would often be recognizable as coast sand, in

its agglutinate state of sandstone. The texture of this rock varies from

an almost imperceptible fineness of grain to great coarseness, and

affords good facilities for microscopic observation of its structure.

There are sandstones, such, for example, as are used for grindstones,

where the grit, as it is called, is of exceeding sharpness; others where

the angles of the grains are so obtuse that they scarcely act at all on

hard metals. The former may be composed of grains of rock, disintegrated

indeed, and re-cemented together, but not, in the meanwhile, much

rolled; the latter, of sands long washed by the sea, and drifted by

land-winds. There is, indeed, so much resemblance between the effects of

driving winds and of rolling water upon light bodies, that there might

be difficulty in distinguishing them; but after all, it is not probable

that sandstone, composed of grains thrown up from the salt sea, and long

tossed by the winds, would be identical in its structure with that

formed from fragments of rock crushed by mechanical force, or

disintegrated by heat, and again agglutinated without much exposure to

the action of moving water.

Dunes of American Coasts.

Upon the Atlantic coast of the United States, the prevalence of western

or off-shore winds is unfavorable to the formation of dunes, and, though

marine currents lodge vast quantities of sand, in the form of banks, on

that coast, its shores are proportionally more free from sand-hills than

some others of lesser extent. There are, however, very important

exceptions. The action of the tide throws much sand upon some points of

the New England coast, as well as upon the Beaches of Long Island and

other more southern shores, and here dunes resembling those of Europe

are formed. There are also extensive ranges of dunes on the Pacific

coast of the United States, and at San Francisco they border some of the

streets of the city.

The dunes of America are far older than her civilization, and the soil

they threaten or protect possesses, in general, too little value to

justify any great expenditure in measures for arresting their progress

or preventing their destruction. Hence, great as is their extent and

their geographical importance, they have, at present, no such intimate

relations to human life as to render them objects of special interest in

the point of view I am taking, and I do not know that the laws of their

formation and motion have been made a subject of original investigation

by any American observer.

Dunes of Western Europe.

Upon the western coast of Europe, on the contrary, the ravages

occasioned by the movement of sand dunes, and the serious consequences

often resulting from the destruction of them, have long engaged the

earnest attention of Governments and of scientific men, and for nearly a

century persevering and systematic effort has been made to bring them

under human control. The subject has been carefully studied in Denmark

and the adjacent duchies, in Western Prussia, in the Netherlands, and in

France; and the experiments in the way of arresting the drifting of the

dunes, and of securing them, and the lands they shelter, from the

encroachments of the sea, have resulted in the adoption of a system of

coast improvement substantially the same in all these countries. The

sands, like the forests, have now their special literature, and the

volumes and memoirs, which describe them and the processes employed to

subdue them, are full of scientific interest and of practical

instruction.

Dunes of Gascony.

In the small kingdom of Denmark, inclusive of the duchies of Schleswig

and Holstein, the dunes cover an area of more than two hundred and sixty

square miles. The breadth of the chain is very various, and in some

places it consists only of a single row of sand-hills, while in others,

it is more than six miles wide. [Footnote: Andersen, Om Klitformationen,

pp. 78, 202, 275.] The dunes of the Prussian coast are vaguely estimated

to cover from eighty-five to one hundred and ten thousand acres; those

of Holland one hundred and forty thousand acres; and those of Gascony

more than two hundred thousand acres. I do not find any estimate of

their extent in other provinces of France, or in the Baltic provinces of

Russia, but it is probable that the entire quantity of dune land upon

the Atlantic and Baltic shores of Europe does not fall much short of a

million of acres. [Footnote: In an article on the dunes of Europe, in

vol. 29 (1864) of Aus der Natur, p. 590, the dunes are estimated to

cover, on the islands and coasts of Schleswig Holstein, in North-west

Germany, Denmark, Holland, and France, one hundred and eighty-one

German, or nearly four thousand English square miles; in Scotland, about

ten German, or two hundred and ten English miles; in Ireland, twenty

German, or four hundred and twenty English miles; and in England, one

hundred and twenty German, or more than twenty-five hundred English

miles. Pannewitz (Anleitung zum Anbau der Sandfluchen), as cited by

Andresen (Om Klitformationen, p. 45), states that the drifting sands of

Europe, including, of course, sand plains as well as dunes, cover an

extent of 21,000 square miles. This is, perhaps, an exaggeration, though

there is, undoubtedly, much more desert-land of this description on the

European continent than has been generally supposed. There is no

question that most of this waste is capable of reclamation by simple

planting, and no mode of physical improvement is better worth the

attention of civilized Governments than this.

There are often serious objections to extensive forest planting on soils

capable of being otherwise made productive, but they do not apply to

sand wastes, which, until covered by woods, are not only a useless

incumbrance, but a source of serious danger to all human improvements in

the neighborhood of them.] This vast deposit of sea-sand extends along

the coasts for a distance of several hundred miles, and from the time of

the destruction of the forests which covered it, to the year 1789, the

whole line was rolling inwards and burying the soil beneath it, or

rendering the fields unproductive by the sand which drifted from it. At

the same time, as the sand-hills moved landwards, the ocean was closely

following their retreat and swallowing up the ground they had covered,

as fast as their movement left it bare.

Age, Character, and Permanence of Dunes.

The origin of most great lines of dunes goes back past all history.

There are on many coasts several distinct ranges of sand-hills which

seem to be of very different ages, and to have been formed under

different relative conditions of land and water. [Footnote: Krause,

speaking of the dunes on the coast of Prussia, says: "Their origin

belongs to three different periods, in which important changes in the

relative level of sea and land have unquestionably taken place....

Except in the deep depressions between them, the dunes are everywhere

sprinkled, to a considerable height, with brown oxydulated iron, which

has penetrated into the sand to the depth of from three to eighteen

inches, and colored it red. ... Above the iron is a stratum of sand

differing in composition from ordinary sea-sand, and on this, growing

woods are always found.... The gradually accumulated forest soil occurs

in beds of from one to three feet thick, and changes, proceeding upward,

from gray sand to black humus."

Even on the third or seaward range, the sand grasses appear and thrive

luxuriantly, at least on the west coast, though Krause doubts whether

the dunes of the east coast were ever thus protected.--Der Dunenbau, pp.

8, 11.] In some cases there has been an upheaval of the coast line since

the formation of the oldest hillocks, and these have become inland

dunes, while younger rows have been thrown up on the new beach laid bare

by elevation of the sea-bed. Our knowledge of the mode of their first

accumulation is derived from observation of the action of wind and water

in the few instances where, with or without the aid of man, new coast

dunes have been accumulated, and of the influence of wind alone in

elevating new sand-heaps inland of the coast tier, when the outer rows

are destroyed by the sea, as also when the sodded surface of ancient

sands has been broken, and the subjacent strata laid open to the air.

It is a question of much interest, in what degree the naked condition of

most dunes is to be ascribed to the improvidence and indiscretion of

man. There are, in Western France, extensive ranges of dunes covered

with ancient and dense forests, while the recently formed sand-hills

between them and the sea are bare of vegetation, and in some cases are

rapidly advancing upon the wooded dunes, which they threaten to bury

beneath their drifts. Between the old dunes and the new there is no

discoverable difference in material or in structure; but the modern

sand-hills are naked and shifting, the ancient, clothed with vegetation

and fixed. It has been conjectured that artificial methods of

confinement and plantation were employed by the primitive inhabitants of

Gaul; and Laval, basing his calculations on the rate of annual movement

of the shifting dunes, assigns the fifth century of the Christian era as

the period when those processes wore abandoned. [Footnote: Laval,

Memoire sur les Dunes de Gascogne, Annales des Ponts et Chaussees, 1847,

2me semestre, p. 231. The same opinion had been expressed by Bremontier,

Annales des Ponts et Chaussees, 1833, 1er semestre, p. 185.]

There is no historical evidence that the Gauls were acquainted with

artificial methods of fixing the sands of the coast, and we have little

reason to suppose that they were advanced enough in civilization to be

likely to resort to such processes, especially at a period when land

could have had but a moderate value.

In other countries, dunes have spontaneously clothed themselves with

forests, and the rapidity with which their surface is covered by various

species of sand-plants, and finally by trees, where man and cattle and

burrowing animals are excluded from them, renders it highly probable

that they would, as a general rule, protect themselves, if left to the

undisturbed action of natural causes. The sand-hills of the Frische

Nehrung, on the coast of Prussia, were formerly wooded down to the

water's edge, and it was only in the last century that, in consequence

of the destruction of their forests, they became moving sands.

[Footnote: "In the Middle Ages," says Willibald Alexis, as quoted by

Muller, Das Buch der Pflanzenwelt, i., p. 16, "the Nebrung was extending

itself further, and the narrow opening near Lochstadt had filled itself

up with sand. A great pine forest bound with its roots the dune sand and

the heath uninterruptedly from Danzig to Pillau. King Frederick William

I. was once in want of money. A certain Herr von Korff promised to

procure it for him, without loan or taxes, if he could be allowed to

remove something quite useless. He thinned out the forests of Prussia,

which then, indeed, possessed little pecuniary value; but he felled the

entire woods of the Frische Nebrung, so far as they lay within the

Prussian territory. The financial operation was a success. The king had

money, but in the material effects which resulted from it, the state

received irreparable injury. The sea-winds rush over the bared hills;

the Frische Haff is half-choked with sand; the channel between Elbing,

the sea, and Konigsberg is endangered, and the fisheries in the Haff

injured. The operation of Herr von Korff brought the king 200,000

thalers. The state would now willingly expend millions to restore the

forests again."] There is every reason to believe that the dunes of the

Netherlands were clothed with trees until after the Roman invasion. The

old geographers, in describing these countries, speak of vast forests

extending to the very brink of the sea; but drifting coast dunes are

first mentioned by the chroniclers of the Middle Ages, and so far as we

know they have assumed a destructive character in consequence of the

improvidence of man. [Footnote: Staring, Voormaals en Thans, p. 231.

Had the dunes of the Netherlandish and French coasts, at the period of

the Roman invasion, resembled the moving sand-hills of the present day,

it is inconceivable that they could have escaped the notice of so acute

a physical geographer as Strabo; and the absolute silence of Caesar,

Ptolemy, and the encyclopaedic Pliny, respecting them, would be not less

inexplicable.] The history of the dunes of Michigan, so far as I have

been able to learn from my own observation, or that of others, is the

same. Thirty years ago, when that region was scarcely inhabited, they

were generally covered with a thick growth of trees, chiefly pines, and

underwood, and there was little appearance of undermining and wash on

the lake side, or of shifting of the sands, except where the trees had

been cut or turned up by the roots. [Footnote: The sands of Cape Cod were

partially, if not completely, covered with vegetation by nature. Dr.

Dwight, describing the dunes as they were in 1800, says: "Some of them

are covered with beach grass; some fringed with whortleberry bushes; and

some tufted with a small and singular growth of oaks. ... The parts of

this barrier which are covered with whortleberry bushes and with oaks,

have been either not at all or very little blown. The oaks,

particularly, appear to be the continuation of the forests originally

formed on this spot. ... They wore all the marks of extreme age; were,

in some instances, already decayed, and in others decaying; were hoary

with moss and were deformed by branches, broken and wasted, not by

violence, but by time."--Travels, iii., p. 91]

Nature, as she builds up dunes for the protection of the seashore,

provides, with similar conservatism, for the preservation of the dunes

themselves; so that, without the interference of man, these hillocks

would be, not perhaps absolutely perpetual, but very lasting in

duration, and very slowly altered in form or position. When once covered

with the trees, shrubs, and herbaceous growths adapted to such

localities, dunes undergo no apparent change, except the slow occasional

undermining of the outer tier, and accidental destruction by the

exposure of the interior, from the burrowing of animals, or the

upturning of trees with their roots, and all these causes of

displacement are very much less destructive when a vegetable covering

exists in the immediate neighborhood of the breach.

Protection of Dunes.

Before the occupation of the coasts by man, dunes, at all points where

they have been observed, seem to have been protected in their rear by

forests, which served to break the force of the winds in both

directions, [Footnote: Bergsoe (Reventlovs Virksomhed, ii., 3) states

that the dunes on the west coast of Jutland were stationary before the

destruction of the forests to the east of them. The felling of the tall

trees removed the resistance to the lower currents of the westerly

winds, and the sands have since buried a great extent of fertile soil.

See also same work, ii., p. 124.] and to have spontaneously clothed

themselves with a dense growth of the various plants, grasses, shrubs,

and trees, which nature has assigned to such soils. It is observed in

Europe that dunes, though now without the shelter of a forest country

behind them, begin to protect themselves as soon as human trespassers

are excluded, and grazing animals denied access to them. Herbaceous and

arborescent plants spring up almost at once, first in the depressions,

and then upon the surface of the sand-hills. Every seed that sprouts,

binds together a certain amount of sand by its roots, shades a little

ground with its leaves, and furnishes food and shelter for still younger

or smaller growths. A succession of a very few favorable seasons

suffices to bind the whole surface together with a vegetable network,

and the power of resistance possessed by the dunes themselves, and the

protection they afford to the fields behind them, are just in proportion

to the abundance and density of the plants they support.

The growth of the vegetable covering can, of course, be much accelerated

by judicious planting and watchful care, and this species of improvement

is now carried on upon a vast scale on the sandy coasts of Western

Europe, wherever the value of land is considerable and the population

dense.

Use of Dunes as a Barrier against the Sea.

Although the sea throws up large quantities of sand on flat lee-shores,

there are many cases where it continually encroaches on those same

shores and washes them away. At all points of the shallow North Sea

where the agitation of the waves extends to the bottom, banks are

forming and rolling eastwards. Hence the sea-sand tends to accumulate

upon the coast of Schleswig-Holstein and Jutland, and were there no

conflicting influences, the shore would rapidly extend itself westwards.

But the same waves which wash the sand to the coast undermine the beach

they cover, and still more rapidly degrade the shore at points where it

is too high to receive partial protection by the formation of dunes upon

it. The earth of the coast is generally composed of particles finer,

lighter, and more transportable by water than the sea-sand. While,

therefore, the billows raised by a heavy west wind may roll up and

deposit along the beach thousands of tons of sand, the same waves may

swallow up even a larger quantity of fine shore-earth. This earth, with

a portion of the sand, is swept off by northwardly and southwardly

currents, and let fall at other points of the coast, or carried off,

altogether, out of the reach of causes which might bring it back to its

former position.

Although, then, the eastern shore of the German Ocean here and there

advances into the sea, it in general retreats before it, and but for the

protection afforded it by natural arrangements seconded by the art and

industry of man, whole provinces would soon be engulfed by the waters.

This protection consists in an almost unbroken chain of sand banks and

dunes, extending from the northernmost point of Jutland to the Elbe, a

distance of not much less than three hundred miles, and from the Elbe

again, though with more frequent and wider interruptions, to the

Atlantic borders of France and Spain. So long as the dunes are

maintained by nature or by human art, they serve, like any other

embankment or dike, as a partial or a complete protection against the

encroachments of the sea; and on the other hand, when their drifts are

not checked by natural processes, or by the industry of man, they become

a cause of as certain, if not of as sudden, destruction as the ocean

itself whose advance they retard. On the whole, the dunes on the coast

of the German Sea, notwithstanding the great quantity of often fertile

land they cover, and the evils which result from their movement, are a

protective and beneficial agent, and their maintenance is an object of

solicitude with the Governments and people of the shores they defend.

[Footnote: "We must, therefore, not be surprised to see the people here

deal as gingerly with their dunes as if treading among eggs. He who is

lucky enough to own a molehill of dune pets it affectionately, and

spends his substance in cherishing and fattening it. That fair, fertile,

rich province, the peninsula of Eiderstadt in the south of Friesland,

has, on the point towards the sea, only a tiny row of dunes, some six

miles long or so; but the people talk of their fringe of sand hills, as

if it were a border set with pearls. They look upon it as their best

defence against Neptune. They have connected it with their system of

dikes, and for years have kept sentries posted to protect it against

wanton injury."--J. G. Kohl, Die Inseln u. Marschen Schleswig-Holsteins,

ii., p. 115.]

The eastward progress of the sea on the Danish, Netherlandish, and

French coasts depends so much on local geological structure, on the

force and direction of tidal and other marine currents, on the volume

and rapidity of coast rivers, on the contingencies of the weather and on

other varying circumstances, that no general rate can be assigned to it.

At Agger, near the western end of the Liimfjord, in Jutland, the coast

was washed away, between the years 1815 and 1839, at the rate of more

than eighteen feet a year. The advance of the sea appears to have been

something less rapid for a century before; but from 1840 to 1857, it

gained upon the land no less than thirty feet a year. At other points of

the shore of Jutland the loss is smaller, but the sea is encroaching

generally upon the whole line of the coast. [Footnote: Andersen, "Om

Klitformationen," pp. 68-72.]

The Liimfjord.

The irruption of the sea into the fresh-water lagoon of Liimfjord in

Jutland, in 1825--one of the most remarkable encroachments of the ocean

in modern times--is expressly ascribed to "mismanagement of the dunes"

on the narrow neck of land which separated the fjord from the North Sea.

At earlier periods the sea had swept across the isthmus, and even burst

through it, but the channel had been filled up again, sometimes by

artificial means, sometimes by the operation of natural causes, and on

all these occasions effects were produced very similar to those

resulting from the formation of the new channel in 1825, which still

remains open. [Footnote: Id., pp. 231, 232. Andresen's work, though

printed in 1861, was finished in 1859. Lyell (Antiquity of Man, 1863, p.

14) says: "Even in the course of the present century, the salt-waters

have made one eruption into the Baltic by the Liimfjord, although they

have been now again excluded."] Within comparatively recent historical

ages, the Liimfjord has thus been several times alternately filled with

fresh and with salt water, and man has produced, by neglecting the

dunes, or at least might have prevented by maintaining them, changes

identical with those which are usually ascribed to the action of great

geological causes, and sometimes supposed to have required vast periods

of time for their accomplishment.

"This breach," says Forchhammer, "which converted the Liimfjord into a

sound, and the northern part of Jutland into an island, occasioned

remarkable changes. The first and most striking phenomenon was the

sudden destruction of almost all the fresh-water fish previously

inhabiting this lagoon, which was famous for its abundant fisheries.

Millions of fresh-water fish were thrown on shore, partly dead and

partly dying, and were carted off by the people. A few only survived,

and still frequent the shores at the mouth of the brooks. The eel,

however, has gradually accommodated itself to the change of

circumstances, and is found in all parts of the fjord, while to all

other fresh-water fish, the salt-water of the ocean seems to have been

fatal. It is more than probable that the sand washed in by the irruption

covers, in many places, a layer of dead fish, and has thus prepared the

way for a petrified stratum similar to those observed in so many older

formations.

"As it seems to be a law of nature that animals whose life is suddenly

extinguished while yet in full vigor, are the most likely to be

preserved by petrification, we find here one of the conditions favorable

to the formation of such a petrified stratum. The bottom of the

Liimfjord was covered with a vigorous growth of aquatic plants,

belonging both to fresh and to salt water, especially Zostera marina.

This vegetation totally disappeared after the irruption, and, in some

instances, was buried by the sand; and here again we have a familiar

phenomenon often observed in ancient strata--the indication of a given

formation by a particular vegetable species--and when the strata

deposited at the time of the breach shall be accessible by upheaval, the

period of eruption will be marked by a stratum of Zostera, and probably

by impressions of fresh-water fishes.

"It is very remarkable that the Zostera marina, a sea-plant, was

destroyed even where no sand was deposited. This was probably in

consequence of the sudden change from brackish to salt water ... It is

well established that the Liimfjord communicated with the German Ocean

at some former period.

To that era belong the deep beds of oyster shells and Cardium edule,

which are still found at the bottom of the fjord. And now, after an

interval of centuries, during which the lagoon contained no salt-water

shell fish, it again produces great numbers of Mytilus edulis. Could we

obtain a deep section of the bottom, we should find beds of Ostrea

edulis and Cardium edule, then a layer of Zostera marina with

fresh-water fish, and then a bed of Mytilus edulis. If, in course of

time, the new channel should be closed, the brooks would fill the lagoon

again with fresh water; fresh-water fish and shell fish would reappear,

and thus we should have a repeated alternation of organic inhabitants of

the sea and of the waters of the land.

"These events have been accompanied with but a comparatively

insignificant change of land surface, while the formations in the bed of

this inland sea have been totally revolutionized in character."

[Footnote: Forchhammer, Geognostiche Studien am Meeres-Ufer, Leonhard

und Bronn, Jahrbuch, 1841, pp. 11, 13.] Coasts of Schleswig-Holstein,

Holland, and France. On the islands on the coast of Schleswig-Holstein,

the advance of the sea has been more unequivocal and more rapid. Near

the beginning of the last century, the dunes which had protected the

western coast of the island of Sylt began to roll to the east, and the

sea followed closely as they retired. In 1757, the church of Rantum, a

village upon that island, was obliged to be taken down in consequence of

the advance of the sand-hills; in 1791, these hills had passed beyond

its site, the waves had swallowed up its foundations, and the sea gained

so rapidly, that, fifty years later, the spot where they lay was seven

hundred feet from the shore. [Footnote: Andresen, Om Klitformationen, pp.

68, 72.] The most prominent geological landmark on the coast of Holland

is the Huis te Britten, Arx Britannica, a fortress built by the Romans,

in the time of Caligula, on the main land near the mouth of the Rhine.

At the close of the seventeenth century, the sea had advanced sixteen

hundred paces beyond it. The older Dutch annalists record, with much

parade of numerical accuracy, frequent encroachments of the sea upon

many parts of the Netherlandish coast. But though the general fact of an

advance of the ocean upon the land is established beyond dispute, the

precision of the measurements which have been given is open to question.

Staring, however, who thinks the erosion of the coast much exaggerated

by popular geographers, admits a loss of more than a million and a half

acres, chiefly worthless morass; [Footnote: Voormaals en Thans, pp. 126,

170.] and it is certain that but for the resistance of man, but for his

erection of dikes and protection of dunes, there would now be left of

Holland little but the name. It is, as has been already seen, still a

debated question among geologists whether the coast of Holland now is,

and for centuries has been, subsiding. I believe most investigators

maintain the affirmative; and if the fact is so, the advance of the sea

upon the land is, in part, due to this cause. But the rate of subsidence

is at all events very small, and therefore the encroachments of the

ocean upon the coast are mainly to be ascribed to the erosion and

transportation of the soil by marine waves and currents.

The sea is fast advancing at several points of the western coast of

France, and unknown causes have given a new impulse to its ravages since

the commencement of the present century. Between 1830 and 1842, the

Point de Grave, on the north side of the Girondo, retreated one hundred

and eighty metres, or fifty feet per year; from the latter year to 1846,

the rate was increased to more than three times that quantity, and the

loss in those four years was about six hundred feet. All the buildings

at the extremity of the peninsula have been taken down and rebuilt

farther landwards, and the lighthouse of the Grave now occupies its

third position. The sea attacked the base of the peninsula also, and the

Point de Grave and the adjacent coasts have been for thirty years the

scene of one of the most obstinately contested struggles between man and

the ocean recorded in the annals of modern engineering.

Movement of Dunes.

Besides their importance as a barrier against the inroads of the ocean,

dunes are useful by sheltering the cultivated ground behind them from

the violence of the sea-wind, from salt spray, and from the drifts of

beach sand which would otherwise overwhelm them. But the dunes

themselves, unless their surface sands are kept moist, and confined by

the growth of plants, or at least by a crust of vegetable earth, are

constantly rolling inwards, and thus, while, on one side, they lay bare

the traces of ancient human habitations or other evidences of the social

life of primitive man, they are, on the other, burying fields, houses,

churches, and converting populous districts into barren and deserted

wastes.

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 shores 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 passed over the space

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 quantities 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, entitled "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 accumulations of water

dammed up by the dunes, have, along their western shore, 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 shore. 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 entitled 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 accumulated 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 quantity. [Footnote: Andresen, On Klitformationen, pp. 56, 79,

82]