Principles of Geology Part 61

Before concluding this chapter, I shall offer a few observations on the influence of man in modifying the physical geography of the globe; for we must cla.s.s his agency among the powers of organic nature.

_Felling of forests._--The felling of forests has been attended, in many countries, by a diminution of rain, as in Barbadoes and Jamaica.[998]

For in tropical countries, where the quant.i.ty of aqueous vapor in the atmosphere is great, but where, on the other hand, the direct rays of the sun are most powerful, any impediment to the free circulation of air, or any screen which shades the earth from the solar rays, becomes a source of humidity; and wherever dampness and cold have begun to be generated by such causes, the condensation of vapor continues. The leaves, moreover, of all plants are alembics, and some of those in the torrid zone have the remarkable property of distilling water, thus contributing to prevent the earth from becoming parched up.

_Distribution of the American forests._--There can be no doubt then, that the state of the climate, especially the humidity of the atmosphere, influences vegetation, and that, in its turn, vegetation re-acts upon the climate: but some writers seem to have attributed too much importance to the influence of forests, particularly those of America, as if they were the primary cause of the moisture of the climate.

The theory of a modern author on this subject "that forests exist in those parts of America only where the predominant winds carry with them a considerable quant.i.ty of moisture from the ocean," seems far more rational. In all countries, he says, "having a summer heat exceeding 70, the presence or absence of natural woods, and their greater or less luxuriance, may be taken as a measure of the amount of humidity, and of the fertility of the soil. Short and heavy rains in a warm country will produce gra.s.s, which, having its roots near to the surface, springs up in a few days, and withers when the moisture is exhausted; but transitory rains, however heavy, will not nourish trees; because, after the surface is saturated, the remainder of the water runs off, and the moisture lodged in the soil neither sinks deep enough, nor is in sufficient quant.i.ty, to furnish the giants of the forests with the necessary sustenance. It may be a.s.sumed that twenty inches of rain falling moderately or at intervals, will leave a greater permanent supply in the soil than forty inches falling, as it sometimes does in the torrid zone, in as many hours."[999]

"In all regions," he continues, "where ranges of mountains intercept the course of the constant or predominant winds, the country on the windward side of the mountains will be moist, and that on the leeward dry; and hence parched deserts will generally be found on the west side of countries within the tropics, and on the east side of those beyond them, the prevailing winds in these cases being generally in opposite directions. On this principle, the position of forests in North and South America may be explained. Thus, for example, in the region within the thirtieth parallel, the moisture swept up by the trade-wind from the Atlantic is precipitated in part upon the mountains of Brazil, which are but low, and so distributed as to extend far into the interior. The portion which remains is borne westward, and, losing a little as it proceeds, is at length arrested by the Andes, where it falls down in showers on their summits. The aerial current, now deprived of all the humidity with which it can part, arrives in a state of complete exsiccation at Peru, where consequently no rain falls. But in the region of America, beyond the thirtieth parallel, the Andes serve as a screen to intercept the moisture brought by the prevailing winds from the Pacific Ocean: rains are copious on their summits, and in Chili on their _western_ declivities; but none falls on the plains to the _eastward_, except occasionally when the wind blows from the Atlantic."[1000]

I have been more particular in explaining these views, because they appear to place in a true light the dependence of vegetation on climate, the humidity being increased, and more uniformly diffused throughout the year, by the gradual spreading of wood.

It has been affirmed, that formerly, when France and England were covered with wood, Europe was much colder than at present; that the winters in Italy were longer, and that the Seine, and many other rivers, froze more regularly every winter than now. M. Arago, in an essay on this subject, has endeavored to show, by tables of observations on the congelation of the Rhine, Danube, Rhone, Po, Seine, and other rivers, at different periods, that there is no reason to believe the cold to have been in general more intense in ancient times.[1001] He admits, however, that the climate of Tuscany has been so far modified, by the removal of wood, as that the winters are less cold; but the summers also, he contends, are less hot than of old; and the summers, according to him, were formerly hotter in France than in our own times. His evidence is derived chiefly from doc.u.ments showing that wine was made three centuries ago in the Vivarais and several other provinces, at an earlier season, at greater elevations, and in higher lat.i.tudes, than are now found suitable to the vine.

There seems little doubt that in the United States of North America the rapid _clearing_ of the country has rendered the winters less severe and the summers less hot; in other words, the extreme temperatures of January and July have been observed from year to year to approach somewhat nearer to each other. Whether in this case, or in France, the _mean_ temperature has been raised, seems by no means as yet decided; but there is no doubt that the climate has become, as Buffon would have said, "less excessive."

I have before shown, when treating of the excavation of new estuaries in Holland by inroads of the ocean, as also of the changes on our own coasts, that although the conversion of sea into land by artificial labors may be great, yet it must always be in subordination to the power of the tides and currents, or to the great movements which alter the relative level of the land and sea, (Chap. XX.) If, in addition to the a.s.sistance obtained by parliamentary grants for defending Dunwich from the waves, all the resources of Europe had been directed to the same end, the existence of that port might perhaps have been prolonged for several centuries (p. 310.) But in the mean time, the current would have continued to sweep away portions from the adjoining cliffs on each side, giving to the whole line of coast its present form, until at length the town, projecting as a narrow promontory, must have become exposed to the irresistible fury of the waves.

It is scarcely necessary to observe, that the control which man can obtain over the igneous agents is less even than that which he may exert over the aqueous. He cannot modify the upheaving or depressing force of earthquakes, or the periods or degree of violence of volcanic eruptions; and on these causes the inequalities of the earth's surface, and, consequently, the shape of the sea and land, appear mainly to depend.

The utmost that man can hope to effect in this respect is occasionally to divert the course of a lava-stream, and to prevent the burning matter, for a season, from overwhelming a city, or some other of the proudest works of human industry.

If all the nations of the earth should attempt to quarry away the lava which flowed during one eruption from the Icelandic volcanoes in 1783, and the two following years, and should attempt to consign it to the deepest abysses of the ocean, they might toil for thousands of years and not accomplish their task. Yet the matter borne down to the sea by two great rivers, the Ganges and Burrampooter, in each quarter of a century, probably equals in weight and volume the ma.s.s of Icelandic lava produced by that great eruption (p. 282). So insignificant is the aggregate force exerted by man, when contrasted with the ordinary operations of aqueous or igneous agents in the natural world.

No application, perhaps, of human skill and labor tends so greatly to vary the state of the habitable surface, as that employed in the drainage of lakes and marshes, since not only the _stations_ of many animals and plants, but the general climate of a district, may thus be modified. It is also a kind of alteration to which it is difficult to find anything a.n.a.logous in the agency of inferior beings; for we ought always, before we decide that any part of the influence of man is novel and anomalous, carefully to consider the powers of all other animated agents which may be limited or superseded by him.[1002] Many who have reasoned on these subjects seem to have forgotten that the human race often succeeds to the discharge of functions previously fulfilled by other species. Suppose the growth of some of the larger terrestrial plants, or, in other words, the extent of forest, to be diminished by man, and the climate to be thereby modified, it does not follow that this kind of innovation is unprecedented. It is a change in the state of vegetation, and such may often have been the result of the appearance of new species upon the earth. The multiplication, for example, of certain insects in parts of Germany, during the last century, destroyed more trees than man, perhaps, could have felled during an equal period.

It would be rash, however, to affirm that the power of man to modify the surface may not differ in kind or degree from that of other living beings; although the problem is certainly more complex than many who have speculated on such topics have imagined. If land be raised from the sea, the greatest alteration in its physical condition, which could ever arise from the influence of organic beings, would probably be produced by the first immigration of terrestrial plants, whereby the new tract would become covered with vegetation. The change next in importance would seem to be when animals first enter, and modify the proportionate numbers of certain species of plants. If there be any anomaly in the intervention of man, in farther varying the relative numbers in the vegetable kingdom, it may not so much consist in the kind or absolute quant.i.ty of alteration, as in the circ.u.mstance that a _single species_, in this case, would exert, by its superior power and universal distribution, an influence equal to that of hundreds of other terrestrial animals.

If we inquire whether man, by his direct power, or by the changes which he may give rise to indirectly, tends, upon the whole, to lessen or increase the inequalities of the earth's surface, we shall incline, perhaps, to the opinion that he is a levelling agent. In mining operations he conveys upwards a certain quant.i.ty of materials from the bowels of the earth; but, on the other hand, much rock is taken annually from the land, in the shape of ballast, and afterwards thrown into the sea, and by this means, in spite of prohibitory laws, many harbors, in various parts of the world, have been blocked up. We rarely transport heavy materials to higher levels, and our pyramids and cities are chiefly constructed of stone brought down from more elevated situations.

By ploughing up thousands of square miles, and exposing a surface for part of the year to the action of the elements, we a.s.sist the abrading force of rain, and diminish the conservative effects of vegetation.

CHAPTER XLV.

INCLOSING OF FOSSILS IN PEAT, BROWN SAND, AND VOLCANIC EJECTIONS.

Division of the subject--Imbedding of organic remains in deposits on emerged land--Growth of peat--Site of ancient forests in Europe now occupied by peat--Bog iron-ore--Preservation of animal substances in peat--Miring of quadrupeds--Bursting of the Solway moss--Great Dismal Swamp--Imbedding of organic bodies and human remains in blown sand--Moving sands of African deserts--De Luc on their recent origin--Buried temple of Ipsambul--Dried carcases in the sands--Towns overwhelmed by sand-floods--Imbedding of organic and other remains in volcanic formations on the land.

_Division of the subject._--The next subject of inquiry is the mode in which the remains of animals and plants become fossil, or are buried in the earth by natural causes. M. Constant Prevost has observed, that the effects of geological causes are divisible into two great cla.s.ses; those produced during the submersion of land beneath the waters, and those which take place after its emersion. Agreeably to this cla.s.sification, I shall consider, first, in what manner animal and vegetable remains become included and preserved in deposits on emerged land, or that part of the surface which is not _permanently_ covered by water, whether of seas or lakes; secondly, the manner in which organic remains become imbedded in subaqueous deposits.

Under the first division, I shall treat of the following topics:--1st, the growth of peat, and the preservation of vegetable and animal remains therein;--2dly, the burying of organic remains in blown sand;--3dly, of the same in the ejections and alluviums of volcanoes;--4thly, in alluviums generally, and in the ruins of landslips;--5thly, in the mud and stalagmite of caves and fissures.

_Growth of Peat, and Preservation of Vegetable and Animal Remains therein._

The generation of peat, when not completely under water, is confined to moist situations, where the temperature is low, and where vegetables may decompose without putrefying. It may consist of any of the numerous plants which are capable of growing in such _stations_; but a species of moss (_Sphagnum_) const.i.tutes a considerable part of the peat found in marshes of the north of Europe; this plant having the property of throwing up new shoots in its upper part, while its lower extremities are decaying.[1003] Reeds, rushes, and other aquatic plants may usually be traced in peat; and their organization is often so entire that there is no difficulty in discriminating the distinct species.

_a.n.a.lysis of peat._--In general, says Sir H. Davy, one hundred parts of dry peat contain from sixty to ninety-nine parts of matter destructible by fire; and the residuum consists of earths usually of the same kind as the substratum of clay, marl, gravel, or rock, on which they are found, together with oxide of iron. "The peat of the chalk counties of England," observes the same writer, "contains much gypsum: but I have found very little in any specimens from Ireland or Scotland, and in general these peats contain very little saline matter."[1004] From the researches of Dr. MacCulloch, it appears that peat is intermediate between simple vegetable matter and lignite, the conversion of peat to lignite being gradual, and being brought about by a prolonged action of water.[1005]

_Peat abundant in cold and humid climates._--Peat is sometimes formed on a declivity in mountainous regions, where there is much moisture; but in such situations it rarely, if ever, exceeds four feet in thickness. In bogs, and in low grounds into which alluvial peat is drifted, it is found forty feet thick, and upwards; but in such cases it generally owes one half of its volume to the water which it contains. It has seldom, if ever, been discovered within the tropics; and it rarely occurs in the valleys, even in the south of France and Spain. It abounds more and more, in proportion as we advance farther from the equator, and becomes not only more frequent but more inflammable in northern lat.i.tudes.[1006]

The same phenomenon is repeated in the southern hemisphere. No peat is found in Brazil, nor even in the swampy parts of the country drained by the La Plata on the east side of South America, or in the island of Chiloe on the west; yet when we reach the 45th degree of lat.i.tude and examine the Chonos Archipelago or the Falkland Islands, and Tierra del Fuego, we meet with an abundant growth of this substance. Almost all plants contribute here by their decay to the production of peat, even the gra.s.ses; but it is a singular fact, says Mr. Darwin, as contrasted with what occurs in Europe, that no kind of moss enters into the composition of the South American peat, which is formed by many plants, but chiefly by that called by Brown _Astelia pumila_.[1007]

I learn from Dr. Forchhammer (1849) that water charged with vegetable matter in solution does not throw down a deposit of peat in countries where the mean temperature of the year is above 43 or 44 Fahrenheit.

Frost causes the precipitation of such peaty matter, but in warm climates the attraction of the carbon for the oxygen of the air mechanically mixed with the water increases with the increasing temperature, and the dissolved vegetable matter or humic acid (which is organic matter in a progressive state of decomposition) being converted into carbonic acid, rises and is absorbed into the atmosphere, and thus disappears.

_Extent of surface covered by peat._--There is a vast extent of surface in Europe covered with peat, which, in Ireland, is said to extend over a tenth of the whole island. One of the mosses on the Shannon is described as being fifty miles long, by two or three broad; and the great marsh of Montoire, near the mouth of the Loire, is mentioned, by Blavier, as being more than fifty leagues in circ.u.mference. It is a curious and well-ascertained fact, that many of these mosses of the north of Europe occupy the place of forests of pine and oak, which have, many of them, disappeared within the historical era. Such changes are brought about by the fall of trees and the stagnation of water, caused by their trunks and branches obstructing the free drainage of the atmospheric waters, and giving rise to a marsh. In a warm climate, such decayed timber would immediately be removed by insects, or by putrefaction; but, in the cold temperature now prevailing in our lat.i.tudes, many examples are recorded of marshes originating in this source. Thus, in Mar forest, in Aberdeens.h.i.+re, large trunks of Scotch fir, which had fallen from age and decay, were soon immured in peat, formed partly out of their peris.h.i.+ng leaves and branches, and in part from the growth of other plants. We also learn, that the overthrow of a forest by a storm, about the middle of the seventeenth century, gave rise to a peat-moss near Lochbroom, in Ross-s.h.i.+re, where, in less than half a century after the fall of the trees, the inhabitants dug peat.[1008] Dr. Walker mentions a similar change, when, in the year 1756, the whole wood of Drumlaurig in Dumfries-s.h.i.+re was overset by the wind. Such events explain the occurrence, both in Britain and on the Continent, of mosses where the trees are all broken within two or three feet of the original surface, and where their trunks all lie in the same direction.[1009]

It may however be suggested in these cases, that the soil had become exhausted for trees, and that, on the principle of that natural rotation which prevails in the vegetable world, one set of plants died out and another succeeded. It is certainly a remarkable fact that in the Danish islands, and in Jutland and Holstein, fir wood of various species, especially Scotch fir, is found at the bottom of the peat-mosses, although it is well ascertained that for the last five centuries no Coniferae have grown wild in these countries; the coniferous trees which now flourish there having been all planted towards the close of the last century.

Nothing is more common than the occurrence of buried trees at the bottom of the Irish peat-mosses, as also in most of those of England, France, and Holland; and they have been so often observed with parts of their trunks standing erect, and with their roots fixed to the subsoil, that no doubt can be entertained of their having generally grown on the spot.

They consist, for the most part, of the fir, the oak, and the birch: where the subsoil is clay, the remains of oak are the most abundant; where sand is the substratum, fir prevails. In the marsh of Curragh, in the Isle of Man, vast trees are discovered standing firm on their roots, though at the depth of eighteen or twenty feet below the surface.

Some naturalists have desired to refer the imbedding of timber in peat-mosses to aqueous transportation, since rivers are well known to float wood into lakes; but the facts above mentioned show that, in numerous instances, such an hypothesis is inadmissible. It has, moreover, been observed, that in Scotland, as also in many parts of the Continent, the largest trees are found in those peat-mosses which lie in the least elevated regions, and that the trees are proportionally smaller in those which lie at higher levels; from which fact De Luc and Walker have both inferred that the trees grew on the spot, for they would naturally attain a greater size in lower and warmer levels. The leaves, also, and fruits of each species, are continually found immersed in the moss along with the parent trees; as, for example, the leaves and acorns of the oak, the cones and leaves of the fir, and the nuts of the hazel.

_Recent origin of some peat-mosses._--In Hatfield moss, in Yorks.h.i.+re, which appears clearly to have been a forest eighteen hundred years ago, fir-trees have been found ninety feet long, and sold for masts and keels of s.h.i.+ps; oaks have also been discovered there above one hundred feet long. The dimensions of an oak from this moss are given in the Philosophical Transactions, No. 275, which must have been larger than any tree now existing in the British dominions.

In the same moss of Hatfield, as well as in that of Kincardine, in Scotland, and several others, Roman roads have been found covered to the depth of eight feet by peat. All the coins, axes, arms, and other utensils found in British and French mosses, are also Roman; so that a considerable portion of the peat in European peat-bogs is evidently not more ancient than the age of Julius Caesar. Nor can any vestiges of the ancient forests described by that general, along the line of the great Roman way in Britain, be discovered, except in the ruined trunks of trees in peat.

De Luc ascertained that the very sites of the aboriginal forests of Hercinia, Semana, Ardennes, and several others, are now occupied by mosses and fens; and a great part of these changes have, with much probability, been attributed to the strict orders given by Severus, and other emperors, to destroy all the wood in the conquered provinces.

Several of the British forests, however, which are now mosses, were cut at different periods, by order of the English parliament, because they harbored wolves or outlaws. Thus the Welsh woods were cut and burned, in the reign of Edward I.; as were many of those in Ireland, by Henry II., to prevent the natives from harboring in them, and hara.s.sing his troops.

It is curious to reflect that considerable tracts have, by these accidents, been permanently sterilized, and that, during a period when civilization has been making great progress, large areas in Europe have, by human agency, been rendered less capable of administering to the wants of man. Rennie observes,[1010] with truth, that in those regions alone which the Roman eagle never reached--in the remote circles of the German empire, in Poland and Prussia, and still more in Norway, Sweden, and the vast empire of Russia--can we see what Europe was before it yielded to the power of Rome. Desolation now reigns where stately forests of pine and oak once flourished, such as might now have supplied all the navies of Europe with timber.

_Sources of bog iron-ore._--At the bottom of peat-mosses there is sometimes found a cake, or "pan," as it is termed, of oxide of iron, and the frequency of bog iron-ore is familiar to the mineralogist. The oak, which is so often dyed black in peat, owes its color to the same metal.

From what source the iron is derived has often been a subject of discussion, until the discoveries of Ehrenberg seem at length to have removed the difficulty. He had observed in the marshes about Berlin a substance of a deep ochre yellow pa.s.sing into red, which covered the bottom of the ditches, and which, where it had become dry after the evaporation of the water, appeared exactly like oxide of iron. But under the microscope it was found to consist of slender articulated threads or plates, partly siliceous and partly ferruginous, of what he considered an animalcule, _Gaillonella ferruginea_, but which most naturalists now regard as a plant.[1011] There can be little doubt, therefore, that bog iron-ore consists of an aggregate of millions of these organic bodies invisible to the naked eye.[1012]

[Ill.u.s.tration: Fig. 101.

_Gaillonella ferruginea._

_a._ 2000 times magnified.]

_Preservation of animal substances in peat._--One interesting circ.u.mstance attending the history of peat mosses is the high state of preservation of animal substances buried in them for periods of many years. In June, 1747, the body of a woman was found six feet deep, in a peat-moor in the Isle of Axholm, in Lincolns.h.i.+re. The antique sandals on her feet afforded evidence of her having been buried there for many ages: yet her nails, hair, and skin, are described as having shown hardly any marks of decay. On the estate of the Earl of Moira, in Ireland, a human body was dug up, a foot deep in gravel, covered with eleven feet of moss; the body was completely clothed and the garments seemed all to be made of hair. Before the use of wool was known in that country the clothing of the inhabitants was made of hair, so that it would appear that this body had been buried at that early period; yet it was fresh and unimpaired.[1013] In the Philosophical Transactions we find an example recorded of the bodies of two persons having been buried in moist peat, in Derbys.h.i.+re, in 1674, about a yard deep, which were examined twenty-eight years and nine months afterwards; "the color of their skin was fair and natural, their flesh soft as that of persons newly dead."[1014]

Among other a.n.a.logous facts we may mention, that in digging a pit for a well near Dulverton, in Somersets.h.i.+re, many pigs were found in various postures, still entire. Their shape was well preserved, the skin, which retained the hair, having a.s.sumed a dry, membranous appearance. Their whole substance was converted into a white, friable, laminated, inodorous, and tasteless substance; but which, when exposed to heat, emitted an odor precisely similar to broiled bacon.[1015]

_Cause of the antiseptic property of peat._--We naturally ask whence peat derives this antiseptic property? It has been attributed by some to the carbonic and gallic acids which issue from decayed wood, as also to the presence of charred wood in the lowest strata of many peat-mosses, for charcoal is a powerful antiseptic, and capable of purifying water already putrid. Vegetable gums and resins also may operate in the same way.[1016]

The tannin occasionally present in peat is the produce, says Dr.

MacCulloch, of tormentilla, and some other plants; but the quant.i.ty he thinks too small, and its occurrence too casual, to give rise to effects of any importance. He hints that the soft parts of animal bodies, preserved in peat-bogs, may have been converted into adipocire by the action of water merely; an explanation which appears clearly applicable to some of the cases above enumerated.[1017]

_Miring of quadrupeds._--The manner, however, in which peat contributes to preserve, for indefinite periods, the harder parts of terrestrial animals, is a subject of more immediate interest to the geologist. There are two ways in which animals become occasionally buried in the peat of marshy grounds; they either sink down into the semifluid mud, underlying a turfy surface upon which they have rashly ventured, or, at other times, as we shall see in the sequel, a bog "bursts," and animals may be involved in the peaty alluvium.

In the extensive bogs of Newfoundland, cattle are sometimes found buried with only their heads and necks above ground; and after having remained for days in this situation, they have been drawn out by ropes and saved.

In Scotland, also, cattle venturing on the "quaking moss" are often mired, or "laired," as it is termed; and in Ireland, Mr. King a.s.serts that the number of cattle which are lost in sloughs is quite incredible.[1018]