Outlines of the Earth's History Part 14

There is another element in the movement of the soils which, though less appreciable, is still of great importance. The agents of decay which produce and remove the detritus, the chemical changes of the bed rock, and the mechanical action which roots apply to them, along with the solutional processes, are constantly lowering the surface of the ma.s.s. In this way we can often prove that a soil continuously existing has worked downward through many thousand feet of strata. In this process of downgoing the country on which the layer rests may have greatly changed its form, but the deposit, under favourable conditions, may continue to retain some trace of the materials which it derived from beds which have long since disappeared, their position having been far up in the s.p.a.ces now occupied by the air. Where the slopes are steep and streams abound, we rarely find detritus which belonged in rock more than a hundred feet above the present surface of the soil. Where, however, as on those isolated table-lands or b.u.t.tes which abound in certain portions of the Mississippi Valley, as well as in many other countries, we find a patch of soil lying on a nearly level surface, which for geologic ages has not felt the effect of streams, we may discover, commingled in the _debris_, the harder wreckage derived from the decay of a thousand feet or more of vanished strata.

When we consider the effect of organic life on the processes which go on in the soil, we first note the large fact that the development of all land vegetation depends upon the existence of this detritus--in a word, on the slow movement of the decaying rocky matter from the point where it is disrupted to its field of rest in the depths of the sea.

The plants take their food from the portion of this rocky waste which is brought into solution by the waters which penetrate the ma.s.s. On the plants the animals feed, and so this vast a.s.semblage of organisms is maintained. Not only does the land life maintain itself on the soil, and give much to the sea, but it serves in various ways to protect this detrital coating from too rapid destruction, and to improve its quality. To see the nature of this work we should visit a region where primeval forests still lie upon the slopes of a hilly region. In the body of such a wood we find next the surface a coating of decayed vegetable matter, made up of the falling leaves, bark, branches, and trunks which are constantly descending to the earth.

Ordinarily, this layer is a foot or more in thickness; at the top it is almost altogether composed of vegetable matter; at the bottom it verges into the true soil. An important effect of this decayed vegetation is to restrain the movement of the surface water. Even in the heaviest rains, provided the ma.s.s be not frozen, the water is taken into it and delivered in the manner of springs to the larger streams. We can better note the measure of this effect by observing the difference in the ground covered by this primeval forest and that which we find near by which has been converted into tilled fields.

With the same degree of rapidity in the flow, the distinct stream channels on the tilled ground are likely to be from twenty to a hundred times in length what they are on the forest bed. The result is that while the brook which drains the forested area maintains a tolerably constant flow of clean water, the other from the tilled ground courses only in times of heavy rain, and then is heavily charged with mud. In the virgin conditions of the soil the downwear is very slow; in its artificial state this wearing goes on so rapidly that the sloping fields are likely to be worn to below the soil level in a few score years.

Not only does the natural coating of vegetation, such as our forests impose upon the country, protect the soil from washing away, but the roots of the larger plants are continually at work in various ways to increase the fertility and depth of the stratum. In the form of slender fibrils these underground branches enter the joints and bed planes of the rock, and there growing they disrupt the materials, giving them a larger surface on which decay may operate. These bits, at first of considerable size, are in turn broken up by the same action. Where the underlying rocks afford nutritious materials, the branches of our tap-rooted trees sometimes find their way ten feet or more below the base of the true soil. Not only do they thus break up the stones, but the nutrition which they obtain in the depths is brought up and deposited in the parts above the ground, as well as in the roots which lie in the true soil, so that when the tree dies it becomes available for other plants. Thus in the forest condition of a country the amount of rock material contributed to the deposit in general so far exceeds that which is taken away to the rivers by the underground water as to insure the deepening of the soil bed to the point where only the strongest roots--those belonging to our tap-rooted trees--can penetrate through it to the bed rocks.

Almost all forests are from time to time visited by winds which uproot the trees. When they are thus rent from the earth, the underground branches often form a disk containing a thick tangle of stones and earth, and having a diameter of ten or fifteen feet. The writer has frequently observed a hundred cubic feet of soil matter, some of it taken from the depth of a yard or more, thus uplifted into the air. In the path of a hurricane or tornado we may sometimes find thousands of acres which have been subjected to this rude overturning--a natural ploughing. As the roots rot away, the _debris_ which they held falls outside of the pit, thus forming a little hillock along the side of the cavity. After a time the thrusting action of other roots and the slow motion of the soil down the slope restore the surface from its hillocky character to its original smoothness; but in many cases the naturalist who has learned to discern with his feet may note these irregularities long after it has been recovered with the forest.

Great as is the effect of plants on the soil, that influence is almost equalled by the action of the animals which have the habit of entering the earth, finding there a temporary abiding place. The number of these ground forms is surprisingly great. It includes, indeed, a host of creatures which are efficient agents in enriching the earth. The species of earthworms, some of which occupy forested districts as well as the fields, have the habit of pa.s.sing the soil material through their bodies, extracting from the ma.s.s such nutriment as it may contain. In this manner the particles of mineral matter become pulverized, and in a measure affected by chemical changes in the bodies of the creatures, and are thus better fitted to afford plant food. Sometimes the amount of the earth which the creatures take in in moving through their burrows and void upon the surface is sufficient to form annually a layer on the surface of the ground having a depth of one twentieth of an inch or more. It thus may well happen that the soil to the depth of two or three feet is completely overturned in the course of a few hundred years. As the particles which the creatures devour are rather small, the tendency is to acc.u.mulate the finer portions of the soil near the surface of the earth, where by solution they may contribute to the needs of the lowly plants. It is probably due to the action of these creatures that small relics of ancient men, such as stone tools, are commonly found buried at a considerable depth beneath the earth, and rarely appear upon the surface except where it has been subjected to deep ploughing or to the action of running streams.

Along with the earthworms, the ants labour to overturn the soil; frequently they are the more effective of the two agents. The common species, though they make no permanent hillocks, have been observed by the writer to lay upon the surface each year as much as a quarter of an inch of sand and other fine materials which they have brought up from a considerable depth. In many regions, particularly in those occupied by glacial drift, and pebbly alluvium along the rivers, the effect of this action, like that of earthworms, is to bring to the surface the finer materials, leaving the coa.r.s.er pebbles in the depths. In this way they have changed the superficial character of the soil over great areas; we may say, indeed, over a large part of the earth, and this in a way which fits it better to serve the needs of the wild plants as well as the uses of the farmer.

Many thousand species of insects, particularly the larger beetles, have the habit of pa.s.sing their larval state in the under earth. Here they generally excavate burrows, and thus in a way delve the soil. As many of them die before reaching maturity, their store of organic matter is contributed to the ma.s.s, and serves to nourish the plants.

If the student will carefully examine a section of the earth either in its natural or in its tilled state, he will be surprised to find how numerous the grubs are. They may often be found to the number of a score or more of each cubic foot of material. Many of the species which develop underground come from eggs which have carefully been encased in organic matter before their deposition in the earth. Thus some of the carrion beetles are in the habit of laying their eggs in the bodies of dead birds or field mice, which they then bury to the depth of some inches in the earth. In this way nearly all the small birds and mammals of our woods disappear from view in a few hours after they are dead. Other species make b.a.l.l.s from the dung of cattle in which they lay their eggs, afterward rolling the little spheres, it may be for hundreds of feet, to the chambers in the soil which they have previously prepared. In this way a great deal of animal matter is introduced into the earth, and contributes to its fertility.

Many of our small mammals have the habit of making their dwelling places in the soil. Some of them, such as the moles, normally abide in the subterranean realm for all their lives. Others use the excavations as places of retreat. In any case, these excavations serve to move the particles of the soil about, and the materials which the animals drag into the earth, as well as the excrement of the creatures, act to enrich it. This habit of taking food underground is not limited to the mammals; it is common with the ants, and even the earthworms, as noted by Charles Darwin in his wonderful essay on these creatures, are accustomed to drag into their burrows bits of gra.s.s and the slender leaves of pines. It is not known what purpose they attain by these actions, but it is sufficiently common somewhat to affect the conditions of the soil.

The result of these complicated works done by animals and plants on the soil is that the material to a considerable depth are constantly being supplied with organic matter, which, along with the mineral material, const.i.tutes that part of the earth which can support vegetation. Experiment will readily show that neither crushed rock nor pure vegetable mould will of itself serve to maintain any but the lowliest vegetation. It requires that the two materials be mixed in order that the earth may yield food for ordinary plants, particularly for those which are of use to man, as crops. On this account all the processes above noted whereby the waste of plant and animal life is carried below the surface are of the utmost importance in the creation and preservation of the soil. It has been found, indeed, in almost all cases, necessary for the farmer to maintain the fertility of his fields to plough-in quant.i.ties of such organic waste. By so doing he imitates the work which is effected in virgin soil by natural action.

As the process is costly in time and material, it is often neglected or imperfectly done, with the result that the fields rapidly diminish in fertility.

The way in which the buried organic matter acts upon the soil is not yet thoroughly understood. In part it accomplishes the results by the materials which on its decay it contributes to the soil in a state in which they may readily be dissolved and taken up by the roots into their sap; in part, however, it is believed that they better the conditions by affording dwelling places for a host of lowly species, such as the forms which are known as bacteria. The organisms probably aid in the decomposition of the mineral matter, and in the conversion of nitrogen, which abounds in the air or the soil, into nitrates of potash and soda--substances which have a very great value as fertilizers. Some effect is produced by the decay of the foreign matter brought into the soil, which as it pa.s.ses away leaves channels through which the soil water can more readily pa.s.s.

By far the most general and important effect arising from the decay of organic matter in the earth is to be found in the carbon dioxide which is formed as the oxygen of the air combines with the carbon which all organic material contains. As before noted, water thus charged has its capacity for taking other substances into solution vastly increased, and on this solvent action depends in large part the decay of the bed rocks and the solution of materials which are to be appropriated by the plants.

Having now sketched the general conditions which lead to the formation of soils, we must take account of certain important variations in their conditions due to differences in the ways in which they are formed and preserved. These matters are not only of interest to the geologist, but are of the utmost importance to the life of mankind, as well as all the lower creatures which dwell upon the lands. First, we should note that soils are divisible into three great groups, which, though not sharply parted from each other, are sufficiently peculiar for the purposes of cla.s.sification. Where the earth material has been derived from the rocks which nearly or immediately underlie it, we have a group of soils which may be ent.i.tled those of immediate derivation--that is, derived from rocks near by, or from beds which once overlaid the level and have since been decayed away. Next, we have alluvial soils, those composed of materials which have been transported by streams, commonly from a great distance, and laid down on their flood plains. Third, the soils the mineral matters of which have been brought into their position by the action of glaciers; these in a way resemble those formed by rivers, but the materials are generally imperfectly sorted, coa.r.s.e and fine being mingled together.

Last of all, we have the soils due to the acc.u.mulation of blown dust or blown sand, which, unlike the others, occupy but a small part of the land surface. It would be possible, indeed, to make yet another division, including those areas which when emerging from the sea were covered with fine, uncemented detritus ready at once to serve the purposes of a soil. Only here and there, and but seldom, do we find soils of this nature.

It is characteristic of soils belonging to the group to which we have given the t.i.tle of immediate derivation that they have acc.u.mulated slowly, that they move very gradually down the slopes on which they lie, and that in all cases they represent, with a part of their ma.s.s at least, levels of rock which have disappeared from the region which they occupied. The additions made to their ma.s.s are from below, and that ma.s.s is constantly shrinking, generally at a pretty rapid rate, by the mineral matter which is dissolved and goes away with the spring water. They also are characteristically thin on steep slopes, thickening toward the base of the incline, where the diminished grade permits the soil to move slowly, and therefore to acc.u.mulate.

In alluvial soils we find acc.u.mulations which are characterized by growth on their upper surfaces, and by the distant transportation of the materials of which they are composed. In these deposits the outleaching removes vast amounts of the materials, but so long as the floods from time to time visit their surfaces the growth of the deposits is continued. This growth rarely takes place from the waste of the bed rocks on which the alluvium lies. It is characteristic of alluvial soils that they are generally made up of _debris_ derived from fields where the materials have undergone the change which we have noted in the last paragraph; therefore these latter deposits have throughout the character which renders the mineral materials easily dissolved. Moreover, the ma.s.s as it is constructed is commonly mingled with a great deal of organic waste, which serves to promote its fertility. On these accounts alluvial grounds, though they vary considerably in fertility, commonly afford the most fruitful fields of any region. They have, moreover, the signal advantage that they often may be refreshed by allowing the flood waters to visit them, an action which but for the interference of man commonly takes place once each year. Thus in the valley of the Nile there are fields which have been giving rich grain harvests probably for more than four thousand years, without any other effective fertilizing than that derived from the mud of the great river.

The group of glaciated soils differs in many ways from either of those mentioned. In it we find the mineral matter to have been broken up, transported, and acc.u.mulated without the influence of those conditions which ordinarily serve to mix rock _debris_ with organic matter during the process by which it is broken into bits. When vegetation came to preoccupy the fields made desolate by glacial action, it found in most places more than sufficient material to form soils, but the greater part of the matter was in the condition of pebbles of very hard rock and sand grains, fragments of silex. Fortunately, the broken-up state of this material, by exposing a great surface of the rocky matter to decay, has enabled the plants to convert a portion of the ma.s.s into earth fit for the uses of their roots. But as the time which has elapsed since the disappearance of the glaciers is much less than that occupied in the formation of ordinary soil, this decay has in most cases not yet gone very far, so that in a cubic foot of glaciated waste the amount of material available for plants is often only a fraction of that held in the soils of immediate derivation.

In the greater portion of the fields occupied by glacial waste the processes which lead to the introduction of organic matter into the earth have not gone far enough to set in effective work the great laboratory which has to operate in order to give fertile soil. The pebbles hinder the penetration of the roots as well as the movement of insects and other animals. There has not been time enough for the overturning of trees to bring about a certain admixture of vegetable matter with the soil--in a word, the process of soil-making, though the first condition, that of broken-up rock, has been accomplished, is as yet very incomplete. It needs, indeed, care in the introduction of organic matter for its completion.

It is characteristic of glacial soils that they are indefinitely deep.

This often is a disadvantageous feature, for the reason that the soil water may pa.s.s so far down into the earth that the roots are often deprived of the moisture which they need, and which in ordinary soils is retained near the surface by the hard underlayer. On the other hand, where the glacial waste is made up of pebbles formed from rocks of varied chemical composition, which contain a considerable share of lime, potash, soda, and other substances which are required by plants, the very large surface which they expose to decay provides the soil with a continuous enrichment. In a cubic foot of pebbly glacial earth we often find that the ma.s.s offers several hundred times as much surface to the action of decay as is afforded by the underlying solid bed rock from which a soil of immediate derivation has to win its mineral supply. Where the pebbly glacial waste is provided with a mixture of vegetable matter, the process of decay commonly goes forward with considerable rapidity. If the supply of such matter is large, such as may be produced by ploughing in barnyard manure or green crops, the nutritive value of the earth may be brought to a very high point.

It is a familiar experience in regions where glacial soils exist that the earth beneath the swamps when drained is found to be extraordinarily well suited for farming purposes. On inspecting the pebbles from such places, we observe that they are remarkably decayed.

Where the ma.s.ses contain large quant.i.ties of feldspar, as is the case in the greater part of our granitic and other crystalline rocks, this material in its decomposition is converted into kaolin or feldspar clay, and gives the stones a peculiar white appearance, which marks the decomposition, and indicates the process by which a great variety of valuable soil ingredients are brought into a state where they may be available for plants.

In certain parts of the glacial areas, particularly in the region near the margin of the ice sheet, where the glacier remained in one position for a considerable time, we find extensive deposits of silicious sand, formed of the materials which settled from the under-ice stream, near where they escaped from the glacial cavern.

These kames and sand plains, because of the silicious nature of their materials and the very porous nature of the soil which they afford, are commonly sterile, or at most render a profit to the tiller by dint of exceeding care. Thus in Ma.s.sachusetts, although the first settlers seized upon these grounds, and planted their villages upon them because the forests there were scanty and the ground free from enc.u.mbering boulders, were soon driven to betake themselves to those areas where the drift was less silicious, and where the pebbles afforded a share of clay. Very extensive fields of this sandy nature in southeastern New England have never been brought under tillage.

Thus on the island of Martha's Vineyard there is a connected area containing about thirty thousand acres which lies in a very favourable position for tillage, but has been found substantially worthless for such use. The farmers have found it more advantageous to clear away the boulders from the coa.r.s.er drift in order to win soil which would give them fair returns.

Those areas which are occupied by soil materials which have been brought into their position by the action of the wind may, as regards their character, be divided into two very distinct groups--the dunes and loess deposits. In the former group, where, as we have noted (see page 123), the coa.r.s.e sea sands or those from the sh.o.r.es of lakes are driven forward as a marching hillock, the grains of the material are almost always silicious. The fragments in the motion are not taken up into the air, but are blown along the surface. Such dune acc.u.mulations afford an earth which is even more sterile than that of the glacial sand plains, where there is generally a certain admixture of pebbles from rocks which by their decomposition may afford some elements of fertility. Fortunately for the interests of man, these wind-borne sands occupy but a small area; in North America, in the aggregate, there probably are not more than one thousand square miles of such deposits.

Where the rock material drifted by the winds is so fine that it may rise into the air in the form of dust, the acc.u.mulations made of it generally afford a fertile soil, and this for the reason that they are composed of various kinds of rock, and not, as in the case of dunes, of nearly pure silica. In some very rare cases, where the seash.o.r.e is bordered by coral reefs, as it is in parts of southern Florida, and the strand is made up of limestone bits derived from the hard parts which the polyps secrete, small dunes are made of limy material.

Owing, however, in part to the relatively heavy nature of this substance, as well as to the rapid manner in which its grains become cemented together, such limestone dunes never attain great size nor travel any distance from their point of origin.

As before noted, dust acc.u.mulations form the soil in extended areas which lie to the leeward of great deserts. Thus a considerable part of western China and much of the United States to the west of the Mississippi is covered by these wind-blown earths. Wherever the rainfall is considerable these loess deposits have proved to have a high agricultural value.

Where a region has an earth which has recently pa.s.sed from beneath the sea or a great lake, the surface is commonly covered by incoherent detritus which has escaped consolidation into hard rock by the fact that it has not been buried and thus brought into the laboratory of the earth's crust. When such a region becomes dry land, the materials are immediately ready to enter into the state of soil. They commonly contain a good deal of waste derived from the organic life which dwelt upon the sea bottom and was embedded in the strata as they were formed. Where these acc.u.mulations are made in a lake, the land vegetation at once possesses the field, even a single year being sufficient for it to effect its establishment. Where the lands emerge from the sea, it requires a few years for the salt water to drain away so that the earth can be fit for the uses of plants. In a general way these sea-bottom soils resemble those formed in the alluvial plains.

They are, however, commonly more sandy, and their substances less penetrated by that decay which goes on very freely in the atmosphere because of the abundant supply of oxygen, and but slowly on the sea floor. Moreover, the marine deposits are generally made up in large part of silicious sand, a material which is produced in large quant.i.ties by the disruption of the rocks along the sea coast. The largest single field of these ocean-bottom soils of North America is found in the lowland region of the southern United States, a wide belt of country extending along the coast from the Rio Grande to New York.

Although the streams have channelled shallow valleys in the beds of this region, the larger part of its surface still has the peculiar features of form and composition which were impressed upon it when it lay below the surface of the sea.

Local variations in the character of the soil covering are exceedingly numerous, and these differences of condition profoundly affect the estate of man. We shall therefore consider some of the more important of these conditions, with special reference to their origin.

The most important and distinctly marked variation in the fertility of soils is that which is produced by differences in the rainfall. No parts of the earth are entirely lacking in rain, but over considerable areas the precipitation does not exceed half a foot a year. In such realms the soil is sterile, and the natural coating of vegetation limited to those plants which can subsist on dew or which can take on an occasional growth at such times as moisture may come upon them.

With a slight increase in precipitation, the soil rapidly increases in productivity, so that we may say that where as much as about ten inches of water enters the earth during the summer half of the year, it becomes in a considerable measure fit for agriculture. Observations indicate that the conditions of fertility are not satisfied where the rainfall is just sufficient to fill the pores of the soil; there must be enough water entering the earth to bring about a certain amount of outflow in the form of springs. The reason of this need becomes apparent when we study the evident features of those soils which, though from season to season charged with water, do not yield springs, but send the moisture away through the atmosphere. Wherever these conditions occur we observe that the soil in dry seasons becomes coated with a deposit of mineral matter, which, because of its taste, has received the name of alkali. The origin of this coating is as follows: The pores of the soil, charged from year to year with sufficient water to fill them, become stored with a fluid which contains a very large amount of dissolved mineral matter--too much, indeed, to permit the roots of plants, save a few species which have become accustomed to the conditions, to do their appointed work. In fact, this water is much like that of the sea, which the roots of only a few of our higher plants can tolerate. When the dry season comes on, the heat of the sun evaporates the water at the surface, leaving behind a coating composed of the substances which the water contains.

The soil below acts in the manner of a lamp-wick to draw up fluid as rapidly as the heat burns it away. When the soil water is as far as possible exhausted, the alkali coating may represent a considerable part of the soluble matter of the soil, and in the next rainy season it may return in whole or in part to the under-earth, again to be drawn in the manner before described to the upper level. It is therefore only when a considerable share of the ground water goes forth to the streams in each year that the alkaline materials are in quant.i.ty kept down to the point where the roots of our crop-giving plants can make due use of the soil. Where, in an arid region, the ground can be watered from the enduring streams or from artificial reservoirs, the main advantage arising from the process is commonly found in the control which it gives the farmer in the amount of the soil water. He can add to the rainfall sufficient to take away the excess of mineral matter. When such soils are first brought under tillage it is necessary to use a large amount of water from the ca.n.a.ls, in order to wash away the old store of alkali. After that a comparatively small contribution will often keep the soil in excellent condition for agriculture. It has been found, however, in the irrigated lands beside the Nile that where too much saving is practised in the irrigation, the alkaline coating will appear where it has been unknown before, and with it an unfitness of the earth to bear crops.

Although the crust of mineral matters formed in the manner above described is characteristic of arid countries, and in general peculiar to them, a similar deposit may under peculiar conditions be formed in regions of great rainfall. Thus on the eastern coast of New England, where the tidal marshes have here and there been diked from the sea and brought under tillage, the dissolved mineral matters of the soil, which are excessive in quant.i.ty, are drawn to the surface, forming a coating essentially like that which is so common in arid regions. The writer has observed this crust on such diked lands, having a thickness of an eighth of an inch. In fact, this alkali coating represents merely the extreme operation of a process which is going on in all soils, and which contributes much to their fertility. When rain falls and pa.s.ses downward into the earth, it conveys the soluble matter to a depth below the surface, often to beyond the point where our ordinary crop plants, such as the small grains, can have access to it, and this for the reason that their roots do not penetrate deeply. When dry weather comes and evaporation takes place from the surface, the fluid is drawn up to the upper soil layer, and there, in process of evaporation, deposits the dissolved materials which it contains. Thus the mineral matter which is fit for plant food is constantly set in motion, and in its movement pa.s.ses the rootlets of the plants. It is probably on this account--at least in part--that very wet weather is almost as unfavourable to the farmer as exceedingly dry, the normal alternation in the conditions being, as is well known, best suited to his needs.

So long as the earth is subjected to conditions in which the rainfall may bring about a variable amount of water in the superficial detrital layer, we find normal fruitful soils, though in their more arid conditions they may be fit for but few species of plants. When, by increasing aridity, we pa.s.s to conditions where there is no tolerably permanent store of water in the _debris_, the material ceases to have the qualities of a soil, and becomes mere rock waste. At the other extreme of the scale we pa.s.s to conditions where the water is steadfastly maintained in the interstices of the detritus, and there again the characteristic of the soil and its fitness for the uses of land vegetation likewise disappear. In a word, true soil conditions demand the presence of moisture, but that in insufficient quant.i.ties, to keep the pores of the earth continually filled; where they are thus filled, we have the condition of swamps. Between these extremes the level at which the water stands in the soil in average seasons is continually varying. In rainy weather it may rise quite to the surface; in a dry season it may sink far down. As this water rises and falls, it not only moves, as before noted, the soluble mineral materials, but it draws the air into and expels it from the earth with each movement. This atmospheric circulation of the soil, as has been proved by experiment, is of great importance in maintaining its fertility; the successive charges of air supply the needs of the microscopic underground creatures which play a large part in enriching the soil, and the direct effect of the oxygen in promoting decay is likewise considerable. A part of the work which is accomplished by overturning the earth in tillage consists in this introduction of the air into the pores of the soil, where it serves to advance the actions which bring mineral matters into solution.

[Ill.u.s.tration: _Mountain gorge, Himalayas, India. Note the difference in the slope of the eroded rocks and the effect of erosion upon them; also the talus slopes at the base of the cliffs which the torrent is cutting away. On the left of the foreground there is a little bench showing a recent higher line of the water._]

In the original conditions of any country which is the seat of considerable rainfall, and where the river system is not so far developed as to provide channels for the ready exit of the waters, we commonly find very extensive swamps; these conditions of bad drainage almost invariably exist where a region has recently been elevated above the level of the sea, and still retains the form of an irregular rolling plain common to sea floors, and also in regions where the work done by glaciers has confused the drainage which the antecedent streams may have developed. In an old, well-elaborated river system swamps are commonly absent, or, if they occur, are due to local accidents of an unimportant nature.

For our purpose swamps may be divided into three groups--climbing bogs, lake bogs, and marine marshes. The first two of these groups depend on the movements of the rain water over the land; the third on the action of the tides. Beginning our account with the first and most exceptional of these groups, we note the following features in their interesting history:

Wherever in a humid region, on a gentle slope--say with an inclination not exceeding ten feet to the mile--the soil is possessed by any species of plants whose stems grow closely together, so that from their decayed parts a spongelike ma.s.s is produced, we have the conditions which favour the development of climbing bogs. Beginning usually in the sh.o.r.es of a pool, these plants, necessarily of a water-loving species, retain so much moisture in the spongy ma.s.s which they form that they gradually extend up the slope. Thus extending the margin of their field, and at the same time thickening the deposit which they form, these plants may build a climbing bog over the surface until steeps are attained where the inclination is so great that the necessary amount of water can not be held in the spongy ma.s.s, or where, even if so held, the whole coating will in time slip down in the manner of an avalanche.

The greater part of the climbing bogs of the world are limited to the moist and cool regions of high lat.i.tudes, where species of moss belonging to the genus _Sphagnum_ plentifully flourish. These plants can only grow where they are continuously supplied with a bath of water about their roots. They develop in lake bogs as far south as Mexico, but in the climbing form they are hardly traceable south of New England, and are nowhere extensively developed within the limits of the United States. In more northern parts of this continent, and in northwestern Europe, particularly in the moist climate of Ireland, climbing bogs occupy great areas, and hold up their lakes of interst.i.tially contained water over the slopes of hills, where the surface rises at the rate of thirty feet or more to the mile. So long as the deposit of decayed vegetable matter which has acc.u.mulated in this manner is thin, therefore everywhere penetrated by the fibrous roots of the moss, it may continue to cling to its sloping bed; but when it attains a considerable thickness, and the roots in the lower part decay, the pulpy ma.s.s, water-laden in some time of heavy rain, break away in a vast torrent of thick, black mud, which may inundate the lower lands, causing widespread destruction.

In more southern countries, other water-loving plants lead to the formation of climbing bogs. Of these, the commonest and most effective are the species of reeds, of which our Indian cane is a familiar example. Brakes of this vegetation, plentifully mingled with other species of aquatic growth, form those remarkable climbing bogs known as the Dismal and other swamps, which numerously occur along the coast line of the United States from southern Maryland to eastern Texas.

Climbing bogs are particularly interesting, not only from the fact that they are eminently peculiar effects of plant growth, but because they give us a vivid picture of those ancient mora.s.ses in which grew the plants that formed the beds of vegetable matter now appearing in the state of coal. Each such bed of buried swamp material was, with rare exceptions, where the acc.u.mulation took place in lakes, gathered in climbing bogs such as we have described.

Lake bogs occur in all parts of the world, but in their best development are limited to relatively high lat.i.tudes, and this for the reason that the plants which form vegetable matter grow most luxuriantly in cool climates and in regions where the level of the basin is subject to less variation than occurs in the alternating wet and dry seasons which exist in nearly all tropical regions. The fittest conditions are found in glaciated regions, where, as before noted, small lakes are usually very abundant. On the sh.o.r.es of one of these pools, of size not so great that the waves may attain a considerable height, or in the sheltered bay of a larger lake, various aquatic plants, especially the species of pond lilies, take root upon the bottom, and spread their expanded leaves on the surface of the water. These flexible-leaved and elastic-stemmed plants can endure waves which attain no more than a foot or two of height, and by the friction which they afford make the swash on the sh.o.r.e very slight. In the quiet water, rushes take root, and still further protect the strand, so that the very delicate vegetation of the mosses, such as the _Sphagnum_, can fix itself on the sh.o.r.e.

As soon as the _Sphagnum_ mat has begun its growth, the strength given by its interlaced fibres enables it to extend off from the sh.o.r.e and float upon the water. In this way it may rapidly enlarge, if not broken up by the waves, so that its front advances into the lake at the rate of several inches each year. While growing outwardly it thickens, so that the bottom of the ma.s.s gradually works down toward the floor of the basin. At the same time the lower part of the sheet, decaying, contributes a shower of soft peat mud to the floor of the lake. In this way, growing at its edge, deepening, and contributing to an upgrowth from the bottom, a few centuries may serve entirely to fill a deep basin with peaty acc.u.mulation. In general, however, the surface of the bog closes over the lake before the acc.u.mulation has completely filled the sh.o.r.eward portions of the area. In these conditions we have what is familiarly known as a quaking bog, which can be swayed up and down by a person who quickly stoops and rises while standing on the surface. In this state the tough and thick sheet of growing plants is sufficient to uphold a considerable weight, but so elastic that the underlying water can be thrown into waves. Long before the bog has completely filled the lake with the peaty acc.u.mulations the growth of trees is apt to take place on its surface, which often reduces the area to the appearance of a very level wet wood.

[Ill.u.s.tration: Fig. 17.--Diagram showing beginning of peat bog: A, lake; B, lilies and rushes; C, lake bog; D, climbing bog.]

Climbing and lake bogs in the United States occupy a total area of more than fifty thousand square miles. In all North America the total area is probably more than twice as great. Similar deposits are exceedingly common in the Eurasian continent and in southern Patagonia. It is probable that the total amount of these fields in different parts of the world exceeds half a million square miles.

These two groups of fresh-water swamps have an interest, for the reason that when reduced to cultivation by drainage and by subsequent removal of the excess of peaty matter, by burning or by natural decay, afford very rich soil. The fairest fields of northern Europe, particularly in Great Britain and Ireland, have been thus won to tillage. In the first centuries of our era a large part of England--perhaps as much as one tenth of the ground now tilled in that country--was occupied by these lands, which retained water in such measure as to make them unfit for tillage, the greater portion of this area being in the condition of thin climbing bog. For many centuries much of the energy of the people was devoted to the reclamation of these valuable lands. This task of winning the swamp lands to agriculture has been more completely accomplished in England than elsewhere, but it has gone far on the continent of Europe, particularly in Germany. In the United States, owing to the fact that lands have been cheap, little of this work of swamp-draining has as yet been accomplished. It is likely that the next great field of improvement to be cultivated by the enterprising people will be found in these excessively humid lands, from which the food-giving resources for the support of many million people can be won.

[Ill.u.s.tration: Fig. 18.--Diagram showing development of swamp: A, remains of lake; B, surface growth; c, peat.]

The group of marine marshes differs in many important regards from those which are formed in fresh water. Where the tide visits any coast line, and in sheltered positions along that sh.o.r.e, a number of plants, mostly belonging to the group of gra.s.ses, species which have become accustomed to having their roots bathed by salt water, begin the formation of a spongy mat, which resembles that composed of _Sphagnum_, only it is much more solid. This mat of the marine marshes soon attains a thickness of a foot or more, the upper or growing surface lying in a position where it is covered for two or three hours at each visit of the tide. Growing rapidly outward from the sh.o.r.e, and having a strength which enables it to resist in a tolerably effective manner waves not more than two or three feet high, this acc.u.mulation makes head against the sea. To a certain extent the waves undermine the front of the sheet and break up ma.s.ses of it, which they distribute over the shallow bottom below the level at which these plants can grow. In this deeper water, also, other marine animals and plants are continually developing, and their remains are added to the acc.u.mulations which are ever shallowing the water, thus permitting a further extension of the level, higher-lying marsh. This process continues until the growth has gone as far as the scouring action of the tidal currents will permit. In the end the bay, originally of wide-open water, is only such at high tide. For the greater part of the time it appears as broad savannas, whose brilliant green gives them the aspect of rare fertility.

Owing to the conditions of their growth, the deposits formed in marine marshes contain no distinct peat, the nearest approach to that substance being the tangle of wirelike roots which covers the upper foot or so of the acc.u.mulation. The greater part of the ma.s.s is composed of fine silt, brought in by the streams of land water which discharge into the basin, and by the remains of animals which dwelt upon the bottom or between the stalks of the plants that occupy the surface of the marshes. These inters.p.a.ces afford admirable shelter to a host of small marine forms. The result is, that the tidal marshes, as well as the lower-lying mud flats, which have been occupied by the mat of vegetation, afford admirable earth for tillage. Unfortunately, however, there are two disadvantages connected with the redemption of such lands. In the first place, it is necessary to exclude the sea from the area, which can only be accomplished by considerable engineering work; in the second place, the exclusion of the tide inevitably results in the silting up of the pa.s.sage by which the water found its way to the sea. As these openings are often used for harbours, the effect arising from their destruction is often rather serious. Nevertheless, in some parts of the world very extensive and most fertile tracts of land have thus been won from the sea; a large part of Holland and sh.o.r.e-land districts in northern Europe are made up of fields which were originally covered by the tide. Near the mouth of the Rhine, indeed, the people have found these sea-bottom soils so profitable that they have gone beyond the zone of the marshes, and have drained considerable seas which of old were permanently covered, even at the lowest level of the waters.

On the coast of North America marine marshes have an extensive development, and vary much in character. In the Bay of Fundy, where the tides have an alt.i.tude of fifty feet or more, the energy of their currents is such that the marsh mat rarely forms. Its place, however, is taken by vast and ever-changing mud flats, the materials of which are swept to and fro by the moving waters. The people of this region have learned an art of a peculiar nature, by which they win broad fields of excellent land from the sea. Selecting an area of the flats, the surface of which has been brought to within a few feet of high tide, they inclose it with a stout barrier or dike, which has openings for the free admission of the tidal waters. Entering this basin, the tide, moving with considerable velocity, bears in quant.i.ties of sediment. In the basin, the motion being arrested, this sediment falls to the bottom, and serves to raise its level. In a few months the sheet of sediment is brought near the plane of the tidal movement, then the gates are closed at times when the tide has attained half of its height, so that the ground within the dike is not visited by the sea water, and can be cultivated.