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diff --git a/77792-0.txt b/77792-0.txt new file mode 100644 index 0000000..b447658 --- /dev/null +++ b/77792-0.txt @@ -0,0 +1,11686 @@ +*** START OF THE PROJECT GUTENBERG EBOOK 77792 *** + + + TRANSCRIBER’S NOTE + + Italic text is denoted by _underscores_. + + Footnote anchors are denoted by [number], and the footnotes have + been placed at the end of the book. + + Basic fractions are displayed as ½ ⅓ ¼ etc; other fractions were + of the form a-b in the original book, for example 1-3000th and + 7-100ths, and have been left unchanged. + + The text of the heading of Part I of the book (I.--THE EARTH’S + CRUST) has been moved to the next page to be directly above the + heading of the first Chapter. + + Chapter headings have been made consistent, with the title on a + single line and the author on the following line. + + Some minor changes to the text are noted at the end of the book. + + Volume I of this set of four volumes can be found in Project + Gutenberg at: https://www.gutenberg.org/ebooks/74571 + + + + +[Illustration: Jupiter and Minerva Terraces, Hot Springs, Yellowstone +Park] + + + + + THE STORY OF + THE UNIVERSE + + _Told by Great Scientists + and Popular Authors_ + + COLLECTED AND EDITED + + _By_ ESTHER SINGLETON + + Author of “Turrets, Towers and Temples,” “Wonders of Nature,” + “The World’s Great Events,” “Famous Paintings,” Translator + of Lavignac’s “Music Dramas of Richard Wagner” + + _FULLY ILLUSTRATED_ + + + VOLUME II + + THE EARTH: + LAND AND + SEA + + + P. F. COLLIER AND SON + NEW YORK + + + + + COPYRIGHT 1905 + + BY P. F. COLLIER & SON + + + + +ILLUSTRATIONS + + + Hot Springs, Yellowstone Park _Frontispiece_ + + Fingal’s Cave, Staffa _Opposite p._ 475 + + A Forest of the Carboniferous Period ” 523 + + The Giant’s Causeway, Ireland ” 595 + + Stag-Horn Coral Reef, Australia ” 643 + + The Matterhorn ” 691 + + Forms of Snowflakes ” 739 + + Forms of Clouds ” 787 + + Chart of Winds and Tides ” 835 + + + + +CONTENTS + + + FORMATION OF THE EARTH. Élisée Reclus 433 + + CLASSES OF ROCKS. Sir Charles Lyell 439 + + GEOLOGICAL CHRONOLOGY. Sir J. William Dawson 450 + + THE SILURIAN BEACH. Louis Agassiz 456 + + CARBONIFEROUS PERIOD. Louis Figuier 464 + + THE PALÆONTOLOGICAL HISTORY OF ANIMALS. Hugh Miller 480 + + EUROPEAN AND ASIATIC DELUGES. Louis Figuier 493 + + GLACIERS. Louis Agassiz 502 + + VOLCANIC ACTION. Sir Archibald Geikie 516 + + THOUGHTS ABOUT KRAKATOA. Sir Robert S. Ball 527 + + VOLCANOES. Sir Archibald Geikie 536 + + EARTHQUAKES. William Hughes 559 + + MOUNTAINS. A. Keith 566 + + LAKES--FRESH, SALT, AND BITTER. Sir Archibald Geikie 573 + + UNDERGROUND WATER: SPRINGS, CAVES, RIVERS, AND LAKES. + Élisée Reclus 588 + + RIVERS. A. Keith Johnston 621 + + SWAMPS AND MARSHES. Élisée Reclus 628 + + LOWLAND PLAINS. William Hughes 634 + + THE SMELL OF EARTH. G. Clarke Nuttall 648 + + DESERTS. Élisée Reclus 654 + + THE PRIMITIVE OCEAN. G. Hartwig 666 + + THE FLOOR OF THE OCEAN. John James Wild 676 + + CORAL FORMATIONS. Charles Darwin 689 + + MAGNITUDE AND COLOR OF THE SEA. G. Hartwig 707 + + TIDAL ACTION. Sir Robert S. Ball 713 + + THE GULF STREAM. Lord Kelvin 727 + + THE PHOSPHORESCENCE OF THE SEA. G. Hartwig 750 + + THE SEASHORE. P. Martin Duncan 763 + + THE OCEAN OF AIR. Agnes Giberne 773 + + WEATHER. Sir Ralph Abercromby 784 + + THE ROMANCE OF A RAINDROP. Arthur H. Bell 792 + + THE RAINBOW. John Tyndall 799 + + SNOW, HAIL, AND DEW. Alexander Buchan 807 + + THE AURORA BOREALIS. Richard A. Proctor 813 + + CLOUDS. D. Wilson Barker 819 + + WINDS. William Hughes 828 + + SQUALLS, WHIRLWINDS, AND TORNADOES. Sir Ralph Abercromby 845 + + + + + THE STORY OF THE UNIVERSE + VOLUME II + + + THE EARTH: LAND, SEA, AND AIR + + + + + THE + STORY OF THE UNIVERSE + + + + + _I.--THE EARTH’S CRUST_ + + + + + FORMATION OF THE EARTH + --ÉLISÉE RECLUS + + +According to Laplace’s ideas, the whole planetary system formed, +in long past ages, a portion of the sun. This luminary, composed +solely of gaseous particles much lighter than hydrogen, pervaded +with its enormous rotundity the whole of the space in which the +planets, including Neptune, are now describing their immense orbits. +The diameter of the solar spheroid must then have been 6,500 times +greater than it now is, and its bulk must have surpassed its present +volume by more than 860,000 millions of times. In the same way, the +earth, before it began to get cool and solidify, would have embraced +the moon within its limits, and its diameter would have been nearly +six times greater than that of the planet Jupiter. But, unsubstantial +and aerial as it was, our earth had then nothing but a cosmical +life which could hardly be called material; it was not until it +became more solid and its outer crust was hardened that it actually +commenced its real existence. + +This brilliant hypothesis accounts better than any other for the +uniform translatory motion of the planets in the direction of west +to east; it also apparently agrees in a remarkable way with certain +facts in the subsequent history of the earth, as disclosed to us +by geology; finally, the marvelous rings which surround the planet +Saturn seem to proclaim the truth of the theory devised by Laplace. +There have been some experiments on a small scale which appeared to +reproduce in miniature the magnificent spectacle presented in the +primitive ages by the origin of the planets. M. Plateau, a Belgian +_savant_, managed to make a globe of oil revolve in a mixture of +water and spirits of wine which was of exactly the same specific +gravity as the oil. When the revolution of the little globe was +sufficiently rapid, it was noticed to flatten at the poles and to +swell at the equator; after a time it threw off rings which suddenly +assumed the shape of globules actuated by a rotatory motion of their +own, and turning round the central globe. + +Another hypothesis connected with Laplace’s brilliant astronomical +theory must be added, in order to describe the formation of the +planetary crust. When the gaseous ring became condensed into a globe, +it would not cease to contract, owing to the continued radiation +of its caloric. The whole mass, having become liquid through the +gradual cooling of its molecules, would be changed into a sea of lava +whirling round in space; but this state was only one of transition. +After an indefinite term of centuries, the loss of heat was +sufficient to cause the formation of a light _scoria_, like a thin +sheet of ice over the surface of the fiery sea, perhaps just at one +of the poles where nowadays the extreme cold produces icebergs and +a frost-bound sea. This first _scoria_ was succeeded by a second, +and then by others; next they would unite into continents floating +on the surface of the lava, and, finally, would cover the whole +circumference of the planet with a continuous layer. A thin but solid +crust would then have imprisoned within it an immense burning sea. + +This crust was frequently broken through by the lava boiling beneath +it, and then, by means of the solidification of the _scoriæ_, +was again united; the cooling process would tend also to slowly +thicken it. After a lapse of time, which must have been immensely +protracted--since the interval during which the temperature of +the terrestrial crust would be lowered from 2,000° to 200° has +been estimated, at the very least, at three and a half millions of +centuries--the pellicle at last became firm, and the eruptions of +the liquid mass within ceased to be a general phenomenon, localizing +themselves at those points where the firm crust was the thinnest. The +surrounding atmosphere, replete with vapors and various substances +maintained by the extreme heat in a gaseous state, would gradually +get rid of its burden; all kinds of matter, one after the other, +would become disengaged from the luminous and burning aerial mass, +and precipitate themselves on the solid crust of the planet. When +the temperature was lowered sufficiently to enable them to pass +from a gaseous to a liquid state, metals and other substances would +fall down in a fiery rain on the terrestrial lava. Next, the steam, +confined entirely to higher regions of the gaseous mass, would be +condensed into an immense layer of clouds, incessantly furrowed by +lightning. Drops of water, the commencement of the atmospheric ocean, +would begin to fall down toward the ground, but only to volatilize +on their way and again ascend. Finally these little drops reached +the surface of the terrestrial _scoria_, the temperature of the +water much exceeding 100°, owing to the enormous pressure exercised +by the heavy air of these ages; and the first pool, the rudiment of +a great sea, was collected in some fissure of the lava. This pool +was constantly increased by fresh falls of water, and ultimately +surrounded nearly the whole of the terrestrial crust with a liquid +covering; but, at the same time, it brought with it fresh elements +for the constitution of future continents. The numerous substances +which the water held in solution formed various combinations with the +metals and soils of its bed; the currents and tempests which agitated +it destroyed its shores only to form new ones; the sediment deposited +at the bottom of the water commenced the series of rocks and strata +which follow one another above the primitive crust. + +Henceforward the igneous planet was externally clothed with a triple +covering, solid, liquid, and gaseous; it might therefore become the +theatre of life. Vegetables and lowly forms of animals were called +into existence in the water, and on the land which had emerged from +it; and, finally, when the temperature of the surface of the globe +had become less than 50°, allowing albumen to liquefy and blood to +flow in the veins, the fauna and the flora would be developed, the +remains of which are found in the earliest fossil strata. The era +of chaos was succeeded by that of vital harmony; but in the immense +series of ages we are dealing with, the life which appeared on the +refrigerated planet was little else than the “mouldiness formed in a +day.” + +According to the theory generally propounded, the solid crust was not +very completely formed; it is, indeed, much thinner than the layer +of air surrounding the globe; for, following the common estimate, +which, however, is purely hypothetical, at 22 to 25, or, at most, 50 +miles below the surface of the earth, the terrestrial heat would be +sufficient to melt granite. Compared to the diameter of the earth, +which is about 250 times greater, this crust is nothing more than +a thin skin, a just idea of which may be given by a sheet of thin +cardboard surrounding a liquid sphere a yard in diameter. In the case +of the earth, this liquid is a sea of lava and molten rocks, having, +like the ocean above it, its currents, its tides, and perhaps its +storms. + +It is, in fact, very probable that a great part of the rocks which +form the outer portion of our planet, especially the most ancient +formations, existed in former times in a state of fusion like that +of volcanic lava. As most geologists are of opinion, granite and +other similar rocks, forming the principal building-blocks in the +architecture of continents, existed once in a soft or semi-soft state. + +Neither must it be forgotten that, under the hypothesis admitted by +those who assume the existence of a central fire, our planet is to +be considered as actually a liquid mass, as the external crust is +in comparison but a thin skin. Under these conditions, it would +be difficult to believe that this great ocean of lava is not, like +the watery ocean, agitated by the alternating motion of tides, and +that it does not move twice every day the raft, as it were, which is +floating on its surface. It is difficult to understand how it is that +the earth is not much more depressed at the poles than it now is, and +has not been transformed into a real disk. This flattening of the +poles is not more considerable than the mere superficial inequalities +in the equatorial zone between the summits of the Himalayas and +the abysses of the Indian Ocean. M. Liais attributes the slight +flattening of the two poles to the erosion which the water and ice +in those parts, irresistibly drawn as they are toward the equator, +incessantly cause, year after year and century after century, by the +enormous quantity of _débris_ torn away from the surface of the soil, +which they bear with them. + +The principal argument of those who look upon the existence of a +central fire as a demonstrated fact is that, in the external strata +of the earth, so far as they have been explored by miners, the heat +keeps on increasing in proportion to the depth of the excavation. In +descending the shaft of a mine we invariably pass through zones of +increasing temperature; only the rate of increase varies in different +parts of the earth, and according to the strata through which the +shaft is sunk. The heat increases more rapidly in schist than in +granite, and in metallic veins more even than in schist; in lodes +of copper more than in those of tin, and in beds of coal more than +in metallic veins. M. Cordier, being struck by all the objections +which presented themselves to his mind as to the thinness of the +terrestrial crust, has admitted that this covering could not be +stable without having at least from 75 to 175 miles of thickness. + + + + + CLASSES OF ROCKS + --SIR CHARLES LYELL + + +Of what materials is the earth composed, and in what manner are these +materials arranged? These are the first inquiries with which geology +is occupied, a science which derives its name from the Greek _ge_, +the earth, and _logos_, a discourse. Previously to experience we +might have imagined that investigations of this kind would relate +exclusively to the mineral kingdom, and to the various rocks, soils, +and metals which occur upon the surface of the earth, or at various +depths beneath it. But, in pursuing such researches, we soon find +ourselves led on to consider the successive changes which have taken +place in the former state of the earth’s surface and interior, and +the causes which have given rise to these changes; and, what is still +more singular and unexpected, we soon become engaged in researches +into the history of the animate creation, or of the various tribes +of animals and plants which have, at different periods of the past, +inhabited the globe. + +By the “earth’s crust” is meant that small portion of the exterior +of our planet which is accessible to human observation. It comprises +not merely all of which the structure is laid open in mountain +precipices, or in cliffs overhanging a river or the sea, or whatever +the miner reveals in artificial excavation; but the whole of that +outer covering of the planet on which we are enabled to reason by +observations made at or near the surface. + +The materials of this crust are not thrown together confusedly; +but distinct mineral masses, called rocks, are found to occupy +definite spaces, and to exhibit a certain order of arrangement. The +term _rock_ is applied indifferently by geologists to all these +substances, whether they be soft or strong, for clay and sand are +included in the term, and some have even brought peat under this +denomination. + +The most natural and convenient mode of classifying the various rocks +which compose the earth’s crust is to refer, in the first place, to +their origin, and in the second to their relative age. + +The first two divisions, which will at once be understood as natural, +are the aqueous and volcanic, or the products of watery and those of +igneous action at or near the surface. The aqueous rocks, sometimes +called the sedimentary or fossiliferous, cover a larger part of the +earth’s surface than any others. They consist chiefly of mechanical +deposits (pebbles, sand, and mud), but are partly of chemical and +some of them of organic origin, especially the limestones. These +rocks are _stratified_, or divided into distinct layers or strata. +The term _stratum_ means simply a bed, or anything spread out or +_strewed_ over a given surface; and we infer that these strata have +been generally spread out by the action of water, from what we daily +see taking place near the mouths of rivers, or on the land during +temporary inundations. For, whenever a running stream, charged with +mud or sand, has its velocity checked, as when it enters a lake +or sea, or overflows a plain, the sediment, previously held in +suspension by the motion of the water, sinks, by its own gravity, to +the bottom. In this manner layers of mud and sand are thrown down one +upon another. + +If we drain a lake which has been fed by a small stream, we +frequently find at the bottom a series of deposits, disposed with +considerable regularity, one above the other; the uppermost, perhaps, +may be a stratum of peat, next below a more dense and solid variety +of the same material; still lower a bed of shell-marl, alternating +with peat or sand, and then other beds of marl, divided by layers +of clay. Now, if a second pit be sunk through the same continuous +lacustrine _formation_ at some distance from the first, nearly the +same series of beds is commonly met with, yet with slight variations; +some, for example, of the layers of sand, clay, or marl may be +wanting, one or more of them having thinned out and given place to +others, or sometimes one of the masses first examined is observed to +increase in thickness to the exclusion of other beds. + +The term _formation_, which I have used in the above explanation, +expresses in geology any assemblage of rocks which have some +character in common, whether of origin, age, or composition. Thus we +speak of stratified and unstratified, fresh-water and marine, aqueous +and volcanic, ancient and modern, metalliferous and non-metalliferous +formations. + +In the estuaries of large rivers, such as the Ganges and the +Mississippi, we may observe, at low water, phenomena analogous to +those of the drained lakes above mentioned, but on a grander scale, +and extending over areas several hundred miles in length and breadth. +When the periodical inundations subside, the river hollows out a +channel to the depth of many yards through horizontal beds of clay +and sand, the ends of which are seen exposed in perpendicular cliffs. +These beds vary in their mineral composition, or color, or in the +fineness or coarseness of their particles, and some of them are +occasionally characterized by containing driftwood. At the junction +of the river and the sea, especially in lagoons nearly separated by +sand bars from the ocean, deposits are often formed in which brackish +and salt-water shells are included. + +In Egypt, where the Nile is always adding to its delta by filling +up part of the Mediterranean with mud, the newly deposited sediment +is _stratified_, the thin layer thrown down in one season differing +slightly in color from that of a previous year, and being separable +from it, as has been observed in Cairo and other places. + +When beds of sand, clay, and marl containing shells and vegetable +matter are found arranged in a similar manner in the interior of the +earth, we ascribe to them a similar origin; and the more we examine +their characters in minute detail, the more exact do we find the +resemblance. Thus, for example, at various heights and depths in +the earth, and often far from seas, lakes, and rivers, we meet with +layers of rounded pebbles composed of flint, limestone, granite, or +other rocks, resembling the shingles of a sea-beach or the gravel in +a torrent’s bed. Such layers of pebbles frequently alternate with +others formed of sand or fine sediment, just as we may see in the +channel of a river descending from hills bordering a coast, where the +current sweeps down at one season coarse sand and gravel, while at +another, when the waters are low and less rapid, fine mud and sand +alone are carried seaward. + +If a stratified arrangement and the rounded form of pebbles are alone +sufficient to lead us to the conclusion that certain rocks originated +under water, this opinion is further confirmed by the distinct and +independent evidences of _fossils_, so abundantly included in the +earth’s crust. By a _fossil_ is meant any body, or the traces of the +existence of any body, whether animal or vegetable, which has been +buried in the earth by natural causes. Now the remains of animals, +especially of aquatic species, are found almost everywhere imbedded +in stratified rocks, and sometimes, in the case of limestone, they +are in such abundance as to constitute the entire mass of the rock +itself. Shells and corals are the most frequent, and with them are +often associated the bones and teeth of fishes, fragments of wood, +impressions of leaves, and other organic substances. Fossil shells +of forms such as now abound in the sea are met with far inland, +both near the surface and at great depths below it. They occur at +all heights above the level of the ocean, having been observed at +elevations of more than 8,000 feet in the Pyrenees, 10,000 in the +Alps, 13,000 in the Andes, and above 18,000 feet in the Himalayas. + +These shells belong mostly to marine testacea, but in some places +exclusively to forms characteristic of lakes and rivers. Hence it is +concluded that some ancient strata were deposited at the bottom of +the sea, and others in lakes and estuaries. + +The division of rocks, which we may next consider, are the volcanic, +or those which have been produced at or near the surface, whether in +ancient or modern times, not by water, but by the action of fire or +subterranean heat. These rocks are for the most part unstratified, +and are devoid of fossils. They are more partially distributed than +aqueous formations, at least in respect to horizontal extension. +Among those parts of Europe where they exhibit characters not to +be mistaken, I may mention not only Sicily and the country round +Naples, but Auvergne, Velay, and Vivarais, now the departments of Puy +de Dôme, Haute Loire, and Ardêche, toward the centre and south of +France, in which are several hundred conical hills having the forms +of modern volcanoes, with craters more or less perfect on many of +their summits. These cones are composed, moreover, of lava, sand, +and ashes similar to those of active volcanoes. Streams of lava may +sometimes be traced from the cones into the adjoining valleys, where +they have choked up the ancient channels of rivers with solid rock, +in the same manner as some modern flows of lava in Iceland have +been known to do, the rivers either flowing beneath or cutting out +a narrow passage on one side of the lava. Although none of these +French volcanoes has been in activity within the period of history +or tradition, their forms are often very perfect. Some, however, +have been compared to the mere skeletons of volcanoes, the rains and +torrents having washed their sides, and removed all the loose sand +and scoriæ, leaving only the harder and more solid materials. By this +erosion and by earthquakes their internal structure has occasionally +been laid open to view, in fissures and ravines; and we then behold +not only many successive beds and masses of porous lava, sand, and +scoriæ, but also perpendicular walls, or _dikes_, as they are called, +of volcanic rock, which have burst through the other materials. Such +dikes are also observed in the structure of Vesuvius, Etna, and other +active volcanoes. They have been formed by the pouring of melted +matter, whether from above or below, into open fissures, and they +commonly traverse deposits of _volcanic tuff_, a substance produced +by the showering down from the air, or incumbent waters, of sand +and cinders, first shot up from the interior of the earth by the +explosions of volcanic gases. + +Besides the parts of France above alluded to, there are other +countries, as the north of Spain, the south of Sicily, the Tuscan +territory of Italy, the lower Rhenish provinces, and Hungary, where +spent volcanoes may be seen, still preserving in many cases a conical +form, and having craters and often lava streams connected with them. + +There are also other rocks in England, Scotland, Ireland, and almost +every country in Europe, which we infer to be of igneous origin, +although they do not form hills with cones and craters. Thus, for +example, we feel assured that the rock of Staffa and that of the +Giant’s Causeway, called basalt, is volcanic, because it agrees in +its columnar structure and mineral composition with streams of lava +which we know to have flowed from the craters of volcanoes. + +The absence of cones and craters, and long narrow streams of +superficial lava in England and many other countries, is principally +to be attributed to the eruptions having been submarine, just as +a considerable proportion of volcanoes in our own times burst out +beneath the sea. The igneous, as well as the aqueous rocks may be +classed as a chronological series of monuments, throwing light on a +succession of events in the history of the earth. + +We have now pointed out the existence of two distinct orders of +mineral masses, the aqueous and the volcanic; but if we examine a +large portion of a continent, especially if it contain within it a +lofty mountain range, we rarely fail to discover two other classes +of rocks, very distinct from either of those above alluded to, +and which we can neither assimilate to deposits such as are now +accumulated in lakes or seas, nor to those generated by ordinary +volcanic action. The members of both these divisions of rocks agree +in being highly crystalline and destitute of organic remains. The +rocks of one division have been called plutonic, comprehending all +the granites and certain porphyries, which are nearly allied in +some of their characters to volcanic formations. The members of the +other class are stratified and often slaty, and have been called by +some the _crystalline schists_, in which group are included gneiss, +micaceous-schist (or mica-slate), hornblende-schist, statuary marble, +the finer kinds of roofing-slate, and other rocks afterward to be +described. + +All the various kinds of granites which constitute the plutonic +family are supposed to be of igneous or aqueo-igneous origin, and to +have been formed under great pressure, at a considerable depth in +the earth, or sometimes perhaps under a certain weight of incumbent +ocean. Like the lava of volcanoes, they have been melted, and +afterward cooled and crystallized, but with extreme slowness, and +under conditions very different from those of bodies cooling in the +open air. Hence they differ from the volcanic rocks, not only by +their more crystalline texture, but also by the absence of tuffs and +breccias, which are the products of eruptions at the earth’s surface, +or beneath seas of inconsiderable depth. They differ also by the +absence of pores or cellular cavities, to which the expansion of the +entangled gases gives rise in ordinary lava. + +The fourth and last great division of rocks are the crystalline +strata and slates, or schists, called gneiss, mica-schist, +clay-slate, chlorite-schist, marble, and the like, the origin +of which is more doubtful than that of the other three classes. +They contain no pebbles, or sand, or scoriæ, or angular pieces of +imbedded stone, and no traces of organic bodies, and they are often +as crystalline as granite, yet are divided into beds, corresponding +in form and arrangement to those of sedimentary formations, and +are therefore said to be stratified. The beds sometimes consist +of an alternation of substances varying in color, composition, and +thickness, precisely as we see in stratified fossiliferous deposits. +According to the Huttonian theory, which I adopt as the most +probable, the materials of these strata were originally deposited +from water in the usual form of sediment, but they were subsequently +so altered by subterranean heat as to assume a new texture. It is +demonstrable, in some cases at least, that such a complete conversion +has actually taken place, fossiliferous strata having exchanged an +earthy for a highly crystalline texture for a distance of a quarter +of a mile from their contact with granite. In some cases, dark +limestones, replete with shells and corals, have been turned into +white statuary marble, and hard clays, containing vegetable or other +remains, into slates called mica-schist or hornblende-schist, every +vestige of the organic bodies having been obliterated. + +Although we are in a great degree ignorant of the precise nature of +the influence exerted in these cases, yet it evidently bears some +analogy to that which volcanic heat and gases are known to produce; +and the action may be conveniently called plutonic, because it +appears to have been developed in those regions where plutonic rocks +are generated, and under similar circumstances of pressure and depth +in the earth. Intensely heated water or steam permeating stratified +masses under great pressure have no doubt played their part in +producing the crystalline texture and other changes, and it is clear +that the transforming influence has often pervaded entire mountain +masses of strata. + +In accordance with the hypothesis above alluded to, I proposed in +the first edition of the _Principles of Geology_ (1833), the term +Metamorphic, for the altered strata, a term derived from meta, +_trans_, and morphe, _forma_. + +Hence there are four great classes of rocks considered in reference +to their origin--the aqueous, the volcanic, the plutonic, and the +metamorphic. Portions of each of these four distinct classes have +originated at many successive periods. They have all been produced +contemporaneously, and may even now be in the progress of formation +on a large scale. It is not true, as was formerly supposed, that all +granites, together with the crystalline or metamorphic strata, were +first formed, and therefore entitled to be called “primitive,” and +that the aqueous and volcanic rocks were afterward superimposed, +and should, therefore, rank as secondary in the order of time. This +idea was adopted in the infancy of the science, when all formations, +whether stratified or unstratified, earthy or crystalline, with or +without fossils, were alike regarded as of aqueous origin. + +From what has now been said, the reader will understand that each of +the four great classes of rocks may be studied under two distinct +points of view; first, they may be studied simply as mineral masses +deriving their origin from particular causes, and having a certain +composition, form, and position in the earth’s crust, or other +characters, both positive and negative, such as the presence or +absence of organic remains. In the second place, the rocks of each +class may be viewed as a grand chronological series of monuments, +attesting a succession of events in the former history of the globe +and its living inhabitants. + + + + + GEOLOGICAL CHRONOLOGY + --SIR J. WILLIAM DAWSON + + +The crust of the earth, as we somewhat modestly term that portion +of its outer shell which is open to our observation, consists of +many beds of rock superimposed on each other, and which must have +been deposited successively, beginning with the lowest. This is +proved by the structure of the beds themselves, by the markings on +their surfaces, and by the remains of animals and plants which they +contain; all these appearances indicating that each successive bed +must have been the surface before it was covered by the next. + +As these beds of rock were mostly formed under water, and of material +derived from the waste of land, they are not universal, but occur +in those places where there were extensive areas of water receiving +detritus from the land. Further, as the distinction of land and +water arises primarily from the shrinkage of the mass of the earth, +and from the consequent collapse of the crust in some places and +ridging of it up in others, it follows that there have, from the +earliest geological periods, been deep ocean-basins, ridges of +elevated land, and broad plateaus intervening between the ridges, +and which were at some times under water and at other times land, +with many intermediate phases. The settlement and crumpling of the +crust were not continuous, but took place at intervals; and each +such settlement produced not only a ridging up along certain lines, +but also an emergence of the plains or plateaus. Thus at all times +there have been ridges of folded rock constituting mountain ranges, +flat expansions of continental plateau, sometimes dry and sometimes +submerged, and deep ocean-basins, never except in some of their +shallower portions elevated into land. + +By the study of the successive beds, more especially of those +deposited in the times of continental submergence, we obtain a +table of geological chronology which expresses the several stages +of the formation of the earth’s crust, from that early time when a +solid shell first formed on our nascent planet to the present day. +By collecting the fossil remains imbedded in the several layers +and placing these in chronological order, we obtain in like manner +histories of animal and plant life parallel to the physical changes +indicated by the beds themselves. The facts as to the sequence we +obtain from the study of exposures in cliffs, cuttings, quarries, +and mines; and by correlating these local sections in a great number +of places, we obtain our general table of succession; though it is +to be observed that in some single exposures or series of exposures, +like those in the great cañons of Colorado, or on the coasts of Great +Britain, we can often in one locality see nearly the whole sequence +of beds. + +The evidence is similar to that obtained by Schliemann on the site +of Troy, where, in digging through successive layers of _débris_, +he found the objects deposited by successive occupants of the site, +from the time of the Roman Empire back to the earliest tribes, whose +flint weapons and the ashes of their fires rest on the original +surface of the ground. + +Let us now tabulate the whole geological succession with the history +of animals and plants associated with it: + + --------------------+---------------------+-------------------------- + ANIMALS |SYSTEMS OF FORMATIONS| PLANTS + --------------------+---------------------+-------------------------- + | Kainozoic | + | Modern | + Age of Man and | Pleistocene | Angiosperms and + Mammalia | Pliocene | Palms dominant + | Miocene | + | Eocene | + --------------------+---------------------+-------------------------- + | Mesozoic | + | Cretaceous | Cycads and Pines + Age of Reptiles | Jurassic | dominant + | Triassic | + --------------------+---------------------+-------------------------- + | Palæozoic | + | Permian | + | Carboniferous | + Age of Amphibians | Erian | Acrogens and Gymnosperms + and Fishes | Silurian | dominant + Age of Invertebrates| Ordovician | + | Cambrian | + | Huronian (Upper) | + --------------------+---------------------+-------------------------- + | Eozoic | + | Huronian (Lower) | + Age of Protozoa | Upper Laurentian | Protogens and Algæ + | Middle Laurentian | + | Lower Laurentian | + --------------------+---------------------+-------------------------- + +It will be observed, since only the latest of the systems of +formations in this table belongs to the period of human history, that +the whole lapse of time embraced in the table must be enormous. If +we suppose the modern period to have continued for say ten thousand +years, and each of the others to have been equal to it, we shall +require two hundred thousand years for the whole. There is, however, +reason to believe, from the great thickness of the formations and the +slowness of the deposition of many of them in the older systems, that +they must have required vastly greater time. Taking these criteria +into account, it has been estimated that the time-ratios for the +first three great ages may be as one for the Kainozoic to three for +the Mesozoic and twelve for the Palæozoic, with as much for the +Eozoic as for the Palæozoic. This is Dana’s estimate. Another, by +Hull and Houghton, gives the following ratios: Azoic, 34.3 per cent; +Palæozoic, 42.5 per cent; Mesozoic and Kainozoic, 23.3 per cent. It +is further held that the modern period is much shorter than the other +periods of the Kainozoic, so that our geological table may have to be +measured by millions of years instead of thousands. + +We can not, however, attach any certain and definite value in years +to geological time, but must content ourselves with the general +statement that it has been vastly long in comparison to that covered +by human history. + +Bearing in mind this great duration of geological time, and the fact +that it probably extends from a period when the earth was intensely +heated, its crust thin, and its continents as yet unformed, it will +be evident that the conditions of life in the earlier geologic +periods may have been very different from those which obtained +later. When we further take into account the vicissitudes of land +and water which have occurred, we shall see that such changes must +have produced very great differences of climate. The warm equatorial +waters have in all periods, as superficial oceanic currents, been +main agents in the diffusion of heat over the surface of the earth, +and their distribution to north and south must have been determined +mainly by the extent and direction of land, though it may also have +been modified by the changes in the astronomical relations and period +of the earth, and the form of its orbit. We know by the evidence of +fossil plants that changes of this kind have occurred so great as, +on the one hand, to permit the plants of warm temperate regions to +exist within the Arctic Circle; and, on the other, to drive these +plants into the tropics and to replace them by Arctic forms. It is +evident also that in those periods when the continental areas were +largely submerged there might be an excessive amount of moisture in +the atmosphere, greatly modifying the climate in so far as plants are +concerned. + +Let us now consider the history of the vegetable kingdom as indicated +in the few notes in the right-hand column of the table. + +The most general subdivision of plants is into the two great series +of Cryptogams, or those which have no manifest flowers, and produce +minute spores instead of seeds; and Phænogams, or those which possess +flowers and produce seeds containing an embryo of the future plant. + +The Cryptogams may be subdivided into the following three groups: + +1. _Thallogens_, cellular plants not distinctly distinguishable into +stem and leaf. These are the Fungi, the Lichens, and the Algæ, or +sea-weeds. + +2. _Anogens_, having stem and foliage, but wholly cellular. These are +the Mosses and Liverworts. + +3. _Acrogens_, which have long tubular fibres as well as cells +in their composition, and thus have the capacity of attaining a +more considerable magnitude. These are the Ferns (_Filices_), the +Mare’s-tails (_Equisetaceæ_), and the Club-mosses (_Lycopodiaceæ_), +and a curious little group of aquatic plants called Rhizocarps +(_Rhizocarpeæ_). + +The Phænogams are all vascular, but they differ much in the +simplicity or complexity of their flowers or seeds. On this ground +they admit of a twofold division: + +1. _Gymnosperms_, or those which bear naked seeds not inclosed in +fruits. They are the Pines and their allies, and the Cycads. + +2. _Angiosperms_, which produce true fruits inclosing the seeds. In +this group there are two well-marked subdivisions differing in the +structure of the seed and stem. They are the _Endogens_, or inside +growers, with seeds having one seed-leaf only, as the grasses and +the palms; and the _Exogens_, having outside-growing woody stems and +seeds with two seed-leaves. Most of the ordinary forest trees of +temperate climates belong to this group. + +On referring to the geological table, it will be seen that there is +a certain rough correspondence between the order of rank of plants +and the order of their appearance in time. The oldest plants that we +certainly know are Algæ, and with these there are plants apparently +with the structures of Thallophytes but the habit of trees, and +which, for want of a better name, I may call _Protogens_. Plants +akin to the Rhizocarps also appear very early. Next in order we +find forests in which gigantic Ferns and Lycopods and Mare’s-tails +predominate, and are associated with pines. Succeeding these we have +a reign of Gymnosperms, and in the later formations we find the +higher Phænogams dominant. + + + + + THE SILURIAN BEACH + --LOUIS AGASSIZ + + +The crust of our earth is a great cemetery where the rocks are +tombstones on which the buried dead have written their own epitaphs. +They tell us not only who they were and when and where they have +lived, but much also of the circumstances under which they lived. +We ascertain the prevalence of certain physical conditions at +special epochs by the presence of animals and plants whose existence +and maintenance requires such a state of things, more than by any +positive knowledge respecting it. Where we find the remains of +quadrupeds corresponding to our ruminating animals, we infer not +only land, but grassy meadows and an extensive vegetation; where we +find none but marine animals, we know the ocean must have covered +the earth; the remains of large reptiles, representing, though in +gigantic size, the half aquatic, half terrestrial reptiles of our +own period, indicate to us the existence of spreading marshes still +soaked by retreating waters; while the traces of such animals as live +now in sand and shoal waters, or in mud, speak to us of shelving +sandy beaches and mud flats. The eye of the Trilobite tells us that +the sun shone on the old beach where he lived; for there is nothing +in nature without a purpose, and when so complicated an organ was +made to receive the light there must have been light to enter it. +The immense vegetable deposits in the Carboniferous period announce +the introduction of an extensive terrestrial vegetation; and the +impressions left by the wood and leaves show that these first forests +must have grown in a damp soil and a moist atmosphere. In short, all +the remains of animals and plants hidden in the rocks have something +to tell of the climatic conditions and the general circumstances +under which they lived, and the study of fossils is to a naturalist +a thermometer by which he reads the variation of temperature in +past times, a plummet by which he sounds the depths of the ancient +oceans--a register, in fact, of all the important physical changes +the earth has undergone. + +The Silurian beach was a shelving one, and covered, of course, +with shoal waters; but the parallel ridges trending east to west +across the State of New York, considered by some geologists as the +successive shores of a receding ocean, are believed by others to be +the inequalities on the bottom of a shallow sea. Not only, however, +does the general character of these successive terraces suggest the +idea that they must have been shores, but the ripple marks upon +them are as distinct as upon any modern beach. The regular rise and +fall of the water is registered there in waving, undulating lines +as clearly as on the sand beaches of Newport or Nahant; and we can +see on any of those ancient shores the track left by the waves as +they rippled back at ebb of the tide thousands of centuries ago. +One can often see where some obstacle interrupted the course of the +water, causing it to break around it; and such an indentation even +retains the soft, muddy, plastic look that we observe on the present +beaches, where the resistance made by any pebble or shell to the +retreating wave has given it greater force at that point, so that the +sand around the spot is soaked and loosened. There is still another +sign familiar to those who have watched the action of water on a +beach. Where a shore is very shelving and flat, so that the waves do +not recede in ripples from it, but in one unbroken sheet, the sand +and small pebbles are dragged and form lines which diverge whenever +the water meets an obstacle, thus forming sharp angles on the sand. +Such marks are as distinct on the oldest Silurian rocks as if they +had been made yesterday. Nor are these the only indications of the +same fact. There are certain animals living always on sandy or muddy +shores which require for their well-being that the beach should be +left dry for a part of the day. These animals, moving about in the +sand or mud from which the water has retreated, leave their tracks +there; and if, at such a time, the wind is blowing dust over the +beach and the sun is hot enough to bake it upon the impressions so +formed, they are left in a kind of mold. Such trails and furrows made +by small shells and crustacea are also found in plenty on the oldest +deposits. + +Admitting it, then, to be a beach, let us begin with the lowest type +of the Animal Kingdom and see what _Radiates_ are to be found there. +There are plenty of _Corals_, but they are not the same kind of +_Corals_ as those that build up our reefs and islands now. The modern +Coral animals are chiefly _Polyps_, but the prevailing _Corals_ +of the _Silurian_ age were _Acalephian Hydroids_, animals which +indeed resemble _Polyps_ in certain external features, and have been +mistaken for them, but which are, nevertheless, _Acalephs_ by their +internal structure. + +Of the _Echinoderms_, the class of _Radiates_ represented now by our +_Star-Fishes_ and _Sea-Urchins_, we may gather any quantity, though +the old-fashioned forms are very different from the living ones. +The _Mollusks_ were also represented then, as now, by their three +classes, _Acephala_, _Gasteropoda_, and _Cephalopoda_. The _Acephala_ +or _Bivalves_ we find in great numbers, but of a very different +pattern from the _Oysters_, _Clams_, and _Mussels_ of recent times. + +Of the _Silurian Univalves_ or _Gasteropods_, there is not much +to tell, for their spiral shells were so brittle that scarcely +any perfect specimens are known, though their broken remains are +found in such quantities as to show that this class also was +very fully represented in the earliest creation. But the highest +class of _Mollusks_, the _Cephalopods_ or _Chambered Shells_, or +_Cuttle-Fishes_, as they are called when the animal is unprotected by +a shell, are, on the contrary, very well preserved, and they are very +numerous. + +Of _Articulates_ we find only two classes, _Worms_ and _Crustacea_. +Insects there were none--for, as we have seen, this early world +was wholly marine. There is little to be said of the _Worms_, for +their soft bodies, unprotected by any hard covering, could hardly +be preserved; but, like the marine _Worms_ of our own times, they +were in the habit of constructing envelopes for themselves, built of +sand, or sometimes from a secretion of their own bodies, and these +cases we find in the earliest deposits, giving us the assurance that +the _Worms_ were represented there. I should add, however, that many +impressions described as produced by _Worms_ are more likely to have +been the tracks of _Crustacea_. But by far the most characteristic +class of _Articulates_ in ancient times were the _Crustaceans_. The +_Trilobites_ stand in the same relation to the modern _Crustacea_ as +the _Crinoids_ do to the modern _Echinoderms_. They were then the +sole representatives of their class, and the variety and richness of +the type are most extraordinary. They were of nearly equal breadth +for the whole length of the body, and rounded at the two ends, so as +to form an oval outline. + +We have found _Radiates_, _Mollusks_, and _Articulates_ in plenty; +and now what is to be said of _Vertebrates_ in these old times--of +the highest and most important division of the Animal Kingdom, that +to which we ourselves belong. They were represented by Fishes alone; +and the fish chapter in the history of the early organic world is +a curious and, as it seems to me, a very significant one. We shall +find no perfect specimens; and he would be a daring, not to say a +presumptuous, thinker who would venture to reconstruct a fish of +the _Silurian_ age from any remains that are left to us. But still +we find enough to indicate clearly the style of those old fishes, +and to show, by comparison with the living types, to what group of +modern times they belong. We should naturally expect to find the +_Vertebrates_ introduced in their simplest form; but this is by no +means the case: the common fishes, as _Cod_, _Herring_, _Mackerel_, +and the like, were unknown in those days. + +I have spoken of the _Silurian_ beach as if there were but one, not +only because I wished to limit my sketch and to attempt, at least, +to give it the vividness of a special locality, but also because a +single such shore will give us as good an idea of the characteristic +fauna of the time as if we drew our material from a wider range. +There are, however, a great number of parallel ridges belonging to +the _Silurian_ and _Devonian_ periods running from east to west, +not only through the State of New York, but far beyond, through the +States of Michigan and Wisconsin into Minnesota; one may follow nine +or ten such successive shores in unbroken lines from the neighborhood +of Lake Champlain to the Far West. + +Although the early geological periods are more legible in North +America, because they are exposed over such extensive tracts of land, +yet they have been studied in many parts of the globe. In Norway, in +Germany, in France, in Russia, in Siberia, in Kamtchatka, in parts of +South America, in short, wherever the civilization of the white race +has extended, _Silurian_ deposits have been observed, and everywhere +they bear the same testimony to a profuse and varied creation. The +earth was teeming then with life as now, and in whatever corner of +its surface the geologist finds the old strata, they hold a dead +fauna as numerous as that which lives and moves above it. Nor do we +find that there was any gradual increase or decrease of any organic +forms at the beginning or close of the successive periods. + +I think the impression that the faunæ of the early geological periods +were more scanty than those of later times arises partly from the +fact that the present creation is made a standard of comparison for +all preceding creations. Of course, the collection of living types +in any museum must be more numerous than those of fossil forms, for +the simple reason that almost the whole of the present surface of +the earth, with the animals and plants inhabiting it, is known to +us, whereas the deposits of the _Silurian_ and _Devonian_ periods +are exposed to view only over comparatively limited tracts and in +disconnected regions. But let us compare a given extent of _Silurian_ +or _Devonian_ seashore with an equal extent of seashore belonging +to our own time, and we shall soon be convinced that the one is as +populous as the other. On the New England Coast there are about one +hundred and fifty different kinds of fishes; in the Gulf of Mexico +two hundred and fifty; in the Red Sea about the same. We may allow +in present times an average of two hundred or two hundred and fifty +different kinds of fishes to an extent of ocean covering about four +hundred miles. Now, I have made a special study of the _Devonian_ +rocks of Northern Europe, in the Baltic, and along the shore of the +German Ocean. I have found in those deposits alone one hundred and +ten kinds of fossil fishes. To judge of the total number of species +belonging to those early ages by the number known to exist now is +about as reasonable as to infer that because Aristotle, familiar only +with the waters of Greece, recorded less than three hundred kinds +of fishes in his limited fishing-ground, therefore these were all +the fishes then living. The fishing-ground of the geologist in the +_Silurian_ and _Devonian_ periods is even more circumscribed than +his, and belongs, besides, not to a living but to a dead world, far +more difficult to decipher. + +Extinct animals exist all over the world; heaped together under the +snows of Siberia, lying thick beneath the Indian soil, found wherever +English settlers till the ground or work the mines in Australia, +figured in the old encyclopedias of China, where the Chinese +philosophers have drawn them with the accuracy of their nation, built +into the most beautiful temples of classic lands--for even the stones +of the Parthenon are full of the fragments of these old fossils, and +if any chance had directed the attention of Aristotle toward them, +the science of Paleontology would not have waited for its founder +till Cuvier was born--in short, in every corner of the earth where +the investigations of civilized men have penetrated, from the Arctic +to Patagonia and the Cape of Good Hope, these relics tell us of +successive populations lying far behind our own, and belonging to +distinct periods of the world’s history. + + + + + CARBONIFEROUS PERIOD + --LOUIS FIGUIER + + +In the history of our globe the Carboniferous period succeeds to +the Devonian. It is in the formations of this latter epoch that we +find the fossil fuel which has done so much to enrich and civilize +the world in our own age. This period divides itself into two great +sub-periods: 1. The _Coal-measures_; and 2. The _Carboniferous +Limestone_. The first, a period which gave rise to the great deposits +of coal; the second, to most important marine deposits, most +frequently underlying the coal-fields in England, Belgium, France, +and America. + +The limestone mountains, which form the base of the whole system, +attain in places, according to Professor Phillips, a thickness +of 2,500 feet. They are of marine origin, as is apparent by the +multitude of fossils they contain of Zoophytes, Radiata, Cephalopoda, +and Fishes. But the chief characteristic of this epoch is its +strictly terrestrial flora--remains of plants now become as common +as they were rare in all previous formations, announcing a great +increase of dry land. + +The monuments of this era of profuse vegetation reveal themselves in +the precious Coal-measures of England and Scotland. These give us +some idea of the rich verdure which covered the surface of the earth, +newly risen from the bosom of its parent waves. It was the paradise +of terrestrial vegetation. The grand _Sigillaria_, the _Stigmaria_, +and other fern-like plants, were especially typical of this age, +and formed the woods, which were left to grow undisturbed; for as +yet no living Mammals seem to have appeared; everything indicates +a uniformly warm, humid temperature, the only climate in which +the gigantic ferns of the Coal-measures could have attained their +magnitude. Conifers have been found of this period with concentric +rings, but these rings are more slightly marked than in existing +trees of the same family, from which it is reasonable to assume that +the seasonal changes were less marked than they are with us. + +Everything announces that the time occupied in the deposition of the +Carboniferous Limestone was one of vast duration. Professor Phillips +calculates that, at the ordinary rate of progress, it would require +122,400 years to produce only sixty feet of coal. Geologists believe, +moreover, that the upper Coal-measures, where bed has been deposited +upon bed for ages upon ages, were accumulated under conditions of +comparative tranquillity, but that the end of this period was marked +by violent convulsions--by ruptures of the terrestrial crust, when +the carboniferous rocks were upturned, contorted, dislocated by +faults, and subsequently partially denuded, and thus appear now in +depressions or basin-shaped concavities; and that upon this deranged +and disturbed foundation a fourth geological system, called Permian, +was constructed. + +Coal, as we shall find, is composed of the mineralized remains of +the vegetation which flourished in remote ages of the world. Buried +under an enormous thickness of rocks, it has been preserved to +our days, after being modified in its inward nature and external +aspect. Having lost a portion of its elementary constituents, it has +become transformed into a species of carbon, impregnated with those +bituminous substances which are the ordinary products of the slow +decomposition of vegetable matter. + +Thus, coal is the substance of the plants which formed the forests, +the vegetation, and the marshes of the ancient world, at a period +too distant for human chronology to calculate with anything like +precision. + +It is a remarkable circumstance that conditions of equable and warm +climate, combined with humidity, do not seem to have been limited to +any one part of the globe, but the temperature of the whole globe +seems to have been nearly the same in very different latitudes. From +the equatorial regions up to Melville Island, in the Arctic Ocean, +where in our days eternal frost prevails--from Spitzbergen to the +centre of Africa, the carboniferous flora is identically the same. +When nearly the same plants are found in Greenland and Guinea; when +the same species, now extinct, are met with of equal development at +the equator as at the pole, we can not but admit that at this epoch +the temperature of the globe was nearly alike everywhere. What we now +call _climate_ was unknown in these geological times. There seems to +have been then only one climate over the whole globe. It was at a +subsequent period, that is, in later Tertiary times, that the cold +began to make itself felt at the terrestrial poles. Whence, then, +proceeded this general superficial warmth, which we now regard with +so much surprise? It was a consequence of the greater or nearer +influence of the interior heat of the globe. The earth was still so +hot in itself that the heat which reached it from the sun may have +been inappreciable. + +Another hypothesis, which has been advanced with much less certainty +than the preceding, relates to the chemical composition of the +air during the Carboniferous period. Seeing the enormous mass of +vegetation which then covered the globe, and extended from one pole +to the other; considering, also, the great proportion of carbon and +hydrogen which exists in the bituminous matter of coal, it has been +thought, and not without reason, that the atmosphere of the period +might be richer in carbonic acid than the atmosphere of the present +day. It has even been thought that the small number of (especially +air-breathing) animals, which then lived, might be accounted for by +the presence of a greater proportion of carbonic acid gas in the +atmosphere than is the case in our own times. This, however, is +pure assumption, totally deficient in proof. What we can remark, +with certainty, as a striking characteristic of the vegetation of +the globe during this phase of its history, was the prodigious +development which it assumed. The Ferns, which in our days and in +our climate are most commonly only small perennial plants, in the +Carboniferous age sometimes presented themselves under lofty and even +magnificent forms. + +Every one knows those marsh-plants with hollow, channeled, and +articulated cylindrical stems; whose joints are furnished with a +membranous, denticulated sheath, and which bear the vulgar name of +“mare’s-tail”; their fructification forming a sort of catkin composed +of many rings of scales, carrying on their lower surface sacs full of +_spores_ or seeds. These humble _Equiseta_ were represented during +the coal-period by herbaceous trees from twenty to thirty feet +high and four to six inches in diameter. Their trunks, channeled +longitudinally, and divided transversely by lines of articulation, +have been preserved to us: they bear the name of _Calamites_. + +The _Lycopods_ of our age are humble plants, scarcely a yard in +height, and most commonly creepers; but the Lycopodiaceæ of the +ancient world were trees of eighty or ninety feet in height. It +was the _Lepidodendrons_ which filled the forests. Their leaves +were sometimes twenty inches long, and their trunks a yard in +diameter. Such are the dimensions of some specimens of _Lepidodendron +carinatum_ which have been found. Another Lycopod of this period, the +_Lomatophloyos crassicaule_, attained dimensions still more colossal. +The _Sigillarias_ sometimes exceeded 100 feet in height. Herbaceous +Ferns were also exceedingly abundant, and grew beneath the shade of +these gigantic trees. It was the combination of these lofty trees +with such shrubs (if we may so call them) which formed the forests of +the Carboniferous period. + +How this vegetation, so imposing, both on account of the dimensions +of the individual trees and the immense space which they occupied, so +splendid in its aspect, and yet so simple in its organization, must +have differed from that which now embellishes the earth and charms +our eyes! It certainly possessed the advantage of size and rapid +growth; but how poor it was in species--how uniform in appearance! No +flowers yet adorned the foliage or varied the tints of the forests. +Eternal verdure clothed the branches of the Ferns, the Lycopods, and +Equiseta, which composed to a great extent the vegetation of the age. +The forests presented an innumerable collection of individuals, but +very few species, and all belonging to the lower types of vegetation. +No fruit appeared fit for nourishment; none would seem to have been +on the branches. Suffice it to say that few terrestrial animals +seem to have existed yet; animal life was apparently almost wholly +confined to the sea, while the vegetable kingdom occupied the land, +which at a later period was more thickly inhabited by air-breathing +animals. Probably a few winged insects (some coleoptera, orthoptera, +and neuroptera) gave animation to the air while exhibiting their +variegated colors; and it was not impossible but that many +pulmoniferous mollusca (such as land-snails) lived at the same time. + +The vegetation which covered the numerous islands of the +Carboniferous sea consisted, then, of Ferns, of Equisetaceæ, of +Lycopodiaceæ, and dicotyledonous Gymnosperms. The Annularia and +Sigillariæ belong to families of the last-named class, which are now +completely extinct. + +The _Annulariæ_ were small plants which floated on the surface of +fresh-water lakes and ponds; their leaves were verticillate, that +is, arranged in a great number of whorls, at each articulation of +the stem with the branches. The _Sigillariæ_ were, on the contrary, +great trees, consisting of a simple trunk, surmounted with a bunch or +panicle of slender drooping leaves, with the bark often channeled, +and displaying impressions or scars of the old leaves, which, from +their resemblance to a seal, _sigillum_, gave origin to their name. + +The _Stigmariæ_, according to palæontologists, were roots of +Sigillariæ, with a subterranean fructification; all that is known of +them is the long roots which carry the reproductive organs, and in +some cases are as much as sixteen feet long. + +Two other gigantic trees grew in the forests of this period: these +were _Lepidodendron carinatum_ and _Lomatophloyos crassicaule_, both +belonging to the family of Lycopodiaceæ, which now includes only very +small species. The trunk of the Lomatophloyos threw out numerous +branches, which terminated in thick tufts of linear and fleshy +leaves. The Ferns composed a great part of the vegetation of the +Coal-measure period. + +The seas of this epoch included an immense number of Zoophytes, +nearly 400 species of Mollusca, and a few Crustaceans and Fishes. +Among the Fishes, _Psammodus_ and _Coccosteus_, whose massive +teeth inserted in the palate were suitable for grinding; and the +_Holoptychius_ and _Megalichthys_, are the most important. The +Mollusca are chiefly Brachiopods of great size. The _Bellerophon_, +whose convoluted shell in some respects resembles the Nautilus of our +present seas, but without its chambered shell, were then represented +by many species. + +Crustaceans are rare in the Carboniferous Limestone strata; the genus +Phillipsia is the last of the Trilobites, all of which became extinct +at the close of this period. As to the Zoophytes, they consist +chiefly of Crinoids and Corals. We also have in these rocks many +Polyzoa. + +Among the corals of the period we may include the genera +_Lithostrotion_ and _Lonsdalea_. Among the Polyzoa are the genera +_Fenestrella_ and _Polypora_. Lastly, to these we may add a group +of animals which will play a very important part and become +abundantly represented in the beds of later geological periods, but +which already abounded in the seas of the Carboniferous period. We +speak of the _Foraminifera_, microscopic animals, which clustered +either in one body or divided into segments, and covered with a +calcareous, many-chambered shell, as _Fusulina cylindrica_. These +little creatures, which, during the Jurassic and Cretaceous periods, +formed enormous banks and entire masses of rock, began to make their +appearance in the period which now engages our attention. + +This terrestrial period is characterized, in a remarkable manner, by +the abundance and strangeness of the vegetation which then covered +the islands and continents of the whole globe. Upon all points of the +earth, as we have said, this flora presented a striking uniformity. +In comparing it with the vegetation of the present day, the learned +French botanist, M. Brongniart, who has given particular attention +to the flora of the Coal-measures, has arrived at the conclusion +that it presented considerable analogy with that of the islands +of the equatorial and torrid zone, in which a maritime climate and +elevated temperature exist in the highest degree. It is believed that +islands were very numerous at this period; that, in short, the dry +land formed a sort of vast archipelago upon the general ocean, of no +great depth, the islands being connected together and formed into +continents as they gradually emerged from the ocean. + +This flora, then, consists of great trees, and also of many +smaller plants, which would form a close, thick turf, or sod, +when partially buried in marshes of almost unlimited extent. M. +Brongniart indicates, as characterizing the period, 500 species +of plants which now attain a prodigious development. The ordinary +dicotyledons and monocotyledons--that is, plants having seeds with +two lobes in germinating and plants having one seed-lobe--are +almost entirely absent; the cryptogamic, or flowerless plants, +predominate; especially Ferns, Lycopodiaceæ, and Equisetaceæ--but +of forms insulated and actually extinct in these same families. A +few dicotyledonous gymnosperms, or naked-seed plants forming genera +of Conifers, have completely disappeared, not only from the present +flora, but since the close of the period under consideration, there +being no trace of them in the succeeding Permian flora. Such is a +general view of the features most characteristic of the coal-period, +and of the Primary epoch in general. It differs, altogether and +absolutely, from that of the present day; the climatic condition of +these remote ages of the globe, however, enables us to comprehend the +characteristics which distinguish its vegetation. A damp atmosphere, +of an equable rather than an intense heat like that of the tropics, +a soft light veiled by permanent fogs, were favorable to the growth +of this peculiar vegetation, of which we search in vain for anything +strictly analogous in our own days. The nearest approach to the +climate and vegetation proper to the geological period which now +occupies our attention would probably be found in certain islands, +or on the littoral of the Pacific Ocean--the island of Chloë, for +example, where it rains during 300 days in the year, and where the +light of the sun is shut out by perpetual fogs; where arborescent +Ferns form forests, beneath whose shade grow herbaceous Ferns, which +rise three feet and upward above a marshy soil; which gives shelter +also to a mass of cryptogamic plants, greatly resembling, in its +main features, the flora of the Coal-measures. This flora was, as +we have said, uniform and poor in its botanic genera, compared to +the abundance and variety of the flora of the present time; but the +few families of plants which existed then included many more species +than are now produced in the same countries. The fossil Ferns of the +coal-series in Europe, for instance, comprehend about 300 species, +while all Europe now only produces fifty. The gymnosperms, which now +muster only twenty-five species in Europe, then numbered more than +120. + +Calamites are among the most abundant fossil plants of the +Carboniferous period, and occur also in the Devonian. They are +preserved as striated, jointed, cylindrical, or compressed stems, +with fluted channels or furrows at their sides, and sometimes +surrounded by a bituminous coating, the remains of a cortical +integument. They were originally hollow, but the cavity is usually +filled up with a substance into which they themselves have been +converted. + +If, during the coal-period, the vegetable kingdom had reached its +maximum, the animal kingdom, on the contrary, was poorly represented. +Some remains have been found, both in America and Germany, consisting +of portions of the skeleton and the impressions of the footsteps +of a Reptile, which has received the name of Archegosaurus. Among +the animals of this period we find a few Fishes, analogous to +those of the Devonian formation. These are the _Holoptychius_ and +_Megalichthys_, having jawbones armed with enormous teeth. Scales +of _Pygopterus_ have been found in the Northumberland Coal-shale at +Newsham Colliery, and also in the Staffordshire Coal-shale. Some +winged insects would probably join this slender group of living +beings. It may then be said with truth that the immense forests and +marshy plains, crowded with trees, shrubs, and herbaceous plants, +which formed on the innumerable isles of the period a thick and +tufted sward, were almost destitute of animals. + +Coal, as we have said, is only the result of a partial decomposition +of the plants which covered the earth during a geological period +of immense duration. No one, now, has any doubt that this is its +origin. In coal-mines it is not unusual to find fragments of the very +plants whose trunks and leaves characterize the Coal-measures, or +Carboniferous era. Immense trunks of trees have also been met with +in the middle of a seam of coal. In order to explain the presence +of coal in the depths of the earth, there are only two possible +hypotheses. This vegetable débris may either result from the burying +of plants brought from afar and transported by river or maritime +currents, forming immense rafts, which may have grounded in different +places and been covered subsequently by sedimentary deposits; or the +trees may have grown on the spot where they perished, and where they +are now found. + +Can the coal-beds result from the transport by water, and burial +under ground, of immense rafts formed of the trunks of trees? The +hypothesis has against it the enormous height which must be conceded +to the raft, in order to form coal-seams as thick as some of those +which are worked in our collieries. If we take into consideration the +specific gravity of wood, and the amount of carbon it contains, we +find that the coal-deposits can only be about seven-hundredths of the +volume of the original wood and other vegetable materials from which +they are formed. If we take into account, besides, the numerous voids +necessarily arising from the loose packing of the materials forming +the supposed raft, as compared with the compactness of coal, this may +fairly be reduced to five-hundredths. A bed of coal, for instance, +sixteen feet thick, would have required a raft 310 feet high for its +formation. These accumulations of wood could never have arranged +themselves with sufficient regularity to form those well-stratified +coal-beds, maintaining a uniform thickness over many miles, and that +are seen in most coal-fields to lie one above another in succession, +separated by beds of sandstone or shale. And even admitting the +possibility of a slow and gradual accumulation of vegetable débris, +like that which reaches the mouth of a river, would not the plants +in that case be buried in great quantities of mud and earth? Now, +in most of our coal-beds the proportion of earthy matter does not +exceed fifteen per cent of the entire mass. If we bear in mind, +finally, the remarkable parallelism existing in the stratification +of the coal-formation, and the state of preservation in which the +impressions of the most delicate vegetable forms are discovered, it +will, we think, be proved to demonstration that those coal-seams have +been formed in perfect tranquillity. We are, then, forced to the +conclusion that coal results from the mineralization of plants which +has taken place on the spot; that is to say, in the very place where +the plants lived and died. + +It was suggested long ago by Bakewell, from the occurrence of the +same peculiar kind of fireclay under each bed of coal, that it was +the soil proper for the production of those plants from which coal +has been formed. + +[Illustration: Fingal’s Cave, Staffa, Coast of Scotland] + +The clay-beds, “which vary in thickness from a few inches to more +than ten feet, are penetrated in all directions by a confused and +tangled collection of the roots and leaves, as they may be, of the +_Stigmaria ficoides_, these being frequently traceable to the main +stem (_Sigillaria_), which varies in diameter from about two inches +to half a foot. The main stems are noticed as occurring nearer +the top than the bottom of the bed, as usually of considerable +length, the leaves or roots radiating from them in a tortuous +irregular course to considerable distances, and as so mingled with +the under-clay that it is not possible to cut out a cubic foot of it +which does not contain portions of the plant.” + +It is a natural inference to suppose that the present indurated +under-clay is only another condition of that soft, silty soil, or +of that finely levigated muddy sediment--most likely of still and +shallow water--in which the vegetation grew, the remains of which +were afterward carbonized and converted into coal. + +In order thoroughly to comprehend the phenomena of the transformation +into coal of the forests and of the herbaceous plants which +filled the marshes and swamps of the ancient world, there is +another consideration to be presented. During the coal-period, the +terrestrial crust was subjected to alternate movements of elevation +and depression of the internal liquid mass, under the impulse of the +solar and lunar attractions to which they would be subject, as our +seas are now, giving rise to a sort of subterranean tide, operating +at intervals, more or less widely apart, upon the weaker parts of +the crust, and producing considerable subsidences of the ground. +It might, perhaps, happen that, in consequence of a subsidence +produced in such a manner, the vegetation of the coal-period would +be submerged, and the shrubs and plants which covered the surface +of the earth would finally become buried under water. After this +submergence new forests sprung up in the same place. Owing to another +submergence, the second forests were depressed in their turn, and +again covered by water. It is probably by a series of repetitions +of this double phenomenon--this submergence of whole regions of +forest, and the development upon the same site of new growths of +vegetation--that the enormous accumulations of semi-decomposed +plants, which constitute the Coal-measures, have been formed in a +long series of ages. + +But, has coal been produced from the larger plants only--for example, +from the great forest-trees of the period, such as the Lepidodendra, +Sigillariæ, Calamites, and Sphenophylla? That is scarcely probable, +for many coal-deposits contain no vestiges of the great trees of the +period, but only of Ferns and other herbaceous plants of small size. +It is, therefore, presumable that the larger vegetation has been +almost unconnected with the formation of coal, or, at least, that it +has played a minor part in its production. In all probability there +existed in the coal-period, as at the present time, two distinct +kinds of vegetation: one formed of lofty forest-trees, growing on the +higher grounds; the other, herbaceous and aquatic plants, growing on +marshy plains. It is the latter kind of vegetation, probably, which +has mostly furnished the material for the coal; in the same way +that marsh-plants have, during historic times and up to the present +day, supplied our existing peat, which may be regarded as a sort of +contemporaneous incipient coal. + +To what modification has the vegetation of the ancient world been +subjected to attain that carbonized state which constitutes coal? +The submerged plants would, at first, be a light, spongy mass, in +all respects resembling the peat-moss of our moors and marshes. +While under water, and afterward, when covered with sediment, +these vegetable masses underwent a partial decomposition--a moist, +putrefactive fermentation, accompanied by the production of much +carbureted hydrogen and carbonic acid gas. In this way, the hydrogen +escaping in the form of carbureted hydrogen, and the oxygen in the +form of carbonic acid gas, the carbon became more concentrated, and +coal was ultimately formed. This emission of carbureted hydrogen gas +would, probably, continue after the peat-beds were buried beneath +the strata which were deposited and accumulated upon them. The mere +weight and pressure of the superincumbent mass, continued at an +increasing ratio during a long series of ages, have given to the coal +its density and compact state. + +The heat emanating from the interior of the globe would also +exercise a great influence upon the final result. It is to these two +causes--that is to say, to pressure and to the central heat--that we +may attribute the differences which exist in the mineral characters +of various kinds of coal. The inferior beds are _drier_ and more +compact than the upper ones; or less bituminous, because their +mineralization has been completed under the influence of a higher +temperature, and at the same time under a greater pressure. + + + + + THE PALÆONTOLOGICAL HISTORY OF ANIMALS + --HUGH MILLER + + +However much the faunas of the various geologic periods may have +differed from each other, or from the fauna which now exists, in +their general aspect and character, they were all, if I may so speak, +equally underlaid by the great leading ideas which still constitute +the master types of animal life. And these leading ideas are four in +number. _First_, there is the _star-like_ type of life--life embodied +in a form that, as in the corals, the sea-anemones, the sea-urchins, +and the star-fishes, radiates outward from a centre; _second_, +there is the _articulated_ type of life--life embodied in a form +composed, as in the worms, crustaceans, and insects, of a series of +rings united by their edges, but more or less movable on each other; +_third_, there is the bilateral or _molluscan_ type of life--life +embodied in a form in which there is a duality of corresponding +parts, ranged, as in the cuttle-fishes, the clams, and the snails, +on the sides of a central axis or plane; and _fourth_, there is +the _vertebrate_ type of life--life embodied in a form in which +an internal skeleton is built up into two cavities placed the one +over the other; the upper for the reception of the nervous centres, +cerebral and spinal--the lower for the lodgment of the respiratory, +circulatory, and digestive organs. Such have been the four central +ideas of the faunas of every succeeding creation, except, perhaps, +the earliest of all, that of the Lower Silurian System, in which, so +far as is yet known, only three of the number existed--the radiated, +articulated, and molluscan ideas or types. + +The fauna of the Silurian System bears in all its three great types +the stamp of a fashion peculiarly antique, and which, save in a few +of the mollusca, has long since become obsolete. Its radiate animals +are chiefly corals, simple or compound, whose inhabitants may have +somewhat resembled the sea-anemones; with zoophytes, akin mayhap to +the sea-pens, though the relationship must have been a remote one; +and numerous crinoids, or stone lilies, some of which consisted of +but a sculptured calyx without petals, while others threw off a +series of long flexible arms, that divided and subdivided like the +branches of a tree, and were thickly fringed by hair-like fibres. + +The articulata of the Silurian period bore a still more peculiar +character. They consisted mainly of the Trilobites--a family in +whose nicely jointed shells the armorer of the Middle Ages might +have found almost all the contrivances of his craft anticipated, +with not a few besides which he had failed to discover; and which, +after receiving so immense a development during the middle and later +times of the Silurian period that whole rocks were formed almost +exclusively of their remains, gradually died out in the times of +the Old Red Sandstone, and disappeared forever from creation after +the Carboniferous Limestone had been deposited. The mollusca of +the Silurians ranged from the high cephalopoda, represented in our +existing seas by the nautili and the cuttle-fishes, to the low +brachiopods, some of whose cogeners may still be detected in the +terebratulæ of the Highland lochs and bays, and some in the lingulæ +of the Southern Hemisphere. The cephalopods of the system are all of +an obsolete type, that disappeared myriads of ages ago. At length, in +an upper bed of the system, immediately under the base of the Old Red +Sandstone, the remains of the earliest known fishes appear, blended +with what also appears for the first time--the fragmentary remains +of a terrestrial vegetation. The rocks beneath this ancient bone-bed +have yielded no trace of any plant higher than the Thallogens, +or at least not higher than the Zosteraceæ--plants whose proper +habitat is the sea; but, through an apparently simultaneous advance +of the two kingdoms, animal and vegetable--though, of course, the +simultaneousness may be but merely apparent--the first land-plants +and the first vertebrates appear together in the same deposit. The +earliest fishes--first-born of their family--seem to have been all +placoids. The Silurian System has not yet afforded trace of any other +vertebral animal. With the Old Red Sandstone the ganoids were ushered +upon the scene in amazing abundance; and for untold ages, comprising +mayhap millions of years, the entire ichthyic class consisted, so far +as is yet known, of but these two orders. During the times of the Old +Red Sandstone, of the Carboniferous, of the Permian, of the Triassic, +and of the Oolitic Systems, all fishes, though apparently as numerous +individually as they are now, were comprised in the ganoidal and +placoidal orders. The period of these orders seems to have been +nearly correspondent with the reign, in the vegetable kingdom, of the +Acrogens and Gymnogens, with the intermediate classes, their allies. +At length, during the ages of the Chalk, the Cycloids and Ctenoids +were ushered in, and were gradually developed in creation until the +human period, in which they seem to have reached their culminating +point, and now many times exceed in number and importance all other +fishes. The delicate Salmonidæ and the Pleuronectidæ--families to +which the salmon and turbot belong--were ushered into being as early +as the times of the Chalk; but the Gadidæ or cod family--that family +to which the cod proper, the haddock, the dorse, the whiting, the +coal-fish, the pollock, the hake, the torsk, and the ling belong, +with many other useful and wholesome species--did not precede man by +at least any period of time appreciable to the geologist. No trace of +the family has yet been detected in even the Tertiary rocks. + +Of the ganoids of the second age of vertebrate existence--that of +the Old Red Sandstone--some were remarkable for the strangeness +of their forms, and some for constituting links of connection, +which no longer exist in nature, between the ganoid and placoid +orders. The Acanth family, which ceased with the Coal-measures, was +characterized, especially in its Old Red species, by a combination +of traits common to both orders; and among the extremer forms, in +which palæontologists for a time failed to detect that of the fish +at all, we reckon those of the genera Coccosteus, Pterichthys, and +Cephalaspis. The more aberrant genera, however, even while they +consisted each of several species, were comparatively short-lived. +The Coccosteus and Cephalaspis were restricted to but one formation +apiece; while the Pterichthys, which appears for the first time in +the lower deposits of the Old Red Sandstone, becomes extinct at its +close. On the other hand, some of the genera that exemplified the +general type of their class were extremely long-lived. The Celacanths +were reproduced in many various species, from the times of the Lower +Old Red Sandstone to those of the Chalk; and the Cestracions, which +appear in the Upper Ludlow Rocks as the oldest of fishes, continue in +at least one species to exist still. + +The ancient fishes seem to have received their fullest development +during the Carboniferous period. Their number was very great: some +of them attained to an enormous size, and, though the true reptile +had already appeared, they continued to retain till the close of +the System the high reptilian character and organization. Nothing, +however, so impresses the observer as the formidable character of the +offensive weapons with which they were furnished, and the amazing +strength of their defensive armature. I need scarce say that the +palæontologist finds no trace in nature of that golden age of the +world, of which the poets delighted to sing, when all creatures lived +together in unbroken peace, and war and bloodshed were unknown. Ever +since animal life began upon our planet there existed, in all the +departments of being, carnivorous classes, who could not live but by +the death of their neighbors, and who were armed, in consequence, +for their destruction, like the butcher with his axe and knife, and +the angler with his hook and spear. But there were certain periods +in the history of the past during which these weapons assumed a more +formidable aspect than at others; and never were they more formidable +than in the times of the Coal-measures. The teeth of the Rhizodus--a +ganoidal fish of our coal-fields--were more sharp and trenchant than +those of the crocodile of the Nile, and in the larger specimens fully +four times the bulk and size of the teeth of the hugest reptile of +this species that now lives. The dorsal spine of its contemporary, +the Gyracanthus, a great placoid, much exceeded in size that of any +existing fish; it was a mighty spearhead, ornately carved like that +of a New Zealand chief, but in a style that, when he first saw a +specimen in my collection, greatly excited the admiration of Mr. +Ruskin. But one of the most remarkable weapons of the period was +the sting of the Pleuracanthus, another great placoid of the age of +gigantic fishes. It was sharp and polished as a stiletto, but, from +its rounded form and dense structure, of great strength; and along +two of its sides, from the taper point to within a few inches of the +base, there ran a thickly set row of barbs, hooked downward, like +the thorns that bristle on the young shoots of the wild rose, and +which must have rendered it a weapon not merely of destruction, but +also of torture. The defensive armor of the period, especially that +of its ganoids, seems to have been as remarkable for its powers of +resistance as the offensive must have been for their potency in the +assault; and it seems probable that in the great strength of the bony +and enameled armature of this order of fishes we have the secret of +the extremely formidable character of the teeth, spines, and stings +that coexisted along with it. + +The oldest known reptiles appear just a little before the close of +the Old Red Sandstone, just as the oldest known fishes appeared +just a little before the close of the Silurian System. What seems +to be the Upper Old Red of Great Britain, though there still hangs +a shade of doubt on the subject, has furnished the remains of a +small reptile, equally akin, it would appear, to the lizards and the +batrachians; and what seems to be the Upper Old Red of the United +States has exhibited the foot-tracks of a larger animal of the same +class, which not a little resemble those which would be impressed on +recent sand or clay by the alligator of the Mississippi, did not the +alligator of the Mississippi efface its own footprints (a consequence +of the shortness of its legs) by the trail of its abdomen. In the +Coal-measures the reptiles hitherto found are all allied, though +not without a cross of the higher crocodilian or lacertian nature, +to the batrachian order--that lowest order of the reptiles to which +the frogs, newts, and salamanders belong. It was not, however, until +the Permian and Triassic Systems had come to a close, and even the +earlier ages of the Oolitic System had passed away, that the class +received its fullest development in creation. And certainly very +wonderful was the development which it then did receive. Reptiles +became everywhere the lords and masters of this lower world. When +any class of the air-breathing vertebrates is very largely developed, +we find it taking possession of all the three old terrestrial +elements--earth, air, and water. The human period, for instance, +like that which immediately preceded it, is peculiarly a period of +mammals; and we find the class _free_, if I may so express myself, of +the three elements, disputing possession of the sea with the fishes, +in its Cetaceans, its seals and its sea-lions, and of the air with +the birds, in its numerous genera of the bat family. Further, not +until the great mammaliferous period is fairly ushered in do either +the bats or the whales make their appearance in creation. Remains of +Oolitic reptiles have been mistaken in more than one instance for +those of Cetacea; but it is now generally held that the earliest +known specimens of the family belong to the Tertiary ages, while +those of the oldest bats occur in the Eocene of the Paris basin, +associated with the bones of dolphins, lamantines, and morses. Now, +in the times of the Oolite it was the reptilian class that possessed +itself of all the elements. Its gigantic enaliosaurs, huge reptilian +whales mounted on paddles, were the tyrants of the ocean, and must +have reigned supreme over the already reduced class of fishes; its +pterodactyles--dragons as strange as were ever feigned by romancer +of the Middle Ages, and that to the jaws and teeth of the crocodile +added the wings of a bat and the body and tail of an ordinary +mammal--had the “power of the air,” and, pursuing the fleetest +insects in their flight, captured and bore them down; its lakes and +rivers abounded in crocodiles and fresh-water tortoises of ancient +type and fashion; and its woods and plains were the haunts of a +strange reptilian fauna of what has been well termed “fearfully great +lizards”--some of which, such as the iguanodon, rivaled the largest +elephant in height, and greatly more than rivaled him in length and +bulk. Judging from what remains, it seems not improbable that the +reptiles of this Oolitic period were quite as numerous individually, +and consisted of wellnigh as many genera and species as all the +mammals of the present time. In the cretaceous ages, the class, +though still the dominant one, is visibly reduced in its standing: +it had reached its culminating point in the Oolite and then began to +decline; and with the first dawn of the Tertiary division we find it +occupying, as now, a very subordinate place in creation. Curiously +enough, it is not until its times of humiliation and decay that +one of the most remarkable of its orders appears--an order itself +illustrative of extreme degradation, and which figures largely in +every scheme of mythology that borrowed through traditional channels +from Divine revelation, as a meet representative of man’s great +enemy, the Evil One. I, of course, refer to the ophidian or serpent +family. The earliest ophidian remains known to the palæontologist +occur in that ancient deposit of the Tertiary division known as the +London Clay, and must have belonged to serpents, some of them allied +to the Pythons, some to the sea-snakes, which, judging from the +corresponding parts of recent species, must have been from fourteen +to twenty feet in length. + +Birds make their first appearance in a Red Sandstone deposit of the +United States in the valley of the Connecticut, which was at one time +supposed to belong to the Triassic System, but which is now held to +be at least not older than the times of the Lias. No fragments of +the skeletons of birds have yet been discovered in formations older +than the Chalk; the Connecticut remains are those of footprints +exclusively; and yet they tell their extraordinary story, so far as +it extends, with remarkable precision and distinctness. They were +apparently all of the Grallæ or stilt order of birds--an order to +which the cranes, herons, and bustards belong, with the ostriches and +cassowaries, and which is characterized by possessing but three toes +on each foot (one species of ostrich has but two), or, if a fourth +toe be present, so imperfectly is it developed in most of the cases +that it fails to reach the ground. And in almost all the footprints +of the primeval birds of the Connecticut there are only three toes +exhibited. The immense size of some of these footprints served to +militate for a time against belief in their ornithic origin. The +impressions that are but secondary in point of size greatly exceed +those of the hugest birds which now exist; while those of the +largest class equal the prints of the bulkier quadrupeds. There are +tridactyle footprints in the Red Sandstones of Connecticut that +measure eighteen inches in length from the heel to the middle claw, +nearly thirteen inches in breadth from the outer to the inner toe, +and which indicate, from their distance apart in the straight line, +a stride of about six feet in the creature that impressed them in +these ancient sands--measurements that might well startle zoologists +who had derived their experience of the ornithic class from existing +birds exclusively. In a deposit of New Zealand that dates little +if at all in advance of the human period, there have been detected +the remains of birds scarce inferior in size to those of America in +the Liassic ages. The bones of the _Dinornus giganteus_, exhibited +by Dr. Mantell in Edinburgh in 1850, greatly exceeded in bulk those +of the largest horse. The larger thigh-bone referred to must have +belonged, it was held, to a bird that stood from eleven to twelve +feet high--the extreme height of the great African elephant. Such +were the monster birds of a comparatively recent period; and their +remains serve to render credible the evidence furnished by the great +footprints of their remote predecessors of the Lias. The huge feet of +the greatest Dinornus would have left impressions scarcely an inch +shorter than those of the still huger birds of the Connecticut. + +With the Stonesfield slates--a deposit which lies above what is known +as the Inferior Oolite--the remains of mammaliferous animals first +appear. + +The Eocene ages were peculiarly the ages of the Palæotheres--strange +animals of, that pachydermatous or thick-skinned order to which the +elephants, the tapirs, the hogs, and the horses belong. It had been +remarked by naturalists that there are fewer families of this order +in living nature than of almost any other, and that of the existing +genera not a few are widely separated in their analogies from the +others. But in the Palæotheres of the Eocene, which ranged in size +from a large horse to a hare, not a few of the missing links have +been found--links connecting the tapirs to the hogs, and the hogs to +the Palæotheres proper; and there is at least one species suggestive +of a union of some of the more peculiar traits of the tapirs and the +horses. It was among these extinct Pachydermata of the Paris basin +that Cuvier effected his wonderful restorations, and produced those +figures in outline which are now as familiar to the geologist as +any of the forms of the existing animals. The London Clay and the +Eocene of the Isle of Wight have also yielded numerous specimens of +these pachyderms, whose identity with the Continental ones has been +established by Owen; but they are more fragmentary, and their state +of keeping less perfect than those furnished by the gypsum quarries +of Velay and Montmartre. + +In the Middle or Miocene Tertiary, pachyderms, though of a wholly +different type from their predecessors, are still the prevailing +forms. The Dinotherium, one of the greatest quadrupedal mammals that +ever lived, seems to have formed a connecting link in this middle age +between the Pachydermata and the Cetaceæ. Each ramus of the under +jaw, which in the larger specimens are fully four feet in length, +bore at the symphysis a great bent tusk turned downward, which +appears to have been employed as a pick-axe in uprooting the aquatic +plants and liliaceous roots on which the creature seems to have +lived. The head, which measured about three feet across--a breadth +sufficient, surely, to satisfy the demands of the most exacting +phrenologist--was provided with muscles of enormous strength, +arranged so as to give potent effect to the operations of this +strange tool. The hinder part of the skull not a little resembled +that of the Cetaceæ; while, from the form of the nasal bones, the +creature was evidently furnished with a trunk like the elephant. It +seems not improbable, therefore, that this bulkiest of mammaliferous +quadrupeds constituted, as I have said, a sort of uniting tie between +creatures still associated in the human mind, from the circumstances +of their massive proportions, as the greatest that swim the sea +or walk the land--the whale and the elephant The Mastodon, an +elephantoid animal, also furnished, like the elephant, with tusks +and trunk, but marked by certain peculiarities which constitute it +a different genus, seems in Europe to have been contemporary with +the Dinotherium; but in North America (the scene of its greatest +numerical development) it appears to belong to a later age. In +height it did not surpass the African elephant, but it considerably +exceeded it in length--a specimen which could not have stood above +twelve feet high indicating a length of about twenty-five feet: it +had what the elephants want--tusks fixed in its lower jaw, which the +males retained through life, but the females lost when young; its +limbs were proportionally shorter, but more massive, and its abdomen +more elongated and slim; its grinder teeth, too, some of which have +been known to weigh from seventeen to twenty pounds, had their cusps +elevated into great mammæ-like protuberances, to which the creature +owes its name, and wholly differ in their proportions and outline +from the grinders of the elephant. The much greater remoteness of the +mastodontic period in Europe than in America is a circumstance worthy +of notice, as it is one of many facts that seem to indicate a general +transposition of at least the later geologic ages on the opposite +sides of the Atlantic. + + + + + EUROPEAN AND ASIATIC DELUGES + --LOUIS FIGUIER + + +The Tertiary formations, in many parts of Europe of more or less +extent, are covered by an accumulation of heterogeneous deposits, +filling up the valleys, and composed of very various materials, +consisting mostly of fragments of the neighboring rocks. The erosions +which we remark at the bottoms of the hills, and which have greatly +enlarged already existing valleys; the mounds of gravel accumulated +at one point, and which is formed of rolled materials, that is +to say, of fragments of rocks worn smooth and round by continual +friction during a long period, in which they have been transported +from one point to another--all these signs indicate that these +denudations of the soil, these displacements and transports of very +heavy bodies to great distances, are due to the violent and sudden +action of large currents of water. An immense wave has been thrown +suddenly on the surface of the earth, making great ravages in its +passage, furrowing the earth and driving before it débris of all +sorts in its disorderly course. + +To what cause are we to attribute these sudden and apparently +temporary invasions of the earth’s surface by rapid currents of +water? In all probability to the upheaval of some vast extent of dry +land, to the formation of some mountain or mountain range in the +neighborhood of the sea, or even in the bed of the sea itself. The +land, suddenly elevated by an upward movement of the terrestrial +crust, or by the formation of ridges and furrows at the surface, has, +by its reaction, violently agitated the waters, that is to say, the +more mobile portion of the globe. By this new impulse the waters have +been thrown with great violence over the earth, inundating the plains +and valleys, and for the moment covering the soil with their furious +waves, mingled with the earth, sand, and mud, of which the devastated +districts have been denuded by their abrupt invasion. + +There have been, doubtless, during the epochs anterior to the +Quaternary period many deluges such as we are considering. Mountains +and chains of mountains were formed by upheaval of the crust into +ridges, where it was too elastic or too thick to be fractured. Each +of these subterranean commotions would be provocative of momentary +irruptions of the waves. + +But the visible testimony to this phenomenon--the living proofs +of this denudation, of this tearing away of the soil--is found +nowhere so strikingly as in the beds superimposed, far and near, +upon the Tertiary formations, and which bear the geological name of +_diluvium_. This term was long employed to designate what is now +better known as the “bowlder” formation, a glacial deposit which is +abundant in Europe north of the 50th, and in America north of the +40th, parallel, and reappearing again in the Southern Hemisphere; +but altogether absent in tropical regions. It consists of sand and +clay, sometimes stratified, mixed with rounded and angular fragments +of rock, generally derived from the same district; and their origin +has generally been ascribed to a series of diluvial waves raised +by hurricanes, earthquakes, or the sudden upheaval of land from +the bed of the sea, which had swept over continents, carrying with +them vast masses of mud and heavy stones, and forcing these stones +over rocky surfaces so as to polish and impress them with furrows +and striæ. Other circumstances occurred, however, to establish a +connection between this formation and the glacial drift. The size and +number of the erratic blocks increase as we travel toward the Arctic +regions; some intimate association exists, therefore, between this +formation and the accumulations of ice and snow which characterize +the approaching glacial period. + +There is very distinct evidence of two successive deluges in our +hemisphere during the Quaternary epoch. The two may be distinguished +as the _European Deluge_ and the _Asiatic_. The two European deluges +occurred prior to the appearance of man; the Asiatic deluge happened +after that event; and the human race, then in the early days of its +existence, certainly suffered from this cataclysm. + +The first occurred in the north of Europe, where it was produced by +the upheaval of the mountains of Norway. Commencing in Scandinavia, +the wave spread and carried its ravages into those regions which +now constitute Sweden, Norway, European Russia, and the north of +Germany, sweeping before it all the loose soil on the surface, and +covering the whole of Scandinavia--all the plains and valleys of +Northern Europe--with a mantle of transported soil. As the regions +in the midst of which this great mountainous upheaval occurred--as +the seas surrounding these vast spaces were partly frozen and covered +with ice, from their elevation and neighborhood to the pole--the wave +which swept these countries carried along with it enormous masses of +ice. + +The physical proof of this _deluge of the north of Europe_ exists +in the accumulation of unstratified deposits which covers all the +plains and low grounds of Northern Europe. On and in this deposit +are found numerous blocks which have received the characteristic +and significant name of erratic blocks, and which are frequently of +considerable size. These become more characteristic as we ascend to +higher latitudes, as in Norway, Sweden, and Denmark, the southern +borders of the Baltic, and in the British Islands generally, in all +of which countries deposits of marine fossil shells occur, which +prove the submergence of large areas of Scandinavia, of the British +Isles, and other regions during parts of the glacial period. Some of +these rocks, characterized as _erratic_, are of very considerable +volume; such, for instance, is the granite block which forms the +pedestal of the statue of Peter the Great at St. Petersburg. This +block was found in the interior of Russia, where the whole formation +is _Permian_, and its presence there can only be explained by +supposing it to have been transported by some vast iceberg, carried +by a diluvial current. This hypothesis alone enables us to account +for another block of granite, weighing about 340 tons, which was +found on the sandy plains in the north of Prussia, an immense model +of which was made for the Berlin Museum. The last of these erratic +blocks deposited in Germany covers the grave of King Gustavus +Adolphus, of Sweden, killed at the battle of Lutzen, in 1632. He was +interred beneath the rock. Another similar block has been raised in +Germany into a monument to the geologist Leopold von Buch. + +These erratic blocks, which are met with in the plains of Russia, +Poland, and Prussia, and in the eastern parts of England, are +composed of rocks entirely foreign to the region where they are +found. They belong to the primary rocks of Norway; they have been +transported to their present sites, protected by a covering of ice, +by the waters of the northern deluge. + +The second European deluge is supposed to have been the result of +the formation and upheaval of the Alps. It has filled with débris +and transported material the valleys of France, Germany, and Italy +over a circumference which has the Alps for its centre. The proofs +of a great convulsion at a comparatively recent geological date are +numerous. The Alps may be from eighty to one hundred miles across, +and the probabilities are that their existence is due, as Sir Charles +Lyell supposes, to a succession of unequal movements of upheaval and +subsidence; that the Alpine region had been exposed for countless +ages to the action of rain and rivers, and that the larger valleys +were of pre-glacial times, is highly probable. In the eastern part +of the chain some of the Primary fossiliferous rocks, as well as +Oolitic and Cretaceous rocks, and even Tertiary deposits, are +observable; but in the central Alps these disappear, and more recent +rocks, in some places even Eocene strata, graduate into metamorphic +rocks, in which Oolitic, Cretaceous, and Eocene strata have been +altered into granular marble, gneiss, and other metamorphic schists; +showing that eruptions continued after the deposit of the Middle +Eocene formations. Again, in the Swiss and Savoy Alps, Oolitic and +Cretaceous formations have been elevated to the height of 12,000 +feet, and Eocene strata 10,000 feet above the level of the sea; while +in the Rothal, in the Bernese Alps, occurs a mass of gneiss 1,000 +feet thick between two strata containing Oolitic fossils. + +Besides these proofs of recent upheaval, we can trace effects of two +different kinds, resulting from the powerful action of masses of +water violently displaced by this gigantic upheaval. At first broad +tracks have been hollowed out by the diluvial waves, which have, at +these points, formed deep valleys. Afterward these valleys have been +filled up by materials derived from the mountain and transported +into the valley, these materials consisting of rounded pebbles, +argillaceous and sandy mud, generally calcareous and ferriferous. +This double effect is exhibited, with more or less distinctness, in +all the great valleys of the centre and south of France. The valley +of the Garonne is, in respect to these phenomena, classic ground, as +it were. + +The small valleys, tributary to the principal valley, would appear to +have been excavated secondarily, partly out of diluvial deposits, and +their alluvium, essentially earthy, has been formed at the expense of +the Tertiary formation, and even of the diluvium itself. Among other +celebrated sites, the diluvial formation is largely developed in +Sicily. The ancient temple of the Parthenon at Athens is built on an +eminence formed of diluvial earth. + +In the valley of the Rhine, in Alsace, and in many isolated parts of +Europe, a particular sort of _diluvium_ forms thick beds; it consists +of a yellowish-gray mud, composed of argillaceous matter mixed with +carbonate of lime, quartzose and micaceous sand, and oxide of iron. +This mud, termed by geologists _loess_, attains in some places +considerable thickness. It is recognizable in the neighborhood of +Paris. It rises a little both on the right and left, above the base +of the mountains of the Black Forest and of the Vosges; and forms +thick beds on the banks of the Rhine. + +The fossils contained in diluvial deposits consist, generally, of +terrestrial, lacustrine, or fluviatile shells, for the most part +belonging to species still living. In parts of the valley of the +Rhine, between Bingen and Basle, the fluviatile loam or _loess_, +now under consideration, is seen forming hills several hundred feet +thick, and containing, here and there, throughout that thickness, +land and fresh-water shells; from which it seems necessary to +suppose, according to Lyell, first, a time when the _loess_ was +slowly accumulated, then a later period, when large portions of it +were removed--and followed by movements of oscillation, consisting, +first, of a general depression, and then of a gradual re-elevation of +the land. + +The Asiatic deluge--of which sacred history has transmitted to us +the few particulars we know--was the result of the upheaval of a +part of the long chain of mountains which are a prolongation of +the Caucasus. The earth opening by one of the fissures made in its +crust, in course of cooling, an eruption of volcanic matter escaped +through the enormous crater so produced. Volumes of watery vapor or +steam accompanied the lava discharged from the interior of the globe, +which, being first dissipated in clouds and afterward condensing, +descended in torrents of rain, and the plains were drowned with the +volcanic mud. The inundation of the plains over an extensive radius +was the immediate effect of this upheaval, and the formation of the +volcanic cone of Mount Ararat, with the vast plateau on which it +rests, altogether 17,323 feet above the sea, the permanent result. +The event is graphically detailed in the seventh chapter of Genesis. + +All the particulars of the Biblical narrative here recited are only +to be explained by the volcanic and muddy eruption which preceded the +formation of Mount Ararat. The waters which produced the inundation +of these countries proceeded from a volcanic eruption accompanied +by enormous volumes of vapor, which in due course became condensed +and descended on the earth, inundating the extensive plains which +now stretch away from the foot of Ararat. The expression, “the +earth,” or “all the earth,” as it is translated in the Vulgate, which +might be implied to mean the entire globe, is explained by Marcel +de Serres, in a learned book entitled _La Cosmogonie de Moïse_, +and other philologists, as being an inaccurate translation. He has +proved that the Hebrew word _haarets_, incorrectly translated “all +the earth,” is often used in the sense of _region_ or _country_, and +that, in this instance, Moses used it to express only the part of +the globe which was then peopled, and not its entire surface. In the +same manner “_the mountains_” (rendered “_all the mountains_” in the +Vulgate) only implies all the mountains known to Moses. + +Of this deluge many races besides the Jews have preserved a +tradition. Moses dates it from 1,500 to 1,800 years before the epoch +in which he wrote. Berosus, the Chaldean historian, who wrote at +Babylon in the time of Alexander, speaks of a universal deluge, the +date of which he places immediately before the reign of Belus, the +father of Ninus. + +The _Vedas_, or sacred books of the Hindus, supposed to have been +composed about the same time as Genesis, that is, about 3,300 years +ago, make out that the deluge occurred 1,500 years before their time. +The _Guebers_ speak of the same event as having occurred about the +same date. + +Confucius, the celebrated Chinese philosopher and lawgiver, born +toward the year 551 before Christ, begins his history of China by +speaking of the Emperor named Jas, whom he represents as making the +waters flow back, which, being _raised to the heavens_, washed the +feet of the highest mountains, covered the less elevated hills, and +inundated the plains. Thus the Biblical deluge is confirmed in many +respects; but it was local, like all phenomena of the kind, and was +the result of the upheaval of the mountains of western Asia. + + + + + GLACIERS + --LOUIS AGASSIZ + + +The long summer was over. For ages a tropical climate had prevailed +over a great part of the earth, and animals whose home is now beneath +the equator roamed over the world from the far south to the very +borders of the Arctics. The gigantic quadrupeds, the mastodons, +elephants, tigers, lions, hyenas, bears, whose remains are found in +Europe from its southern promontories to the northernmost limits of +Siberia and Scandinavia, and in America from the Southern States +to Greenland and the Melville Islands, may indeed be said to have +possessed the earth in those days. But their reign was over. A +sudden intense winter, that was also to last for ages, fell upon +our globe; it spread over the very countries where these tropical +animals had their homes, and so suddenly did it come upon them that +they were embalmed beneath masses of snow and ice, without time even +for the decay which follows death. The elephant was by no means a +solitary specimen; upon further investigation it was found that the +disinterment of these large tropical animals in Northern Russia and +Asia was no unusual occurrence. Indeed, their frequent discoveries +of this kind had given rise among the ignorant inhabitants to the +singular superstition that gigantic moles lived under the earth which +crumbled away and turned to dust as soon as they came to the upper +air. This tradition, no doubt, arose from the fact that, when in +digging they came upon the bodies of these animals, they often found +them perfectly preserved under the frozen ground, but the moment +they were exposed to heat and light they decayed and fell to pieces +at once. Admiral Wrangell, whose Arctic explorations have been so +valuable to science, tells us that the remains of these animals are +heaped up in such quantities in certain parts of Siberia that he and +his men climbed over ridges and mounds consisting entirely of the +bones of elephants, rhinoceroses, etc. + +We have as yet no clew to the source of this great and sudden change +of climate. Various suggestions have been made, among others that +formerly the inclination of the earth’s axis was greater, or that +the submersion of the continents under water might have produced +a decided increase of cold; but none of these explanations is +satisfactory, and science has yet to find any cause which accounts +for all the phenomena connected with it. It seems, however, +unquestionable that, since the opening of the Tertiary age, a cosmic +summer and winter have succeeded each other, during which a tropical +heat and an Arctic cold have alternately prevailed over a great +portion of the present temperate zone. In the so-called drift (a +superficial deposit subsequent to the Tertiaries) there are found far +to the south of their present abode the remains of animals whose +home now is in the Arctics or the coldest parts of the Temperate +Zones. Among them are the musk-ox, the reindeer, the walrus, the +seal, and many kinds of shells characteristic of the Arctic regions. +The northernmost part of Norway and Sweden is at this day the +southern limit of the reindeer in Europe; but their fossil remains +are found in large quantities in the drift about the neighborhood of +Paris, and they have been traced even to the foot of the Pyrenees, +where their presence would, of course, indicate a climate similar to +the one now prevailing in Northern Scandinavia. Side by side with +the remains of the reindeer are found those of the European marmot, +whose present home is in the mountains, about six thousand feet +above the level of the sea. The occurrence of these animals in the +superficial deposits of the plains of Central Europe, one of which +is now confined to the high north and the other to mountain heights, +certainly indicates an entire change of climatic conditions since +the time of their existence. European shells now confined to the +Northern Ocean are found as fossils in Italy, showing that, while +the present Arctic climate prevailed in the Temperate Zone, that of +the Temperate Zone extended much further south to the regions we now +call sub-tropical. In America there is abundant evidence of the same +kind; throughout the recent marine deposits of the Temperate Zone, +covering the low lands above tide water on this Continent, are found +fossil shells whose present home is on the shores of Greenland. It is +not only in the Northern Hemisphere that these remains occur, but in +Africa and in South America, wherever there has been an opportunity +for investigation, the drift is found to contain the traces of +animals whose presence indicates a climate many degrees colder than +that now prevailing there. + +But these organic remains are not the only evidence of the geological +winter. There are a number of phenomena indicating that during +this period two vast caps of ice stretched from the northern pole +southward and from the southern pole northward, extending in each +case far toward the equator, and that ice fields, such as now spread +over the Arctics, covered a great part of the Temperate Zones, while +the line of perpetual ice and snow in the tropical mountain ranges +descended far below its present limits. + +The first essential condition for the formation of glaciers in +mountain ranges is the shape of their valleys. Glaciers are by no +means in proportion to the height and extent of mountains. There +are many mountain chains as high or higher than the Alps which can +boast of but few and small glaciers, if, indeed, they have any. In +the Andes, the Rocky Mountains, the Pyrenees, the Caucasus, the few +glaciers remaining from the great ice period are insignificant in +size. The volcanic, cone-like shape of the Andes gives, indeed, but +little chance for the formation of glaciers, though their summits +are capped with snow. The glaciers of the Rocky Mountains have been +little explored, but it is known that they are by no means extensive. +In the Pyrenees there is but one great glacier, though the height +of these mountains is such that, were the shape of their valleys +favorable to the accumulation of snow, they might present beautiful +glaciers. In the Tyrol, on the contrary, as well as in Norway and +Sweden, we find glaciers as fine as those of Switzerland in mountain +ranges much lower than either of the above-mentioned chains. But they +are of diversified forms, and have valleys widening upward on the +slope of long crests. The glaciers on the Caucasus are very small in +proportion to the height of the range; but on the northern side of +the Himalayas there are large and beautiful ones, while the southern +slope is almost destitute of them. Spitzbergen and Greenland are +famous for their extensive glaciers, coming down to the seashore, +where huge masses of ice, many hundred feet in thickness, break off +and float away into the ocean as icebergs. + +At the Aletsch in Switzerland, where a little lake lies in a deep cup +between the mountains, with the glacier coming down to its brink, we +have these Arctic phenomena on a small scale; a miniature iceberg +may often be seen to break off from the edge of the larger mass and +float out upon the surface of the water. Icebergs were first traced +back to their true origin by the nature of the land ice of which they +were always composed, and which is quite distinct in structure and +consistency from the marine ice produced by frozen sea water, and +called “ice flow” by the Arctic explorers, as well as from the pond +or river ice, resulting from the simple congelation of fresh water, +the laminated structure of which is in striking contrast to the +granular structure of glacier ice. + +Land ice, of which both the ice fields of the Arctics and the +glaciers consist, is produced by slow and gradual transformation of +snow into ice; and though the ice thus formed may eventually be as +clear and transparent as the present pond or river ice, its structure +is, nevertheless, entirely distinct. + +We may compare these different processes during any moderately cold +winter in the ponds and snow meadows immediately about us. We need +not join an Arctic exploring expedition, or even undertake a more +tempting trip to the Alps, in order to investigate these phenomena +ourselves, if we have any curiosity to do so. The first warm day +after a thick fall of light, dry snow, such as occurs in the coldest +of our winter weather, is sufficient to melt its surface. + +As this snow is porous, the water readily penetrates it, having also +a tendency to sink by its own weight, so that the whole mass becomes +more or less filled with moisture in the course of the day. During +the lower temperature of the night, however, the water is frozen +again, and the snow is now filled with new ice particles. Let this +process be continued long enough and the mass of snow is changed into +a kind of ice gravel, or, if the grains adhere together, to something +like what we call pudding-stone, allowing, of course, for the +difference of material; the snow, which has been rendered cohesive +by the process of partial melting and regelation, holding the ice +globules together, just as the loose materials of the pudding-stone +are held together by the cement which unites them. + +Within this mass air is intercepted and held inclosed between the +particles of ice. The process by which snowflakes or snow crystals +are transformed into grains of ice, more or less compact, is easily +understood. It is the result of a partial thawing under a temperature +maintained very nearly at thirty-two degrees, falling sometimes a +little below and then rising a little above the freezing-point, and +thus producing constant alternations of freezing and thawing in the +same mass of snow. This process amounts to a kind of kneading of +the snow, and when combined with the cohesion among the particles +more closely held together in one snowflake, it produces granular +ice. Of course, the change takes place gradually, and is unequal in +its progress at different depths in the same bed of fallen snow. It +depends greatly on the amount of moisture infiltrating the mass, +whether derived from the melting of its own surface, or from the +accumulation of dew, or the falling of rain or mist upon it. + +The amount of water retained within the mass will also be greatly +affected by the bottom on which it rests and by the state of the +atmosphere. Under a certain temperature the snow may only be glazed +at the surface by the formation of a thin, icy crust, an outer +membrane, as it were, protecting the mass below from a deeper +transformation into ice; or it may be rapidly soaked throughout its +whole bulk, the snow being thus changed into a kind of soft pulp, +what we commonly call slush, which, upon freezing, becomes at once +compact ice; or, the water sinking rapidly, the lower layers only +may be soaked, while the upper portion remains comparatively dry. +But, under all these various circumstances, frost will transform the +crystalline snow into more or less compact ice, the mass of which +will be composed of an infinite number of aggregated snow particles, +very unequal in regularity of outline, and cemented by ice of another +kind, derived from the freezing of the infiltrated moisture, the +whole being interspersed with air. + +Let the temperature rise, and such a mass, rigid before, will resolve +itself again into disconnected ice particles, like grains more or +less rounded. The process may be repeated till the whole mass is +transformed into very compact, almost uniformly transparent and blue +ice, broken only by the intervening air-bubbles. Such a mass of ice, +when exposed to a temperature sufficiently high to dissolve it, does +not melt from the surface and disappear by a gradual diminution of +its bulk, like pond ice, but crumbles into its original granular +fragments, each one of which melts separately. This accounts for the +sudden disappearance of icebergs, which, instead of slowly dissolving +into the ocean, are often seen to fall to pieces and vanish at once. + +Ice of this kind may be seen forming every winter on our sidewalks, +on the edge of the little ditches which drain them, or on the summits +of broad gate posts when capped with snow. Of such ice glaciers are +composed; but, in the glacier, another element comes in which we have +not considered as yet--that of immense pressure in consequence of +the vast accumulations of snow within circumscribed spaces. We see +the same effects produced on a small scale when snow is transformed +into a snowball between the hands. Every boy who balls a mass of snow +in his hands illustrates one side of glacial phenomena. Loose snow, +light and porous, and pure white from the amount of air contained in +it, is in this way presently converted into hard, compact, almost +transparent, ice. This change will take place sooner if the snow be +damp at first, but if dry, the action of the hand will presently +produce moisture enough to complete the process. In this case, +mere pressure produces the same effect which, in the cases we have +been considering above, was brought about by alternate thawing and +freezing, only that, in the latter, the ice is distinctly granular, +instead of being uniform throughout, as when formed under pressure. +In the glaciers, we have the two processes combined. But the +investigators of glacial phenomena have considered too exclusively +one or the other: some of them attributing glacial motion wholly to +the dilatation produced by the freezing of infiltrated moisture in +the mass of snow; others accounting for it entirely by weight and +pressure. There is yet a third class, who, disregarding the real +properties of ice, would have us believe that, because tar, for +instance, is viscid when it moves, therefore ice is viscid because it +moves. + +There is no chain of mountains in which the shape of the valleys is +more favorable to the formation of glaciers than the Alps. Contracted +at their lower extremity, these valleys widen upward, spreading +into deep, broad, trough-like depressions. Take, for instance, the +valley of Hassli, which is not more than half a mile wide where you +enter it above Meyringen; it opens gradually upward till, above the +Grimsel, at the foot of the Finster-Aarhorn, it measures several +miles across. These huge mountain-troughs form admirable cradles for +the snow, which collects in immense quantities within them, and as it +moves slowly down from the upper ranges is transformed into ice on +its way, and compactly crowded into the narrower space below. At the +lower extremity of the glacier the ice is pure blue and transparent, +but as we ascend it appears less compact, more porous and granular, +assuming gradually the character of snow, till in the higher regions +the snow is as light, as shifting, as incoherent as the sand of the +desert. A snowstorm on a mountain summit is very different from +a snowstorm on the plain on account of the different degrees of +moisture in the atmosphere. At great heights there is never dampness +enough to allow the fine snow crystals to coalesce and form what are +called snowflakes. I have even stood on the summit of the Jungfrau +when a frozen cloud filled the air with ice-needles, while I could +see the same cloud poring down sheets of rain upon Lauterbrunnen +below. I remember this spectacle as one of the most impressive I +have ever witnessed in my long experience of Alpine scenery. The +air immediately about me seemed filled with rainbow dust, for the +ice-needles glittered with a thousand hues under the decomposition of +light upon them, while the dark storm in the valley below offered +a strange contrast to the brilliancy of the upper region in which I +stood. One wonders where even so much vapor as may be transformed +into the finest snow should come from at such heights. But the warm +winds creeping up the sides of the valley, the walls of which become +heated during the middle of the day, come laden with moisture which +is changed to a dry snow like dust as soon as it comes into contact +with the intense cold above. + +Currents of warm air affect the extent of the glaciers and influence +also the line of perpetual snow, which is by no means at the same +level, even in neighboring localities. The size of glaciers, of +course, determines to a great degree the height at which they +terminate, simply because a small mass of ice will melt more +rapidly, and at a lower temperature, than a larger one. Thus the +small glaciers, such as those of the Rothhorn or of Trift, above the +Grimsel, terminate at a considerable height above the plain, while +the Mer de Glace, fed from the great snow-caldrons of Mont Blanc, +forces its way down to the bottom of the Valley of Chamouni, and the +glacier of Grindelwald, constantly renewed from the deep reservoirs +where the Jungfrau hoards her vast supplies of snow, descends to +about four thousand feet above the sea level. But the glacier of the +Aar, though also very large, comes to a pause at about six thousand +feet above the level of the sea; for the south wind from the other +side of the Alps, the warm sirocco of Italy, blows across it, and it +consequently melts at a higher level than either the Mer de Glace or +the Grindelwald. It is a curious fact that, in the Valley of Hassli, +the temperature frequently rises instead of falling, as you ascend; +at the Grimsel the temperature is at times higher than at Meyringen, +below, where the warmer winds are not felt so directly. The glacier +of Aletsch, on the southern slope of the Jungfrau, and into which +many other glaciers enter, terminates also at a considerable height, +because it turns into the Valley of the Rhone, through which the +southern winds blow constantly. Under ordinary conditions, vegetation +fades in these mountains at the height of six thousand feet, but, +in consequence of prevailing winds and the sheltering influence of +the mountain walls, there is no uniformity in the limit of perpetual +snow and ice. Where currents of warm air are very constant, glaciers +do not occur at all, even where other circumstances are favorable to +their formation. + +There are valleys in the Alps far above six thousand feet which have +no glaciers, and where perpetual snow is seen only on their northern +sides. These contrasts in the temperature lead to the most wonderful +contrasts in the aspect of the soil; summer and winter lie side by +side, and bright flowers look out from the edge of snows that never +melt. Where the warm winds prevail there may be sheltered spots at +a height of ten or eleven thousand feet, isolated nooks opening +southward where the most exquisite flowers bloom in the midst of +perpetual snow and ice; and occasionally I have seen a bright little +flower with a cap of snow over it that seems to be its shelter. +The flowers give, indeed, a most peculiar charm to these high +Alpine regions. Occurring often in beds of the same kind, forming +green, blue, or yellow patches, they seem nestled close together in +sheltered spots, or even in fissures and chasms of the rock, where +they gather in dense quantities. + +Even in the sternest scenery of the Alps some sign of vegetation +lingers; and I remember to have found a tuft of lichen growing on +the only rock which pierced through the ice on the summit of the +Jungfrau. It was a species then unknown to botanists, since described +under the name of Umbelicarus Higinis. The absolute solitude, the +intense stillness of the upper Alps is most impressive; no cattle, +no pasturage, no bird, nor any sound of life--and, indeed, even if +there were, the rarity of the air in these high regions is such that +sound is hardly transmissible. The deep repose, the purity of aspect +of every object, the snow, broken only by ridges of angular rocks, +produce an effect no less beautiful than solemn. Sometimes, in the +midst of the wide expanse, one comes upon a patch of the so-called +red snow of the Alps. At a distance one would say that such a spot +marked some terrible scene of blood, but as you come nearer the hues +are so tender and delicate, as they fade from deep red to rose, and +so die into the pure colorless snow around, that the first impression +is completely dispelled. This red snow is an organic growth, a plant +springing up in such abundance that it colors extensive surfaces, +just as the microscopic plants dye our pools with green in the +spring. It is an Alga (Protocoites nivalis), well known in the +Arctics, where it forms wide fields in the summer. + +In ordinary times, layers from six to eight feet deep are regularly +added annually to the accumulation of snow in the higher regions--not +taking into account, of course, the heavy drifts heaped up in +particular localities, but estimating the uniform average over wide +fields. This snow is gradually transformed into more or less compact +ice, passing through an intermediate condition analogous to the slush +of our roads, and in that condition chiefly occupies the upper part +of the extensive troughs into which these masses descend from the +loftier heights. This region is called the region of the _névé_. It +is properly the birthplace of the glaciers, for it is here that the +transformation of the snow into ice begins. The _névé_ ice, though +varying in the degree of its compactness and solidity, is always very +porous and whitish in color, resembling somewhat frozen slush, while +lower down in the region of the glacier proper the ice is close, +solid, transparent, and of a bluish tint. + +In consequence of the greater or less rapidity in the movement of +certain portions of the mass, its centre progressing faster than its +sides, and the upper, middle and lower regions of the same glacier +advancing at different rates, the strata, which in the higher ranges +of the snow fields were evenly spread over wide expanses, become +bent and folded to such a degree that the primitive stratification +is nearly obliterated, while the internal mass of the ice has also +assumed new features under these new circumstances. There is, indeed, +as much difference between the newly formed beds of snow in the +upper region and the condition of the ice at the lower end of a +glacier as between a recent deposit of coral sand or a mud bed in +an estuary and the metamorphic limestone or clay slate twisted and +broken as they are seen in the very chains of mountains from which +the glacier descended. + + + + + VOLCANIC ACTION + --SIR ARCHIBALD GEIKIE + + +Large quantities of water accompany many volcanic eruptions. In some +cases, where ancient crater-lakes or internal reservoirs, shaken by +repeated detonations, have been finally disrupted, the mud which has +thereby been liberated has issued from the mountain. Such “mud-lava” +(_lava d’aqua_), on account of its liquidity and swiftness of motion, +is more dreaded for destructiveness than even the true melted lavas. +On the other hand, rain or melted snow or ice, rushing down the cone +and taking up loose volcanic dust, is converted into a kind of mud +that grows more and more pasty as it descends. The mere sudden rush +of such large bodies of water down the steep declivity of a volcanic +cone can not fail to effect much geological change. Deep trenches +are cut out of the loose volcanic slopes, and sometimes large areas +of woodland are swept away, the débris being strewn over the plains +below. + +One of these mud-lavas invaded Herculaneum during the great eruption +of 79, and by quickly enveloping the houses and their contents, +has preserved for us so many precious and perishable monuments of +antiquity. In the same district, during the eruption of 1622, a +torrent of this kind poured down upon the villages of Ottajano and +Massa, overthrowing walls, filling up streets, and even burying +houses with their inhabitants. During the great eruption of Cotopaxi, +in June, 1877, enormous torrents of water and mud, produced by +the melting of the snow and ice of the cone, rushed down from the +mountain. Huge portions of the glaciers of the mountain were detached +by the heat of the rocks below them and rushed down bodily, breaking +up into blocks. The villages all round the mountain to a distance +of sometimes more than ten geographical miles were left deeply +buried under a deposit of mud mixed with blocks of lava, ashes, +pieces of wood, lumps of ice, etc. Many of the volcanoes of Central +and South America discharge large quantities of mud directly from +their craters. Thus, in the year 1691, Imbaburu, one of the Andes of +Quito, emitted floods of mud so largely charged with dead fish that +pestilential fevers arose from the subsequent effluvia. Seven years +later (1698), during an explosion of another of the same range of +lofty mountains, Carguairazo (14,706 feet), the summit of the cone +is said to have fallen in, while torrents of mud containing immense +numbers of the fish _Pymelodus Cyclopum_ poured forth and covered +the ground over a space of four square leagues. The carbonaceous mud +(locally called _moya_) emitted by the Quito volcanoes sometimes +escapes from lateral fissures, sometimes from the craters. Its +organic contents, and notably its siluroid fish, which are the same +as those found living in the streams above ground, prove that the +water is derived from the surface, and accumulates in craters or +underground cavities until discharged by volcanic action. Similar but +even more stupendous and destructive outpourings have taken place +from the volcanoes of Java, where wide tracts of luxuriant vegetation +have at different times been buried under masses of dark gray mud, +sometimes 100 feet thick, with a rough hillocky surface from which +the top of a submerged palm-tree would here and there protrude. + +A volcano, as its activity wanes, may pass into the Solfatara +stage, when only volatile emanations are discharged. The well-known +Solfatara near Naples, since its last eruption in 1198, has +constantly discharged steam and sulphurous vapors. The island of +Volcano has now passed also into this phase, though giving vent to +occasional explosions. Numerous other examples occur among the old +volcanic tracts of Italy, where they have been termed _soffioni_. + +Another class of gaseous emanations betokens a condition of volcanic +activity further advanced toward final extinction. In these, the +gas is carbon-dioxide, either issuing directly from the rock or +bubbling up with water which is often quite cold. The old volcanic +districts of Europe furnish many examples. Thus on the shores of the +Laacher See--an ancient crater-lake of the Eifel--the gas issues +from numerous openings called _moffette_, round which dead insects, +and occasionally mice and birds, may be found. In the same region +occur hundreds of springs more or less charged with this gas. The +famous Valley of Death in Java contains one of the most remarkable +gas-springs in the world. It is a deep, bosky hollow, from one small +space on the bottom of which carbon-dioxide issues so copiously as +to form the lower stratum of the atmosphere. Tigers, deer, and wild +boars, enticed by the shelter of the spot, descend and are speedily +suffocated. Many skeletons, including those of man himself, have been +observed. + +As a distinct class of gas-springs, we may group and describe here +the emanations of volatile hydrocarbons which, when they take fire, +are known as Fire-wells. These are not of volcanic origin, but arise +from changes within the solid rocks underneath. They occur in many of +the districts where mud-volcanoes appear, as in northern Italy, on +the Caspian, in Mesopotamia, in southern Kurdistan, and in many parts +of the United States. + +In the oil regions of Pennsylvania, certain sandy strata occur at +various geological horizons whence large quantities of petroleum and +gas are obtained. In making the borings for oil-wells, reservoirs +of gas as well as subterranean courses or springs of water are met +with. When the supply of oil is limited, but that of gas is large, a +contest for possession of the bore-hole sometimes takes place between +the gas and water. When the machinery is removed and the boring is +abandoned, the contest is allowed to proceed unimpeded, and results +in the intermittent discharge of columns of water and gas to heights +of 130 feet or more. At night, when the gas has been lighted, the +spectacle of one of these “fire-geysers” is inconceivably grand. + +Eruptive fountains of hot water and steam, to which the general name +of Geysers (_i. e._, gushers) is given, from the examples in Iceland, +which were the first to be seen and described, mark a declining phase +of volcanic activity. The Great and Little Geysers, the Strokkr, +and other minor springs of hot water in Iceland, have long been +celebrated examples. More recently another series has been discovered +in New Zealand. But probably the most remarkable and numerous +assemblage is that which has been brought to light in the northwest +part of the Territory of Wyoming, and which has been included within +the “Yellowstone National Park.” In this singular region the ground +in certain tracts is honeycombed with passages which communicate with +the surface by hundreds of openings, whence boiling water and steam +are emitted. In most cases, the water remains clear, tranquil, and +of a deep green-blue tint, though many of the otherwise quiet pools +are marked by patches of rapid ebullition. These pools lie on mounds +or sheets of sinter, and are usually edged round with a raised rim +of the same substance, often beautifully fretted and streaked with +brilliant colors. The eruptive openings usually appear on small, low, +conical elevations of sinter, from each of which one or more tubular +projections rise. It is from these irregular tube-like excrescences +that the eruptions take place. + +The term geyser is restricted to active openings whence columns of +hot water and steam are from time to time ejected; the non-eruptive +pools are only hot springs. A true geyser should thus possess an +underground pipe or passage, terminating at the surface in an opening +built round with deposits of sinter. + +At more or less regular intervals, rumblings and sharp detonations +in the pipe are followed by an agitation of the water in the basin, +and then by the violent expulsion of a column of water and steam to a +considerable height in the air. In the Upper Fire Hole basin of the +Yellowstone Park, one of the geysers, named “Old Faithful,” has ever +since the discovery of the region sent out a column of mingled water +and steam every sixty-three minutes or thereabout. The column rushes +up with a loud roar to a height of more than 100 feet, the whole +eruption not occupying more than about five or six minutes. The other +geysers of the same district are more capricious in their movements, +and some of them more stupendous in the volume of their discharge. +The eruptions of the Castle, Giant, and Beehive vents are marvelously +impressive. + +In course of time, the network of underground passages undergoes +alteration. Orifices that were once active cease to erupt, and even +the water fails to overflow them. Sinter is no longer formed round +them, and their surfaces, exposed to the weather, crack into fine +shaly rubbish like comminuted oyster-shells. Or the cylinder of +sinter grows upward until, by the continued deposit of sinter and the +failing force of the geyser, the tube is finally filled up, and then +a dry and crumbling white pillar is left to mark the site of the +extinct geyser. + +Mud-Volcanoes are of two kinds: 1st, where the chief source of +movement is the escape of gaseous discharges; 2d, where the active +agent is steam. + +Although not volcanic in the proper sense of the term, certain +remarkable orifices of eruption may be noticed here, to which the +names of _mud-volcanoes_, _salses_, _air-volcanoes_, and _maccalubas_ +have been applied (Sicily, the Apennines, Caucasus, Kertch, Tamar). +These are conical hills formed by the accumulation of fine and +usually saline mud, which, with various gases, is continuously or +intermittently given out from the orifice or crater in the centre. +They occur in groups, each hillock being sometimes less than a yard +in height, but ranging up to elevations of 100 feet or more. Like +true volcanoes, they have their periods of repose, when either no +discharge takes place at all, or mud oozes out tranquilly from the +crater, and their epochs of activity, when large volumes of gas, +and sometimes columns of flame, rush out with considerable violence +and explosion, and throw up mud and stones to a height of several +hundred feet. The gases play much the same part, therefore, in these +phenomena that steam does in those of true volcanoes. They consist +of marsh-gas and other hydrocarbons, carbon-dioxide, sulphureted +hydrogen, and nitrogen, with petroleum vapors. The mud is usually +cold. In the water occur various saline ingredients, among which +common salt generally appears; hence the names _Salses_. Naphtha is +likewise frequently present. Large pieces of stone, differing from +those in the neighborhood, have been observed among the ejections, +indicative doubtless of a somewhat deeper source than in ordinary +cases. Heavy rains may wash down the minor mud-cones and spread out +the material over the ground; but gas-bubbles again appear through +the sheet of mud, and by degrees a new series of mounds is once more +thrown up. + +The second class of mud-volcano presents itself in true volcanic +regions, and is due to the escape of hot water and steam through +beds of tuff or some other friable kind of rock. The mud is kept +in ebullition by the rise of steam through it. As it becomes more +pasty and the steam meets with greater resistance, large bubbles are +formed which burst, and the more liquid mud from below oozes out from +the vent. In this way, small cones are built up, many of which have +perfect craters atop. In the Geyser tracts of the Yellowstone region, +there are instructive examples of such active and extinct mud-vents. +Some of the extinct cones there are not more than a foot high, and +might be carefully removed as museum specimens. + +Mud-volcanoes occur in Iceland, Sicily (Maccaluba), in many districts +of northern Italy, at Tamar and Kertch, at Baku on the Caspian, near +the mouth of the Indus, and in other parts of the globe. + +It is not only on the surface of the land that volcanic action shows +itself. It takes place likewise under the sea, and as the geological +records of the earth’s past history are chiefly marine formations, +the characteristics of submarine volcanic action have no small +interest for the geologist. In a few instances, the actual outbreak +of a submarine eruption has been witnessed. Thus, in the early summer +of 1783, a volcanic eruption took place about thirty miles from Cape +Reykjanaes on the west coast of Iceland. An island was built up, from +which fire and smoke continued to issue, but in less than a year the +waves had washed the loose pumice away, leaving a submerged reef +from five to thirty fathoms below sea-level. About a month after +this eruption, the frightful outbreak of Skaptar-Jökull began, the +distance of this mountain from the submarine vent being nearly 200 +miles. A century afterward, viz., in July, 1884, another volcanic +island is said to have been thrown up near the same spot, having at +first the form of a flattened cone, but soon yielding to the power +of the breakers. Many submarine eruptions have taken place within +historic times in the Mediterranean. The most noted of these occurred +in the year 1831, when a new volcanic island (Graham’s Island, Ile +Julia) was thrown up, with abundant discharge of steam and showers of +scoriæ, between Sicily and the coast of Africa. It reached an extreme +height of 200 feet or more above the sea-level (800 feet above +sea-bottom), with a circumference of 3 miles, but on the cessation of +the eruptions was attacked by the waves and soon demolished, leaving +only a shoal to mark its site. In the year 1811, another island +was formed by submarine eruption of the coast off St. Michael’s in +the Azores. Consisting, like the Mediterranean example, of loose +cinders, it rose to a height of about three hundred feet, with a +circumference of about a mile, but subsequently disappeared. In +the year 1796 the island of Johanna Bogoslawa, in Alaska, appeared +above the water, and in four years had grown into a large volcanic +cone, the summit of which was 3,000 feet above sea-level. + +[Illustration: Ideal Landscape of the Carboniferous Period + +Showing Lepidodendra and other Giant Ferns and Mosses whose remains +are found in the Coal-Measures] + +Unfortunately, the phenomena of recent volcanic eruptions under the +sea are for the most part inaccessible. Here and there, as in the +Bay of Naples, at Etna, among the islands of the Greek Archipelago, +and at Tahiti, elevation of the sea-bed has taken place, and brought +to the surface beds of tuff or of lava, which have consolidated +under water. Both Vesuvius and Etna began their career as submarine +volcanoes. The Islands of Santorin and Therasia form the unsubmerged +portions of a great crater-rim rising round a crater which descends +1,278 feet below sea-level. + +Confining attention to vents now active, of which the total number +may be about 300, the chief facts regarding their distribution over +the globe may be thus summarized. (1) Volcanoes occur along the +margins of the ocean-basins, particularly along lines of dominant +mountain ranges, which either form part of the mainland of the +continents or extend as adjacent lines of islands. The vast hollow +of the Pacific is girdled with a wide ring of volcanic foci. (2) +Volcanoes rise, as a striking feature, from the submarine ridges +that traverse the ocean basins. All the oceanic islands are either +volcanic or formed of coral, and the scattered coral-islands have +in all likelihood been built upon the tops of submarine volcanic +cones. (3) Volcanoes are situated not far from the sea. The only +exceptions to this rule are certain vents in Manchuria and in the +tract lying between Tibet and Siberia; but of the actual nature of +these vents very little is yet known. (4) The dominant arrangement +of volcanoes is in series along subterranean lines of weakness, +as in the chain of the Andes, the Aleutian Islands, and the Malay +Archipelago. A remarkable zone of volcanic vents girdles the globe +from Central America eastward by the Azores and Canary Islands to +the Mediterranean, thence to the Red Sea, and through the chains of +islands from the south of Asia to New Zealand and the heart of the +Pacific. (5) On a smaller scale the linear arrangement gives place to +one in groups, as in Italy, Iceland, and the volcanic islands of the +great oceans. + +In the European area there are six active volcanoes--Vesuvius, Etna, +Stromboli, Volcano, Santorin, and Nisyros. Asia contains twenty-four, +Africa ten, North America twenty, Central America twenty-five, and +South America thirty-seven. By much the larger number, however, occur +on islands in the ocean. In the Arctic Ocean rises the solitary Jan +Mayen. On the ridge separating the Arctic and Atlantic basins, the +group of Icelandic volcanoes is found. Along the great central ridge +of the Atlantic bottom, numerous volcanic vents have risen above +the surface of the sea--the Azores, Canary Islands, and the extinct +degraded volcanoes of St. Helena, Ascension, and Tristan d’Acunha. +On the eastern border lie the volcanic vents of the islands off the +African coast, and to the west those of the West Indian Islands. +Still more remarkable is the development of volcanic energy in the +Pacific area. From the Aleutian Islands southward, a long line of +volcanoes, numbering upward of a hundred active vents, extends +through Kamtchatka and the Kurile Islands to Japan, whence another +numerous series carries the volcanic band far south toward the +Malay Archipelago, which must be regarded as the chief centre of +the present volcanic activity of our planet. In Sumatra, Java, and +adjoining islands, no fewer than fifty active vents occur. The chain +is continued through New Guinea and the groups of islands to New +Zealand. Even in the Antarctic regions, Mounts Erebus and Terror are +cited as active vents; while in the centre of the Pacific Ocean rise +the great lava cones of the Sandwich Islands. In the Indian Ocean, +the Red Sea, and off the east coast of Africa a few scattered vents +appear. + + + + + THOUGHTS ABOUT KRAKATOA + --SIR ROBERT S. BALL + + +Midway between Sumatra and Java lies a group of small islands, +which, prior to 1883, were beautified by the dense forests and +glorious vegetation of the tropics. Of these islands Krakatoa was +the chief, though even of it but little was known. Its appearance +from the sea must, indeed, have been familiar to the crews of the +many vessels that navigated the Straits of Sunda, but it was not +regularly inhabited. Glowing with tropical verdure, such an island +seemed an unlikely theatre for the display of an unparalleled effect +of plutonic energy, but yet there were certain circumstances which +may tend to lessen our surprise at the outbreak. In the first place, +as Professor Judd has so clearly pointed out, not only is Krakatoa +situated in a region famous, or perhaps infamous, for volcanoes and +earthquakes, but it actually happens to lie at the intersection of +two main lines, along which volcanic phenomena are, in some degree, +perennial. In the second place, history records that there have been +previous eruptions at Krakatoa. The last of these appears to have +occurred in May, 1680, but unfortunately only imperfect accounts +of it have been preserved. It seems, however, to have annihilated +the forests of the island, and to have ejected vast quantities of +pumice, which cumbered the seas around. Krakatoa then remained active +for a year and a half, after which the mighty fires subsided. The +irrepressible tropical vegetation again resumed possession. The +desolated islet again became clothed with beauty, and for a couple of +centuries reposed in peace. + +It was one o’clock in the afternoon of Sunday, August 26, 1883, when +Krakatoa commenced a series of gigantic volcanic efforts. Detonations +were heard which succeeded each other at intervals of about ten +minutes. These were loud enough to penetrate as far as Batavia and +Buitenzorg, distant 96 and 100 miles respectively from the volcano. +A vast column of steam, smoke, and ashes ascended to a prodigious +elevation. It was measured at two P. M. from a ship 76 miles away, +and was then judged to be 17 miles high--that is, three times the +height of the loftiest mountain in the world. As the Sunday afternoon +wore on, the volcanic manifestations became ever fiercer. At 3 P. M. +the sounds were loudly heard in a town 150 miles away. At 5 P. M. +every ear in the island of Java was engaged in listening to volcanic +explosions, which were considered to be of quite unusual intensity +even in that part of the world. These phenomena were, however, only +introductory. Krakatoa was gathering strength. Between 5 and 6 P. +M. the British ship _Charles Bal_, commanded by Captain Watson, +was about ten miles south of the volcano. The ship had to shorten +sail in the darkness, and a rain of pumice, in large pieces and +quite warm, fell upon her decks. At 7 P. M. the mighty column of +smoke is described as having the shape of a pine tree, and as being +brilliantly illuminated by electric flashes. The sulphurous air is +laden with fine dust, while the lead dropped from a ship in its +anxious navigation astounds the leadsman by coming up hot from the +bottom of the sea. From sunset on Sunday till midnight the tremendous +detonations followed each other so quickly that a continuous roar +may be said to have issued from the island. The full terrors of the +eruption were now approaching. The distance of 96 miles between +Krakatoa and Batavia was not sufficient to permit the inhabitants of +the town to enjoy their night’s sleep. All night long the thunders of +the volcano sounded like the discharges of artillery at their very +doors, while the windows rattled with aerial vibrations. + +On Monday morning, August 27, the eruption culminated in four +terrific explosions, of which the third, shortly after 10 A. M. +Krakatoa time, was by far the most violent. The quantity of material +ejected was now so great that darkness prevailed even as far as +Batavia soon after 11 A. M., and there was a rain of dust until +three in the afternoon. The explosions continued with more or less +intensity all the afternoon of Monday and throughout Monday night. +They finally ceased at about 2:30 A. M. on Tuesday, August 28. The +entire series of grand phenomena thus occupied a little more than +thirty-six hours. + +It seems to be certain that if all the materials poured forth from +Krakatoa during the critical period could be collected together, +the mass they would form would be considerably over a cubic mile +in volume. It is in the other standards of comparison that the +importance of the explosion of Krakatoa is to be sought. The +intensity of this outbreak in its last throes was such that mighty +sounds were heard and mighty waves arose in the sea for which we can +find no parallel. Every part of our globe’s surface felt the pulse of +the air-waves, and beautiful optical phenomena made the circuit of +the globe even more than once or twice. In these last respects the +eruption of Krakatoa is unique. + +It appears to me that the most remarkable incident connected with +the eruption of Krakatoa was the production of the great air-wave by +that particular explosion that occurred at ten o’clock on the morning +of Monday, August 27. The great air-wave was truly of cosmical +importance, affecting as it did every particle of the atmosphere on +our globe. + +The comprehensive series of phenomena wherein the atmosphere of the +entire globe participates in an organized vibration has, so far as +we know, only once been witnessed, and that was after the greatest +outbreak at Krakatoa, at ten o’clock on the morning of August 27. But +the ebb and the flow of these mighty undulations are not immediately +appreciable to the senses. The great wave, for instance, passed +and re-passed and passed again over London, and no inhabitant was +conscious of the fact. But the automatic records of the barometer at +Greenwich show that the vibration from Krakatoa to its antipodes, and +from the antipodes back to Krakatoa, was distinctly perceptible over +London not less than six or seven times. + +From all parts of Europe, from Berlin to Palermo, from St. +Petersburg to Valencia, we obtain the same indications. Fortunately +self-recording barometric instruments are now to be found all over +the world. Almost all the instruments show distinctly the first great +wave from Krakatoa to its antipodes in Central America, and the +return wave from the antipodes to Krakatoa. They also all show the +second great wave which sped from Krakatoa, as well as the second +great wave which returned from the antipodes. Thus, the first four of +the oscillations are depicted on upward of forty of the barograms. +The fifth and sixth oscillations are also to be distinguished on +several of the curves, and even the seventh is certainly established +at some few places, of which Kew is one. Then the gradually +increasing faintness of the indications renders them unrecognizable, +from which we conclude that after seven pulsations our atmosphere +had sensibly regained its former condition ere it was disturbed by +Krakatoa. + +In the whole annals of noise there is nothing which can be compared +to the records. Lloyd’s agent at Batavia, 94 miles distant, says +that on the morning of August 27 the reports and concussions were +deafening. At Carimon, Java Island, reports were heard which led to +the belief that some vessel offshore was making signals of distress, +and boats were accordingly put out to render succor, but no vessel +was found, as the reports were from Krakatoa, at a distance of 355 +miles. At Macassar, in Celebes, explosions were heard all over the +province. Two steamers were sent out to discover the cause, for +the authorities did not then know that what they heard came from +Krakatoa, 969 miles away. But mere hundreds of miles will not suffice +to exemplify the range of this stupendous siren. In St. Lucia Bay, +in Borneo, a number of natives, who had been guilty of murder, +thought they heard the sounds of vengeance in the approach of an +attacking force. They fled from their village, little fancying that +what alarmed them really came from Krakatoa, 1,116 miles distant. +All over the island of Timor alarming sounds were heard, and so +urgent did the situation appear that the government was aroused, and +sent off a steamer to ascertain the cause. The sounds had, however, +come 1,351 miles, all the way from Krakatoa. In the Victoria Plains +of West Australia the inhabitants were startled by the discharge +of artillery--an unwonted noise in that peaceful district--but the +artillery was at Krakatoa, 1,700 miles distant. The inhabitants of +Daly Waters, in South Australia, were rudely awakened at midnight on +Sunday, August 26, by an explosion resembling the blasting of a rock, +which lasted for a few minutes. The time and other circumstances +show that here again was Krakatoa heard, this time at the monstrous +distance of 2,023 miles. But there is undoubted testimony that to +distances even greater than 2,023 miles the waves of sound conveyed +tidings of the mighty convulsion. Diego Garcia, in the Chagos +Islands, is 2,267 miles from Krakatoa, but the thunders traversed +even this distance, and created the belief that there must be some +ship in distress, for which a diligent but necessarily ineffectual +search was made. To pass at once to the most remarkable case of all, +we have a report from Mr. James Wallis, chief of police in Rodriguez, +that “several times during the night of August 26-27, 1883, reports +were heard coming from the eastward, like the distant roar of heavy +guns. These reports continued at intervals of between three and four +hours.” We have thus the astounding fact that almost across the +whole wide extent of the Indian Ocean, that is, to a distance of +nearly 3,000 miles (2,968), the sound of the throes of Krakatoa was +propagated. + +I shall content myself with the mention of three facts in +illustration of the great sea waves which accompanied the eruption +of Krakatoa. Of these, probably the most unusual is the magnitude +of the area over which the undulations were perceived. Thus, to +mention but a single instance, and that not by any means an extreme +one, we find that the tide gauge at Table Bay reveals waves which, +notwithstanding that they have traveled 5,100 miles from Krakatoa, +have still a range of eighteen inches when they arrive at the +southern coast of Africa. The second fact that I mention illustrates +the magnitude of the seismic waves by the extraordinary inundations +that they produced on the shores of the Straits of Sunda. Captain +Wharton shows that the waves, as they deluged the land, must have +been fifty feet, or, in one well authenticated case, seventy-two +feet high. It was, of course, these vast floods which caused the +fearful loss of life. The third illustrative fact concerns the fate +of a man-of-war, the _Berouw_. This unhappy vessel was borne from +its normal element and left high and dry in Sumatra, a mile and +three-quarters inland, and thirty feet above the level of the sea. + +During the crisis on August 26-27, the volume of material blown into +the air was sufficiently dense to obscure the coasts of Sumatra +to such a degree that at 10 A. M. the darkness there is stated to +have been more intense than it is even in the blackest of nights. +The fire-dust ascended to an elevation which, as we have already +mentioned, is estimated to have been as much as seventeen miles. +Borne aloft into these higher regions of our atmosphere, the clouds +of dust at once became the sport of the winds and the currents which +may be found there. If we had not previously known the prevailing +tendency of the winds at these elevations and in these latitudes, the +journey of the Krakatoa dust would have taught us. + +It seems certain that, having attained their lofty elevation, the +mighty clouds of dust were seized by easterly winds, and were swept +along with a velocity which may not improbably be normal at a height +of twenty miles above the earth’s surface. + +It appears that this cloud of dust started immediately from Krakatoa +for a series of voyages round the world. The highway which it at +first pursued may, for our present purpose, be sufficiently defined +by the Tropic of Cancer and the Tropic of Capricorn, though it hardly +approached these margins at first. Westward the dust of Krakatoa +takes its way. In three days it had crossed the Indian Ocean and +was rapidly flying over the heart of Equatorial Africa; for another +couple of days it was making a transatlantic journey; and then it +might be found, for still a couple of days more, over the forests of +Brazil ere it commenced the great Pacific voyage which brought it +back to the East Indies. The dust of Krakatoa had put a girdle round +the earth in thirteen days! The shape of the cloud appears to have +been elongated, so that it took two or three days to complete the +passage over any stated place. + +It remains to give some brief account of the optical phenomena due +to the presence of dust, unusual both in quantity and in character, +in the upper atmosphere. Beautiful pictures show the twilight and +after-glow effects as seen by Mr. W. Ascroft on the bank of the +Thames a little west of London, on the evening of November 26, 1883. +Analogous phenomena were seen almost universally during November and +December in the same year. Who is there that does not remember the +wondrous loveliness of the twilights and the after-glows during that +remarkable winter! These appearances at sunrise and sunset are only +the more generally recognized of a whole system of strange optical +phenomena. One of the most striking indications of the presence of +the dust-stream in its first voyage round the earth was given by the +strange blue hue it imparted to the sun. The dust-stream was also +visible in its rapid voyages as a lofty haze or extensive cloud of +cirro-stratus. Then, too, strange halos were often seen, there were +occasional blue or green moons, and the sun was sometimes glorified +by a corona that had its origin in our atmosphere. Everywhere in the +world there were remarkable features in the sky that winter: from +Tierra del Fuego to Lake Superior; from China to the Gulf of Guinea; +from Panama to Australia. Wherever on land there were inhabitants +with sufficient intelligence to note the unusual, wherever on the sea +there were mariners who kept a careful log, from all such observers +we learn that in the autumn and winter months following the great +eruption of Krakatoa, there were extraordinary manifestations +witnessed in the heavens. + + + + + VOLCANOES + --SIR ARCHIBALD GEIKIE + + +The term volcanic action (volcanism or volcanicity) embraces all the +phenomena connected with the expulsion of heated materials from the +interior of the earth to the surface. Among these phenomena, some +possess an evanescent character, while others leave permanent proofs +of their existence. It is naturally to the latter that the geologist +gives chief attention, for it is by their means that he can trace +former phases of volcanic activity in regions where, for many ages, +there have been no volcanic eruptions. In the operations of existing +volcanoes, he can observe only superficial manifestations of volcanic +action. But examining the rocks of the earth’s crust, he discovers +that amid the many terrestrial revolutions which geology reveals, +the very roots of former volcanoes have been laid bare, displaying +subterranean phases of volcanism which could not be studied in any +modern volcano. Hence an acquaintance only with active volcanoes +will not afford a complete knowledge of volcanic action. It must +be supplemented and enlarged by an investigation of the traces of +ancient volcanoes preserved in the crust of the earth. + +The word “volcano” is applied to a conical hill or mountain (composed +mainly or wholly of erupted materials), from the summit and often +also from the sides of which hot vapors issue, and ashes and streams +of molten rock are intermittently expelled. The term “volcanic” +designates all the phenomena essentially connected with one of these +channels of communication between the surface and the heated interior +of the globe. Yet there is good reason to believe that the active +volcanoes of the present day do not afford by any means a complete +type of volcanic action. The first effort in the formation of a new +volcano is to establish a fissure in the earth’s crust. A volcano is +only one vent or group of vents established along the line of such +a fissure. But in many parts of the earth, alike in the Old World +and the New, there have been periods in the earth’s history when +the crust was rent into innumerable fissures over areas thousands of +square miles in extent, and when the molten rock, instead of issuing, +as it does at a modern volcano, in narrow streams from a central +elevated cone, welled out from numerous small vents along the rents, +and flooded enormous tracts of country without forming any mountain +or conspicuous volcanic cone in the usual sense of these terms. Of +these “fissure-eruptions,” apart from central volcanic cones, no +examples appear to have occurred within the times of human history, +except in Iceland, where vast lava-floods issued from a fissure in +1783. They can best be studied from the remains of former convulsions. + +The materials erupted from volcanic vents may be classed as (1) gases +and vapors, (2) water, (3) lava, (4) fragmentary substances. + +Gases and vapors exist dissolved in the molten magma within the +earth’s crust. They play an important part in volcanic activity, +showing themselves in the earliest stages of a volcano’s history, +and continuing to appear for centuries after all other subterranean +action has ceased. By much the most abundant of them all is +water-gas, which, ultimately escaping as steam, has been estimated +to form 999-1000ths of the whole cloud that hangs over an active +volcano. In great eruptions, steam rises in prodigious quantities, +and is rapidly condensed into a heavy rainfall. M. Fouqué calculated +that, during 100 days, one of the parasitic cones on Etna had +ejected vapor enough to form, if condensed, 2,100,000 cubic metres +(462,000,000 gallons) of water. But even from volcanoes which, like +the Solfatara of Naples, have been dormant for centuries, steam +sometimes still rises without intermission and in considerable +volume. Jets of vapor rush out from clefts in the sides and bottom +of a crater with a noise like that made by the steam blown off by a +locomotive. The number of these funnels or “fumaroles” is often so +large, and the amount of vapor so abundant, that only now and then, +when the wind blows the dense cloud aside, can a momentary glimpse +be had of a part of the bottom of the crater; while at the same +time the rush and roar of the escaping steam remind one of the din +of some vast factory. Aqueous vapor rises likewise from rents on +the outside of the volcanic cone. It issues so copiously from some +flowing lavas that the stream of rock may be almost concealed from +view by the cloud; and it continues to escape from fissures of the +lava, far below the point of exit, for a long time after the rock has +solidified and come to rest. + +Abundant discharges of water accompany some volcanic explosions. +Three sources of this water may be assigned: (1) from the melting of +snow by a rapid accession of temperature previous to or during an +eruption; this takes place from time to time on Etna, in Iceland, +and among the snowy ranges of the Andes, where the cone of Cotopaxi +is said to have been entirely divested of its snow in a single night +by the heating of the mountain; (2) from the condensation of the +vast clouds of steam which are discharged during an eruption; this +undoubtedly is the chief source of the destructive torrents so +frequently observed to form part of the phenomena of a great volcanic +explosion; and (3) from the disruption of reservoirs of water filling +subterranean cavities, or of lakes occupying crater-basins; this has +several times been observed among the South American volcanoes, where +immense quantities of dead fish, which inhabited the water, have +been swept down with the escaping torrents. The volcano of Agua in +Guatemala received its name from the disruption of a crater-lake at +its summit by an earthquake in 1540, whereby a vast and destructive +debacle of water was discharged down the slopes of the mountain. In +the beginning of the year 1817, an eruption took place at the large +crater of Idjèn, one of the volcanoes of Java, whereby a steaming +lake of hot acid water was discharged with frightful destruction down +the slopes of the mountain. After the explosion, the basin filled +again with water, but its temperature was no longer high. + +The term lava is applied generally to all the molten rocks of +volcanoes. The use of the word in this broad sense is of great +convenience in geological descriptions, by directing attention to the +leading character of the rocks as molten products of volcanic action, +and obviating the confusion and errors which are apt to arise from an +ill-defined or incorrect lithological terminology. + +While still flowing or not yet cooled, lavas differ from each other +in the extent to which they are impregnated with gases and vapors. +Some appear to be saturated, others contain a much smaller gaseous +impregnation; and hence arise important distinctions in their +behavior. After solidification, lavas present some noticeable +characters, then easily ascertainable. (1) Their average specific +gravity may be taken as ranging between 2.37 and 3.22. (2) The +heavier varieties contain much magnetic or titaniferous iron, +with augite and olivine, their composition being basic, and their +proportion of silica averaging about 45 to 55 per cent. (3) Lavas +differ much in structure and texture. (4) Lavas vary greatly in +color and general external aspect. The heavy basic kinds are usually +dark gray, or almost black, though, on exposure to the weather, +they acquire a brown tint from the oxidation and hydration of their +iron. Their surface is commonly rough and ragged, until it has been +sufficiently decomposed by the atmosphere to crumble into soil which, +under favorable circumstances, supports a luxuriant vegetation. The +less dense lavas, such as phonolites and trachytes, are frequently +paler in color, sometimes yellow or buff, and decompose into light +soils; but the obsidians present rugged black sheets of rock, +roughened with ridges and heaps of gray froth-like pumice. Some of +the most brilliant surfaces of color in any rock-scenery on the globe +are to be found among volcanic rocks. The walls of active craters +glow with endless hues of red and yellow. The Grand Cañon of the +Yellowstone River has been dug out of the most marvelously tinted +lavas and tuffs. + +Volcanic action may be either constant or periodic. Stromboli, in the +Mediterranean, so far as we know, has been uninterruptedly emitting +hot stones and steam, from a basin of molten lava, since the earliest +period of history. Among the Moluccas, the volcano Sioa, and in the +Friendly Islands, that of Tofua, have never ceased to be in eruption +since their first discovery. The lofty cone of Sangay, among the +Andes of Quito, is always giving off hot vapors; Cotopaxi, too, is +ever constantly active. But, though examples of unceasing action may +thus be cited from widely different quarters of the globe, they are +nevertheless exceptional. The general rule is that a volcano breaks +out from time to time with varying vigor, and after longer or shorter +intervals of quiescence. + +It is usual to class volcanoes as _active_, _dormant_, and _extinct_. +This arrangement, however, often presents considerable difficulty in +its application. An active volcano can not of course be mistaken, for +even when not in eruption, it shows by its discharge of steam and +hot vapors that it might break out into activity at any moment. But +in many cases it is impossible to decide whether a volcano should +be called extinct or only dormant. The volcanoes of Silurian age +in Wales, of Carboniferous age in Ireland, of Permian age in the +Harz, of Miocene age in the Hebrides, of younger Tertiary age in +the Western States and Territories of North America, are certainly +all extinct. But the older Tertiary volcanoes of Iceland are still +represented there by Skaptar-Jökull, Hecla, and their neighbors. +Somma, in the First Century of the Christian era, would have been +naturally regarded as an extinct volcano. Its fires had never been +known to have been kindled; its vast crater was a wilderness of wild +vines and brushwood, haunted, no doubt, by wolf and wild boar. Yet in +a few days, during the autumn of the year 79, the half of the crater +walls was blown out by a terrific series of explosions, the present +Vesuvius was then formed within the limits of the earlier crater, and +since that time volcanic action has been intermittently exhibited up +to the present day. Some of the intervals of quietude, however, have +been so considerable that the mountain might then again have been +claimed as an extinct volcano. Thus, in the 131 years between 1500 +and 1631, so completely had eruptions ceased that the crater had once +more become choked with copse-wood. A few pools and springs of very +salt and hot water remained as memorials of the former condition of +the mountain. But this period of quiescence closed with the eruption +of 1631--the most powerful of all the known explosions of Vesuvius, +except the great one of 79. + +In short, no essential distinction can be drawn between dormant and +extinct volcanoes. Volcanic action is apt to show itself again and +again, even at vast intervals, within the same regions and over the +same sites. The dormant or waning condition of a volcano, when only +steam and various gases and sublimates are given off, is sometimes +called the Solfatara phase, from the well-known dormant crater of +that name near Naples. + +The interval between two eruptions of an active volcano shows a +gradual augmentation of energy. The crater, emptied by the last +discharge, has its floor slowly upraised by the expansive force +of the lava-column underneath. Vapors rise in constant outflow, +accompanied sometimes by discharges of dust or stones. Through +rents in the crater-floor red-hot lava may be seen only a few feet +down. Where the lava is maintained at or above its fusion-point and +possesses great liquidity, it may form boiling lakes, as in the +great crater of Kilauea, where acres of seething lava may be watched +throwing up fountains of molten rock, surging against the walls +and re-fusing large masses that fall into the burning flood. The +lava-column inside the pipe of a volcano is all this time gradually +rising, until some weak part of the wall allows it to escape, or +until the pressure of the accumulated vapors becomes great enough to +burst through the hardened crust of the crater-floor and give rise to +the phenomena of an eruption. + +Kluge has sought to trace a connection between the years of maximum +and minimum sun-spots and those of greatest and feeblest volcanic +activity, and has constructed lists to show that years which +have been specially characterized by terrestrial eruptions have +coincided with those marked by few sun-spots and diminished magnetic +disturbance. Such a connection can not be regarded as having yet +been satisfactorily established. Again, the same author has called +attention to the frequency and vigor of volcanic explosions at or +near the time of the August meteoric shower. But in this case, +likewise, the cited examples can hardly yet be looked upon as more +than coincidences. + +At many volcanic vents the eruptive energy manifests itself with +more or less regularity. At Stromboli, which is constantly in an +active state, the explosions occur at intervals varying from three or +four to ten minutes and upward. A similar rhythmical movement has +been often observed during the eruptions at other vents which are +not constantly active. Volcano, for example, during its eruption of +September, 1873, displayed a succession of explosions which followed +each other at intervals of from twenty to thirty minutes. At Etna and +Vesuvius a similar rhythmical series of convulsive efforts has often +been observed during the course of an eruption. Among the volcanoes +of the Andes a periodic discharge of steam has been observed; Mr. +Whymper noticed outrushes of steam to proceed at intervals of from +twenty to thirty minutes from the summit of Sangai, while during his +inspection of the great crater of Cotopaxi, this volcano was seen to +blow off steam at intervals of about half an hour. At the eruption of +the Japanese volcano, Oshima, in 1877, Mr. Milne observed that the +explosions occurred nearly every two seconds, with occasional pauses +of 15 or 20 seconds. Kilauea, in Hawaii, seems to show a regular +system of grand eruptive periods. Dana has pointed out that outbreaks +of lava have taken place from that volcano at intervals of from eight +to nine years, this being the time required to fill the crater up to +the point of outbreak, or to a depth of 400 or 500 feet. + +The approach of an eruption is not always indicated by any +premonitory symptoms, for many tremendous explosions are recorded to +have taken place in different parts of the world without perceptible +warning. Much in this respect would appear to depend upon the +condition of liquidity of the lava, and the amount of resistance +offered by it to the passage of the escaping vapors through its mass. +In Hawaii, where the lavas are remarkably liquid, vast outpourings of +them have taken place quietly without earthquakes during the present +century. But even there the great eruption of 1868 was accompanied by +violent earthquakes. + +The eruptions of Vesuvius are often preceded by failure or diminution +of wells and springs. But more frequent indications of an approaching +outburst are conveyed by sympathetic movements of the ground. +Subterranean rumblings and groanings are heard; slight tremors +succeed, increasing in frequency and violence till they become +distinct earthquake shocks. The vapors from the crater grow more +abundant as the lava-column in the pipe or funnel of the volcano +ascends, forced upward and kept in perpetual agitation by the passage +of elastic vapors through its mass. After a long previous interval of +quiescence, there may be much solidified lava toward the top of the +funnel, which will restrain the ascent of the still molten portion +underneath. A vast pressure is thus exercised on the sides of the +cone, which, if too weak to resist, will open in one or more rents, +and the liquid lava will issue from the outer slope of the mountain; +or the energies of the volcano will be directed toward clearing the +obstruction in the chief throat, until with tremendous explosions, +and the rise of a vast cloud of dust and fragments, the bottom and +sides of the crater are finally blown out, and the top of the cone +disappears. The lava may now escape from the lowest part of the lip +of the crater, while, at the same time, immense numbers of red-hot +bombs, scoriæ, and stones are shot up into the air. The lava at first +rushes down like one or more rivers of melted iron, but, as it cools, +its rate of motion lessens. Clouds of steam rise from its surface, as +well as from the central crater. Indeed, every successive paroxysmal +convulsion of the mountain is marked, even at a distance, by the rise +of huge ball-like wreaths or clouds of steam, mixed with dust and +stones, forming a column which towers sometimes a couple of miles +or more above the summit of the cone. By degrees these eructations +diminish in frequency and intensity. The lava ceases to issue, the +showers of stones and dust decrease, and after a time, which may +vary from hours to days or months, even in the _régime_ of the same +mountain, the volcano becomes once more tranquil. + +The convulsions which culminate in the formation of a volcano usually +split open the terrestrial crust by a more or less nearly rectilinear +fissure, or by a system of fissures. In the subsequent progress of +the mountain, the ground at and around the focus of action is liable +to be again and again rent open by other fissures. These tend to +diverge from the focus; but around the vent where the rocks have been +most exposed to concussion, the fissures sometimes intersect each +other in all directions. In the great eruption of Etna, in the year +1669, a series of six parallel fissures opened on the side of the +mountain. One of these, with a width of two yards, ran for a distance +of 12 miles, in a somewhat winding course, to within a mile of the +top of the cone. + +In the deeper portions of a volcanic vent the convulsive efforts of +the lava-column to force its way upward must often produce lateral as +well as vertical rifts, and into these the molten material will rush, +exerting as it goes an enormous upward pressure on the mass of rock +overlying it. At a modern volcano these subterranean manifestations +can not be seen, but among the volcanoes of Tertiary and older times +they have been revealed by the progress of denudation. + +Though lava very commonly issues from the lateral fissures on a +volcanic cone, it may sometimes approach the surface in them without +actually flowing out. The great fissure on Etna in 1669, for example, +was visible even from a distance, by the long line of vivid light +which rose from the incandescent lava within. Again, it frequently +happens that minor volcanic cones are thrown up on the line of a +fissure, either from the congelation of the lava round the point +of emission, or from the accumulation of ejected scoriæ round the +fissure-vent. One of the most remarkable examples of this kind is +that of the Laki fissure in Iceland, the whole length of which (12 +miles) bristles with small cones and craters almost touching each +other. + +Apart from the appearance of visible fissures, volcanic energy may +be, as it were, concentrated on a given point, which will usually be +the weakest in the structure of that part of the terrestrial crust, +and from which the solid rock, shattered into pieces, is hurled into +the air by the enormous expansive energy of the volcanic vapors. +The history of the cone of Vesuvius brings before us a long series +of such explosions, beginning with that of A. D. 79, and coming +down to the present day. Even now, in spite of all the lava and +ashes poured out during the last eighteen centuries, it is easy to +see how stupendous must have been that earliest explosion by which +the southern half of the ancient crater was blown out. At every +successive important eruption, a similar but minor operation takes +place within the present cone. The hardened cake of lava forming the +floor is burst open, and with it there usually disappears much of the +upper part of the cone, and sometimes, as in 1872, a large segment +of the crater-wall. The islands of Santorin bring before us evidence +of a prehistoric catastrophe of a similar nature, by which a large +volcanic cone was blown up. The existing outer islands are a chain of +fragments of the periphery of the cone, the centre of which is now +occupied by the sea. In the year 1538 a new volcano, Monte Nuovo, was +formed in twenty-four hours on the margin of the Bay of Naples. An +opening was drilled by successive explosions, and such quantities of +stones, scoriæ, and ashes were thrown out from it as to form a hill +that rose 440 English feet above the sea-level, and was more than a +mile and a half in circumference. + +A communication having been opened, either by fissuring or explosion, +between the heated interior and the surface, fragmentary materials +are commonly ejected from it, consisting at first mainly of the rocks +through which the orifice has been opened, afterward of volcanic +substances. In a great eruption, vast numbers of red-hot stones +are shot up into the air, and fall back partly into the crater +and partly on the outer slopes of the cone. According to Sir W. +Hamilton, cinders were thrown by Vesuvius, during the eruption of +1779, to a height of 10,000 feet. Instances are known where large +stones, ejected obliquely, have described huge parabolic curves +in the air, and fallen at a great distance. Stones eight pounds +in weight occur among the ashes which buried Pompeii. The volcano +of Antuco in Chili is said to send stones flying to a distance of +thirty-six miles, Cotopaxi is reported to have hurled a 200-ton block +nine miles, and the Japanese volcano, Asama, is said to have ejected +many blocks of stone measuring from 40 to more than 100 feet in +diameter. + +But in many great eruptions, besides a constant shower of stones +and scoriæ, a vast column of exceedingly fine dust rises out of the +crater, sometimes to a height of several miles, and then spreads +outward like a sheet of cloud. The remarkable fineness of this dust +may be understood from the fact that during great volcanic explosions +no boxes, watches, or close-fitting joints have been found to be able +to exclude it. Mr. Whymper collected some dust that fell sixty-five +miles away from Cotopaxi, and which was so fine that from 4,000 to +25,000 particles were required to weigh a grain. So dense is the +dust-cloud as to obscure the sun, and for days together the darkness +of night may reign for miles around the volcano. The eruption of +Cotopaxi, on 26th June, 1877, began by an explosion that sent up +a column of fine ashes to a prodigious height into the air, where +it rapidly spread out and formed so dense a canopy as to throw the +region below it into total darkness. So quickly did it diffuse +itself, that in an hour and a half a previously bright morning became +at Quito, thirty-three miles distant, a dim twilight, which in the +afternoon passed into such darkness that the hand placed before +the eye could not be seen. At Guayaquil, on the coast, 150 miles +distant, the shower of ashes continued till the 1st of July. Dr. +Wolf collected the ashes daily, and estimated that at that place +there fell 315 kilogrammes on every square kilometre during the first +thirty hours, and on the 30th of June, 209 kilogrammes in twelve +hours. + +One of the most stupendous outpourings of volcanic ashes on record +took place, after a quiescence of twenty-six years, from the volcano +Coseguina, in Nicaragua, during the early part of the year 1835. On +that occasion, utter darkness prevailed over a circle of thirty-five +miles radius, the ashes falling so thickly that, even eight leagues +from the mountain, they covered the ground to a depth of about ten +feet. It was estimated that the rain of dust and sand fell over an +area at least 270 geographical miles in diameter. Some of the finer +materials, thrown so high as to come within the influence of an upper +air-current, were borne away eastward, and fell, four days afterward, +at Kingston, in Jamaica--a distance of 700 miles. During the great +eruption of Sumbawa, in 1815, the dust and stones fell over an area +of nearly one million square miles, and were estimated by Zollinger +to amount to fully fifty cubic miles of material, and by Junghuhn +to be equal to one hundred and eighty-five mountains like Vesuvius. +Toward the end of the Eighteenth Century, during a time of great +disturbance among the Japanese volcanoes, one of them, Sakurajima, +threw out so much pumiceous material that it was possible to walk a +distance of twenty-three miles upon the floating débris in the sea. + +The varying degree of liquidity or viscosity of the lava probably +modifies the force of explosions, owing to the different amounts +of resistance offered to the upward passage of the absorbed gases +and vapors. Thus explosions and accompanying scoriæ are abundant at +Vesuvius, where the lavas are comparatively viscid; they are almost +unknown at Kilauea, where the lava is remarkably liquid. + +In tranquil conditions of a volcano, the steam, whether collecting +into larger or smaller vesicles, works its way upward through +the substance of the molten lava, and as the elasticity of this +compressed vapor overcomes the pressure of the overlying lava, +it escapes at the surface, and there the lava is thus kept in +ebullition. But this comparatively quiet operation, which may be +watched within the craters of many active volcanoes, does not produce +clouds of fine dust. The collision or friction of millions of stones +ascending and descending in the dark column above the crater must +doubtless cause much dust and sand. But the explosive action of steam +is probably also an immediate cause of much trituration. The aqueous +vapor or water-gas which is so largely dissolved in many lavas must +exist within the lava-column, under an enormous pressure, at a +temperature far above its critical point, even at a white heat, and +therefore possibly in a state of dissociation. The sudden ascent of +lava so constituted relieves the pressure rapidly without sensibly +affecting the temperature of the mass. Consequently, the white-hot +gases or vapors at length explode, and reduce the molten mass to the +finest powder, like water shot out of a gun. + +As every shower of dust and sand adds to the height of the ground +on which it falls, thick volcanic accumulations may be formed far +beyond the base of the mountain. The volcano of Sangay, in Ecuador, +for instance, has buried the country around it to a depth of 4,000 +feet under its ashes. In such loose deposits are entombed trees and +other kinds of vegetation, together with the bodies of animals, as +well as the works of man. In some cases, where the layer of volcanic +dust is thin, it may merely add to the height of the soil, without +sensibly interfering with the vegetation. But it has been observed at +Santorin that though this is true in dry weather, the fall of rain +with the dust at once acts detrimentally. On the 3d of June, 1866, +the vines were there withered up, as if they had been burned, along +the track of the smoke cloud. By the gradual accumulation of volcanic +ashes, new geological formations arise which, in their component +materials, not only bear witness to the volcanic eruptions that +produced them, but preserve a record of the land-surfaces over which +they spread. In the third place, besides the distance to which the +fragments may be hurled by volcanic explosions, or to which they may +be diffused by the ordinary aerial movements, we have to take into +account the vast spaces across which the finer dust is sometimes +borne by upper air-currents. In the instance already cited, ashes +from Coseguina fell 700 miles away, having been carried all that +long distance by a high counter-current of air, moving apparently at +the rate of about seven miles an hour in an opposite direction to +that of the wind which blew at the surface. By the Sumbawa eruption, +also referred to above, the sea west of Sumatra was covered with a +layer of ashes two feet thick. On several occasions ashes from the +Icelandic volcanoes have fallen so thickly between the Orkney and +Shetland Islands, that vessels passing there have had the unwonted +deposit shoveled off their decks in the morning. In the year 1783, +during the memorable eruption of Skaptar-Jökull, so vast an amount +of fine dust was ejected that the atmosphere over Iceland continued +loaded with it for months afterward. It fell in such quantities +over parts of Caithness--a distance of 600 miles--as to destroy +the crops; that year is still spoken of by the inhabitants as the +year of “the ashie.” Traces of the same deposit have been observed +in Norway, and even as far as Holland. Hence it is evident that +volcanic accumulations may take place in regions many hundreds +of miles distant from any active volcano. A single thin layer of +volcanic detritus in a group of sedimentary strata would not thus of +itself prove the existence of contemporaneous volcanic action in its +neighborhood. + +At its exit from the side of a volcano, lava glows with a white heat, +and flows with a motion which has been compared to that of honey or +of melted iron. It soon becomes red, and like a coal fallen from a +hot fireplace rapidly grows dull as it moves along, until it assumes +a black, cindery aspect. At the same time the surface congeals, and +soon becomes solid enough to support a heavy block of stone. The +aspect of the stream varies with the composition and fluidity of +the lava, form of the ground, angle of slope, and rapidity of flow. +Viscous lavas, like those of Vesuvius, break up along the surface +into rough brown or black cinder-like slags and irregular ragged +cakes, bristling with jagged points, which, in their onward motion, +grind and grate against each other with a harsh, metallic sound, +sometimes rising into rugged mounds or becoming seamed with rents and +gashes, at the bottom of which the red-hot glowing lava may be seen. +In lavas possessing somewhat greater fluidity, the surface presents +froth-like, curving lines, as in the scum of a slowly flowing river, +or is arranged in curious ropy folds, as the layers have successively +flowed over each other and congealed. A large area which has been +flooded with lava is perhaps the most hideous and appalling scene of +desolation anywhere to be found on the surface of the globe. + +A lava-stream usually spreads out as it descends from its point +of escape, and moves more slowly. Its sides look like huge +embankments, or like some of the long mounds of “clinkers” in a great +manufacturing district. The advancing end is often much steeper, +creeping onward like a great wall or rampart, down the face of which +the rough blocks of hardened lava are ever rattling. + +In a lofty volcano, lava occasionally rises to the lip of the crater +and flows out there; but more frequently it escapes from some fissure +or orifice in a weak part of the cone. In minor volcanoes, on the +other hand, where the explosions are less violent, and where the +thickness of the cone in proportion to the diameter of the funnel is +often greater, the lava very commonly rises into the crater. Should +the crater-walls be too weak to resist the pressure of the molten +mass, they give way, and the lava rushes out from the breach. This is +seen to have happened in several of the puys of Auvergne. But if the +crater be massive enough to withstand the pressure, the lava may at +last flow out from the lowest part of the rim. + +As soon as the molten rock reaches the surface, the superheated +water-vapor or gas dissolved within its mass escapes copiously, +and hangs as a dense white cloud over the moving current. The +lava-streams of Vesuvius sometimes appear with as dense a steam-cloud +at their lower ends as that which escapes at the same time from the +main crater. Even after the molten mass has flowed several miles, +steam continues to rise abundantly both from its end and from +numerous points along its surface, and continues to do so for many +weeks, months, or it may be for several years. + +Should the point of escape of a lava-stream lie well down on the +cone, far below the summit of the lava-column in the funnel, the +molten rock, on its first escape, driven by hydrostatic pressure, +will sometimes spout up high into the air--a fountain of molten rock. +This was observed in 1794 on Vesuvius, and in 1832 on Etna. In the +eruption of 1852 at Mauna Loa, an unbroken fountain of lava, from 200 +to 700 feet in height and 1,000 feet broad, burst out at the base of +the cone. Similar “geysers” of molten rock have subsequently been +noticed in the same region. Thus in March and April, 1868, four fiery +fountains, throwing lava to heights varying from 500 to 1,000 feet, +continued to play for several weeks. According to Mr. Coan, such +outbursts take place from the bottom of a column of lava 3,000 feet +high. The volcano of Mauna Loa strikingly illustrates another feature +of volcanic dynamics in the position and outflow of lava. It bears +upon its flanks at a distance of 20 miles, but 10,000 feet lower, +the huge crater Kilauea. As Dana has pointed out, these orifices +form part of one mountain, yet the column of lava stands 10,000 feet +higher in one conduit than in the other. On a far smaller scale the +same independence occurs among the several pipes of some of the +geysers in the Yellowstone region of North America. + +The rate of movement is regulated by the fluidity of the lava, by +its volume, and by the form and inclination of the ground. Hence, as +a rule, a lava-stream moves faster at first than afterward, because +it has not had time to stiffen, and its slope of descent is usually +steeper than further down the mountain. One of the most fluid and +swiftly flowing lava-streams ever observed on Vesuvius was that +erupted on 12th August, 1805. It is said to have rushed down a space +of 3 Italian (3⅔ English) miles in the first four minutes, but to +have widened out and moved more slowly as it descended, yet finally +to have reached Torre del Greco in three hours. A lava erupted by +Mauna Loa in 1852 went as fast as an ordinary stage-coach, or fifteen +miles in two hours; but some of the lavas from that mountain have in +parts of their course moved with double that rapidity. + +In some cases, lava escaping from craters or fissures comes to rest +before reaching the base of the slopes, like the obsidian current +which has congealed on the side of the little volcanic island of +Volcano. In other instances, the molten rock not only reaches the +plains, but flows for many miles away from the point of eruption. +Sartorius von Waltershausen computed the lava emitted by Etna in 1865 +at 92 millions of cubic metres, that of 1852 at 420 millions, that +of 1669 at 980 millions, and that of a prehistoric lava-stream near +Randazzo at more than 1,000 millions. The most stupendous outpouring +of lava on record was that which took place in Iceland in the year +1783. Successive streams issued from a fissure about 12 miles long, +filling up river gorges which were sometimes 600 feet deep and 200 +feet broad, and advancing into the alluvial plains in lakes of +molten rock 12 to 15 miles wide and 100 feet deep. Two currents of +lava which, filling up the valley of the Skapta, escaped in nearly +opposite directions, extended for 45 and 50 miles respectively, their +usual thickness being 100 feet. Bischof estimated that the total +amount of lava poured forth during this single eruption “surpassed in +magnitude the bulk of Mont Blanc.” + +The varying degrees of liquidity are manifested in a characteristic +way on the surface of lava. Thus, in the great lava-pools of Hawaii, +the rock exhibits a remarkable liquidity, throwing up fountains of +molten rock to a height of 300 feet or more. During its ebullition in +the crater-pools, jets and driblets a quarter of an inch in diameter +are tossed up, and, falling back on one another, make “a column of +hardened tears of lava,” one of which was found to have attained +a height of 40 feet, while in other places the jets thrown up and +blown aside by the wind give rise to long threads of glass which +lie thickly together like mown grass, and are known by the natives +under the name of “Pele’s Hair,” after one of their divinities. Yet, +although the ebullition is caused by the uprise and escape of highly +heated vapors, there is no cloud over the boiling lake itself, heavy +white vapor only escaping at different points along the edge. + + + + + EARTHQUAKES + --WILLIAM HUGHES + + +It appears, from the accurate records of such phenomena which have +been kept within recent periods, that earthquakes are of much more +frequent occurrence than is commonly supposed. Upward of three +thousand earthquakes are recorded as having occurred within the first +half of the Nineteenth Century--an average of more than one for every +week throughout the entire period. But not more than one in forty is +of considerable importance, by far the greater number consisting of +such slight shocks as are occasionally experienced in Great Britain +and other countries favored with a like immunity in this regard. An +important earthquake, however, in some part of the world or other, +appears, from the above average, to occur once in every eight months. +In Europe alone, where a more complete record of such occurrences is +obtainable than in other parts of the world, as many as 320 distinct +earthquakes are recorded to have occurred within a period of ten +years (1833-42)--an average of thirty-two annually, and of one such +shock for every ten days throughout the period. + + [The _geographical area_ within which shocks of earthquakes + are experienced is a widely spread one, and does not appear to + undergo any material change (if, indeed, any change whatever) + as to its limits. At any rate, the regions in which violent + earthquakes are recorded to have occurred in former times are + those in which such disturbances are of most frequent recurrence + at the present day. One of the most striking evidences in favor + of the supposition that the volcanic eruption is due to the same + deeply seated cause which produces the shock of the earthquake, + is afforded by the fact, that all the volcanoes which have been + in eruption within the modern period of geology are found within + regions liable to earthquakes, and, for the most part, to violent + shocks.] + +Regarding the earthquake and the volcanic eruption as the +manifestation, under different conditions, of the earth’s internal +fires, we readily mark out upon the globe the great regions of +geographical distribution in the case of such phenomena. The most +widely extended of these coincides with the circuit of the Pacific +Ocean. Along the entire western coast of the New World, from Tierra +del Fuego to the peninsula of Alaska and the neighborhood of the +Aleutian Islands, shocks of earthquakes are known to occur; and, +within a large portion of the space, vents of active eruption +are found. The subterranean igneous force is, indeed, much more +powerfully displayed in the southern than in the northern half of +the American continent, and the active volcanoes that occur within +the limits referred to are nearly all found amid the cordilleras of +the Andes, or upon the plateaus of the Mexican isthmus. One of the +Mexican volcanoes--Jorullo--is especially deserving of notice, from +the circumstance of its having first risen above the surrounding +plain by the accumulation of volcanic matter during an eruption in +the year 1759. + +The Aleutian Islands connect the volcanic region of the eastern +Pacific with that which extends along its western shores. In the +latter case, however, it is upon the peninsular regions, or in the +chains of islands that adjoin the mainland, that the igneous force +is displayed. Kamtchatka, the Kurile Islands, Yesso, the Japanese +group, and the entire region of the Malay Archipelago, exhibit the +presence of igneous force below the ground. Seven active volcanoes +occur in Kamtchatka. The Japanese Archipelago is said to contain at +least twenty-seven active volcanoes, eight of them upon Yesso and +the adjacent islets. Between Japan and the Loo-choo group is Sulphur +Island, an insular volcano, from which smoke is constantly emitted. + +The Philippine Islands, in which earthquakes are of frequent +occurrence, prolong the volcanic chain to the southward. Thence it +is traced, at intervals, along the northern shores of New Guinea, +and through the prolonged chains of the Solomon Islands, and the +New Hebrides, to the North Island of New Zealand. Slight shocks +of earthquake have also been experienced within the southern and +eastwardly portions of the Australian mainland. + +The numerous volcanoes of the Malay Archipelago, the whole area of +which is liable to frequent earthquake shocks, often of the most +destructive violence, belong to the eastern portion of this region, +and display the agency of subterranean heat on the grandest scale. +The island of Java alone contains forty-three active volcanoes, +ranging in a linear direction throughout its length. The volcanic +chain of Java is prolonged to the eastward through the Lesser Sunda +Islands (Sumbawa, etc.), in which direction it is united with +that which borders the Pacific waters. There are active volcanoes +on an island in the Gulf of Siam, besides the well-known crater +of Barren Island, in the Bay of Bengal. The region adjoining the +last-named body of water, together with the whole of northern India, +is of frequent liability to earthquakes, some of them (as that of +Cutch, in 1819) of the most destructive violence. The volcanic +island of Mayotta (Comoro group), the active Piton of Réunion or +Bourbon Island, and the hot springs and extinct craters of St. Paul +and Amsterdam Islands, in a high southern latitude of the Indian +Ocean, constitute points which indicate, at distant intervals, the +continuity of the volcanic chain. + +The southwestern portion of Asia, the southern shores of Europe, and +the northwestwardly portion of the African mainland, fall within +this region on the one side, as the islands of the West Indies do +upon the other. The entire breadth of the Atlantic Ocean, as well +as the circuit of the Mediterranean, is thus included within its +limits. To the northward, the numerous volcanoes of Iceland, and +the more distant cone of Jan Mayen Island, lying within the Arctic +circle, must be regarded as within its area; together with, in an +opposite direction, the still-burning peak of the Cameroon Mountains, +adjoining the upper extremity of the Gulf of Guinea. The volcanic +peaks found within the widely detached groups of the Azores and the +Cape Verde Islands, with Tenerife, in the Canary group, are among its +outlying members. + +Throughout the wide region thus indicated, earthquakes are of +frequent occurrence. There are fewer active vents of eruption than +in the case of the Pacific circuit. But the cones of Etna and +Vesuvius, with the island of Santorin, in the Mediterranean, and +the numerous volcanoes of Iceland, attest the destructive violence +of the subterranean fires. Western Asia, from the Caspian to the +shores of the Archipelago (including Armenia, Syria, and the Lesser +Asia), Greece, southern Italy, the Spanish peninsula, and the region +of Mount Atlas, in Northwestern Africa, are all liable to the +frequent repetition of such convulsions. The only portion of the +Mediterranean coasts exempt from such disturbing phenomena is on its +southern shores, embracing that part of the North African coast which +stretches from the Lesser Syrtis to the valley of the Nile. We have +no record of the experience of any shocks of earthquake in Egypt. +Had it been otherwise, perhaps the pyramids of that land of wonders +might have proved less enduring monuments of the past. + +The movement imparted to the ground during an earthquake may be +either horizontal or vertical. In the former case, the phenomenon +consists in an undulating, wave-like movement; in the latter, in +an upheaval or subsidence of land. The vertical shock affects most +the relative levels of adjacent objects, and produces the most +striking permanent changes in the natural aspect of the region in +which it is experienced. But the undulatory movement is attended +by more serious consequences to man, since it at once shakes the +foundations of the strongest edifices, and may overthrow in the +space of a few seconds the accumulated labors of prior ages. Whole +tracts of land, with their cities or villages, may be elevated or +depressed with comparatively little injury to life; but nothing can +withstand the force of a motion which rocks the solid strata of the +earth itself. The most solidly constructed buildings are not proof +against the earthquake any more than the weakest. Indeed, it has in +many instances been observed that those erections which displayed +the strongest masonry have suffered more from the effects of an +earthquake than buildings of slighter structure. The cracking of +walls, the falling-in of roofs, and the crash of tumbling houses on +every side, burying their inmates beneath the ruins, are among the +characteristics of the earthquake in its most violent and frightful +form. + +It has been asserted that a third kind of movement--viz., in a +rotatory direction--sometimes occurs, and certain phenomena by +which earthquakes have been attended have favored this belief. +Thus, isolated columns or statues have been found, after such +an occurrence, to face a different quarter from that which they +previously did. This, however, would be sufficiently accounted for +by a vibratory movement, acting upon a column which was _unequally_ +attached to its base; _i.e._, the fastening of which was of unequal +strength relatively to the central point of junction. During the +Chilian earthquake of 1835, vessels moored alongside of one another +in the harbor of Concepcion were afterward found with their cables +twisted together. + +The duration of any single earthquake shock is seldom more than a few +seconds, though the terror which it inspires naturally tends to make +it seem of longer continuance; but in the case of the more violent +movements, even a few moments serve to destroy the work of ages. In +the Chilian earthquake of 1835, the great shock which destroyed the +city of Concepcion was preceded by several tremulous movements of +minor intensity. During the first half-minute, many persons remained +in their houses; but the convulsive motion of the earth then became +so strong that all rushed into the open streets for safety. The +horrid motion (writes an eye-witness of the scene) increased; people +could hardly stand; buildings waved and tottered; suddenly an awful +and overpowering shock caused universal destruction. In less than six +seconds the city was in ruins! + +The earthquake is propagated to enormous distances from the region +in which the shock originates, the rate at which the motion travels +varying not merely with the violence of the originating impulse, but +also with the nature of the formations through which it passes. Rocks +of solid and homogeneous texture, as granite, favor the transmission +of the shock; while formations of loose texture, such as sand, most +retard its speed. The well-known Lisbon earthquake of 1755, by which +sixty thousand persons are said to have perished within the brief +space of six minutes, was felt in the British Islands, as well as +upon the coast of Barbary, and even among the islands of the West +Indies, on the opposite side of the Atlantic. + + + + + MOUNTAINS + --A. KEITH JOHNSTON + + +The number and altitude of the mountains of the globe are so great +that they form almost everywhere prominent objects, and operate to a +large extent in modifying the climatic conditions of every country +in the world. Yet the amount of solid material so raised above the +ordinary level of the land is not so much as might be expected. +Remembering that elevated plateaus of great extent occur in several +regions, and that the general surface of the earth is considerably +higher than the sea-level, it has been estimated that were the whole +dry land reduced to a uniform level, it would form a plain having an +elevation of 1,800 feet above the sea. And were these solid materials +scattered over the whole surface of the globe, so as to fill up the +bed of the ocean, the resulting level would be considerably below +the present surface of the sea, inasmuch as the main height of the +dry land most probably does not exceed one-fifteenth of the mean +depth of the bed of the ocean. + +Mountains, and especially mountain-chains, subserve important uses in +the economy of nature, especially in connection with the water system +of the world. They are at once the great collectors and distributers +of water. In the passage of moisture-charged winds across them the +moisture is precipitated as rain or snow. When mountain ranges +intersect the course of constant winds by thus abstracting the +moisture, they produce a moist country on the windward side, and a +comparatively dry and arid one on the leeward. This is exemplified in +the Andes, the precipitous western surface of which has a different +aspect from the sloping eastern plains; and so also the greater +supply of moisture on the southern sides of the Himalayas brings the +snow-line 5,000 feet lower than on the northern side. + +Above a certain height the moisture falls as snow, and a range of +snow-clad summits would form a more effectual separation between +the plains on either side than would the widest ocean, were it not +that transverse valleys are of frequent occurrence, which open up a +pass, or way of transit, at a level below the snow-line. But even +these would not prevent the range being an impassable barrier, if +the temperate regions contained as lofty mountains as the tropics. +Mountain ranges, however, decrease in height from the equator to the +poles in relation to the snow-line. + +The numerous attempts that have been made to generalize on the +distribution of mountains on the globe have hitherto been almost +unsuccessful. In America, the mountains take a general direction more +or less parallel to the meridian, and for a distance of 8,280 miles, +from Patagonia to the Arctic Ocean, form a vast and precipitous range +of lofty mountains, which follow the coast-line in South America, +and spread somewhat out in North America, presenting everywhere +throughout their course a tendency to separate into two or more +parallel ridges, and giving to the whole continent the character of +a precipitous and lofty western border, gradually lowering into an +immense expanse of eastern lowlands. In the Old World, on the other +hand, there is no single well-defined continuous chain connected +with the coast-line. The principal ranges are grouped together in a +Y-shaped form, the general direction of which is at right angles to +the New World chain. The centre of the system in the Himalayas is +the highest land in the hemisphere. From this, one arm radiates in +a northeast direction, and terminates in the high land at Behring +Strait: the other two take a westerly course; the one a little to the +north, through the Caucasus, Carpathians, and Alps, to the Pyrenees; +the other more to the south, through the immense chain of Central +African mountains, and terminating at Sierra Leone. Most of the +principal secondary ranges have generally a direction more or less at +right angles to this great mountain tract. + +The inquiry into the origin of mountains is one that has received +not a little attention. Geologists have shown that the principal +agents in altering the surface of the globe are denudation, which is +always abrading and carrying to a lower level the exposed surfaces, +and an internal force which is rising or depressing the existing +strata, or bringing unstratified rocks to the surface. Whether the +changes are the small and almost imperceptible alterations now taking +place, or those recorded in the mighty mountains and deep valleys +everywhere existing, denudation and internal force are the great +producing causes. These give us two great classes of mountains. + +The extent to which denudation has altered the surface of the globe +can scarcely be imagined. All the stratified rocks are produced by +its action; but these do not measure its full amount, for many of +these beds have been deposited and denuded, not once or twice, but +repeatedly, before they reach their present state. Masses of rock +more indurated, or better defended from the wasting currents than +those around, serve as indices of the extent of denudation. The most +remarkable case of this kind with which we are acquainted is that of +the three insulated mountains in Ross-shire--Suil Veinn, Coul Beg, +and Coul More--which are about 3,000 feet high. The strata of the +mountains are horizontal, like the courses of masonry in a pyramid, +and their deep red color is in striking contrast with the cold bluish +hue of the gneiss which forms the plain, and on whose upturned edges +the mountain-beds rest. It seems very probable, as Hugh Miller +suggests, that when the formation of which these are relics (at one +time considered as Old Red Sandstone, but now determined by Sir +Roderick Murchison as being older than Silurian) was first raised +above the waves, it covered with an amazing thickness the whole +surface of the Highlands of Scotland, from Ben Lomond to the Maiden +Paps of Caithness, but that subsequent denudation swept it all away, +except in circumscribed districts, and in detached localities like +these pyramidal hills. + +Mountains produced by internal force are of several kinds. (_a_) +Mountains of ejection, in which the internal force is confined to a +point, so to speak, having the means of exhausting itself through an +opening in the surface. The lava, scoriæ, and stones ejected at this +opening form a conical projection which, at least on the surface, +is composed of strata sloping away from the crater. Volcanoes are +mostly isolated conical hills, yet they chiefly occur in a somewhat +tortuous linear series, on the mainland and islands which inclose +the great Pacific Ocean. Vesuvius and the other European volcanoes +are unconnected with this immense volcanic tract. (_b_) But the +internal force may be diffused under a large tract or zone, which, if +it obtain no relief from an opening, will be elevated in the mass. +When the upheaval occurs to any extent, the strata are subjected to +great tension. If they can bear it, a soft rounded mountain-chain +is the result; but generally one or more series of cracks are +formed, and into them igneous rocks are pushed, which, rising up +into mountain-chains, elevate the stratified rocks on their flanks, +and perhaps as parallel ridges. Thus, the Andes consist of the +stratified rocks of various ages, lying in order on the granite and +porphyry of which the mass of the range is composed. + +The position of the strata on such mountains supplies the means of +determining, within definite limits, the period of upheaval. The +newest strata that have been elevated on the sides of the mountain +when it was formed, give a date antecedent to that at which the +elevation took place, while the horizontal strata at the base +of the mountains supply one subsequent to that event. Thus, the +principal chain of the Alps was raised during the period between the +deposition of the Tertiary and that of the older recent deposits. +(_c_) But there is yet another way in which the upheaving internal +force operates, viz., where it does not act at right angles to the +surface, but rather obliquely, and, as it were, pushes the solid +strata forward, causing them to rise in huge folds, which, becoming +permanent, form parallel ranges of mountains. + +The crust of the earth, in its present solid and brittle condition, +is thus curved, in a greater or less degree, by the shock of every +earthquake; it is well known that the trembling of the earth is +produced by the progress of a wave of the solid crust; that the +destruction of buildings is caused by the undulation; and that the +wave has been so evident that it has been described as producing +a sickening feeling on the observer, as if the land were but thin +ice heaving over water. The Appalachians were thus formed. Many +other ranges have had a similar origin, as some in Belgium and in +the Southern Highlands of Scotland, as has been suggested by Mr. +Carruthers. + +It is evident that in the last two classes the parallel ridges +were produced at the same time. Elie de Beaumont generalized this, +maintaining that all parallel ridges or fissures are synchronous; +and on this he based a system of mountain structure which is too +universal and too geometrical to be true. The synchronism of parallel +fissures had been noticed by Werner, and it is now received as a +first principle in mining. The converse is also held to be generally +true, that fissures differing in direction differ also in age; +yet divergence from a centre, and consequent want of parallelism, +as in the case of volcanoes, may be an essential characteristic +of contemporaneity. Nevertheless, Elie de Beaumont classified the +mountains of the world according to this parallelism, holding that +the various groups are synchronous. The parallelism does not consist +in having the same relations to the points of the compass--for these, +as regards north and south, would be far from parallel--but is +estimated in its relation to some imaginary great circle, which being +drawn round the globe would divide it into equal hemispheres. Such +circles he calls Great Circles of Reference. But beyond this, he went +a step further, and proposed a more refined classification, depending +on a principle of geometrical symmetry, which he believed he had +discovered among his great circles of reference. It is to be feared, +however, that his geometrical speculations have little foundation in +nature. + + + + + LAKES--FRESH, SALT, AND BITTER + --SIR ARCHIBALD GEIKIE + + +Depressions filled with water on the surface of the land, and known +as Lakes, occur abundantly in the northern parts of both hemispheres, +and more sparingly, but often of large size, in warmer latitudes. For +the most part, they do not belong to the normal system of erosion in +which running water is the prime agent, and to which the excavation +of valleys and ravines must be attributed. On the contrary, they are +exceptional to that system, for the constant tendency of running +water is to fill them up. Their origin, therefore, must be sought +among some of the other geological processes. + +Lakes are conveniently classed as fresh or salt. Those which possess +an outlet contain in almost all cases fresh water; those which have +none are usually salt. + +In the northern parts of Europe and America, as first emphasized by +Sir Andrew C. Ramsay, lakes are prodigiously abundant on ice-worn +rock-surfaces, irrespective of dominant lines of drainage. They +seem to be distributed as it were at random, being found now on the +summits of ridges, now on the sides of hills, and now over broad +plains. They lie for the most part in rock-basins, but many of them +have barriers of detritus. In the mountainous regions of temperate +and polar latitudes, lakes abound in valleys, and are connected with +main drainage-lines. In North America and in Equatorial Africa, vast +sheets of fresh water occur in depressions of the land, and are +rather inland seas than lakes. + +The water of many lakes has been observed to rise above its normal +level for a few minutes or for more than an hour, then to descend +beneath that level, and to continue this vibration for some time. In +the Lake of Geneva, where these movements, locally known there as +_Seiches_, have long been noticed, the amplitude of the oscillation +ranges up to a metre or even sometimes to two metres. These +disturbances may sometimes be due to subterranean movements; but +probably they are mainly the effect of atmospheric perturbations, +and, in particular, of local storms with a vertical descending +movement. + +Among the geological functions discharged by lakes the following may +be noticed: + +1st. Lakes equalize the temperature of the localities in which they +lie, preventing it from falling as much in winter and rising as much +in summer as it would otherwise do.[1] The mean annual temperature of +the surface water at the outflow of the Lake of Geneva is nearly 4° +warmer than that of the air. + +2d. Lakes regulate the drainage of the area below their outfall, +thereby preventing or lessening the destructive effects of floods. + +3d. Lakes filter river-water and permit the undisturbed accumulation +of new deposits, which in some modern cases may cover thousands +of square miles of surface, and may attain a thickness of nearly +3,000 feet (Lake Superior has an area of 32,000 square miles; +Lago Maggiore is 2,800 feet deep). How thoroughly lakes can filter +river-water is typically displayed by the contrast between the muddy +river which flows in at the head of the Lake of Geneva, and the +“blue rushing of the arrowy Rhone,” which escapes at the foot. The +mouths of small brooks entering lakes afford excellent materials +for studying the behavior of silt-bearing streams when they reach +still water. Each rivulet may be observed pushing forward its delta +composed of successive sloping layers of sediment. On a shelving +bank, the coarser detritus may repose directly upon the solid rock of +the district. But as it advances into the lake, it may come to rest +upon some older lacustrine deposit. The river Linth since 1860 has +annually discharged into Lake Wallenstadt some 62,000 cubic metres of +detritus. + +A river which flows through a succession of lakes can not carry much +sediment to the sea, unless it has a long course to run after it has +passed the lowest lake, and receives one or more muddy tributaries. +Let us suppose, for example, that, in a hilly region, a stream passes +through a series of lakes. As the highest lake will intercept much, +perhaps all, of this sediment, the next in succession will receive +little or none until the first is either filled up or has been +drained by the cutting of a gorge through the intervening rock. The +same process will be repeated until the lakes are effaced, and their +places are taken by alluvial meadows. Examples of this sequence of +events are of frequent occurrence in Britain. + +Besides the detrital accumulations due to the influx of streams, +there are some which may properly be regarded as the work of lakes +themselves. Even on small sheets of water, the eroding influence +of wind-waves may be observed; but on large lakes the wind throws +the water into waves which almost rival those of the ocean in size +and destructive power. Beaches, sand-dunes, shore-cliffs, and other +familiar features of the meeting-line between land and sea, reappear +along the margins of such great fresh-water seas as Lake Superior. +Beneath the level of the water a terrace or platform is formed, of +which the distance from shore and depth vary with the energy of the +waves by which it is produced. This platform is well developed in the +Lake of Geneva. + +Some of the distinctive features of the erosion and deposition that +take place in lake-basins have been admirably laid open for study in +those basins of vanished lakes which have been so well described by +Gilbert, Dutton, Russell, and Upham in the Western Territories of the +United States. They have been treated of in a masterly way by Gilbert +in his essay on _The Topographic Features of Lake-Shores_. + +4th. Lakes serve as basins in which chemical deposits may take place. +Of these the most interesting and extensive are those of iron-ore, +which chiefly occur in northern latitudes. + +5th. Lakes furnish an abode for a lacustrine fauna and flora, +receive the remains of the plants and animals washed down from +the surrounding country, and entomb these organisms in the +growing deposits, so as to preserve a record of the lacustrine +and terrestrial life of the period during which they continue. +Besides the more familiar pond-snails and fishes, lakes possess a +peculiar pelagic fauna, consisting in large measure of entomostracous +crustaceans, distinguished more especially by their transparency. +These, as well as the organisms of shallower water, doubtless furnish +calcareous materials for the mud or marl of the lake bottoms. But +it is as receptacles of sediment from the land, and as localities +for the preservation of a portion of the terrestrial fauna and +flora, that lakes present their chief interest to a geologist. Their +deposits consist of alternations of sand, silt, mud, gravel, and +occasional irregular seams of vegetable matter, together with layers +of calcareous marl formed of lacustrine shells, _Entomostraca_, etc. +In lakes receiving much sediment, little or no marl can accumulate +during the time when sediment is being deposited. In small, clear, +and not very deep lakes, on the other hand, where there is little +sediment, or where it only comes occasionally at intervals of flood, +thick beds of white marl, formed entirely of organic remains, may +gather on the bottom, as has happened in numerous districts of +Scotland and Ireland. The fresh-water limestones and clays of some +old lake-basins (those of Miocene time in Auvergne and Switzerland, +and of Eocene age in Wyoming, for example) cover areas occasionally +hundreds of square miles in extent, and attain a thickness of +hundreds, sometimes even thousands, of feet. + +Existing lakes are of geologically recent origin. Their disappearance +is continually in progress by infilling and erosion. Besides the +displacement of their water by alluvial accumulations, they are +lowered and eventually drained by the cutting down of the barrier at +their outlets. Where they are effaced merely by erosion, it must be +an excessively slow process, owing to the filtered character of the +water; but where it is performed by the retrocession of a waterfall +at the head of an advancing gorge, it may be relatively rapid after +it has once begun. In a river-course it is usual to find a lake-like +expansion of alluvial land above each gorge. These plains may be +regarded as old lake-bottoms, which have been drained by the cutting +out of the ravines. Successive terraces often fringe a lake and mark +former levels of its waters. It is when we reflect upon the continued +operation of the agencies which tend to efface them, that we can best +realize why the lakes now extant must necessarily be of comparatively +modern date. + +Saline lakes, considered chemically, may be grouped as _salt lakes_, +where the chief constituents are sodium and magnesium chlorides +with magnesium and calcium sulphates; and _bitter lakes_, which are +usually distinguished by their large percentage of sodium carbonate +as well as chloride and sulphate (natron-lakes), sometimes by their +proportion of borax (borax lakes). From a geological point of view +they may be divided into two classes--(1) those which owe their +saltness to the evaporation and concentration of water poured into +them by their feeders; and (2) those which were originally parts of +the ocean. + +Salt and bitter lakes of terrestrial origin are abundantly scattered +over inland areas of drainage in the heart of continents, as in Utah +and adjacent territories of North America, and on the great plateau +of Central Asia. These sheets of water were doubtless fresh at first, +but they have progressively increased in salinity, because, though +the water is evaporated, there is no escape for its dissolved salts, +which consequently remain in the increasing concentrated liquid. In +Ladâkh, extensive lakes formed by the ponding back of valley waters +by alluvial fans have grown saline and bitter, and have become the +site of deposits of rock-salt and soda. + +The Great Salt Lake of Utah, which has now been so carefully studied +by Gilbert and other geologists, may be taken as a typical example +of an inland basin, formed by unequal subterranean movement that +has intercepted the drainage of a large area, wherein rainfall +and evaporation on the whole balance each other, and where the +water becomes increasingly salt from evaporation, but is liable to +fluctuations in level, according to oscillations of meteorological +conditions. The present lake occupies an area of rather more than +2,000 square miles, its surface being at a height of 4,250 feet above +the sea. It is, however, merely the shrunk remnant of a once far more +extensive sheet of water, to which the name of Lake Bonneville has +been given by Gilbert. It is partly surrounded with mountains, along +the sides of which well-defined lines of terrace mark former levels +of the water. The highest of these terraces lies about 940 feet +above the present surface of the lake, so that when at its greatest +dimensions this vast sheet of water must have stood at a level of +about 5,200 feet above the sea, and covered an area of 300 miles from +north to south, and 180 miles in extreme width from east to west. +It was then certainly fresh, for, having an outlet to the north, it +drained into the Pacific Ocean, and in its stratified deposits an +abundant lacustrine molluscan fauna has been found. According to +Gilbert there are proofs that, previous to the great extension of +Lake Bonneville, there was a dry period, during which considerable +accumulations of subaerial detritus were formed along the slopes of +the mountains. A great meteorological change then took place, and the +whole vast basin, not only that termed Lake Bonneville, but a second +large basin, Lake Lahontan of King, lying to the west and hardly +inferior in area, was gradually filled with fresh water. Again, +another meteorological revolution supervened and the climate once +more became dry. The waters shrank back, and in so doing, when they +had sunk below the level of their outlet, began to grow increasingly +saline. The decrease of the water and the increase of salinity +were in direct relation to each other until the present degree of +concentration has been reached. The Great Salt Lake, at present +having an extreme depth of less than 50 feet, is still subject to +oscillations of level. When surveyed by the Stansbury Expedition +in 1849, its level was 11 feet lower than in 1877, when the Survey +of the 40th Parallel examined the ground. From 1866, however, a +slow subsidence of the lake has been in progress, consequent upon a +diminution of the rainfall. Large tracts of flat land, formerly under +water, are being laid bare. As the water recedes from them and they +are exposed to the remarkably dry atmosphere of these regions, they +soon become crusted with a white saliferous and alkaline deposition, +which likewise permeates the dried mud underneath. So strongly +saline are the waters of the lake, and so rapid the evaporation, +as I found on trial, that one floats in spite of one’s self, and +the under surfaces of the wooden steps leading into the water at +the bathing-places are hung with short stalactites of salt from the +evaporation of the drip of the emergent bathers. + +Some of the smaller lakes in the great arid basin of North America +are intensely bitter, and contain large quantities of carbonate +and sulphate as well as chloride of sodium. The Big Soda Lake near +Ragtown in Nevada contains 129.015 grammes of salts in the litre of +water. These salts consist largely of chloride of sodium (55.42 per +cent of the whole), sulphate of soda (14.86 per cent), carbonate of +soda (12.96 per cent), and chloride of potassium (3.73 per cent). +Soda is obtained from this lake for commercial purposes. + +Salt lakes of oceanic origin are comparatively few in number. In +their case, portions of the sea have been isolated by movements of +the earth’s crust, and these detached areas, exposed to evaporation, +which is only partially compensated by inflowing rivers, have shrunk +in level, and at the same time have sometimes grown much salter than +the parent ocean. + +The Caspian Sea, 180,000 square miles in extent, and with a maximum +depth of from 2,000 to 3,000 feet, is a magnificent example. The +shells living in its waters are chiefly the same as those of the +Black Sea. Banks of them may be traced between the two seas, with +salt lakes, marshes, and other evidences to prove that the Caspian +was once joined to the Black Sea, and had thus communication with +the main ocean. In this case, also, there are proofs of considerable +changes of water-level. At present the surface of the Caspian is 85½ +feet below that of the Black Sea. The Sea of Aral, also sensibly salt +to the taste, was once probably united with the Caspian, but now +rests at a level of 242.7 feet above that sheet of water. The steppes +of southeastern Russia are a vast depression with numerous salt lakes +and abundant saline and alkaline deposits. It has been supposed that +this depression continued far to the north, and that a great firth, +running up between Europe and Asia, stretched completely across what +are now the steppes and plains of the Tundras, till it merged into +the Arctic Sea. Seals of a species (_Phoca caspica_) which may be +only a variety of the common northern form (_Ph. fætida_) abound in +the Caspian, which is the scene of one of the chief seal-fisheries of +the world.[2] On the west side of the Ural chain, even at present, by +means of canals connecting the rivers Volga and Dwina, vessels can +pass from the Caspian into the White Sea.[3] + +The cause of the isolation of the Caspian and the other saline +basins of that region is to be sought in underground movements +which, according to Helmersen, are still in progress, but partly, +and, in the case of the smaller basins, probably chiefly in a +general diminution of the water supply all over Central Asia and the +neighboring regions. The rivers that flow from the north toward Lake +Balkash, and that once doubtless emptied into it, now lose themselves +in the wastes and are evaporated before reaching that sheet of water, +which is fed only from the mountains to the south. The channels of +the Amur Darya, Syr Darya, and other streams bear witness also to the +same general desiccation. At present, the amount of water supplied +by rivers to the Caspian Sea appears on the whole to balance that +removed by evaporation, though there are slight yearly or seasonal +fluctuations. In the Aral basin, however, there can be no doubt that +the waters are progressively diminishing. + +Owing to the enormous volume of fresh water poured into it by its +rivers, the Caspian Sea is not as a whole so salt as the main +ocean, and still less so than the Mediterranean Sea. Nevertheless +the inevitable result of evaporation is there manifested. Along the +shallow pools which border this sea, a constant deposition of salt +is taking place, forming sometimes a pan or layer of rose-colored +crystals on the bottom, or gradually getting dry and covered with +drift-sand. This concentration of the water is particularly marked +in the great offshoot called the Karaboghaz, which is connected with +the middle basin of the Caspian Sea by a channel 150 yards wide and +5 feet deep. Through this narrow mouth there flows from the main sea +a constant current, which Von Baer estimated to carry daily into +the Karaboghaz 350,000 tons of salt. An appreciable increase of the +saltness of that gulf has been noticed; seals, which once frequented +it, have forsaken its barren shores. Layers of salt are gathering on +the mud at the bottom, where they have formed a salt bed of unknown +extent, and the sounding-line, when scarcely out of the water, is +covered with saline crystals. + +The study of the precipitations which take place on the floors of +modern salt lakes is important in throwing light upon the history +of a number of chemically formed rocks. The salts in these waters +accumulate until their point of saturation is reached, or until +by chemical reactions they are thrown down. The least soluble are +naturally the first to appear, the water becoming progressively +more and more saline till it reaches a condition like that of the +mother-liquor of a salt work. Gypsum begins to be thrown down from +sea-water, when 37 per cent of water has been evaporated, but 93 +per cent of water must be driven off before chloride of sodium can +begin to be deposited. Hence the concentration and evaporation of +the water of a salt lake having a composition like that of the sea +would give rise first to a layer or sole of gypsum, followed by one +of rock-salt. This has been found to be the normal order among the +various saliferous formations in the earth’s crust. But gypsum may be +precipitated without rock-salt, either because the water was diluted +before the point of saturation for rock-salt was reached, or because +the salt, if deposited, has been subsequently dissolved and removed. +In every case where an alternation of layers of gypsum and rock-salt +occurs, there must have been repeated renewals of the water-supply, +each gypsum zone marking the commencement of a new series of +precipitates. + +But from what has now been adduced it is obvious that the composition +of many existing saline lakes is strikingly unlike that of the sea +in the proportions of the different constituents. Some of them +contain carbonate of sodium; in others the chloride of magnesium +is enormously in excess of the less soluble chloride of sodium. +These variations modify the effects of evaporation of additional +supplies of water now poured into the lakes. The presence of the +sodium-carbonate causes the decomposition of lime salts, with the +consequent precipitation of calcium-carbonate accompanied with a +slight admixture of magnesium-carbonate, while by further addition of +the sodium-carbonate a hydrated magnesium-carbonate may be eventually +precipitated. Hunt has shown that solutions of bicarbonate of lime +decompose sulphate of magnesia with the consequent precipitation of +gypsum, and eventually also of hydrated carbonate of magnesia, which, +mingling with carbonate of lime, may give rise to dolomite. By such +processes the marls or clays deposited on the floors of inland seas +and salt lakes may conceivably be impregnated and interstratified +with gypseous and dolomitic matter, though in the Trias and other +ancient formations which have been formed in inclosed saline waters, +the magnesium-chloride has probably been the chief agent in the +production of dolomite. + +The Dead Sea, Elton Lake, and other very salt waters of the +Aralo-Caspian depression, are interesting examples of salt lakes +far advanced in the process of concentration. The great excess of +the magnesium-chloride shows, as Bischof pointed out, that the +waters of these basins are a kind of mother-liquor, from which most +of the sodium-chloride has already been deposited. The greater the +proportion of the magnesium-chloride, the less sodium-chloride can +be held in solution. Hence, as soon as the waters of the Jordan and +other streams enter the Dead Sea, their proportion of sodium-chloride +(which in the Jordan water amounts to from .0525 to .0603 per cent) +is at once precipitated. With it gypsum in crystals goes down, +also the carbonate of lime which, though present in the tributary +streams, is not found in the waters of the Dead Sea. In spring, the +rains bring large quantities of muddy water into this sea. Owing +to dilution and diminished evaporation, a check must be given to +the deposition of common salt, and a layer of mud is formed over +the bottom. As the summer advances and the supply of water and mud +decreases, while evaporation increases, the deposition of salt +and gypsum begins anew. As the level of the Dead Sea is liable to +variations, parts of the bottom are from time to time exposed, and +show a surface of bluish-gray clay or marl full of crystals of common +salt and gypsum. Beds of similar saliferous and gypsiferous clays, +with bands of gypsum, rise along the slopes for some height above +the present surface of the water, and mark the deposits left when +the Dead Sea covered a larger area than it now does. Save occasional +impressions of drifted terrestrial plants, these strata contain no +organic remains. Interesting details regarding saliferous deposits of +recent origin, on the site of the Bitter Lakes, were obtained during +the construction of the Suez Canal. Beds of salt, interleaved with +laminæ of clay and gypsum-crystals, were found to form a deposit +upward of 30 feet thick extending along 21 miles in length by about +8 miles in breadth. No fewer than 42 layers of salt, from 3 to 18 +centimetres thick, could be counted in a depth of 2.46 metres. A +deposit of earthy gypsum and clay was ascertained to have a thickness +of 367 feet (112 metres), and another bed of nearly pure crumbling +gypsum to be about 230 feet (70 metres) deep. + +The desiccated floors of the great saline lakes of Utah and Nevada +have revealed some interesting facts in the history of saliferous +deposits. The ancient terraces marking former levels of these lakes +are cemented by tufa, which appears to have been abundantly formed +along the shores where the brooks, on mingling with the lake, +immediately parted with their lime. Even at present, oolitic grains +of carbonate of lime are to be found in course of formation along +the margin of Great Salt Lake, though carbonate of lime has not been +detected in the water of the lake, being at once precipitated in the +saline solution. The site of the ancient salt lake which has been +termed Lake Lahontan displays areas several square miles in extent +covered with deposits of calcareous tufa, 20 to 60 and even 150 +feet thick. This tufa, however, presents a remarkable peculiarity. +It is sometimes almost wholly composed of what have been determined +to be calcareous pseudo-morphs after gaylussite (a mineral composed +of carbonates of calcium and sodium with water)--the sodium of the +mineral having been replaced by calcium. When this variety of tufa, +distinguished by the name of _thïnolite_, was originally formed, the +waters of the vast lake must have been bitter, like those of the +little soda-lakes which now lie on its site--a dense solution in +which carbonate of soda predominated. On the margin of one of the +present Soda Lakes, crystals of gaylussite now form in the drier +season of the year. Yet no trace of carbonate of lime has been +detected in the water. The carbonate of lime in the crystals must +be derived from water which on entering the saline lakes is at once +deprived of its lime. + + + + + UNDERGROUND WATER: SPRINGS, CAVES, RIVERS, AND LAKES + --ÉLISÉE RECLUS + + +If all soils were absolutely impervious, there would be no springs, +and the whole of the liquid mass furnished by rain and snow would +flow away over the surface of the ground like the torrents and +flood-waters of the mountains. The greater part, however, of the +water which falls upon the ground sinks in the first place into the +depths of the earth. There it becomes more or less perfectly purified +from the foreign bodies with which it was charged, gradually rising +to the temperature of the strata through which it passes, and +becoming impregnated with the soluble salts which it meets with. +Ultimately, when the water, in sinking down, encounters impervious +beds, it can penetrate no further, and, flowing laterally to the +outcrop of the beds, makes its escape in the form of springs. + +The absorption of the rain and melted snow takes place in various +ways, according to the nature of the soil. Ordinary vegetable +earth only allows the water to penetrate to a very slight depth, +especially when the rain falls in showers and the slope of the +ground is favorable for drainage. As mould is capable of absorbing +a very large quantity--indeed, more than half its own weight, it +prevents the strata beneath from receiving its due share of moisture, +retaining almost the whole of it for the use of the vegetation which +it nourishes. In fact, it requires an altogether exceptional rainfall +to saturate any ordinary arable soil to the extent of a yard below +the surface. Water passes with much more facility through sandy +and gravelly beds; but compact loams and clay will not allow it to +penetrate through them, retaining it in the form of pools or ponds on +the surface of the ground. + +The action of vegetation is not confined merely to imbibing the water +falling from the clouds; it often, also, assists the superabundant +moisture in penetrating the interior of the ground. Trees, after +they have received the water upon their foliage, let it trickle down +drop by drop on the gradually softened earth, and thus facilitate +the gentle permeation of the moisture into the substratum; another +part of the rain-water, running down the trunk and along the roots, +at once finds its way to lower strata. On mountain slopes, the +mosses and the freshly growing carpet of Alpine plants swell like +sponges when they are watered with rain or melted snow, and retain +the moisture in the interstices of their leaves and stalks until the +vegetable mass is thoroughly saturated and the liquid surplus flows +away. Peat-mosses especially absorb a very considerable quantity of +water, and form great feeding reservoirs for the springs which gush +out at a lower level. The immense fields of peat which cover hundreds +and thousands of acres on the mountain slopes of Ireland and Scotland +may, notwithstanding their elevation and inclined position, be +considered as actual lacustrine basins containing millions of tons of +water dispersed among their innumerable leaflets. The superabundant +water of these tracts of peat-mosses issues forth in springs in the +plains below. + +Rocks, like vegetable earth, also absorb water in greater or less +quantities, according to their fissures and the density of their +particles. If the soil is formed of volcanic scoriæ, or porous beds +of pebbles, gravel, or sand, the water rapidly descends toward the +underlying strata. Some of the harder rocks, especially certain kinds +of granite, absorb but a very small quantity of water, on account of +the small number of their clefts; others, on the contrary, as most +of the calcareous masses, imbibe every drop of water which falls on +their surface. There are some rocks which have their layers broken +and cracked to such an extent that they resemble enormous walls of +rubble-work; the rain instantly disappears on them as if it had +fallen into a sieve. But the greater part of the calcareous rocks +belonging to various geological periods are formed of thick and +regular strata, cleft at intervals by long vertical crevices. Below +the surface-beds, perhaps, are layers of soft marl, which the water +penetrates with difficulty, although it can soften and carry away its +particles. Here are formed, rill by rill, the subterranean rivulets +which ultimately spread all over the substratum of marl, following +the general slope of the bed. After a more or less considerable lapse +of time, the stratum of marl ultimately becomes saturated, and the +water then flows out through caverns which are variously modified by +subsidences--faults in the strata and the perpetual action of the +streams. The springs which proceed from calcareous rocks of this +nature are in general the most abundant, owing to the length of their +subterranean course. The water which falls on vast areas on the +surface of plateaus is ultimately united in one bed. A liquid mass of +this kind, which springs up suddenly into sight, just as if it merely +issued from the soil, drains perhaps an extent of country of many +hundreds or thousands of square miles. + +Thus, according to the nature of the rock on which the rain +falls, the latter finds its way again to the surface, either at a +considerable distance from the spot where it fell, or else springs +out in little rivulets immediately below the place where its drops +were first gathered. On a great many mountains we are surprised to +meet with springs gushing out at a few yards from the summit. These +jets have, indeed, often been considered as the evidence of some +miraculous intervention. Among others, we may mention the “Sorcerers’ +Spring,” which gushes out on one of the highest points of the +Brocken, the culminating peak in the Hartz Mountains. + +The springs which cause the most astonishment are those which for a +time flow plentifully, and then all at once cease running, but, after +an uncertain lapse of time, again make their appearance. One might +almost fancy that some invisible hand alternately opened and shut the +secret flood-gate which gave an outlet to the subterranean stream. +The cause for this phenomenon of intermission is easily explained. +When the water brought by the underground stream is collected in a +capacious cavity in the rock, which communicates with the exterior +surface through a siphon-shaped channel, the liquid mass gradually +rises in the stone reservoir before it rushes out into the air. It is +necessary that the reservoir should be filled up to the level of the +siphon, in order that the latter should be primed, and that the water +should flow out as a spring into the external basin. If the water in +the reservoir is not replenished with sufficient rapidity, and is +unable to keep at least on a level with the external outlet, the jet +of water will immediately cease, and can not recommence until the +upper part of the liquid mass has again risen up to the highest point +of the siphon. After an indefinite period of repose, the spring then +enters on a new phase of activity. + +There are many of these subterranean streams which, before they break +forth in springs, do not flow over beds continuously sloping in the +direction of their current, as in the case with the water-courses on +the surface of the ground. There are some indeed which first descend +into the bowels of the earth, either by a uniform declivity or by a +series of cascades or rapids, and ultimately reascend from the depths +toward the surface, or jet out vertically from the ground. + +In obedience to the law which compels liquids to seek the same +level in all connected reservoirs, a rivulet of water will never +fail to dart forth as a spring as soon as it finds an outlet below +the caverns in which the water is collected from which it proceeds. +Likewise, if the spot where the gushing out takes place is on a much +lower level than that of the feeding reservoirs situated above, the +liquid jet must necessarily shoot up in a column above the surface +of the ground. This is the case at Châtagna, in the department of +the Jura, where a natural _jet d’eau_ springs up to a height of +10 or 12 feet. In the grotto of Male-Mort, near Saint-Etienne, in +Dauphiné, the jet of water is not less than 23 to 26 feet in height. +But the water of the fountains being always more or less charged with +sediment, the deposit accumulates in the form of a circular hillock +around the orifice, thus almost always ultimately raising it to the +level of the top of the liquid column. As an instance of these rising +fountains, we may mention the famous springs of Moses (_Aïn Musa_), +which gush out in a charming oasis not far from the shores of the +Gulf of Suez. These springs, the temperature of which varies from +70° to 84° (Fahr.), now flow from the top of several small cones of +sandy and slimy débris which they have gradually thrown up above the +level of the plain. They are also shaded by olive and tamarind trees. + +In the innumerable multitude of springs, either cold or thermal, +which rise from the earth, we may observe the whole range of possible +temperatures from freezing-point up to the boiling-point. A spring +which flows from the side of the Hangerer, in the Oetzthal, at a +height of 6,742 feet, is only 1° warmer than ice. On the Alps, the +Pyrenees, and all the other chains of snow-clad mountains, near +the summits small rills of water are very frequently met with, +the temperature of which is scarcely higher than that of melting +snow. Even at the bases of mountains, and especially those of a +calcareous nature, a great number of springs are found which are +much colder than the surrounding soil. This is so because, in +addition to the water, the air also enters the subterranean channels +and circulates in all the network of clefts and crevices, and, by +incessantly gliding over the wet sides of the channels, produces a +rapid evaporation of moisture, and, in consequence, refrigerates the +surface of the rocks and even the stream itself. The temperature, +therefore, of springs which proceed from the interior of cavernous +mountains is always several degrees lower than the normal temperature +of the soil. + +Springs which have a higher temperature than the soil are called +_thermal_ springs. + +It is to be remarked that nearly all thermal springs which do not owe +their high temperature to the vicinity of volcanoes issue forth from +faults which open on the surface of masses of a crystalline nature, +and principally at the side of modern eruptive rocks which have been +thrust up through older strata. + +The influence of rains and seasons has much less effect upon thermal +waters than upon cold springs which proceed from the upper layers +of the soil. A great number of warm springs, however, undergo +certain changes in their yield of water, which must be without +doubt attributable, at least partially, to the same causes as the +variations in the discharges of superficial streams. In Auvergne, +in the Pyrenees, and in Switzerland, several springs, perfectly +protected against any infiltration of rain-water, flow in much +greater abundance at the very same period when the adjacent torrents +become swollen. At Brig-Baden, in the Valais, the water, the mean +temperature of which is in autumn and winter from 71° to 72° (Fahr.), +rises to 113° and 122° (Fahr.) when the breath of spring melts the +ice on the Jungfrau. + +Most thermal springs contain mineral substances in solution; +there are, however, a certain number which are almost as pure +as rain-water--such as, for instance, the celebrated waters of +Plombières, also that of Gastein, Pfeffers, Wildbad, and Badenweiler. +The springs of Chaudes-Aigues--those in France which have the highest +temperature, 158° to 176° (Fahr.)--contain only a small amount of +mineral substances. The inhabitants of Chaudes-Aigues use the water +to prepare their food, to wash their linen, and to warm their houses. +Wooden conduits, erected in all the streets of the town, supply, on +the ground floor of each house, a reservoir which serves to heat it +during cold weather, and thus dispenses with fires and chimneys. + +Among the various substances which spring-water brings to the +surface, those which are most common proceed from the strata +which serve to constitute the very framework of the globe. Chalk, +especially, occurs in different proportions in most springs, either +under the form of sulphate of lime, or, more often, as carbonate of +lime. Water which contains carbonic acid in solution is charged with +calcareous matter dissolved away from the sides of the rocks through +which it passes; then, by means of evaporation, it redeposits the +stony substances which it previously held in solution. Hence arise +all those calcareous concretions which form around so many springs; +also the stalactites in caverns. + +[Illustration: The Giant’s Causeway, Antrim, Ireland] + +Nearly all countries of the world possess some of these curious +springs, which cover with a calcareous crust any object placed +in their waters. Among these incrusting springs, those of Saint +Allyre, near Clermont, Rivoli, and San Filippo, not far from Rome, +have justly become celebrated. These latter have, in a space of +twenty years, filled up a pond with a bed of travertin 30 feet +thick, and, in the neighborhood, entire strata of this same rock +may be seen having a depth of more than 328 feet. The springs of +Hammam-Mes-Khoutine, in the province of Constantine, are also +very remarkable on account of the considerable amount of their +deposits. This water, which rises at a temperature of 203° (Fahr.), +and from which a high column of steam always rises, is frequently +compelled to change its point of issue on account of the dense beds +of travertin which are gradually deposited upon the soil. Most of +these deposits are of a dazzling white hue, striped here and there +with bright colors, and are developed in mammillated strata; other +concretions, accumulating gradually round an orifice, have taken +the form of cones, and are like small craters near a volcano, some +of them rising to a height of as much as 33 feet; lastly, there are +masses of travertin which stretch out in a kind of wall below the +flow which deposits them. One of these walls, which is interrupted at +intervals by heaps of earth upon which large trees grow, is not less +than 4,921 feet long, 66 feet high, and, on an average, from 33 to 49 +feet wide. + +The thermal waters of Algeria are, however, surpassed in grandeur and +beauty by the springs of the ancient Ionian city of Hierapolis (holy +city), which at the present time flow in the solitary plateau called +Panbouk-Kelessi (Castle of Cotton), on account of the cotton-like +aspect of the white masses of travertin of which it is composed. On +reaching this spot from Smyrna, something like an immense cataract +may be seen in the distance, 328 feet high and 2½ miles wide; this +is formed by the walls which the water has gradually constructed, +column after column, and layer after layer, by flowing over the +edges of the plateau and gushing out on the slopes. Here and there, +real cascades glitter in the sun, and their sparkling surfaces +light up the dead whiteness of the crystal walls. As a spectator +ascends the declivities, the masses deposited and carved out by the +water appear in all their strange beauty; one might fancy that they +were colonnades, groups of figures, and rude bas-reliefs which the +chisel had not yet perfectly set free from their rough coverings of +stone. And all these calcareous deposits which have been fashioned +by the cascades during a succession of ages open a multitude of +cup-like hollows with fluted edges fringed with stalactites; these +graceful reservoirs--some of which are shaded with yellow or veined +with red, brown, and violet, like jasper or agate--are filled with +pure water. Higher still follow two steps of the plateau on which +stood the ancient thermal edifice and the Necropolis of Hierapolis. +There whitish masses cover the ancient tombstones and fill up the +conduits. The ground is crossed in various directions by the former +beds of rivulets, which have gradually stopped up their own courses +by depositing concretions upon them. Above one of the widest of +these dried-up channels, the magnificent span of a natural bridge +displays its graceful form, like an arch of alabaster, streaming with +innumerable stalactites. At what date did this majestic structure +take its rise, and how many years and centuries did the process of +its formation last? No one knows. According to Strabo, the channels +of the baths of Hierapolis were soon filled up by solid masses, and +if Vitruvius can be believed, when the proprietors of the environs +wished to inclose their domain, they caused a current of water to run +along the boundary-line, and in the space of a year the walls had +risen. + +Silica, which is still more important than chalk in the formation +of terrestrial rocks, is also sometimes deposited on the edge of +springs, but in very small quantities. + +The various dislocations of the terrestrial strata, the cooling of +the waters, and, perhaps, in many instances, the obstruction of +channels by deposits of ore, explain why, in the present period, +so small a number of thermal springs issue from metalliferous +beds. Nevertheless, many localities might be mentioned where these +phenomena take place at the present time. A spring at Badenweiler, +in the Black Forest, issues forth at a few yards from a vein of +sulphuret of lead. In the granitic plateau of central France other +springs are likewise found to be associated with this metal. Various +thermal waters in the Black Forest, like those of Carlsbad and +Marienbad, are in close connection with veins of iron and manganese. +Oligiste iron is found in the fissures of the springs of Plombières +and Chaude-Fontaine. In Tuscany sulphureous fumaroles proceed from +the veins of antimony. In France and Algeria the waters of Sylvanès +and Hammam R’ira issue forth from beds of copper. Lastly, near +Freyberg, a voluminous thermal spring has been discovered in a vein +of silver. + +Among the mineral substances which some springs bring to the surface +of the soil, the most important, in an economical point of view, is +common salt. This substance, being one of those which dissolve most +readily in water, all the liquid veins which pass over saline beds +become saturated with salt; therefore springs of this kind, which +flow in great abundance, give rise to salt-works of more or less +importance. The masses of common salt which make their way every +year from the interior of the earth may be estimated at thousands +of tons. The springs of Halle, which rise on the northern slope of +the Alps of Salzburg (Salt Town), and are managed with the greatest +care, annually produce 15,000 tons of this mineral. The salt springs +of Halle, in Prussia, which have been worked from time immemorial by +a company, furnish 10,000 tons of salt every year. Other parts of +Germany also yield for consumption thousands of tons of white salt, +which is produced by the evaporation of saline springs. The mass +of salt furnished by the single artesian well of Neusalzwerk, near +Minden, in Prussia, represents every year a cube measuring 78 feet on +each side. + +Though not so rich as Germany in saline springs thus turned to +account, most of the civilized countries of the world possess +salt-works which are also very important. France enjoys the springs +of Dieuze, Salins, and Salies; Switzerland, those of Bex; Italy has +the springs in the environs of Modena, and many others besides. In +England, near Chester, there are some mines of rock-salt in which +numerous liquid veins issue forth which are impregnated with salt. +Lastly, the United States have the celebrated springs of Syracuse. + +Not far from the “spot where Troy once stood” is the valley of +Touzla-sou, which owes its name (Salt Water) to its numerous salt +springs. The mountains which rise around its circumference are +variously shaded with blue, red, and yellow, and the rocks are +incessantly decomposing under the action of the liquid salt which +oozes out from and trickles down their sides. The plain itself +is covered with a variegated crust, while jets of boiling water, +saturated with salt, burst forth in every direction. Here and there +pools are found, the moisture of which, by evaporating in the sun, +leaves upon the soil beds of salt as white as snow. Near the mouth +of the valley springs become more and more numerous. Lastly, in +the place where the cliffs approach near together, so as to form a +defile, a magnificent spout of water jets out from one side of the +rock. This jet is not less than a foot in diameter at the orifice, +and falls again after having described a parabola of more than a +yard and a half. Other springs shoot out on both sides, the constant +temperature of which is more than 212° (Fahr.); these, together with +the principal jet, form a rivulet of boiling and steaming water. + +Springs of salt water are used for the treatment of diseases as +well as for the extraction of salt. They constitute one of the most +important groups of medicinal waters, according to the various +substances which they contain in solution. The other springs made +use of, on account of their healing virtues, have been classed under +ferruginous, sulphureous, and acidulous springs. These waters also +contain, in different proportions, a variable quantity of gases and +salts which they have dissolved in their passage over subterranean +beds of every kind. + +Mineral springs are most numerous and abundant in mountain valleys, +and there, consequently, the great thermal institutions are +established. In Europe the chain of the Pyrenees is probably the +richest in mineral, sulphureous, saline, ferruginous, and acidulous +springs. According to Francis, the engineer, in 1860 more than 550 +mineral springs, 187 of which are used, flowed upon the French +slopes of the Pyrenees. These waters supplied 83 hot baths in 53 +localities, the principal of which are Bagnères de Bigorre, Luchon, +Eaux-Bonnes, and Cauterets. The most abundant springs, those of Graus +d’Olette, form a sort of mineral stream, yielding more than four +gallons a second, or 2,322 cubic yards a day. In Algeria the spring +of Hammam-Mes Khoutine yields 6 gallons a second. + +There are regions, some volcanic and some not, in which nearly all +the springs are thermal and mineral; springs of pure and fresh +water being so rare, they are there considered to be most precious +treasures. One of these regions comprehends a large part of the +plateau of Utah. In this place numerous thermal springs issue forth, +to which have been given the vulgar names of the Beer, Steamboat, +Whistle Springs, etc., and into one of which the Mormons plunge their +neophytes. The springs which are not thermal are loaded with saline +and calcareous matter. It is only in spring, at the time when the +snow melts, that the springs, which then become very abundant, yield +comparatively pure water. During the dry season, salt and carbonate +of lime become concentrated in the nearly exhausted springs, and give +to the liquid flow an unpalatable taste. Palgrave, the traveler, +informs us that all the springs of the country of Hasa, in Arabia, +are also thermal. + +It can readily be understood that when all these substances escape +from the interior of the rocks, together with the water which holds +them in solution, they must leave empty spaces in the earth. During +the course of long centuries whole strata are dissolved, and, under +a form more or less chemically modified, are brought up from the +depths and distributed on the surface of the soil. The thermal waters +of Bath, which are far from being remarkable for the proportion of +mineral substances they contain, bring to the surface of the earth +an annual amount of sulphates of lime and soda, and chlorides of +sodium and magnesium, the cubic mass of which is not less than 554 +cubic yards. It has also been calculated that one of the springs of +Louèche, that of Saint Laurent, brings every year to the surface +8,822,400 pounds of gypsum, or about 2,122 cubic yards; this quantity +is enough to lower a bed of gypsum a square mile in extent, more than +five feet in one century. But this is only one spring, and we have +reckoned one century only; if we think of the thousands of mineral +springs which gush from the soil, and of the immensity of time during +which their waters have flowed, some idea may be formed of the +importance of the alterations caused by springs. In time they lower +the whole mass of mountains, and, no doubt, after these sinkings, +violent oscillations of the earth may often have taken place. + +In regions where the strata are pierced with wide and deep caverns, +and especially in calcareous countries, the waters sometimes +accumulate in sufficient quantities to form perfect streams with +long subterranean courses. At their issue from the caverns, these +waters form a contrast with the rocks and hills around, all the more +striking because the latter are completely devoid of moisture, and +fearfully sterile, while on the brink of the limpid stream the fresh +verdure of plants and trees is at once developed. Like a captive, +joyous at seeing the light once more, the water which shoots forth +from the sombre grotto of rocks sparkles in the sun, and careers +along with a light murmur between its flowery banks. + +Among these subterranean streams, the most celebrated, and doubtless +one of the most beautiful, is the Sorgues of Vaucluse. The vaulted +grotto from which the mighty mass of water escapes opens at the mouth +of an amphitheatre of calcareous rocks with perpendicular sides. +Above the spring rises a high white cliff, bearing on its summit +a ruined tower of the Middle Ages; the rock is everywhere sterile +and bare; there is nothing but a miserable fig-tree, clinging to +the stone like a parasitical plant to the bark of a tree, which has +plunged its roots into the fissure of the cave, and greedily absorbs +with its leaves the moisture which floats like a mist above the +cascades of the spring. After heavy rains, the liquid mass, which is +then estimated at 26 or even 32 cubic yards a second, flows in a wide +sheet high above the entrance to the cavern, which is then altogether +inaccessible. When the waters are low, they flow bubbling across the +barrier of rocky débris which obstructs the entrance; at that time +it is quite possible to penetrate under the arch, and to contemplate +the vast basin in which the blue waters of the subterranean stream +spread out before they leap into the open air. Soon after its issue +from the cave and amphitheatre of Vaucluse, the Sorgues is divided +into numerous irrigation channels, which spread fertility in the +country over an area of more than 77 square miles. The subterranean +course of the affluents which form the stream is not ascertained; but +it is known that most of them commence 12 or 15 miles to the east, +in the plateaus of Saint Christol and Lagarde, which are pierced all +over with _avens_ or chasms, into which the rain-water sinks and +disappears. + +In another part of France there is a second important subterranean +stream, which is much less known but no less remarkable than that of +Vaucluse; this is the Touvre of Angoulême, continuing the course of +the Bandiat, the waters of which, like those of the Tardoire, are +swallowed up in several abysses at distances varying from 3 to 7 +miles to the east and northeast. The three principal springs of the +Touvre flow slowly out of a deep cave, hollowed out at the base of +an escarped cliff; another spring bubbles up in a basin of rock; the +third emerges from a sort of boggy meadow intersected by drains. At +the outlet of their subterranean courses these three enormous springs +immediately form three streams, which reunite, leaving between them +two long peninsulas of reeds and other aquatic plants. Below the +junction, the Touvre, which is here more than 100 yards wide, passes +round a rugged hill, and, dividing into several branches, turns +the numerous mill-wheels of the important gun-foundry of Ruelle; +then, after a course of five miles, it flows into the Charente at a +small distance above Angoulême. Among the hundreds and thousands of +travelers whom steam annually conveys over the bridge of the Touvre, +there are few who are aware of the curious nature of the source of +the river of limpid water over which the train passes in its noisy +career. + +Omitting to mention the streams which accidentally pass under the +strata of rocks during a small part of their course, or of the +subterranean outlets of certain lakes, a multitude of other instances +might be brought forward of masses of water, more or less abundant, +which appear above ground after having traversed a considerable +distance under the earth. Of this kind is the graceful spring of +Nîmes, the blue transparent water of which, reflecting the foliage +of pines and chestnut trees, glides in its gentle ripples over the +semicircular steps of an old Roman staircase. Of this kind, too, is +the spring of Vénéran, near Saintes: this spring, which was formerly +sacred to the Goddess of Love, gushes from the ground in a gorge of +rocks, and, passing through a mill, the wheel of which it turns, it +suddenly disappears, being swallowed up in an abyss; thus it appears +on the earth to work but for an instant. + +Numbers of water-courses do not reappear on the surface of the soil +after being swallowed up in the earth, but flow straight to the sea +by means of subterranean channels. On nearly the whole extent of the +continental shores, and principally in localities where the coasts +are of a calcareous nature, the outlets of submarine tributaries may +be noticed, some of which are perfect rivers. Most of the springs of +the department of Bouches du Rhone jet up from the bottom of the sea, +but at various distances from the shore. One of them, that of Porte +Miou, near Cassis, forms on the surface of the sea a considerable +current, which drifts any floating bodies to a great distance. +At Saint Nazaire, Ciotat, Cannes, San Remo, and Spezzia, other +streams also issue from the midst of the salt waves, and attempts +have even been made to measure approximately their discharge. M. +Villeneuve-Flayosc estimates at 24 cubic yards a second the quantity +of water discharged into the sea by all the hidden affluents of the +Mediterranean between Nice and Genoa. Some of the submarine springs +of Provence and Liguria proceed from enormous depths. The orifice of +the spring of Cannes is 531 feet below the level of the sea; that of +San Remo rises from a depth of 954 feet; lastly, at four miles to +the south of Cape Saint Martin, between Monaco and Mentone, another +stream of fresh water empties itself under a bed of salt water, near +2,296 feet deep. + +The coasts of Algeria, Istria, Dalmatia, and the Herzegovina also +present numerous instances of submarine streams; on the eastern +shores of the Adriatic the traveler may even have the pleasure of +contemplating the delta of a considerable river, the Trebintchitza, +visible through the sea-water at the depth of a yard. The abundant +springs of fresh water which pour out into the open sea to the +southwest of the Cuban port of Batabano are well known, since +Humboldt described them, and it is observed that the lamantins, +or sea-cows, which dread salt water, delight in frequenting these +parts. Lastly, the Red Sea, which does not throughout its immense +circumference receive a single permanent stream flowing on the +surface of the ground, nevertheless receives some which spring +from the bottom of its bed. The shores of the United States, the +calcareous soil of which is probably pierced with caverns from the +very centre of the continent, perhaps are the coasts which pour into +the sea the most abundant subterranean rivers. Near the mouth of the +stream of St. John, a submarine stream of perfectly pure water spouts +in bubbles as far as one to two yards above the level of the sea. +Off the Carolinas, and Florida, salt water has been known to change +into brackish water under the influence of the sudden increase of its +subterranean affluents. In the month of January, 1857, all that part +of the sea which is adjacent to the southern point of Florida was +the scene of an immense eruption of fresh water. Muddy and yellowish +water furrowed the straits, and myriads of dead fish floated on the +surface and accumulated on the shores. Even in the open sea the +saltness diminished by one-half, and in some places the fishermen +drew their drinking-water from the surface of the sea as if from +a well. It is affirmed by all those who witnessed this remarkable +inundation of the subterranean river that, during more than a month, +it discharged at least as much water as the Mississippi itself, and +spread over all the strait, 31 miles wide, which separates Key West +from Florida. + +On the coasts of Yucatan, the fresh waters which take a subterranean +course down to the sea do not appear to flow like rivers which have a +narrow bed and attain considerable speed, but more in the form of a +wide sheet of liquid with a nearly imperceptible current. _Cenotes_ +open here and there over the surface of the country; they are a +kind of natural draining-well or hole, not very deep, into which +the inhabitants descend to draw spring water. At Merida and in the +environs the subterranean water is found at a depth of 26 to 30 feet; +but the nearer we approach to the sea the thinner the layer of rock +becomes which covers the liquid veins; on the seashore fresh water +is found nearly on a level with the soil. The height of the veins +varies several inches, according to the quantity of rain; but in +every season the mass of water descending from the plateau of Yucatan +is poured into the sea through innumerable outlets. Over a great +extent of the shore of the peninsula, these hidden springs furnish +collectively a mass sufficiently large to counterpoise the waters of +the sea. Under the pressure of the marine current which runs along +the coast, there is formed, between the open sea and the liquid mass +which has made its way from the land, a littoral bank like those +barriers which the waves construct before the mouths of rivers. This +embankment, which protects the coasts of Yucatan like a breakwater, +is not less than 171 miles long, and is cut through by the sea at +two or three points. The channel, which stretches like a wide river +between the bank of alluvium and the Yucatan coast, is, not without +reason, designated by the inhabitants by the name of stream or _rio_. + +Among the remarkable phenomena which perhaps owe their existence to +subterranean water-courses, we must mention the sudden or gradual +appearance of those hillocks of clay (“mud-lumps”) which rise, to +the great danger of navigators, either in the middle of the bar of +the Mississippi, or in the immediate vicinity. Like small volcanoes +of mud, the “mud-lumps” generally appear under the form of isolated +cones, allowing a rill of dirty water to escape from their summits. +Some of them are irregular on their surface, on which lateral +orifices here and there show themselves, some in full activity, +others abandoned by the springs which formerly gushed from them. The +water of some “mud-lumps” is loaded with oxide of iron or carbonate +of lime, which, with the agglutinated sands, form hard masses, having +the consistence of perfect rocks. These hillocks vary both in their +height and shape. The greater part remain hidden at the bottom of the +water, and even their summits do not reach the level of the river +or sea; others hardly raise their heads above the waves; the most +considerable, however, rise to a height of 6, 9, or even 19 feet, and +their base covers an area of several acres. The sudden way in which +most of these water-volcanoes make their appearance, the anchors of +vessels, and the remains of cargoes which have been found on their +surface, their conical form, their terminal craters, and all the +springs, “which seem to spout out as if from a subterranean sieve,” +indicate the existence of a subterranean force always at work to +upheave this band of hillocks. + +M. Thomassy is of opinion that the hillocks of these bars are the +orifices of regular artesian wells naturally formed by a sheet of +subterranean water descending from the plateaus of the interior and +flowing below the Mississippi and the clayey levels of Louisiana. +However this may be, the mode in which these mud hillocks are formed +is well enough known to render it easy to clear them away from the +mouths of the Mississippi and to protect the interests of navigation. +When a cone of clay makes its appearance on the bar, a charge of +powder is introduced into it and explodes it. Thus, in the year 1858, +the southwest passage was cleared of a “mud-lump” which formed a +considerable island; a single charge was sufficient to annihilate +the whole. The island suddenly sunk; in its place a wide depression +was formed, the circumference of which resembled that of a volcanic +crater; at the same time an enormous quantity of hydrogen gas was +discharged into the atmosphere. + +Above the springs the course of subterranean rivulets is generally +indicated by a series of chasms or natural wells, which disclose +the stream beneath. The arches of caves not being always strong +enough to support the weight of the superincumbent masses, they +necessarily fall in some places, leaving above them other spaces +into which the upper beds successively sink. The débris of the ruin +is afterward cleared away by the water, or dissolved, atom by atom, +by the carbonic acid contained in the stream, and gradually all the +loose rubbish is carried away. In this manner, above the subterranean +rivulets, a kind of well is formed, which is designated in various +countries by very different names. + +By means of these natural gulfs it is possible to reach the +subterranean streams, and to give some account of their system, which +is exactly like that of rivulets and rivers flowing in the open air. +These streams also have their cascades, their windings, and their +islands; they also erode or cover with alluvium the rocks which +compose their bed, and they are subject to all the fluctuations of +high and low water. The only important difference which superficial +waters and subterranean currents present in their phenomena is that +these streams in some places fill the whole section of the cave, and +are thus kept back by the upper sides, which compress the liquid +mass. In fact, the spaces hollowed out by the waters in the interior +of the earth are only in a few places formed into regular avenues, +which might be compared to our railway tunnels. Where beds of hard +stone oppose the flow of the rivulet, all it has done during the +course of centuries has been to hew out one narrow aperture. This +succession of widenings and contractions, similar to those of the +valleys on the surface, forms a series of chambers, separated one +from the other by partitions of rock. The water spreads widely in +large cavities, then, contracting its stream, rushes through each +defile as if through a sluice. + +On account of these partitions, it is very difficult, or even +impossible, to navigate the course of subterranean rivers to any +considerable distance, even at the time the water is low. When it +is high, the liquid mass, detained by the partitions, rises to a +very high level in the large interior cavities, and often reaches +the roof above. Sometimes when, through the clefts of the rocks, a +communication exists between the cave and some hollow above, the +surplus water from the subterranean streams makes its appearance +there. Thus the Recca, which flows beneath the adjacent plateau +of Trieste, does not always find space enough to flow freely in +its lower channels, and Schmidt has seen it ascend in the chasms +of Trebich to a height of 341 feet. It may be understood that the +pressure of such a column of water often shatters enormous pieces of +rock, and thus modifies the course of underground streams. + +When the water, impelled by force of gravitation, seeks a new bed in +the cavernous depths of the earth, and disappears from its former +channels, these are at first much easier of access than they formerly +were; but ere long, in most caves, a new agent intervenes, which +seeks to contract or even completely obstruct them. This agent is +the snow-water, or rain, which percolates, drop by drop, through +the enormous filter of the upper strata. In passing through the +calcareous mass, each one of these drops dissolves a certain quantity +of carbonate of lime, which is afterward set free on the arch or +the sides of the cave. When the drop of water falls, it leaves +attached to the stone a small ring of a whitish substance; this is +the commencement of a stalactite. Another drop trickles down, and, +trembling on this ring, lengthens it slightly by adding to its edges +a thin circular deposit of lime, and then falls. Thus drop succeeds +drop in an infinite series, each depositing the particles of lime +which it contains, and forming ultimately a number of frail tubes, +round which the calcareous deposit slowly accumulates. But the water +which drops from the stalactites has not yet lost all the lime which +it held in solution; it still retains sufficient to enable it to +elevate the stalagmites and all the mammillated concretions which +roughen or cover the floor of the grotto. It is well known what +fairy-like decorations some caverns owe to this continuous oozing +through the vaults of their roofs. There are few sights in the world +more astonishing than that of these subterranean galleries, with +their dead-white columns, their innumerable pendants and multiform +groups, like veiled statues, all yet unstained by the smoke of the +visitor’s torch. + +When the action of the water is not disturbed, the needles and +other deposits of the calcareous sediment continue to increase with +considerable regularity. In some cases each new layer which is +added to the concretions may be studied as a kind of time-measurer, +indicating the date when the running water abandoned the cave. At +length, however, the soft concentric layers disappear, and are +replaced by forms of a more or less crystalline character; for +in every case where solid particles exist, subject to constant +conditions of imbibition by water, crystals are readily produced. +Sooner or later, the stalactites, increasing gradually in a downward +direction, meet and unite with the needles rising from the surface of +the ground, and, forming by their number a kind of barrier, obstruct +the narrower passages and close up the defiles separating the cavern +into distinct chambers. + +One of these Kentucky caves, called the “Mammoth Cave,” is the +largest which is at present known. The whole of its extent has +not been as yet fully explored, for it may be almost called a +subterranean world, having a system of lakes and rivers, and a +network of galleries and passages without number, which cross and +recross one another, going down to an immense depth. From the +chief entrance to the further recesses of the cave, the distance +is reckoned to be not less than 9¼ miles, and the whole length of +the two hundred alleys that have been traced out in this enormous +labyrinth is 217 miles in extent. This “Mammoth Cave” once served as +a retreat for savage tribes, for skeletons of men of an unknown race +have been found buried in it under layers of stalactite. + +The district which is the most remarkable among all the calcareous +countries of Europe for its caves, its subterranean streams, and +its abysses is unquestionably the region of the Carniolan and +Istrian Alps, which extends to the east of the Adriatic, between +Laibach and Fiume. The whole surface of the country, as in certain +plateaus of the Jura in France, is everywhere pierced with deep +boat-shaped cavities, at the bottom of which the water forms a kind +of whirlpool, like the water flowing out of the hold of a stranded +ship. Many mountains are penetrated in every direction with caverns +and passages, just as if the whole rocky mass was nothing more than +an accumulation of cells. On one steep cliff-side may be noticed +all kinds of perforations at different heights--arched portals and +orifices of fantastic shape; on another there are numbers of springs +of blue water gushing from the caves, or from the rocks heaped up at +the foot of the cliff, and forming rivulets which disappear a little +further on in the fissures of the ground, as if through the holes of +a sieve. The whole surface of the plateaus, whether bare or covered +with forests, is scattered over with wells, or funnel-shaped holes +communicating with subterranean reservoirs. + +One of the Istrian rivers, the subterranean course of which, although +still unknown as regards a great number of points, has given rise to +a most continuous course of investigations, is the celebrated Timavus +(Timavo), which falls into the sea near Duino, about twelve miles +to the north of Trieste. Virgil’s description no longer applies to +the mouths of the Timavo; at present they do not reach the number +of nine, because the extermination of the woods of the Carso has +diminished the mass of the water, or the action of the stream and +the alluvium of the delta have modified the form of the shore. But +still it is a magnificent spectacle to see the outlet of the three +principal torrents of water which rush foaming out of the heart of +the rocks, and are navigable from their mouths to their very source. +A river of this importance must certainly receive the drainage of +a vast basin, and yet all the neighboring valleys seem perfectly +devoid of rivulets, and their surface presents little else but the +bare rock; in fact, the whole of the rain and snow-water runs away +through underground caverns. + +The most remarkable network of caverns in this region of the Alps is +that which spreads out from the southwest to the northeast across +the Adelsberg group of mountains, between Fiume and Laibach. The +principal cave is especially curious on account of its size, the +variety of its calcareous concretions, and the torrent which runs +roaring through it. + +North of the town of Adelsberg the traveler passes along the base of +a hill with steep and bare sides, bringing into view the sharp edges +of its highly pitched calcareous beds. On the right the stream of +the Poik winds peaceably in the valley; and then, its course being +arrested by a headland, turning suddenly, it flows into the interior +of the mountain through a kind of high portal, opening between two +parallel beds of rocks. Unless the water in the stream is very low, +it is impossible to follow it over the accumulation of rocks upon its +bed; but on the right, at a height of a few yards, there is another +entry, through which the traveler may descend dry-shod into a vast +cavity or chamber, where the Poik again appears issuing from its +narrow passage of rocks. + +At this point the cave divides; on the north the stream, the depth +of which varies, according to the season, from a few inches to 30 or +33 feet, buries itself in a winding avenue, which has been traversed +in a boat as far as a point 1,027 yards from the entrance; on the +northeast, a higher avenue, discovered only in 1818, pushes its +way far into the heart of the mountain, branching out in various +directions into narrow passages and wide compartments. This portion +of the grotto, which appears to have been the former bed of the Poik, +is the most curious part of the Adelsberg labyrinth; it affords +wonderful groups of stalactites, especially in the Salle du Calvaire, +the vaulted roof of which, having the enormous span of 210 yards, +has dropped upon a hillock of débris a perfect forest of stalagmitic +columns and white needles. The full length of the principal cave is +not less than 2,575 yards; but very probably some other and still +longer avenues may yet be discovered. + +Although it is impossible to go in a boat along the subterranean +portion of the Poik for a greater distance than 1,027 yards, +by traversing the surface of the calcareous plateaus we can at +all events trace out the subterranean stream by means of the +funnel-shaped holes which open above its course. One of these gulfs, +the Piuka-Jama, is situated about a mile and a half to the north of +the entrance of the Adelsberg caves; the only way to descend into +this is by clinging to the branches of the shrubs and sliding down by +the assistance of a cord fastened to the top of the rocks. By these +means the entrance to a kind of air-hole may be reached, from which +the Poik is visible foaming over its bed of rocks, and only a slope +of débris is to be descended to reach the edge of the stream. It can +only be followed in the downstream direction for about 275 yards; +but it can easily be ascended for a distance of 495 yards by passing +under a high portal with lofty pillars, and in this way a point can +be reached which is less than a mile from the place where the stream +disappeared in the cave of Adelsberg. + +Further down the stream the Poik is not visible again until it +emerges from the mountain, where it is known under the name of the +Planina; it rushes out through a circular arch at the base of a +perpendicular bluff crowned with fir-trees. It really is the Poik, +as is proved by the equal temperature of water and the sudden +increase of its liquid mass after a storm has burst at Adelsberg; +but the stream always issues from the cave much more considerable +in bulk than it is when it enters, owing to the tributaries which +pour into it on both sides during its subterranean course of five +to six miles. One of these rivulets, which comes down from the +plateaus of Kaltenfeld, joins the Poik at a little distance from its +outlet. Above the confluence the principal stream can be ascended +in a boat to a distance of more than 3,500 yards, which, with the +other explored parts of the subterranean river, makes about three +miles. Below the point of outlet the stream is partially lost in the +fissures of its bed, and then, joining the Unz, goes on and empties +itself into the Danubian Save. + +About a dozen miles to the southeast of the Adelsberg and Planina +caves extends a large plain surrounded on all sides by high +calcareous cliffs, at the base of which nestle seven villages. In +this hollow, the most elevated portion of which is under cultivation, +the remainder being covered with rushes and other marsh-plants, there +are to be found more than 400 funnel-shaped holes resembling those +in other parts of Carniola. These _dolinas_, the average depth of +which is from 40 to 60 feet, have each their special name, such as +the “_Grand Crible_” (great sieve), the “_Crible-à-froment_” (corn +sieve), the “_Tambour_” (drum), the “_Cuve_” (tub), the “_Tonneau_” +(cask), pointing out the form or some remarkable peculiarity of +each abyss. During extremely dry seasons there is only one of these +cavities which contains any water; but after continuous and heavy +rain, the water of a stream which is swallowed up in the rocks a +little above the plain rises with a roaring noise in each of these +wells. Torrents escaping from all these open “_cribles_” form in the +wide space hemmed in by the cliffs a sea of blue and transparent +water. This is the lake of Jessero or Zirknitz, the _lacus Lugens_ of +the Romans. The surface of the sheet of water extends over an area of +14,826 acres; at the time of great inundations, this extraordinary +temporary lake, thus vomited out by the underground river, is not +less than 24,711 acres. The water runs away through a subterranean +channel, and, further on, empties itself into the Unz, below the +Planina. + +Lacustrine basins of this sort, first emitted, and then again +absorbed by a subterranean water-course, are rather rare; there are, +however, some other remarkable instances of them in Europe. Thus, +in the Oriental Hartz, in the midst of a beautiful spot surrounded +by fir-trees, the charming lake called Bauerngraben (Peasants’ +Ditch), or sometimes Hungersee (Lake of Famine), sometimes makes its +appearance; but when this mass of blue water has filled but for a few +days its basin of gypsum rock, it is suddenly swallowed up, and flows +away by subterranean channels into the stream of the Helme. The +celebrated lake of Copaïs, in Bœotia, may likewise be compared to the +Zirknitz lake, at least as regards certain portions of its basin. + + + + + RIVERS + --A. KEITH JOHNSTON + + +Rivers are the result of the natural tendency of water, as of all +other bodies, to obey the law of gravitation by moving downward +to the lowest position it can reach. The supply of water for the +formation of rivers, though apparently derived from various sources, +as from rain-clouds, springs, lakes, or from the melting of snow, is +really due only to atmospheric precipitation; for springs are merely +collections of rain-water; lakes are collections of rain or spring +water in natural hollows, and snow is merely rain in a state of +congelation. The rills issuing from springs and from surface-drainage +unite during their downward course with other streams, forming +_rivulets_; these, after a further course, unite to form _rivers_, +which, receiving fresh accessions in their course from _tributaries_ +(subordinate rivers or rivulets) and their _feeders_ (the tributaries +of tributaries), sweep onward through ravines, and over precipices, +or crawl with almost imperceptible motion across wide, flat plains, +till they reach their lowest level in ocean, sea, or lake. The +path of a river is called its _course_; the hollow channel along +which it flows, its _bed_; and the tract of country from which it +and its subordinates draw their supplies of water, its _basin_, or +_drainage-area_. The basin of a river is bounded by an elevated +ridge, part of which is generally mountainous, the crest forming +the watershed; and the size of the basin, and the altitude of its +watershed, determine, _cæteris paribus_, the volume of the river. The +greater or less degree of uniformity in the volume of a river in the +course of a year is one of its chief physical features, and depends +very much on the mode in which its supply of water is obtained. + +In temperate regions, where the mountains do not reach the limit +of perpetual snow, the rivers depend for their increase wholly on +the rains, which, occurring frequently, and at no fixed periods, +and discharging only comparatively small quantities of water at a +time, preserve a moderate degree of uniformity in the volume of the +rivers--a uniformity which is aided by the circumstance that in these +ones only about one-third of the rainfall finds its way directly over +the surface to the rivers; the remaining two-thirds sinking into +the ground, and finding its way to spring-reservoirs, or gradually +oozing through at a lower level in little rills which continue to +flow till the saturated soil becomes drained of its surplus moisture, +a process which continues for weeks, and helps greatly to maintain +the volume of the river till the next rainfall. This process, it +is evident, is only possible where the temperature is mild, the +climate moist, evaporation small, and the soil sufficiently porous; +and under these circumstances great fluctuations can only occur +from long-continued and excessive rains or droughts. In the hotter +tracts of the temperate zones, where little rain falls in summer, +we occasionally find small rivers and mountain torrents becoming +completely exhausted; such is often the case in Spain, Italy, Greece, +and with the Orange, one of the largest rivers of South Africa. + +In tropical and semi-tropical countries, on the other hand, the year +is divisible into one dry and one wet season; and in consequence the +rivers have also a periodicity of rise and fall, the former taking +place first near the source, and, on account of the great length of +course of some of the tropical rivers, and the excessive evaporation +to which they are subjected (which has necessarily most effect where +the current is slow), not making itself felt in the lower part of +their course till a considerable time afterward. Thus, the rise of +the Nile occurs in Abyssinia in April, and is not observed at Cairo +till about mid-summer. The fluctuations of this river were a subject +of perpetual wonderment to the ancient civilized world, and were +of course attributed to superhuman agency; but modern travel and +investigation have not only laid bare the reason of this phenomenon, +but discovered other instances of it, before which this one shrinks +into insignificance. + +The maximum rise of the Nile, which is about 40 feet, floods 2,100 +square miles of ground; while that of the Orinoco, in Guiana, which +is from 30 to 36 feet, lays 45,000 square miles of savannah under +water; the Brahmaputra at flood covers the whole of Upper Assam to a +depth of 10 feet, and the mighty Amazon converts a great portion of +its 500,000 square miles of selvas into one extensive lake. But the +fluctuations in the rise of the flood-waters are surpassed by some +of the comparatively small rivers of Australia, one of which, the +Hawkesbury, has been known to rise 100 feet above its usual level. +This, however, is owing to the river-beds being hemmed in by lofty +abrupt cliffs, which resist the free passage of a swollen stream. + +The increase from the melting of snow in summer most frequently +occurs during the rainy season, so that it is somewhat difficult +to determine, with anything like accuracy, the share of each +in producing the floods; but in some rivers, as the Ganges and +Brahmaputra, the increase from this cause is distinctly observable, +as it occurs some time after the rains have commenced, while in the +case of the Indus it is the principal source of flood. When the +increase from melted snow does not occur during the rainy season, we +have the phenomenon of flooding occurring twice a year, as in the +case of the Tigris, Euphrates, Mississippi, and others; but in most +of these cases the grand flood is that due to the melting of the snow +or ice about the source. + +The advantages of this periodical flooding in bringing down abundance +of rich fertile silt--the Nile bringing down, it is said, no less +than 140 millions of tons, and the Irrawadi 110 millions of tons +annually--are too well known to need exposition here. Islands are +thus frequently formed, especially at a river’s mouth. Permanent and +capacious lakes in a river’s course have a modifying effect owing to +their acting as reservoirs, as is seen in the St. Lawrence; while +the Red River (North) and others in the same tract inundate the +districts surrounding their banks for miles. In tropical countries, +owing to the powerful action of the sun, all rivers whose source is +in the regions of perpetual snow experience a daily augmentation +of their volume; while some in Peru and Chili, being fed only by +snow-water, are dried up regularly during the night. + +The course of a river is necessarily the line of lowest level from +its starting-point, and as most rivers have their sources high up a +mountain slope the velocity of their current is much greater at the +commencement. The courses of rivers seem to be partially regulated +by geological conditions of the country, as in the case of the San +Francisco of Brazil, which forms with the most perfect accuracy the +boundary-line between the granitic and the tertiary and alluvial +formations in that country; and many instances are known of rivers +changing their course from the action of earthquakes, as well as from +the silting up of the old bed. The inclination of a river’s course +is also connected with the geological character of the country; in +primary and transition formations, the streams are bold and rapid, +with deep channels, frequent waterfalls and rapids, and pure waters, +while secondary and alluvial districts present slow and powerful +currents, sloping banks, winding courses, and tinted waters; the +incline of a river is, however, in general very gentle--the average +inclination of the Amazon throughout its whole course being estimated +at little more than six inches per mile, that of the Lower Nile less +than seven inches, and of the Lower Ganges about four inches per mile. + +The average slope of the Mississippi throughout its whole length is +more than seventeen inches per mile, while the Rhone is, with the +exception of some much smaller rivers and torrents, the most rapid +river in the world, its fall from Geneva to Lyons being eighty inches +per mile, and thirty-two inches from Lyons to its mouth. + +The velocity of rivers does not depend wholly on their slope; much is +owing to their depth and volume (the latter being fully proved by the +fact that the beds of many rivers remain unaltered in size and slope +after their streams have received considerable accessions, owing to +the greater rapidity with which the water runs off); while bends in +the course, jutting peaks of rock or other obstacles, whether at the +sides or bottom, and even the friction of the aqueous particles, +which, though slight, is productive of perceptible effect, are +retarding agencies. In consequence, the water of a river flows with +different velocities at different parts of its bed; it moves slower +at the bottom than at the surface, and at the sides than the middle. +The line of quickest velocity is the line drawn along the centre of +the current, and in cases where this line is free from sudden bends +or sharp turns, it also represents the deepest part of the channel. +The average velocity of a river may be estimated approximately by +finding the surface-velocity in the centre of the current by means of +a float which swims just below the surface, and taking four-fifths +of this quantity as a mean. If the mean velocity in feet per minute +be multiplied by the area of the transverse section of the stream in +square feet, the product is the amount of water discharged in cubic +feet per minute. According to Sir Charles Lyell, a velocity of 40 +feet per minute will sweep along coarse sand; one of 60 feet, fine +gravel; one of 120 feet, rounded pebbles; one of 180 feet (a little +more than two miles per hour), angular stones the size of an egg. + +“Rivers are the irrigators of the earth’s surface, adding alike to +the beauty of the landscape and the fertility of the soil; they +carry off impurities and every sort of waste débris; and when of +sufficient volume, they form the most available of all channels of +communication with the interior of continents.... They have ever been +things of vitality and beauty to the poet, silent monitors to the +moralist, and agents of comfort and civilization to all mankind.” By +far the greater portion of them find their way to the ocean, either +directly or by means of semi-lacustrine seas; but others, as the +Volga, Sir-Daria (Jaxartes), Amu-Daria (Oxus), and Kur (Araxes), pour +their waters into inland seas; while many in the interior of Asia +and Africa--as the Murghab in Turkestan, and the Gir in the south of +Morocco--“lose themselves in the sands,” partly, doubtless, owing +to the porous nature of their bed, but much more to the excessive +evaporation which goes on in those regions. + + + + + SWAMPS AND MARSHES + --ÉLISÉE RECLUS + + +Marshes proper are shallow lakes, the waters of which are either +stagnant or actuated by a very feeble current; they are, at least +in the temperate zone, filled with rushes, reeds, and sedge, and +are often bordered by trees, which love to plunge their roots into +the muddy soil. In the tropical zone a large number of marshes are +completely hidden by multitudes of plants or forests of trees, +between the crowded trunks of which the black and stagnant water can +only here and there be seen. Marshes of this kind are inaccessible +to travelers, except where some deep channel, winding in the midst +of the chaos of verdure, allows boats to attempt a passage between +the water-lilies, or under some avenue of great trees with their +long garlands of creepers waving in the shade. Whatever may be the +climate, it would, however, be impossible to draw any distinction, +even the most vague, between lakes and marshes, as the level of these +sheets of water oscillates according to the seasons and years, and +as the greater number of lakes, principally those of the plains, +terminate in shallow bays which are perfect marshes. Some very +important lacustral basins, among others Lake Tchad, one of the +most considerable in all Africa, are entirely surrounded by swamps +and inundated ground, which prohibit access to the lake itself, and +prevent its true dimensions from being known. + +In like manner, a portion of the course of many rivers traverses low +regions in which marshes are formed, either temporary or permanent, +the uncertain limits of which change incessantly with the level of +the current. The borders of great water-courses, when left in their +natural state, are the localities in which these marshy reservoirs +principally exist. The most remarkable marshes of this kind are +perhaps those crossed by the Paraguay and several of its tributaries; +they consist of wet prairies and interminable sheets of water, which +stretch away like a sea from one horizon to the other. They have +received the names of Lakes Xarayes, Pantanal, etc. Further south, +certain tributaries of the Parana, the Maloya, the Batel, and the +Sarandi, which cross the State of Corrientes from northeast to +southwest, are nothing but wide marshes, the water of which overflows +slowly across the grass on the imperceptible slope of the territory. +There is, indeed, one of these marshes, the Laguna Bera, which drains +simultaneously into the two great rivers of Parana and Uruguay. + +In the same way as the low river-shores are frequently converted into +marshes, vast extents of the seacoasts when but slightly inclined +are also covered over by marshes, which are generally separated from +the main sea by tongues of sand gradually thrown up by the waves. +In these marshes, most of which once formed a part of the sea and +still mark its ancient outline, the water presents the most varied +proportions of saline admixture. These half dried-up bays are rarely +deep enough to allow of large vessels sailing in them, and their +banks are generally overrun by the most luxuriant vegetation. The +shore constantly keeps gaining upon them, and thus tends to the +increase of the mainland. + +The coasts which surround the Caribbean Sea and the Gulf of Mexico, +and also the Atlantic shores of North America from the point of +Florida to the mouth of the Chesapeake, are bordered by a very +large number of marine marshes, forming a continued series over +hundreds and thousands of miles in length. In this immense series +of coast-marshes all kinds of vegetation seem to flourish, and +threaten to get the better of the mud and water, and to convert +them into _terra firma_. To the south, upon the shores of Colombia +and Central America, the mangroves and other trees of like species +plunge the terminal points of their aerial roots deep into the mud, +crossing and recrossing in an arch-like form, and retaining all the +débris of plants and animals under the inextricable network of their +natural scaffoldings. The shores of the Gulf of Mexico, in Louisiana, +Georgia, and Florida, are bordered by cypress swamps, or forests of +cypress (_Cupressus disticha_); these strange trees, the roots of +which, entirely buried, throw out above the layer of water which +covers the soil multitudes of little cones, the business of which is +to absorb the air. For millions of acres nearly all the marshy belt +along the seashore is nothing but an immense cypress swamp, with +trees bare of leaves, and fluttering in the wind their long hair-like +fibres of moss. Here and there the trees and muddy soil give place to +bays, lakes, or quaking-meadows, formed by a carpet of grass lying +upon a soil of wet mud, or even upon the hidden water. In Brazil +these buoyant beds of vegetation are frequently met with, and the +significant name of _tremendal_ has been given to them: in Ireland +these are called “quaking-bogs.” The least movement of the traveler +who ventures upon them makes the soil tremble to some yards’ distance. + +To the north of Florida, in the Carolinas and Virginia, the belt of +cypress swamps continues; but in consequence of the change of climate +and vegetation, the quaking-meadows are gradually converted into +peat-mosses. The surface of the marsh is incessantly renewed by a +carpet of green vegetation, while below, the dead plants, deprived +of air, carbonize slowly in the moisture which surrounds them: these +are the beds of peat which form upon the ground just as the layers of +coal were formed in previous geological epochs. + +On the southern side, the first great peat-bog of a well-defined +character is the “Dismal Swamp,” which extends along the frontiers +of North Carolina and Virginia. This spongy mass of vegetation rises +ten feet above the surrounding land. In the centre, and, so to speak, +upon the summit of the marsh, lies Lake Drummond, the clear water +of which is colored reddish-brown by the tannin of the plants. A +canal, which crosses the Dismal Swamp to connect it with the adjacent +streams, is obliged to make its way along the marsh by means of +locks. To the north of Virginia peat-bogs proper become more and more +numerous; and in Canada, Labrador, etc., they cover vast expanses +of country. All the interior of the island of Newfoundland, inside +the inclosure formed by the forests on the shore, is nothing but +a labyrinth--a great part of which is still unknown--of lakes and +peat-bogs; even on the sides of the hills there are marshes on so +steep an incline that the water from them would disappear and run off +in a stream if it was not stopped by the thick carpet of plants which +it saturates. Many a large peat-bog which may be crossed dry-shod +contains more water than many lakes filling a hollow of the valley +with deep water. + +Opposite Newfoundland, on the other side of the Atlantic, Ireland +is hardly less remarkable for the enormous development of its +peat-mosses or bogs. These tracts of saturated vegetation, in +which _Sphagnum palustre_ predominates, comprehend nearly two and +a half millions of acres--the seventh part of the whole island. +The inhabitants continue to extract from them, every year, immense +quantities of fuel. The spaces left by the spade in the vegetable +mass are gradually filled up again by new layers. After a certain +number of years, which vary according to the abundance of rain, the +depth of the bed of water, the force of vegetation, and the slope of +the soil, the turf “quarry” is formed anew. In Ireland it generally +takes about ten years to entirely fill up again the trenches, +measuring from nine to thirteen feet in depth, which are made in the +bogs on the plains, when a fresh digging of turf may be commenced. +In Holland, crops of this fuel may be gathered, on an average, every +thirty years. In other peat-moss districts the period of regeneration +last forty, fifty, and even a hundred years. In France, on the +borders of the Seugne (Charente-Inférieure), it has been ascertained +that ditches five feet deep and nearly seven feet wide are completely +obstructed by vegetation after the lapse of twenty years. As for the +beds of peat which carpet the sides of mountains, they take centuries +to form afresh. + +In Ireland, the Low Countries, the north of Germany and Russia, heaps +of trunks of former forest-trees--oaks, beech, alder, and other +trees--are frequently discovered, which by their decay have made way +for the peat-mosses. The _Sphagnum_, too, often takes possession of +ground of which man had previously made himself master, and in many +places roads, remains of buildings, and other vestiges of human labor +are found below the modern bed of vegetation by which they are now +covered. Certain peat-bogs in Denmark and Sweden may be considered, +on account of the curiosities which have been found in them, as +perfect natural museums, in which the relics of the civilization of +ancient nations have been preserved for the _savants_ of our own day. + +The air above the peat-mosses of Ireland and other countries in +the world is not often unhealthy, either because the heat is not +sufficient to develop miasma, or else because the vegetation, by +absorbing the water into its spongy mass, impedes the corruption of +the liquid, and produces a considerable quantity of oxygen. Further +south, the peat-mosses, which are intermixed with pools of stagnant +water, and especially marshes properly so-called, generate an impure +air, which spreads fever and death over the surrounding country. +Unless marshes are surrounded with dense forests, which arrest +the dispersion of the gases, the latter exercise a most injurious +influence on the general salubrity of the district; for during dry +weather, a vast area of the bed of the marshes becomes exposed, and +the heaps of organic débris lying on the bottom decompose in the heat +and infect the whole atmosphere. The average of life is much shorter +in all marshy countries than in the adjacent regions which are +invigorated by running water. In Brescia, Poland, in the marshes of +Tuscany, and in the Roman plains, the wan and livid complexion of the +inhabitants, their hollow eyes, and their feverish skin, announce at +first sight the vicinity of some centre of infection. There are some +marshes in the torrid zone where the decomposition of organic remains +goes on with a much greater rapidity than in temperate climates; no +one can venture on the edges of these districts without peril to his +life. As Frœbel ascertained in his journey across Central America, +the miasma is occasionally produced in such abundance that not only +can it be smelt, but a distinct impression of it is left upon the +palate. + + + + + LOWLAND PLAINS + --WILLIAM HUGHES + + +The plateaus and mountain regions of the globe occupy a large +portion of its surface--perhaps more than half of the whole extent +of the land--and their influence over its climate and other natural +conditions affecting mankind is very great. The highlands of the Old +World--fitted by their physical attributes to be the home of pastoral +and nomad races--were among the regions earliest occupied by mankind. +From the banks of the Euphrates and the primeval cities of the +Assyrian plain, the course of the shepherd-warrior--whether directed +to the east or the west--led toward some of the elevated regions +which stretched thence within the same (or nearly the same) degrees +of latitude, and which, at least in a general sense, are under like +conditions of climate. The highlands of Persia and Afghanistan, +in the one direction, of Syria and the Lesser Asia, in the other, +display abundant evidence, both in traditional and monumental +records, of their early occupation by man. From the one, the natural +order of advance leads to the fertile plains of India; from the +other, to the shores of the Mediterranean, whence is easy transit to +the peninsula and islands that lie beyond. + +But if the highlands of the earth were early the dwelling-place of +the shepherd-warrior, it was within the adjoining lowland plains and +fertile river basins that the arts of civilization were first called +into being, that towns were built, that population became numerous, +and that systems of social polity were developed. The lowland plains +of Asia and Europe constitute, in the present day, the most populous +regions of the globe, and include by far the more numerous portion +of the human race. The like regions in the New World are fast +filling with inhabitants, as the redundant population of older lands +is directed, in an ever-flowing stream, across the waters of the +Atlantic. + +The most important and extensive among the lowland plains of the Old +World are the following: + +IN ASIA.--Plain of the Euphrates and Tigris (the ancient Mesopotamia +and Babylonia); Plain of Hindustan, or Northern India; Plain +of China, embracing the northeast part of that country; Plain +of Siberia; Plain of Turkestan. Among lowland regions of less +importance are the plains of Pegu, Siam, and Tonquin, all within the +Indo-Chinese peninsula, or India beyond the Ganges. + +IN EUROPE.--The Great Eastern Plain, embracing nearly the whole of +Russia; Plain of Hungary, embracing the middle portion of the valley +of the Danube; Plain of Wallachia and Bulgaria, or the Lower Danube; +Plain of Lombardy, or Northern Italy; Plain of Languedoc, in the +south of France; Plain of Andalusia, in the south of Spain; Plain of +Bohemia, or basin of the Upper Elbe. + +The limits and direction of these regions may be traced upon any +ordinary map, by means of their coincidence with the great river +basins of the Eastern Hemisphere. They include the longer slopes +of the land, which are directed toward the north and northwest, as +well as the less extensive low grounds which border the Indian and +Pacific Oceans. The Siberian plain alone comprehends an area equal +to that of Europe, and the rivers by which it is watered are among +the most considerable in the Old World. So vast an area, under +other conditions of climate, might have become the home of populous +nations, the seat of civilization and empire. But its high latitudes, +which involve the rigor of an Arctic sky, condemn a large portion +of Siberia to the condition of a sterile wilderness, and must +prevent even its more favored districts from being other than thinly +inhabited. The dreary swamps and morasses of the _tundras_, which +replace, during the brief summer of those latitudes, the plain of ice +and snow, stretch along the shores of the Arctic Sea through a vast +extent of this widespread region. + +Conditions hardly more favorable belong to the extreme northern +portion of the great plain of Europe, the slope of which is directed +toward the White Sea and the Arctic basin. But a large portion of +Eastern Europe is inclined toward a southerly sky, and is watered by +rivers which have their outfall into the Black and Caspian Seas. The +Volga, the longest of European rivers, belongs to the Caspian basin, +the most depressed portion of the entire region. + +The southeastern division of the European lowland, and the adjacent +portions of Asia, constitute the region of the _steppes_. These +occupy an immense portion of the empire of Russia, and are among +the most characteristic of the physical features of the Old World. +The steppes are grassy plains--prairies, or meadows, they would be +called in the New World--which occupy a vast belt of the European +and Asiatic continents. They stretch eastward from the banks of +the Dnieper far into the heart of Asia--along the shores of the +Caspian and Aral Seas, and as far as the banks of the great river +Obi. Indeed, in so far as their grassy covering and general level +expanse--among the prime characteristics of the steppe-land--are +concerned, a like region may be said to extend to the eastward +through Central Asia, as far as the Great Wall of China and the +valley of the Amour. This is the “land of grass” of the Mongol +shepherd, the true home of the Tartar nations, whose descendants +yet preserve in their songs the memory of their famous leader +Timour--the Tamerlane of historic record. So vast is the extent of +this grass-covered region, that a mounted horseman, it has been said, +setting out from one of its extremities at the beginning of the year, +and traveling day and night at his utmost speed, would find the +season of spring elapse ere he reached its further limits. + +The southwestern portion only of the steppe-land falls within +the limits of Europe. This exhibits an unbroken expanse of level +plain--fatiguing to the eye from its perfect uniformity--dry and +burned up by excessive heat in summer, a pathless expanse of snow +during the opposite season of the year. The steppe is only productive +during the brief time that the thirsty soil is refreshed by the rains +of spring and early summer. Its aspect is then, for a time, glowing +and verdant; grass and wild flowers cover the earth with a carpet of +varied and attractive hues, and the wild cattle and horses luxuriate +in the abundant pasture. In the autumn, when the herbage has become +dry and withered, the steppe sometimes exhibits a vast sheet of +rolling flame, the grass being occasionally fired by accident, at +other times intentionally, for the sake of the young crop which +springs up through the ashes. The illusive phenomena of _mirage_--the +result of atmospheric refraction, engendered by the intense dryness +of the air--are of frequent occurrence in the steppe. Sometimes the +eye is cheated by the semblage of a lake, which vanishes on approach. +In other instances, the traveler over these wild regions appears to +see rising before him, and glittering through the dense mist which +often prevails during the hours of midday heat, the towers and other +buildings of a distant city. Spires, trees, bridges, rivers, all +appear in picturesque combination, only to sink into confusion as +they are approached. When the spot where the city of enchantment +had seemed to stand is actually reached, there is found only the +long, dry grass, waving as elsewhere in the surrounding waste. The +vast accumulation of dry sand on the surface gives rise to another +phenomenon, of frequent occurrence on the steppe, resembling +waterspouts upon the sea, excepting that the column is filled with +dust instead of water. “Suppose the great flat steppe stretched out +beneath the blue sky--nothing visible--no breath of air apparently +stirring--the whole plain an embodiment of sultriness, silence, and +calmness--when gradually rise in the distance six or eight columns of +dust, like inverted cones, two or three hundred feet high, gliding +and gliding along the plain in solemn company; they approach, they +pass, and vanish again in the distance, like huge genii on some +preternatural errand.” + +Such is the region over which the semi-nomad tribes of Tartar +shepherds, who constitute a fraction of the vast population of the +Russian Empire, pasture their herds. It is only here, within the +limits of Europe, that the camel is successfully reared. Odessa, the +great outport of southern Russia, stands almost on the edge of the +steppe, and the whirlwinds of dust that pass through its streets, and +constitute, during a portion of the year, one of its chief drawbacks +as a place of residence, furnish obvious evidence of this proximity. +The steppe includes two-thirds of the Crimean peninsula, the extreme +south of which, however, is traversed by a hill-range of considerable +elevation, and exhibits widely different features. + +Beyond the Dnieper, the Don, and even the Volga, the same region of +alternate grassy plain and sandy waste stretches far into the Asiatic +continent. To the east and north of the Caspian and the Aral are the +steppes over which roam the hordes of the Khirghiz. The names of +Kara-kum and Kizil-kum, given respectively to the sandy wastes which +extend upon either side of the river Syr, or Jaxartes, are strikingly +indicative of the general character of the tracts to which they are +applied. + +Mr. T. W. Atkinson, in his _Travels in Regions on the Upper and Lower +Amour_, thus describes the journey through these wild regions: “For +many miles the sand was hard, like a floor, over which we pushed on +at a rapid pace. After this we found it soft in places, and raised +into thousands of little mounds by the wind. Our horses were now +changed, and in an hour these mounds were passed, when we were again +on a good surface, riding hard.... Hour after hour went by, and our +steeds had been changed a second time.... In our route there was no +change visible--it was still the same plain; there was not so much +as a cloud floating in the air, that, by casting a shadow over the +steppe, could give a slight variation to the scene.... The whole +horizon was swept with my glass, but neither man, animal, nor bird +could be seen.... We rode on for several hours, but there was no +change of scene. One spot was so like another that we seemed to make +no progress.... No landmark was visible, no rock protruded through +the sterile soil; neither thorny shrub nor flowering plant appeared, +to indicate the approach to a habitable region; all around was +‘kizil-kum’ (_red sand_).” + +The perfect solitude and unbroken silence of the desert are not less +characteristic than its wearisome monotony of surface. No sound of +bird or animal breaks the solemn stillness which reigns around; +no trees expose their foliage to the influence of the wind. The +course of the traveler is still onward, through the same apparently +interminable waste. “Fourteen hours had passed, and still a desert +was before us. The sun was just sinking below the horizon. The +Kirghiz assured me that two hours more would take us to pastures and +to water.... It had now become quite dark, and the stars were shining +brilliantly in the deep blue vault. My guides altered their course, +going more to the south. On inquiring why they made this change, one +of them pointed to a star, intimating that by that they must direct +their course. + +“We traveled onward, sometimes glancing at the planets above, and +then anxiously scanning the gloom around, in the hope of discovering +the fire of some dwelling that would furnish food and water for our +animals. Having ridden on in this manner for many miles, one of our +men stopped suddenly, sprang from his horse, and discovered that we +had reached vegetation. The horses became more lively and increased +their speed, by which the Kirghiz knew that water was not far off. +In less than half an hour they plunged with us into a stream, and +eagerly began to quench their terrible thirst, after their long and +toilsome journey.” + +The features above described are those of the steppe region, regarded +as a whole. But this aspect undergoes considerable variation in +particular localities. The Lower Steppes, as those portions of +the great plains which immediately border the Caspian are termed, +exhibit a soil largely impregnated with saline particles, and +contain numerous salt-water lakes. Some of these lakes furnish a +large quantity of salt, derived by means of evaporation. This region +resembles in aspect the dried-up bed of a sea. The Caspian, upon +which it borders, occupies the lowest part of a depression below the +general level of the earth’s surface, its waters being 81 feet lower +than those of the Black Sea. The extent of the Caspian appears to be +gradually diminishing. + +The features of the steppe-land, however, are exceptional to the +general characteristics of the European plain, regarded as a +whole. Large portions of its middle and western divisions possess +a rich arable soil, and exhibit annually a waving sea of corn. +The geographical limits of the lowland region are marked, in the +direction of north and south, by the Black Sea and the Arctic Ocean. +The eastwardly portions of this vast level expanse stretch into the +heart of Asia. In the west it reaches the shores of the Baltic, and +is thence prolonged, with narrower dimensions, through northern +Germany and the low flats of Holland, until it subsides beneath the +water of the German Ocean. Throughout this vast extent, tertiary +and recent formations prevail, and the abundant clays, sands, and +gravels give their character to the surface-soil. The plain lying +to the south of the Baltic consists principally of sandy heaths, +and contains, toward the seashore, a vast number of small lakes or +_meers_. + +The low shores of Holland--conquered from the sea by the persevering +industry of the Dutch nation--furnish a conspicuous example of the +sand-hills, or _dunes_, which are often found on low sandy coasts, +and which owe their origin to the action of prevailing winds upon +the loose drift-sand. Where no means are adopted to fix them to +the soil, the sand-hills become agents of destruction, sometimes +overwhelming whole villages in their slow but steady advance inland. +But this is not the case in Holland, where the ingenuity of the Dutch +has converted them from instruments of destruction into a means of +national preservation. In some of the provinces of the Netherlands, +a large portion of the land is actually lower than the level of +high-water mark, and is therefore exposed (it might appear) to the +ravages of the adjoining ocean. But from the channel of the Helder +southward, the coast is protected by a line of broad dunes, or +sand-hills, which are partially covered with grass or heath, and +are in some places from forty to fifty feet in height. These have +been formed by the natural process above adverted to, and still in +operation; the prevalent sea-winds raise banks or ridges of sand +at a short distance from the coast, which the inhabitants prevent +from proceeding further inland by sowing them with a kind of grass +(_arundo arenaria_), the long roots of which bind the whole mass +firmly together. + +The district of the _Landes_, in the southwest corner of France, +offers an example of the combined action of sand and sea which is +widely different from the above in its results. The coast here +exhibits a line of shifting sand, backed toward the interior by a +belt of pine-forest. For a length of nearly two hundred miles, from +the mouth of the Garonne to that of the Adour, there stretches along +the extreme edge of the sea a range of hills composed of white sand, +as fine as though it had been sifted for an hour-glass. Every gale +changes the shape of these rolling masses of drift-sand. A strong +wind from the land flings millions of tons of sand per hour into +the sea, to be again washed up by the surf, flung upon the beach, +and with the first Biscay gale blown in whirlwinds inland. A water +hurricane from the west has been known to fill up with sand many +square miles of shallow lake, driving the displaced waters inland, +dispersing them among the pine-woods, flooding and frequently +destroying the scattered hamlets of the people, and burying forever +their fields of millet and rye. The shepherds of the Landes pursue +their avocation mounted upon stilts, which raise them above the +reach of the sand-blasts. The pine-forests yield annually a large +supply of resin, the only harvest of this wild region. Intermixed +with the pine-forests, a chain of shallow and marshy lakes stretches +in a direction parallel to the coast, and at a few miles inland. + +[Illustration: Great Barrier Coral Reef, Queensland, Australia + +This Reef is composed entirely of Stag’s Horn Coral (_Madrepora +Hebes_)] + +The lowland plains of the New World are on a scale of vast magnitude, +and, if not superior in extent to those of the Eastern Hemisphere, +yet bear a much larger proportion to the entire area of the land. +They are watered, moreover, by the longest rivers of the globe, and +enjoy, for the most part, conditions of situation and climate in the +highest degree favorable to man. Both in North and South America, the +whole central expanse of the continent exhibits a vast succession +of lowland plains, the only division between the different portions +of which is that formed by the watersheds of its longer rivers--not +always to be traced without difficulty, owing to the generally +level nature of the entire plain. In North America, the prairies; +in South America, the tracts known as llanos, selvas, and pampas, +are included within the lowland region, and exhibit some of the most +characteristic among the aspects of nature in the Western world. + +The prairies coincide, in a general sense, with the middle and upper +portion of the Mississippi Valley, embracing the vast region which +extends from the Great Lakes to the base of the Rocky Mountains. They +are covered in their natural state with a rich herbage, and exhibit a +waving sea of grass several feet high. At intervals, toward the banks +of the rivers, patches of forest vegetation break the uniformity +of the prospect, but the prairie itself is destitute of trees, and +(as the name implies) is merely a grassy plain, or meadow. Alternate +forest and prairie constitute the great features of natural scenery +in the New World. When the rich soil of the prairie-land is broken +up by the plow--an operation which is rapidly progressing, year by +year, within the Western States of America--it yields abundant crops +of corn. There are, however, within the vast extent of the North +American continent, immense regions which yet retain the aspect of +the wilderness. It was within these regions that the buffalo roamed, +in vast herds, and that the native Indian hunter pursued his game +ere the advancing footsteps of the white man had driven him from his +haunts. + +The llanos, or savannahs, are vast grassy plains, which occupy nearly +the whole basin of the Orinoco River, excepting only toward its +highest portion, when they are succeeded by wooded plains. The llanos +resemble in general features the prairies of the Mississippi Valley, +but have for the most part a lower level, and (owing to the abundant +rains of the torrid zone) are annually inundated by the rivers to +an immense extent. Whole districts, embracing thousands of square +miles, are annually converted, within the interior plains of South +America, into lakes, or temporary seas of fresh water, to be rapidly +evaporated under the burning rays of a vertical sun. At the close of +the rainy season the llanos are covered with grass, and form rich +natural pasture-grounds. During the prolonged season of drought which +ensues, the verdure is entirely destroyed, and the parched earth +opens in wide and deep crevices--again to be laid under water with +the recommencement of the rains. + +The selvas, or forest-plains, belong to the valley of the Amazon, and +include an immense area of Brazil, watered by the lower portion of +the great stream and its chief tributary, the Madera. Vast regions +are here covered by an uninterrupted forest, composed of trees of +giant growth, their boughs interlaced by immense creeping plants, and +the ground beneath thickly covered with a dense growth of underwood. +To the southward of the forest region are vast grassy plains, which +stretch in that direction into the valley of the Paraguay. + +The pampas, or plains of the Paraguay and Paraná valleys, exhibit the +same luxuriant natural growth of herbaceous plants as other lowland +regions of the New World. They include an immense region, which +stretches from the neighborhood of the southern tropic far to the +southward of the river Negro (lat. 39° S.), and from the banks of the +Paraná to the eastern base of the Andes. The pampas are variously +covered with long coarse grass, mixed with wild oats, clover, and +other herbage. The tract of country known by the name of El Gran +Chaco, immediately to the westward of the upper Paraguay--scarcely +tenanted excepting by wild beasts--exhibits a luxuriant covering of +grass, which springs from a soil possessed of the highest natural +capabilities. + +Further south, the plains that extend from Buenos Ayres to the foot +of the Andes are covered, during a great part of the year, with +gigantic thistles, which grow to the height of seven or eight feet, +and are so thick as to render the country almost impassable. For nine +months of the year the thistles are here the predominant (and almost +the sole) feature of the vegetable kingdom; but with the heats of +summer they are burned up, and their tall leafless stems are leveled +to the ground by the powerful blast of the pampero, or southwest +wind, which blows from the snowy ranges of the Andes, after which the +ground is covered for a brief season with herbage. This is destined, +with the returning spring, again to give place to the stronger +vegetation which it had succeeded, and for a time supplanted. + + + + + THE SMELL OF EARTH + --G. CLARKE NUTTALL + + +A bright fine evening after a day of rain is one of Nature’s +compensations. The air is peculiarly sweet and fresh, as though +the rain had washed all evil out of it. The mind, relieved from +the depressing influence of continuous rain, is exhilarated, and, +above all, the strong smell of the earth rises up with a scent more +pleasing than many a fragrant essence. In the town, indeed, this +earthy smell is often obscured by the bricks and mortar which cover +the land, and by the stronger, less wholesome, odors of human life, +but in the country it has full sway, and fills the whole air with +its presence. Even a slight shower, particularly after drought, is +sufficient to bring out the sweet familiar smell of the land and +thrust it upon our notice. + +The smell of freshly turned earth is often regarded by country lovers +as one of the panaceas for the ills of the flesh, and “follow a +plowshare and you will find health at its tail” has proved a sound +piece of advice to many a weakly town-sick one, over whose head the +threatenings of consumption hung like the sword of Damocles, though +it is possible that it is the fresh air, and more especially the +sunshine, which are the saving media, and not the mere smell. + +But what do we know about this characteristic smell of the soil? Can +we regard it as the mere attribute of the soil as a simple substance, +such an attribute as is, for instance, the peculiar smell of leather, +or the odor of india-rubber; or can we go deeper and find that it is +really an expression of complexity below? + +Strangely enough this is the case, for the smell of damp earth is one +of the latest sign-posts we have found which lead us into a world +which, until recently, was altogether beyond our ken. It points us to +the presence, in the ground beneath us, of large numbers of tiniest +organisms, and not merely to their presence only, but to their +activity and life, and reveals quite a new phase of this activity. +A handful of loose earth picked up in a field by the hedgerow, or +from a garden, no longer represents to us a mere conglomeration of +particles of inorganic mineral matter, “simply that and nothing +more”; we realize now that it is the home of myriads of the smallest +possible members of the great kingdom of plants, who are, in +particular, members of the fungus family in that kingdom, plants so +excessively minute that their very existence was undreamed of until a +few years ago. + +Some faint idea of their relative size, and of the numbers in which +they inhabit the earth, may be gleaned from the calculations of an +Italian, Signor A. Magiora, who, a short time ago, made a study of +the question. He took samples of earth from different places round +about Turin and examined them carefully. In ordinary cultivated +agricultural soil he found there would be eleven millions of these +germs in the small quantity of a gramme, a quantity whose smallness +will be appreciated when it is remembered that a thousand grammes +only make up about two and a quarter pounds of our English measure. +Thus, a shovelful of earth would be the home of a thousand times +eleven millions of bacteria--but the finite mind can not grasp +numbers of such magnitude. In soil taken from the street, and, +therefore, presumably more infected with germs, he calculated that +there was the incredible number of seventy-eight million bacteria to +the gramme. Sandy soil is comparatively free from them, only about +one thousand being discovered in the same amount taken from sandy +dunes outside Turin. + +But though the workers were hidden yet their works were known, for +what they do is out of all proportion to what they are; in fact they +perform the deeds of giants, not those of veriest dwarfs. “By their +works shall ye know them” might be a fitting aphorism to describe +the bacteria of the soil. And the nature of their deeds is widely +various, for though the different groups are members of one great +family, yet, like the individuals of a human family that is well +organized, they have each of them their special vocation. In the +spring time, when the sun warms the chilly earth, they act upon the +husks that have protected the seeds against the rigors of the winter, +and crumble them up so that the seedling is free to grow; they +break down the stony wall of the cherry and plum which has hitherto +imprisoned the embryo; and then, when the young plant starts, they +attach themselves to its roots, assist it to take in all sorts of +nutriment from air and soil, and thus help it in its fight through +life, and when its course has run they decently bury it. They turn +the green leaves and the woody stem and the dark root back into the +very elements from which they were built up; they effect its decay +and putrefaction, and resolve it into earth again. “Dust to dust, +ashes to ashes,” is the great life work of the earth bacteria. + +But up to about 1898 the fresh smell of the earth, the smell peculiar +to it, had not been in any way associated with these energetic +organisms, and it was quite a new revelation to find that it was +a direct outcome of their activity. Among the many bacteria which +inhabit the soil, a new one, hitherto unknown, has been isolated and +watched. It lives, as is usual with them, massed into colonies, which +have a chalky-white appearance, and as it develops and increases in +numbers it manifests itself by the familiar smell of damp earth, +hence the name that has been given it--_Cladothrix odorifera_. +Taken singly, it is a colorless thread-like body, which increases +numerically by continuous subdivisions into two in the direction of +its length. It derives its nutriment from substances in the soil, +which either are, or have been, touched by the subtle influence of +life, and in the processes of growth and development it evolves from +these materials a compound whose volatilizing gives the odor in +question. This compound has not yet been fully examined; it is not +named, nor have all its properties been satisfactorily elucidated, +but two facts concerning it stand out clearly. One is that it is the +true origin of the smell that we have hitherto attributed to earth +simply; and the other, that it changes into vapor under the same +conditions as water does. Therefore, when the sun, shining after the +rain, draws up the water from the earth in vapor form, it draws up, +too, the odorous atoms of this newly found compound, and these atoms, +floating in the air, strike on our olfactory nerves, and it is then +we exclaim so often, “How fresh the earth smells after the rain!” + +Though moisture, to a certain extent, is a necessary condition of the +active work of these bacteria, yet the chief reason why the earthy +smell should be specially noticeable after the rain is probably +because this compound has been accumulating in the soil during the +wet period. We only smell substances when they are in vapor form, +and since the compound under consideration has precisely the same +properties in this respect as water, it will only assume gaseous form +when the rain ceases. The bacteria have, however, been hard at work +all the time, and when the sun shines and “drying” begins, then the +accumulated stores commence their transformation into vapor, and the +strong smell strikes upon our senses. For the same reason we notice a +similar sort of smell, though in a lesser degree, from freshly turned +earth. This is more moist than the earth at the surface, and hence, +on exposing it, evaporation immediately begins which quickly makes +itself known to us through our olfactory nerves. + +It may also have been remarked that this particular odor is always +stronger after a warm day than after a cold one, and is much more +noticeable in summer than in winter. This is because moderate warmth +is highly conducive to the greater increase of these organisms, and, +in fact, in the summer they are present in far larger numbers and +exhibit greater vitality than in the winter, when they are often more +or less quiescent. + +Two other characteristics of _Cladothrix odorifera_ are worthy of +notice as showing the tenacity with which it clings to life. It is +capable of withstanding extremely long periods of drought without +injury; its development may be completely arrested (for water in +some degree is a necessity with all living things, from highest to +lowest), but its vitality remains latent, and with the advent of +water comes back renewed activity. But besides drought it is pretty +well proof against poisons. It can even withstand a fairly large +dose of that most harmful poison to the vegetable world, corrosive +sublimate. Hence any noxious matter introduced into the soil would +harm it little ultimately; the utmost it could do would be to retard +it for a time. + +This, then, is the history of the smell of earth as scientists have +declared it unto us, and its recital serves to further point the +moral that the most obvious, the most commonplace things of everyday +life--things that we have always taken simply for granted without +question or interest--may yet have a story hidden beneath them. +Like sign-posts in a foreign land, they may be speaking, though +in a language not always comprehended by us, of most fascinating +regions--regions we may altogether miss to our great loss if we +neglect ignorantly the directions instead of learning to comprehend +them. + + + + + DESERTS + --ÉLISÉE RECLUS + + +The most important group of deserts in the world is that of the +Sahara, which extends across the African continent from the shores +of the Atlantic to the valley of the Nile. This immense area is more +than 3,100 miles from east to west, and is, on an average, more than +600 miles in breadth; it is, in fact, equal in size to two-thirds +of Europe. In this region there is only one season, viz., summer, +burning and merciless. It is but rarely that rain comes to refresh +these regions, on which the solar rays dart vertically down. + +The mean altitude of the Sahara is estimated at 2,000 feet; but the +level of the soil varies singularly in the different districts. To +the south of Algeria, the surface of the Chott Mel-R’ir, the remains +of an ancient sea, which communicated with the Mediterranean, is at +the present time more than 165 feet below the Gulf of Cabes; while to +the south and east, the ground rises into plateaus and mountains of +sandstone or granite to a height varying from 3,300 to 6,660 feet. In +the centre of the Sahara stands the Djebel-Hogger, the sides of which +are covered with snow during three months in the year; from December +to March, its picturesque defiles are traversed by streams which flow +some distance and lose themselves beneath the surrounding plains. +This group of lofty mountains is the great landmark which forms the +boundary between the eastern deserts, or the Sahara proper, and the +group of western deserts, designated under the general name of Sahel. + +The Sahel is very sandy. Throughout the greater part of its extent +the soil is composed of gravel and large-grained sand, which does +not give way even under the foot of the camel. Some of the ranges +of sand-hills which rise in this desert are chains of small hills, +composed of heavy sand which resists the influence of the wind. But +in many districts of the Sahel, the arenaceous particles of the +soil are fine and small. The trade-winds which pass over the desert +distribute these sandy masses into long waves similar to those of +the ocean, and here and there raise them into movable sand-hills, +which overwhelm all the oases which lie across their path. Traveling +toward the southwest, in which direction they are driven by the wind, +the sands reach the northern shores of the Niger and Senegal at many +points of their course, and by their incessant deposits gradually +drive the waters of these rivers toward the south. To the west, the +sand of the desert encroaches also upon the ocean. Off the coast +which stretches between Cape Bojador and Cape Blanco--pointed out +from afar by the highest dunes in the world--a line of sand-banks +extends far out into the sea. A current of sand is, therefore, +constantly passing across the desert from northeast to southwest. +The débris of rocks in a state of decomposition, and the particles +brought to the coast of the Gulf of Cabes by the tide, which is very +powerful at this point, are driven before the wind into the plains of +the Sahel, and thence, after a journey lasting hundreds and perhaps +thousands of years, they at last reach the seashore of the Atlantic, +in order to recommence in the oceanic currents another eventful +odyssey. + +Some parts of the eastern Sahara are equally sandy; but the principal +parts of the surface of this desert are occupied by plateaus of rock +or clay, and by groups of grayish or yellowish mountains. The chains +of sand-hills are numerous, and, like those of the west, they travel +incessantly under the impulse of the wind in a south or southwest +direction. The rocky plateaus are crossed and recrossed here and +there by wide and deep clefts, which are gradually filled by the +drifted sand, and into which the traveler runs the risk of sinking, +like the mountaineer into the _crevasses_ of a glacier. In the +hollows, patches of salt take the place of the lakes which in more +rainy countries would be found there. + +Those districts of the Sahara which are destitute of oases present +a truly formidable aspect, and are fearful places to travel over. +The path which the feet of the camels have marked out in the immense +solitude points in a straight line toward the spot which the caravan +wishes to reach. Sometimes these faint footmarks are again covered +with sand, and the travelers are obliged to consult the compass, or +examine the horizon; a distant sand-hill, a bush, a heap of camels’ +bones, or some other indications which the practiced eye of the +Touareg alone can understand, are the means by which the road is +recognized. + +Terrible stories are told by the side of the watch-fires of caravans +being overtaken when amid the sand-hills by a sudden storm of wind, +and completely buried under the moving masses; they also tell of +whole companies losing their way in the deserts of sand or rocks, +and dying of madness after having undergone all the direst tortures +of heat and thirst. Happily such adventures are rare, even if the +accounts of them are at all authentic. Caravans, when led by an +experienced guide and protected by treaties and tribute against the +attacks of plundering Arabs and Berbers, nearly always arrive at the +end of their journey without having undergone any other sufferings +than those caused by the intolerable heat, the want of good water, +and the coldness of the nights; for the nights which follow the +burning days in the Sahara are in general very cold. In fact, the air +of these countries being entirely destitute of aqueous vapor, the +heat collected during the day on the surface of the desert is, owing +to the nocturnal radiation, again lost in space. The sensation of +cold produced by this waste of heat is most acute, and especially +so to the chilly Arab. Not a year passes without ice forming on the +ground, and white frosts are frequent. + +In all those countries in the Sahara where the water gushes out in +springs or descends in streams from some group of mountains, there is +an oasis formed--a little green island, the beauty of which contrasts +most strikingly with the barrenness of the surrounding sands. These +oases, compared by Strabo to the spots dotted over the skin of +the panther, are very numerous, and perhaps comprehend altogether +an area equal in extent to one-third of the whole Sahara. In the +greater part of this region, the oases, far from being scattered +about irregularly, are, on the contrary, arranged in long lines in +the middle of the desert. The cause of this is either the higher +proportion of moisture contained in the aerial currents which pass in +this direction, or, and perhaps principally, the subterranean water +which follows this slope, and here and there rises to the surface. + +The oases are, _par excellence_, the country of date-trees; in +the neighborhood of Mourzouk there are no less than thirty-seven +varieties. These trees form the riches of the tribe, for their fruit +supplies food to man as well as to beast--to dromedaries, horses, and +dogs. Below the wide fan of leaves, which quiver in the blue air, are +thickly growing clumps of apricot, peach, pomegranate, and orange +trees, their branches loaded with fruit, and vines intertwining round +the trunks; maize, wheat, and barley ripen under the shade of this +forest of fruit-trees, and, lower still, the modest trefoil fills +up the very smallest intervals of the soil which is capable of +irrigation. + +To the east of Egypt, which may be considered as a long oasis +situated on the banks of the Nile, the desert begins again, and +borders the whole extent of the Red Sea. A large part of Arabia +presents nothing but sands and rocks, and toward the southeast, in +the Dahna, there are solitudes which no traveler, either Arab or +Frank, seems yet to have crossed. To the north and east stretch +the _Nefouds_, or “daughters of the great desert,” which are much +smaller than the Dahna, but are nevertheless formidable tracts to +travel over. One of these regions, which was crossed by Palgrave, +is that in which the mass of sand, formerly deposited there by the +marine currents, affords the greatest depth; in certain places it is +330, 400, and even 500 feet deep. It can be measured by the eye by +descending to the bottom of the funnel-shaped cavities, which the +springs of water, spouting out of the adjacent granite or calcareous +rock, have gradually hollowed out in the bed of sand. This enormous +bed of material, which represents chains of pulverized mountains, +does not exhibit an even surface, as one would expect, but, +throughout its whole expanse, presents long symmetrical undulations, +similar to those waves which roll in the Caribbean Sea under the +even influence of the trade-winds. These waves stretch from north +to south, parallel to the meridian; it is probable that they are +owing to the movement of the earth round its axis. The solid rocks +beneath unresistingly obey the impelling force which carries them +toward the east, but the movable sands which are above them do not +allow themselves to be carried away with an equal rapidity; each day +an infinitesimal quantity remains behind and seems to glide toward +the west, like the waves of the ocean, the atmospheric currents, and +everything that is movable on the face of the globe. The parallel +furrows of sand in the Nefouds certainly rise to a greater height +than those of the other deserts, and differ much in their aspect from +the smaller waves of sand formed by the wind; but the reason is, that +the bed of sand in this region is of a very great bulk, and because +at this point the swiftness of the globe nearly attains its maximum +on account of its vicinity to the equator. + +To the east of the Arabian peninsula, the chain of deserts is +prolonged obliquely across Asia. The principal part of the plateau +of Iran, occupying a quadrilateral space, surrounded by mountains +which stop the rains in their passage, consists of sterile solitudes, +some covered with saline beds, the remains of dried-up lakes, +others spread over with shifting sands, which the wind blows up +into eddies, or dotted over with reddish-colored hills, which the +mirage renders either nearer or more distant to the eye than they +really are, incessantly modifying them according to the undulations +of the atmosphere. This plateau is only separated from the steppes +of Turkestan by the Elburz Mountains, and is continued toward the +east by the deserts of Afghanistan and Beloochistan, which are not +so large, and much easier to travel over. Even the rich peninsula of +India is protected by a belt of sterile tracts situated on the right +and left of the Indus. Between each of the five rivers (Punjaub), +which, by the union of their waters, form the great river, stretches +a line of steppes in which the torrent-waters of the mountains are +soon lost. The soil of these steppes is nearly everywhere barren, +except on the edge of the irrigation canals constructed by the +inhabitants at a very heavy outlay. + +Beyond the mighty central group, whence radiate far and wide +the mountain-chains of Asia, the steppes and deserts, mutually +alternating according to the topographical conditions, and the +abundance or scarcity of water, extend over a space of more than +1,850 miles between Siberia and China Proper. The eastern part +of this belt is called, according to the languages, Gobi or +Chamo, that is to say, the desert _par excellence_, and, from +its enormous dimensions, corresponds with the Sahara of Africa, +situated exactly at the opposite extremity of the long chain of +solitudes which stretches right across the Old World. The mirage, +the moving sand-hills blown up into eddies, and many other phenomena +described by African travelers, are found in certain districts of +the Gobi, just the same as in all other deserts. But the cold here +is exceptionally intense, on account of the great height of the +plateaus, which is on an average 4,950 feet, and the vicinity of the +plains of Siberia, which are crossed by the polar wind. It freezes +nearly every night, and often during the day. The dryness of the +atmosphere is extreme; there is hardly any vegetation, and a few +grassy hollows are the only oases of these regions. From Kiahkta to +Pekin, there are only five trees for a distance of 400 to 500 miles, +which is the width of the desert in this part of Mongolia. The Gobi, +however, like the Sahara, was formerly covered by the waters of the +ocean; even on the elevated plateaus, old cliffs may be noticed, the +bases of which are worn away by the waves, and long strands of round +shingle stretch around the area which was formerly occupied by a now +vanished gulf. + +In North, as in South America, the deserts proper lie to the west of +the continent, and occupy the basins commanded by the parallel or +divergent walls of the Rocky Mountains. + +The most northerly of these American deserts occupies, to the west +of Lake Utah, a part of the space called the “Great Basin,” and is +comprised between the principal chain of the Rocky Mountains and +the Sierra Nevada of California. The desert of Utah is an immense +surface of clay, dotted over with thin tufts of artemisia; in certain +places, however, it exhibits no trace of vegetation, and resembles +a causeway of concrete, intersected by innumerable clefts, forming +nearly regular polygons. In the midst of these solitudes no rivulet +flows, and no water-spring gushes forth; only after journeying for +many a long hour the traveler sometimes comes upon some field of +crystallized salt, a white expanse, on which the clouds and blue sky +are reflected as on the surface of a lake. On the extreme horizon +some volcanic rocks may be seen, like great scoriæ, half veiled by +warm atmospheric columns, quivering like the air over the flame of a +hot brazier. Across these vast plains, inhabited only by a prodigious +quantity of extraordinarily shaped lizards, the road employed by the +emigrants used to pass, which was so soon destined to be supplanted +by the Pacific Railway from New York to San Francisco. + +The deserts of North America, crossed here and there by fertile +valleys, extend eastward toward the basins of the Red River and the +Arkansas, where they blend with the savannas, and to the south into +the Mexican states of Chihuahua, Sonora, and Sinaloa. But in the +tropical zone, which commences beyond these points, the heavy summer +rains and the much smaller extent of the Mexican territory between +the two oceans, have prevented the formation of deserts. Regions +destitute of trees and verdure are only again found on the coasts of +Peru, to the south of the Gulf of Guayaquil. The trade-winds, after +having discharged their moisture on the eastern slopes of the Andes, +pass away through the air far above the seashore on the western side +of the mountains, and then sweep far out to sea over the surface of +the Pacific. + +The solitudes of the Andes most resembling the desert regions of +the Old World and of the United States are the elongated plateaus +which rise one above another between the sea and the principal chain +of the Andes, in southern Peru and on the frontiers of Bolivia and +Chili; such as the _pampas_ of Islay and Tamarugal and the desert of +Atacama. The _pampa_ of Tamarugal, so called from the _Tamarugos_, +or tamarisks, which grow in the hollows where some moisture oozes +out of the soil, has a mean altitude of from 2,900 to 3,900 feet. It +is a plain nearly covered with beds of salt, or _salares_, which are +worked like rock quarries. The strata of salt are so thick, and rain +is so rare upon the plateau, that the houses of the village of Noria, +which are inhabited by the workmen, are entirely constructed of +blocks of salt. Some deserts, situated to the east of the Tamarugal, +on more elevated plateaus, contain a still larger quantity of salt. +The _pampa_ of Sal, which is overlooked by the volcano of Isluga, has +a mean altitude of not less than 13,800 feet, and its whole extent, +which is 125 miles long and from nine to twenty-four miles wide, is +perfectly white. The depth of salt deposited upon this plateau varies +from five to sixteen inches, according to the undulations of the +ground. + +Whence do these enormous masses of salt proceed? Doubtless from the +sea or ancient lakes which formerly covered these countries and have +been gradually emptied by the rising of the soil. Saline matter +saturates even the rocks and clays, for a film of salt again forms +by efflorescence on all the ground in the desert from which crops +have previously been taken. The district of Santa-Rosa, which was +completely cleared of salt in 1827, was all white again and fit for +working after a lapse of twenty-three years. Sea-salt is not the +only production of these immense natural laboratories; but nitrates, +sulphates, carbonate of soda, borates of soda and lime, are also +found there and increase every year in thickness, thanks to the +ephemeral torrents which sometimes descend loaded with débris from +the adjacent Cordilleras. Saltpetre is also procured from the _pampa_ +of Tamarugal, and is the article which, during all the wars of Europe +and America, gave such great commercial importance to the town of +Iquique. + +The desert of Atacama, the largest of all those in South America, +occupies a wide belt of plateaus between the shores of the Pacific +and the high rampart of the Andes, which separates Bolivia from +the Argentine Republic. This expanse of reddish-colored rocks, and +crescent-shaped shifting sand-hills, is so repulsively desolate a +place that the conquerors of Chili, whether Incas or Spaniards, +never made up their minds to venture into it, in going along the +sea-coast; they have been obliged to pass far into the interior, by +the plateaus of Bolivia, and to twice cross the Andes before entering +the Chilian valleys. Not long since, men of science were the only +travelers who dared to enter the desert of Atacama. Nevertheless +this formidable-looking country also possesses, like the _pampa_ of +Tamarugal, great natural riches, which will not fail to summon the +labor of man and all the progress of civilization to these desolate +regions. Besides salt and saltpetre, this desert produces guano--that +is, heaps of the almost exhaustless droppings of all the sea-birds +which settle down in clouds upon the seashore. During the course of +centuries the ordure has accumulated into perfect rocks which the sun +dries up, and the surface of which is but rarely softened by rain. +These masses of detritus, which are, to all appearance, useless upon +these barren shores, are life itself to the countries of England, +France, and Belgium, which have become exhausted by the extent of +cultivation; and, consequently, this substance constitutes a most +important element of national commerce. + + + + + II.--THE SEA + + + + + THE PRIMITIVE OCEAN + --G. HARTWIG + + +The greatest of all histories, traced in mighty characters by the +Almighty Himself, is that of the earth-rind. The leaves of this great +volume are the strata which have been successively deposited in the +bosom of the sea or raised by volcanic powers from the depths of +the earth; the wars which it relates are the Titanic conflicts of +two hostile elements, water and fire, each anxious to destroy the +formations of its opponent; and the historic documents which bear +witness to that ancient strife lie before us in the petrified or +carbonified remains of extinct forms of organic existence--the medals +of creation. + +It is only since yesterday that science has attempted to unriddle +the hieroglyphics in which the past history of our planet reveals +itself to man, and it stands to reason that in so difficult a study +truth must often be obscured by error; but although the geologist is +still a mere scholar, endeavoring to decipher the first chapters of a +voluminous work, yet even now the study of the physical revolutions +of our globe distinctly points out a period when the molten earth +wandered, a ball of liquid fire, through the desert realms of space. +In those times, so distant from ours that even the wildest flight +of imagination is unable to carry us over the intervening abyss, +the waters of the ocean were as yet mixed with the air, and formed +a thick and hazy atmosphere through which no radiant sunbeams, no +soft lunar light ever penetrated to the fiery billows of molten +rock, which at that time covered the whole surface of the earth. +What pictures of desolation rise before our fancy at the idea of yon +boundless ocean of fluid stone which rolled from pole to pole without +meeting on its wide way anything but itself. Ever and ever in the +dark-red clouds shone the reflection of that vast conflagration, +witnessed only by the eye of the Almighty, for organic life could +not exist on a globe which exclusively obeyed the physical and +chemical laws of inorganic nature. But while the fiery mass with +its surrounding atmosphere was circling through the icy region of +ethereal space (the temperature of which is computed to be lower than +60° R. below freezing point) it gradually cooled, and its hitherto +fluid surface began to harden to a solid crust. Who can tell how many +countless ages may have dropped one after the other into the abyss of +the past, ere thus much was accomplished; for the dense atmosphere +constantly threw back again upon the fiery earth-ball the heat +radiating from its surface, and the caloric of the vast body could +escape but very slowly into vacant space? + +Thus millions of years may have gone by before the aqueous vapors, +now no longer obstinately repelled by the cooling earth-rind, +condensed into rain, and, falling in showers, gave birth to an +incipient ocean. But it must not be supposed that the waters +obtained at once a tranquil and undisturbed possession of their new +domain, for, as soon as they descended upon the earth, those endless +elementary wars began, which, with various fortunes, have continued +to the present day. + +As soon as the cooling earth-rind began to harden, it naturally +contracted, like all solid bodies when no longer subject to the +influence of expanding heat, and thus in the thin crust enormous +fissures and rents were formed through which the fluid masses below +gushed forth, and, spreading in wide sheets over the surface, once +more converted into vapors the waters they met with in their fiery +path. + +But after all these revolutions and vicissitudes which opposed the +birth of ocean, perpetually destroying its perpetually renewed +formation, we come at last to a period when, in consequence of +the constantly decreasing temperature of the earth-rind and its +increasing thickness, the waters at last conquered a permanent abode +on its surface, and the oceanic empire was definitely founded. + +The scene has now changed; the sea of fire has disappeared, and water +covers the surface of the earth. The rind is still too thin and the +eruptions from below are still too fluid to form higher elevations +above the general surface: all is flat and even, and land nowhere +rises above the mirror of a boundless ocean. + +This new state of things still affords the same spectacle of dreary +uniformity and solitude in all its horrors. The temperature of +the waters is yet too high, and they contain too many extraneous +substances, too many noxious vapors arise from the clefts of the +earth-rind, the dense atmosphere is still too much impregnated with +poisons to allow the hidden germs of life anywhere to awaken. A +strange and awful primitive ocean rises and falls, rolls and rages, +but nowhere does it beat against a coast; no animal, no plant grows +and thrives in its bosom; no bird flies over its expanse. + +But, meanwhile, the hidden agency of Providence is unremittingly +active in preparing a new order of things. The earth-rind increases +in thickness, the crevices become narrower, and the fluid or +semi-fluid masses escaping through the clefts ascend to a more +considerable height. + +Thus the first islands are formed, and the first separation between +the dry land and the waters takes place. At the same time no less +remarkable changes occur, as well in the constitution of the waters +as in that of the atmosphere. The further the glowing internal heat +of the planet retires from the surface, the greater is the quantity +of water which precipitates itself upon it. The ocean, obliged to +relinquish part of its surface to the dry land, makes up for the loss +of extent by an increase of depth, and the clearer atmosphere allows +the enlivening sunbeam to gild here the crest of a wave, there a +naked rock. + +And now also life awakens in the seas, but how often has it changed +its forms, and how often has Neptune displaced his boundaries since +that primordial dawn? + +Alternately rising or subsiding, what was once the bottom of the +ocean now forms the mountain crest, and whole islands and continents +have been gradually worn away and whelmed beneath the waves of +the sea, to arise and to be whelmed again. In every part of the +world we are able to trace these repeated changes in the fossil +remains imbedded in the strata that have been successively deposited +in the sea, and then raised again above its level by volcanic +agencies, and thus, by a wonderful transposition, the history of the +primitive ocean is revealed to us by the tablets of the dry land. +The indefatigable zeal of the geologists has discovered no less +than thirty-nine distinct fossiliferous strata of different ages, +and as many of these are again subdivided into successive layers, +frequently of a thickness of several thousand feet, and each of them +characterized by its peculiar organic remains, we may form some idea +of the vast spaces of time required for their formation. + +The annals of the human race speak of the rise and downfall of +nations and dynasties, and stamp a couple of thousand years with +the mark of high antiquity; but each stratum or each leaf in the +records of our globe has witnessed the birth and the extinction of +numerous families, genera, and species of plants and animals, and +shows us organic Nature as changeable in time as she appears to us in +space. As, when we sail to the Southern Hemisphere, the stars of the +northern firmament gradually sink below the horizon, until finally +entirely new constellations blaze upon us from the nightly heavens; +thus in the organic vestiges of the Palæozoic seas we find no form of +life resembling those of the actual times, but every class + + “Seems to have undergone a change + Into something new and strange.” + +Then spiral-armed Brachiopods were the chief representatives of the +mollusks; then crinoid star-fishes paved the bottom of the ocean; +then the fishes, covered with large, thick rhomboidal scales, were +buckler-headed like the Cephalaspis, or furnished with wing-like +appendages like the Pterichthys; and then the Trilobites, a +crustacean tribe, thus named from its three-lobed skeleton, swarmed +in the shallow littoral waters where the lesser sea-fry afforded them +abundant food. From a comparison of their structures with recent +analogies, it is supposed that these strange creatures swam in an +inverted position close beneath the surface of the water, the belly +upward, and that they made use of their power of rolling themselves +into a ball as a defence against attacks from above. The remains of +seventeen families of Trilobites, including forty-five genera and 477 +species, some of the size of a pea, others two feet long, testify +the once flourishing condition of these remarkable crustaceans, yet +but few of their petrified remains, so numerous in the Silurian +and Devonian strata, are found in the carboniferous or mountain +limestone, and none whatever in formations of more recent date. + +Thus, long before the wind ever moaned through the dense fronds of +the tree ferns and calamites which once covered the swampy lowlands, +and long before that rich vegetation began to which we are indebted +for our inexhaustible coal-fields, now frequently buried thousands of +feet below the surface on which they originally grew, the Trilobites +belonged already to the things of the past. + +In the seas of the Mesozoic or medieval period, new forms of life +appear upon the scene. A remarkable change has taken place in the +cephalopods; for the chambered and straightened Orthoceratites +and many families of the order have passed away, and the spiral +Ammonites, branching out into numerous genera, and more than 600 +species, now flourish in the seas, so that in some places the +rocks seem, as it were, composed of them alone. Some are of small +dimensions, others upward of three feet in diameter. They are met +with in the Alps, and have been found in the Himalaya Mountains +at elevations of 16,000 feet, as eloquent witnesses of the vast +revolutions of which our earth has been the scene. Carnivorous, +and resembling in habits the _Nautili_, their small and feeble +representatives of the present day, their immense multiplication +proves how numerous must have been the mollusks, crustaceans, and +annelides, on which they fed, all like them widely different from +those of the present day. + +Then also flourished the Belemnites (Thunder-stones), supposed by +the ancients to be the thunder-bolts of Jove, but now known to +be the petrified internal bones of a race of voracious ten-armed +cuttle-fishes, whose importance in the Oolitic or cretaceous seas +may be judged by the frequency of their remains and the 120 species +that have been hitherto discovered. Belemnites two feet long have +been discovered, so that, to judge by analogies, the animals to +which they belonged as cuttle-bones must have measured eighteen to +twenty feet from end to end, a size which reduces the rapacious +Onychoteuthis of the present seas to dwarfish dimensions. But of all +the denizens of the Mesozoic seas, none were more formidable than +the gigantic Saurians, whose approach put even the voracious sharks +to flight. The first of these monsters that raises its frightful +head above the waters is the dreadful Ichthyosaurus, a creature +thirty or even fifty feet long, half fish, half lizard, and combining +in strange assemblage the snout of the porpoise, the teeth of the +crocodile, and the paddles of the whale. Singular above all is the +enormous eye, in size surpassing a man’s head. Woe to the fish +that meets its appalling glance! No rapidity of flight, no weapon, +be it sword or saw, avails, for the long-tailed, gigantic Saurian +darts like lightning through the water, and its dense harness bids +defiance to every attack. Not only have fifteen distinct species of +_Ichthyosauri_ been distinguished, but the remains of crushed and +partially digested fish-bones and scales which are found within their +skeleton indicate the precise nature of their food. Their fossil +remains abound along the whole extent of the Lias formation, from the +coast to Dorset, through Somerset and Leicestershire to the coast of +Yorkshire, but the largest specimens have been found in Franconia. +Along with this monster, another and still more singular deformity +makes its appearance, the Plesiosaurus, in which the fabulous +chimæras and hydras of antiquity seem to start into existence. Fancy +a crocodile twenty-seven feet long, with the fins of a whale, the +long and flexible neck of a swan, and a comparatively small head. +With the appearance of this new tyrant, the last hope of escape +is taken from the trembling fishes; for into the shallow waters +inaccessible to the more bulky Ichthyosaurus the slender Plesiosaurus +penetrates with ease. + +A race of such colossal powers seemed destined for an immortal reign, +for where was the visible enemy that could put an end to its tyranny? +But even the giant strength of the Saurians was obliged to succumb to +the still more formidable power of all-changing time, which slowly +but surely modified the circumstances under which they were called +into being, and gave birth to higher and more beautiful forms. + +In the Tertiary period, the dreadful reptiles of the Mesozoic seas +have long since vanished from the bosom of the ocean, and cetaceans, +walruses, and seals, unknown in the primitive deep, now wander +through the waters or bask on the sunny cliffs. With them begins a +new era in the life of the sea. Hitherto it has only brought forth +creatures of base and brutal instinct, but now the Divine spark of +parental affection begins to ennoble its more perfect inhabitants and +to point out the dim outlines of the spiritual world. + +During all these successive changes the surface of the earth has +gradually cooled to its present temperature, and many plants and +animals that formerly enjoyed the widest range must now rest +satisfied with narrower limits. The sea-animals of the North find +themselves forever severed from their brethren of the South by the +impassable zone of the tropical ocean; and all the fishes, mollusks +and zoophytes, whose organization requires a greater warmth, confine +themselves to the equatorial regions. + +As the Tertiary period advances toward the present epoch, the species +which flourished in its prime become extinct, like the numberless +races which preceded them; new modifications of life, more and more +similar to those of the present day, start into existence; and, +finally, creation appears with increasing beauty in her present rich +attire. + +Thus old Ocean, after having devoured so many of his children, has +transformed himself at last into our contemporaneous seas, with their +currents and floods, and the various animals and plants, growing and +thriving in their bosom. + +Who can tell when the last great revolutions of the earth-rind took +place, which, by the upheaving of mighty mountains or the disruption +of isthmuses, drew the present boundaries of land and sea? or who +can pierce the deep mystery which veils the future duration of the +existing phase of planetary life? + +So much is certain, that the ocean of the present day will be +transformed as the seas of the past have been, and that “all that +it inhabit” are doomed to perish like the long line of animal and +vegetable forms which preceded them. + +We know by too many signs that our earth is slowly but unceasingly +working out changes in her external form. Here lands are rising, +while other areas are gradually sinking, here the breakers +perpetually gnaw the cliffs and hollow out their sides, while in +other places alluvial deposits encroach upon the sea’s domain. + +However slowly these changes may be going on, they point to a +time when a new ocean will encircle new lands, and new animal and +vegetable forms arise within its bosom. Of what nature and how gifted +these races yet slumbering in the lap of time may be. He only knows +whose eye penetrates through all eternity; but we can not doubt that +they will be superior to the present denizens of the ocean. + +Hitherto the annals of the earth-rind have shown us uninterrupted +progress; why, then, should the future be ruled by different laws? +At first the sea only produces weeds, shells, crustacea; then the +fishes and reptiles appear; and the cetaceans close the vista. But is +this the last word, the last manifestation of oceanic life, or is it +not to be expected that the future seas will be peopled with beings +ranking as high above the whale or dolphin as these rank above the +giant Saurians of the past? + + + + + THE FLOOR OF THE OCEAN + --JOHN JAMES WILD + + +If we wish to form a perfect idea of the distribution of land and +water, we must consider not only the length and breadth of the areas +occupied, but also the height of the land and the depth of the water; +in other words, the volume of those portions of the solid crust +of the earth which are raised above the level of the sea, and the +volume of the masses of water which fill up the depressed portions +of the earth’s crust. We are thus led to regard the surface of the +solid crust of our planet as composed of heights and hollows, of +areas of elevation and areas of depression, and, as a next step, to +discriminate between these areas--not according to the usual standard +of the level of the sea, but according to their relative distance +from the centre of the earth. In this sense we may conceive an area +of elevation--_i. e._, a raised portion of the earth’s surface, +which may be partially or entirely covered with water, and an area +of depression--_i. e._, a hollow in the same surface, which may be +raised above the level of the sea, and from dry land or the basin of +an inland sea or lake. + +If we examine a chart of the world in the light which has been thrown +upon this question by all the reliable soundings obtained up to the +present, it will be found that continents and islands which we have +been in the habit of considering as separated from each other by +wide seas and deep straits virtually form part of the same area of +elevation; and, in a similar manner, that certain oceans and seas, +which we are accustomed to distinguish by separate names, form part +of the same area of depression. It will also appear that, with the +exception of the islands scattered over the face of the ocean and of +the Antarctic region, all the dry land at present existing may be +reduced to one large area of elevation gravitating to the North Pole, +as the common centre of the principal land masses; similarly, if we +except the Arctic region and other inland basins, all the oceans and +seas compose a single vast area of depression, with the South Pole +for common centre of the larger accumulations of water on this globe. +The Arctic region forms a distinct area of depression placed in the +centre of the great area of elevation, and the Antarctic region, +according to the evidence we at present possess, is an area of +elevation, surrounded on all sides by the above-described great area +of depression. The numerous small islands that crop up in the middle +of the oceanic basins are generally found associated in groups, and +they belong to areas of elevation at the present time submerged, that +is to say, in the condition in which we know the dry land to have +been at an epoch more or less remote in the history of our planet. In +support of the above generalization, we may point to the following +facts as established by recent soundings. The 100-fathom line, as is +well known, joins the whole of the British Islands, including the +Hebrides, Orkneys, and Shetland Islands, to the continent of Europe. +It forms a broad band connecting the Asiatic and American continents +across Behring Strait. It unites Australia, Papua, and Tasmania in +a single area of elevation, which, together with the intervening +archipelago of Java, Sumatra, Borneo, Celebes, the Moluccas, and the +Philippines, may be looked upon as a prolongation of the continent of +Asia. It joins Ceylon to Hindostan and the Falkland Islands to the +South American continent. The 500-fathom line connects North America, +Greenland, Iceland, the Faroe Islands, and the continent of Europe, +the only unexplored space being Denmark Strait, between Iceland +and Greenland, where the soundings may exceed the above depth. The +1,000-fathom line unites New Zealand with Australia, Madagascar with +Africa, and nearly exhausts the depth of the more or less landlocked +seas which lie between Australia and Asia, Africa and Europe, South +and North America, and of the seas situated within the Arctic and +Antarctic Circles. The Cape de Verde Islands and the Canaries belong +to Africa, Madeira to Europe, and less than 500 fathoms divide Norway +from Spitzbergen. + +Depths from 100 to 1,000 fathoms may be considered as shallow in +comparison with the prevailing depths from 2,000 to 3,000 fathoms +of the principal oceanic basins, and sufficient to establish a +connection between islands and continents, the more so as we +generally find one or more islands occupying the intervening space, +thus betraying the common link between them. + +The result of this examination is that all the larger land masses +compose an area of elevation which, after nearly completing the +circuit of the world in the latitude of the Arctic Circle, subdivides +itself into two parts, an eastern and a western one--the former +embracing Europe, Africa, Asia, and Australia, the latter North and +South America. In a similar manner, the different oceans combine +into an area of depression which, after making the circuit of the +world along the parallel of lat. 60° S. under the name of the +Southern Ocean, divides itself into three large basins, respectively +designated as the Pacific, the Atlantic, and the Indian Oceans. Thus +the two elements, land and water, starting from opposite hemispheres, +extend their arms across the equator, holding each other in close +embrace, like two champions wrestling for the mastery of the world. + +A comparison of the deep-sea soundings obtained up to the present +date shows that, if we omit the seas situated beyond the parallels of +lat. 60° N. and lat. 60° S.--no depths exceeding 2,000 fathoms having +as yet been ascertained beyond these latitudes--the average depth of +the ocean between these parallels may be estimated at about 2,500 +fathoms, or more roughly at three English miles, and the average +depth of all seas on the surface of the globe at probably two miles. + +Contrary to the ideas formerly entertained of the enormous depth of +the ocean, the soundings of H.M.S. _Challenger_, S.M.S. _Gazelle_, +and of the U.S.S. _Tuscarora_ and _Gettysburg_, indicate that depths +of five miles, or even 4,000 fathoms, are but seldom met with, and +are as exceptional as heights of the same amount on land. + +One of the greatest depths ascertained in the Atlantic was found by +H.M.S. _Challenger_, about eighty miles north of the island of St. +Thomas in the West Indies. It is 3,875 fathoms, or about four and a +half miles. In May, 1876, the _Gettysburg_ found 3,593 fathoms only +eleven miles south of the _Challenger_ sounding. A depth of 3,370 +fathoms obtained by the American ship shows that the deepest area in +the Atlantic is placed to the northward of the Virgin Islands, and +extends over 400 miles along the meridian of 65° W. + +The greatest depth observed in the Indian Ocean was discovered by the +_Gazelle_ in May, 1875. Two soundings of 3,020 and 3,010 fathoms were +taken in the eastern extremity of this ocean between the northwest +coast of Australia and the line of islands extending from Java to +Timor. + +The greatest of all depths of which we have reliable evidence +was found by the _Challenger_ on the 23d March, 1875, in the +comparatively narrow channel which separates the Caroline Islands +from the Mariana or Ladrone Islands. This sounding amounts to 4,575 +fathoms, or about five miles and a quarter. Several soundings +exceeding 4,000 fathoms were obtained by the _Tuscarora_ to the +eastward of the islands of Nippon and Yesso, and another close to +the most westerly of the Aleutian Islands. Two of these soundings +are over 4,600 fathoms, but as it appears that no sample of the +bottom was brought up, there is no evidence of the latter having +been reached. H.M.S. _Challenger_, shortly after her departure from +Yokohama, sounded 3,950 and 3,625 fathoms, and in doing so seems to +have just touched the southern border of this deep but narrow area of +depression, which runs parallel to the eastern coasts of Japan and +the Kurile Archipelago as far as the entrance to the Behring Sea. + +It will be observed that the above exceptional depths in the +Atlantic, Indian, and Pacific Oceans are not placed, as one might +be inclined to conjecture, in or near the centre of these oceanic +basins, but, on the contrary, upon their confines and in close +proximity to the land. This remarkable circumstance suggests the idea +that such areas of maximum depression may be the effect of a sinking +of the bottom of the sea in compensation for an upward movement of +the land in their immediate vicinity. + +Just as the results of the recent soundings have rendered the +existence of depths from six to nine miles, as formerly reported, +highly improbable, so have they modified our ideas of the shape +of the sea-bottom. The latter was generally represented as a +repetition of the dry land with its combination of mountain, valley, +and plain. No doubt the sea-bottom within a short distance of the +shore naturally forms a continuation of the leading features of the +adjoining land. Thus a large plain or a low-lying country will, as a +rule, continue its almost level slopes to a considerable distance out +to sea, while a range of hills or a chain of mountains often extends +its steep inclines below the surface of the water. + +The alteration of level in mid-ocean between two points as much as +a hundred miles apart is generally so slight that to an observer +standing at the bottom of the sea, the latter would appear a perfect +plain. Thus the bottom of our oceanic basins is composed of gentle +undulations rising and falling from a few fathoms to two or three +miles, in distances extending over many hundred miles. This view +accords with the experience of the geologist who finds that the bulk +of the dry land consists of sedimentary strata originally laid down +in a horizontal, or nearly horizontal, position at the bottom of the +sea, and there can be little doubt but that the depths of the ocean +are at the present time the scene of the formation of sedimentary +strata which some day may be converted into dry land, and contain +imbedded in their folds traces of the animal life with which they +abound. + +One of the most remarkable results in connection with the +exploration of the sea is the discovery of several extensive +submarine plateaus, which interrupt what was until lately supposed +to be an unbroken waste of fathomless abyss. One of these plateaus +traverses the Atlantic Ocean in its whole length from north to south, +repeating in its form the S-shaped contour of the eastern and western +shores of this ocean. After attaching itself by its northern end to +the plateau which connects Europe and Iceland, and separates the +Atlantic from the Arctic basin, it runs southward toward the Azores, +and, gradually contracting in width, sweeps round toward St. Paul’s +Rocks. Reduced, comparatively speaking, to a narrow ridge, it follows +the line of the equator as far as the meridian of Ascension Island, +where, resuming its southward course, it widens out until in lat. +30° S. it occupies nearly half the space between South America and +Africa, uniting the island of Ascension with St. Helena in the east, +Trinidad in the west, and the group of Tristan d’Acunha and Gough +Island at its southern end. + +Considerable portions of this plateau are within 1,500 fathoms, or a +mile and a half, of the surface of the sea, and most of the islands +are of volcanic origin. An extinct volcano, 8,300 feet in height, +forms the island of Tristan d’Acunha; Ascension Island rises to 2,800 +feet, and the summit of Pico in the Azores to 7,600 feet above the +level of the sea. The northern end of the plateau joins the plateau +of Iceland with its still active focus of eruption. + +By this central plateau, the Atlantic Ocean is divided into two +longitudinal areas of depression or channels, one following the +shores of North and South America, the other the shores of Europe +and Africa. The depths vary from 2,000 to nearly 4,000 fathoms, the +average depth being about three miles. The deepest portion of the +eastern channel is situated to the westward of the Cape de Verde +Islands, and forms an area of depression of over 3,000 fathoms. +In the western channel there are two such depressions, one placed +between the Antilles, Bermudas, and the Azores, the other between +Cape St. Roque, Ascension, and Trinidad. They are divided from each +other by a submarine elevation, which apparently connects the central +plateau with the South American continent. + +The soundings taken in the Indian Ocean prove the existence of a +submerged plateau on the limit between the Indian Ocean and the +Southern Ocean. It rises in many parts to within 1,500 fathoms of +the sea-surface, and forms the common foundation of all the islands +situated in this part of the world--viz., Prince Edward Island, the +Crozet Islands, the Kerguelen group, the Heard Islands, and the +islands of St. Paul and Amsterdam. The origin of all these islands is +probably volcanic. + +The main basin of the Indian Ocean with an average depth of over +2,000 fathoms, stretches from the meridian of the Cape of Good Hope +toward the angle between Java and northwestern Australia, where it +attains its greatest depths, forming a depression of over 3,000 +fathoms. It communicates with the Arabian Sea by two narrow channels +situated north and south of the Chagos Archipelago, being nearly +cut off from that sea by a line of islands and shallow soundings +which connect Africa, Madagascar, Bourbon, and Mauritius, the Chagos +Islands and the Maldive Islands with the Asiatic continent. The +2,500-fathom area of the Indian Ocean crosses the parallel of lat. +40° S. between St. Paul and Amsterdam Islands and Cape Leeuwin in +Australia, and forms, between the south coast of Australia and the +forty-fifth parallel, an area of depression which extends beyond the +southern end of Tasmania, includes the deepest portion of the basin +between New South Wales and New Zealand, and probably communicates +with the depths of the Pacific by a channel situated off the southern +extremity of New Zealand. + +If we divide the Pacific Ocean into an eastern and a western half +by a line passing from Honolulu to Tahiti, or by the meridian of +long. 150° W., we observe a remarkable contrast between the two +portions thus formed. While the eastern half, extending toward +America, presents a vast unbroken sheet of water, almost devoid of +islands, the western half, toward Asia and Australia, and inclosed +between the parallels of lat. 30° N. and lat. 30° S., is composed +of a labyrinth of seas, separated from each other by chains of +islands, the projecting points of numerous submarine ridges. Although +extensive tracts in the Pacific Ocean remain as yet untouched by +the sounding-line, the observations made by the _Challenger_, the +_Gazelle_, and the _Tuscarora_ enable us to form an idea of the +general contours of its bottom. From the shores of North and South +America, the depths of the eastern half of the Pacific gradually +increase until, upon the line between Honolulu and Tahiti, they +attain 3,000 fathoms. The latter depth forms extensive areas of +depression in the western half of this ocean, and increases to +4,000 fathoms in the already described hollow extending along the +Japanese and Kurile Islands toward the entrance of the Behring Sea. +Thus the idea formerly entertained of the inferior depths of the +Pacific in comparison with the Atlantic, founded apparently upon the +large number of islands scattered over its surface, is proved to be +erroneous. Many of these islands, especially in the northwestern +half, rise immediately from depths of 3,000 fathoms and more. + +In the southeastern portion of the Pacific there are indications +of a submerged plateau connecting the Society Islands, the Low +Archipelago, the Marqueses, and the intervening islands of Easter +Island and Juan Fernandez with the coast of Chili and Patagonia. +It seems as if an almost uninterrupted area of elevation crossed +the whole basin of the Pacific in a northwesterly direction from +Patagonia to Japan. The tendency of most of the submerged ridges of +this ocean to follow the same direction has been frequently commented +upon, and, as is the case with the submerged plateaus of the Indian +and Atlantic Oceans, their association with centres of volcanic +activity is equally evident. + +A line passing from Kamtchatka over Japan, the Ladrone, Caroline, +Marshall, Gilbert, Ellice, Samoa, Tonga, and Kermadec Islands to +New Zealand, divides the main basin of the Pacific, of an average +depth of 3,000 fathoms, from the much shallower seas lying to the +westward, and may possibly have formed the coast-line of a large +continent which existed at a remote epoch in the history of the +surface of our planet, and the boundaries of which have since been +driven back to the present confines of Asia and Australia. + +The Southern Ocean, which makes the circuit of the world along the +parallel of 60° S., in length equal to half the circumference of +the earth at the equator, may be considered as occupying the space +between the Antarctic Circle and the parallel of lat. 40° S. Owing to +the limited number of soundings as yet obtained within its limits, we +can only form a general idea of the distribution of its depths. + +The boundary-line of the fortieth parallel, which separates the +Southern Ocean from the Pacific, Atlantic, and Indian Oceans, is +occupied alternately by areas of depression, with depths ranging +from 2,500 to nearly 3,000 fathoms, and by areas of elevation, or +submarine plateaus, approaching to within 1,500 fathoms of the +sea-surface. With regard to the general distribution of depth in the +Southern Ocean, its bottom appears to rise gradually from nearly +3,000 fathoms at the fortieth parallel (with the exception of the +intervening plateaus) to little over 1,500 fathoms at the Antarctic +Circle. There are also indications of an area of depression of an +average depth of 2,000 fathoms, making the circuit of the globe +between the parallels of 50° and 60° lat. S. The whole surface of the +Southern Ocean is strewn with masses of floating ice, some of them +forming islands many miles in extent, and rising from 100 to 300 feet +above the level of the sea--an imposing spectacle, but fraught with +much danger to the navigator in these regions. It is this central +ocean which supplies the masses of cold water that fill up nearly +two-thirds of the total depth of the Atlantic, Pacific, and Indian +Oceans. + +We are indebted to Sir James Ross for the first soundings procured +within the Antarctic Circle. They are situated in the wide inlet, +discovered by that illustrious navigator in the year 1840, which +extends along the meridian of New Zealand, and terminates at the foot +of Mount Erebus and Mount Terror. These soundings, which are all +under 500 fathoms, viewed in combination with the above-mentioned +gradual rise of the bottom of the Southern Ocean toward the Antarctic +Circle, justify the assumption that the seas included within the +latter do not exceed 1,500 fathoms in depth, their average depth +probably falling below this estimate. The extensive formation of ice +in this region, as well as the numerous indications of land reported +by the daring sailors who have penetrated so far south, suggest the +hypothesis of the existence, if not of an Antarctic continent, at +all events of a considerable extent of land, rising in the mountain +ranges and volcanoes of Victoria Land to 10,000 and 15,000 feet above +the level of the sea. + +The region inclosed within the Arctic Circle forms an area of +depression, almost completely surrounded by the land-masses of the +great eastern and western continents. A shallow strait of less than +fifty fathoms in depth connects it with the Pacific Ocean, and it is +separated from the depths of the Atlantic by the plateau between the +British Islands and Iceland, which rises to within 500 fathoms of the +sea-surface. Greenland is probably the largest land-mass belonging to +this basin, and next in importance we have the group of Spitsbergen, +of Franz Joseph Land, discovered by the Austrian expedition; Nova +Zembla, the Liakhov Islands, Kellett Land, off Behring Strait, +discovered by the Americans in 1867; and finally the extensive +archipelago, a continuation of the American continent. + +The few soundings taken within the Arctic Circle leave much to +conjecture, but we are tolerably safe in stating that the average +depth of the Arctic basin is probably under 1,000 fathoms. The +immense plains of Northern Asia and America seem to continue beneath +the surface of the Arctic Sea, as indicated by the numerous islands +which skirt the coasts of these continents, and the greatest depths +to be found inside the Arctic Circle are probably confined to the +basin situated between Greenland and Norway, Iceland and Spitzbergen. + + + + + CORAL FORMATIONS + --CHARLES DARWIN + + +I will give a very brief account of the three great classes of +coral-reefs: namely, Atolls, Barrier, and Fringing-Reefs, and will +explain my views on their formation. Almost every voyager who has +crossed the Pacific has expressed his unbounded astonishment at the +lagoon islands, or as I shall for the future call them by their +Indian name of atolls, and has attempted some explanation. Even as +long ago as the year 1605, Pyrard de Laval well exclaimed, “C’est une +meruille de voir chacun de ces atollons, enuironné d’un grand banc de +pierre tout autour, n’y avant point d’artifice humain.” The immensity +of the ocean, the fury of the breakers, contrasted with the lowness +of the land and the smoothness of the bright green water within the +lagoon, can hardly be imagined without having been seen. + +The earlier voyagers fancied that the coral-building animals +instinctively built up their great circles to afford themselves +protection in the inner parts; but so far is this from the truth that +those massive kinds, to whose growth on the exposed outer shores the +very existence of the reef depends, can not live within the lagoon, +where other delicately branching kinds flourish. Moreover, on this +view, many species of distinct genera and families are supposed +to combine for one end; and of such a combination, not a single +instance can be found in the whole of nature. The theory that has +been most generally received is, that atolls are based on submarine +craters; but when we consider the form and size of some, the number, +proximity, and relative positions of others, this idea loses its +plausible character: thus, Suadiva atoll is 44 geographical miles +in diameter in one line, by 34 miles in another line; Rimsky is +54 by 20 miles across, and it has a strangely sinuous margin; Bow +atoll is 30 miles long and on an average only 6 in width; Menchicoff +atoll consists of three atolls united or tied together. This theory, +moreover, is totally inapplicable to the northern Maldive atoll in +the Indian Ocean (one of which is 88 miles in length, and between 10 +and 20 in breadth), for they are not bounded like ordinary atolls by +narrow reefs, but by a vast number of separate little atolls; other +little atolls rising out of the great central lagoon-like spaces. +A third and better theory was advanced by Chamisso, who thought +that from the corals growing more vigorously where exposed to the +open sea, as undoubtedly is the case, the outer edges would grow up +from the general foundation before any other part, and that this +would account for the ring or cup-shaped structure. But we shall +immediately see that in this, as well as in the crater theory, a most +important consideration has been overlooked; namely, on what have +the reef-building corals, which can not live at a great depth, based +their massive structures? + +Numerous soundings were carefully taken by Captain Fitz Roy on the +steep outside of Keeling atoll, and it was found that within ten +fathoms the prepared tallow at the bottom of the lead invariably came +up marked with the impressions of living corals, but as perfectly +clean as if it had been dropped on a carpet of turf; as the depth +increased, the impressions became less numerous, but the adhering +particles of sand more and more numerous, until at last it was +evident that the bottom consisted of a smooth sandy layer: to carry +on the analogy of the turf, the blades of grass grew thinner and +thinner, till at last the soil was so sterile that nothing sprang +from it. From these observations, confirmed by many others, it +may be safely inferred that the utmost depth at which corals can +construct reefs is between 20 and 30 fathoms. Now there are enormous +areas in the Pacific and Indian Oceans in which every single island +is of coral formation, and is raised only to that height to which +the waves can throw up fragments and the winds pile up sand. Thus +the Radack group of atolls is an irregular square, 520 miles long +and 240 broad; the Low Archipelago is elliptic-formed, 840 miles +in its longer, and 420 in its shorter axis: there are other small +groups and single low islands between these two archipelagoes, making +a linear space of ocean actually more than 4,000 miles in length, +in which not one single island rises above the specified height. +Again, in the Indian Ocean there is a space of ocean 1,500 miles in +length, including three archipelagoes, in which every island is low +and of coral formation. From the fact of the reef-building corals +not living at great depths, it is absolutely certain that throughout +these vast areas, wherever there is now an atoll, a foundation must +have originally existed within a depth of from 20 to 30 fathoms from +the surface. It is improbable in the highest degree that broad, +lofty, isolated, steep-sided banks of sediment, arranged in groups +and lines hundreds of leagues in length, could have been deposited in +the central and profoundest parts of the Pacific and Indian Oceans, +at an immense distance from any continent, and where the water is +perfectly limpid. It is equally improbable that the elevatory forces +should have uplifted, throughout the above vast areas, innumerable +great rocky banks within 20 to 30 fathoms, or 120 to 180 feet, of +the surface of the sea, and not one single point above that level; +for where on the whole face of the globe can we find a single chain +of mountains, even a few hundred miles in length, with their many +summits rising within a few feet of a given level, and not one +pinnacle above it? If then the foundations, whence the atoll-building +corals sprang, were not formed of sediment, and if they were not +lifted up to the required level, they must of necessity have subsided +into it; and this at once solves the difficulty. For as mountain +after mountain, and island after island, slowly sank beneath the +water, fresh bases would be successively afforded for the growth of +the corals. It is impossible here to enter into all the necessary +details, but I venture to defy any one to explain in any other manner +how it is possible that numerous islands should be distributed +throughout vast areas--all the islands being low--all being built of +corals. + +[Illustration: Matterhorn, Valais Alps, Switzerland] + +Before explaining how atoll-formed reefs acquire their peculiar +structure, we must turn to the second great class, namely, +Barrier-reefs. These either extend in straight lines in front of the +shores of a continent or of a large island, or they encircle smaller +islands; in both cases, being separated from the land by a broad +and rather deep channel of water, analogous to the lagoon within +an atoll. It is remarkable how little attention has been paid to +encircling barrier-reefs; yet they are truly wonderful structures. + +Encircling barrier-reefs are of all sizes, from three miles to +no less than forty-four miles in diameter; and that which fronts +one side, and encircles both ends, of New Caledonia, is 400 +miles long. Each reef includes one, two, or several rocky islands +of various heights; and in one instance, even as many as twelve +separate islands. The reef runs at a greater or less distance from +the included land; in the Society Archipelago generally from one +to three or four miles; but at Hogoleu the reef is 20 miles on the +southern side, and 14 miles on the opposite or northern side, from +the included islands. The depth within the lagoon-channel also varies +much; from 10 to 30 fathoms may be taken as an average; but at +Vanikoro there are spaces no less than 56 fathoms or 336 feet deep. +Internally the reef either slopes gently into the lagoon-channel, or +ends in a perpendicular wall, sometimes between two and three hundred +feet under water in height; externally the reef rises, like an atoll, +with extreme abruptness out of the profound depths of the ocean. +What can be more singular than these structures? We see an island, +which may be compared to a castle situated on the summit of a lofty +submarine mountain, protected by a great wall of coral-rock, always +steep externally and sometimes internally, with a broad level summit, +here and there breached by narrow gateways, through which the largest +ships can enter the wide and deep encircling moat. + +As far as the actual reef of coral is concerned, there is not the +smallest difference, in general size, outline, grouping, and even in +quite trifling details of structure, between a barrier and an atoll. +The geographer Balbi has well remarked that an encircled island is an +atoll with high land rising out of its lagoon; remove the land from +within, and a perfect atoll is left. + +But what has caused these reefs to spring up at such great distances +from the shores of the included islands? It can not be that the +corals will not grow close to the land; for the shores within the +lagoon-channel, when not surrounded by alluvial soil, are often +fringed by living reefs; and we shall presently see that there is a +whole class, which I have called fringing-reefs, from their close +attachment to the shores both of continents and of islands. Again, +on what have the reef-building corals, which can not live at great +depths, based their encircling structures? This is a great apparent +difficulty, analogous to that in the case of atolls, which has +generally been overlooked. + +Are we to suppose that each island is surrounded by a collar-like +submarine ledge of rock, or by a great bank of sediment, ending +abruptly where the reef ends? If the sea had formerly eaten deeply +into the islands, before they were protected by the reefs, thus +having left a shallow ledge round them under water, the present +shores would have been invariably bounded by great precipices; but +this is most rarely the case. Moreover, on this notion, it is not +possible to explain why the corals should have sprung up, like a +wall, from the extreme outer margin of the ledge, often leaving a +broad space of water within, too deep for the growth of corals. The +accumulation of a wide bank of sediment all round these islands, and +generally widest where the included islands are smallest, is highly +improbable, considering their exposed positions in the central and +deepest parts of the ocean. In the case of the barrier-reef of New +Caledonia, which extends for 150 miles beyond the northern point of +the island, in the same straight line with which it fronts the west +coast, it is hardly possible to believe that a bank of sediment could +thus have been straightly deposited in front of a lofty island, and +so far beyond its termination in the open sea. Finally, if we look +to other oceanic islands of about the same height and of similar +geological constitution, but not encircled by coral-reefs, we may +in vain search for so trifling a circumambient depth as 30 fathoms, +except quite near to their shores; for usually land that rises +abruptly out of water, as do most of the encircled and non-encircled +oceanic islands, plunges abruptly under it. On what then, I repeat, +are these barrier-reefs based? Why, with their wide and deep +moat-like channels, do they stand so far from the included land? We +shall soon see how easily these difficulties disappear. + +We come now to our third class of fringing-reefs, which will require +a very short notice. Where the land slopes abruptly under water, +these reefs are only a few yards in width, forming a mere ribbon or +fringe round the shores: where the land slopes gently under the water +the reef extends further, sometimes even as much as a mile from the +land; but in such cases the soundings outside the reef always show +that the submarine prolongation of the land is gently inclined. In +fact, the reefs extend only to that distance from the shore at which +a foundation within the requisite depth from 20 to 30 fathoms is +found. As far as the actual reef is concerned, there is no essential +difference between it and that forming a barrier or an atoll: it is, +however, generally of less width, and consequently few islets have +been formed on it. From the corals growing more vigorously on the +outside, and from the noxious effect of the sediment washed inward, +the outer edge of the reef is the highest part, and between it and +the land there is generally a shallow sandy channel a few feet in +depth. Where banks of sediment have accumulated near to the surface, +as in parts of the West Indies, they sometimes become fringed with +corals, and hence in some degree resemble lagoon-islands or atolls; +in the same manner as fringing-reefs, surrounding gently sloping +islands, in some degree resemble barrier-reefs. + +No theory on the formation of coral-reefs can be considered +satisfactory which does not include the three great classes. We +have seen that we are driven to believe in the subsidence of those +vast areas, interspersed with low islands, of which not one rises +above the height to which the wind and waves can throw up matter, +and yet are constructed by animals requiring a foundation, and that +foundation to lie at no great depth. Let us then take an island +surrounded by fringing-reefs, which offer no difficulty in their +structure; and let this island with its reef slowly subside. Now +as the island sinks down, either a few feet at a time or quite +insensibly, we may safely infer, from what is known of the conditions +favorable to the growth of coral, that the living masses, bathed by +the surf on the margin of the reef, will soon regain the surface. +The water, however, will encroach little by little on the shore, +the island becoming lower and smaller and the space between the +inner edge of the reef and the beach proportionally broader. Coral +islets are supposed to have been formed on the reef; and a ship +is anchored in the lagoon-channel. This channel will be more or +less deep, according to the rate of subsidence, to the amount of +sediment accumulated in it, and to the growth of the delicately +branched corals which can live there. We can now see why encircling +barrier-reefs stand so far from the shores which they front. We can +also perceive, that a line drawn perpendicularly down from the outer +edge of the new reef, to the foundation of solid rock beneath the +old fringing-reef, will exceed, by as many feet as there have been +feet of subsidence, that small limit of depth at which the effective +corals can live: the little architects having built up their great +wall-like mass, as the whole sank down, upon a basis formed of other +corals and their consolidated fragments. Thus the difficulty on this +head, which appeared so great, disappears. + +If, instead of an island, we had taken the shore of a continent +fringed with reefs, and had imagined it to have subsided, a great +straight barrier, like that of Australia or New Caledonia, separated +from the land by a wide and deep channel, would evidently have been +the result. + +As the barrier-reef slowly sinks down, the corals will go on +vigorously growing upward; but as the island sinks, the water will +gain inch by inch on the shore--the separate mountains first forming +separate islands within one great reef--and finally, the last and +highest pinnacle disappearing. The instant this takes place, a +perfect atoll is formed: I have said, remove the high land from +within an encircling barrier-reef, and an atoll is left, and the land +has been removed. + +We can now perceive how it comes that atolls, having sprung from +encircling barrier-reefs, resemble them in general size, form, in the +manner in which they are grouped together, and in their arrangement +in single or double lines; for they may be called rude outline charts +of the sunken islands over which they stand. We can further see how +it arises that the atolls in the Pacific and Indian Oceans extend in +lines parallel to the generally prevailing strike of the high islands +and great coast-lines of those oceans. I venture, therefore, to +affirm that, on the theory of the upward growth of the corals during +the sinking of the land, all the leading features in those wonderful +structures, the lagoon-islands or atolls, which have so long excited +the attention of voyagers, as well as in the no less wonderful +barrier-reefs, whether encircling small islands or stretching for +hundreds of miles along the shores of a continent, are simply +explained. + +It may be asked whether I can offer any direct evidence of the +subsidence of barrier-reefs or atolls; but it must be borne in mind +how difficult it must ever be to detect a movement the tendency of +which is to hide under water the part affected. Nevertheless, at +Keeling atoll I observed on all sides of the lagoon old cocoanut +trees undermined and falling; and in one place the foundation-posts +of a shed, which the inhabitants asserted had stood seven years +before just above high-water mark, but now was daily washed by +every tide: on inquiry I found that three earthquakes, one of them +severe, had been felt here during the last ten years. At Vanikoro +the lagoon-channel is remarkably deep, scarcely any alluvial soil +has accumulated at the foot of the lofty included mountains, and +remarkably few islets have been formed by the heaping of fragments +and sand on the wall-like barrier-reef; these facts, and some +analogous ones, led me to believe that this island must lately have +subsided and the reef grown upward: here again earthquakes are +frequent and very severe. In the Society Archipelago, on the other +hand, where the lagoon-channels are almost choked up, where much low +alluvial land has accumulated, and where in some cases long islets +have been formed on the barrier-reefs--facts all showing that the +islands have not very lately subsided--only feeble shocks are most +rarely felt. In these coral formations, where the land and water +seem struggling for mastery, it must be ever difficult to decide +between the effects of a change in the set of the tides and of a +slight subsidence: that many of these reefs and atolls are subject +to changes of some kind is certain; on some atolls the islets appear +to have increased greatly within a late period; on others they have +been partially or wholly washed away. The inhabitants of parts of the +Maldive Archipelago know the date of the first formation of some +islets; in other parts the corals are now flourishing on water-washed +reefs, where holes made for graves attest the former existence of +inhabited land. It is difficult to believe in frequent changes in the +tidal currents of an open ocean; whereas we have, in the earthquakes +recorded by the natives on some atolls and in the great fissures +observed on other atolls, plain evidence of changes and disturbances +in progress in the subterranean regions. + +Not only the grand features in the structure of barrier-reefs and +of atolls, and of their likeness to each other in form, size, and +other characters, are explained on the theory of subsidence--which +theory we are independently forced to admit in the very areas in +question, from the necessity of finding bases for the corals within +the requisite depth--but many details in structure and exceptional +cases can thus also be simply explained. I will give only a few +instances. In barrier-reefs it has long been remarked with surprise +that the passages through the reef exactly face valleys in the +included land, even in cases where the reef is separated from the +land by a lagoon-channel so wide and so much deeper than the actual +passage itself that it seems hardly possible that the very small +quantity of water or sediment brought down could injure the corals +on the reef. Now, every reef of the fringing class is breached by +a narrow gateway in front of the smallest rivulet, even if dry +during the greater part of the year, for the mud, sand, or gravel, +occasionally washed down, kills the corals on which it is deposited. +Consequently, when an island thus fringed subsides, though most of +the narrow gateways will probably become closed by the outward and +upward growth of the corals, yet any that are not closed (and some +must always be kept open by the sediment and impure water flowing out +of the lagoon-channel) will still continue to front exactly the upper +parts of those valleys at the mouths of which the original basal +fringing-reef was breached. + +We can easily see how an island fronted only on one side, or on +one side with one end or both ends encircled by barrier-reefs, +might after long-continued subsidence be converted either into a +single wall-like reef, or into an atoll with a great straight spur +projecting from it, or into two or three atolls tied together by +straight reefs--all of which exceptional cases actually occur. As the +reef-building corals require food, are preyed upon by other animals, +are killed by sediment, can not adhere to a loose bottom, and may be +easily carried down to a depth whence they can not spring up again, +we need feel no surprise at the reefs both of atolls and barriers +becoming in parts imperfect. The great barrier of New Caledonia +is thus imperfect and broken in many parts; hence, after long +subsidence, this great reef would not produce one great atoll 400 +miles in length, but a chain or archipelago of atolls, of very nearly +the same dimensions with those in the Maldive Archipelago. Moreover, +in an atoll once breached on opposite sides, from the likelihood of +the oceanic and tidal currents passing straight through the breaches, +it is extremely improbable that the corals, especially during +continued subsidence, would ever be able again to unite the rims: if +they did not, as the whole sank downward one atoll would be divided +into two or more. In the Maldive Archipelago there are distinct +atolls so related to each other in position, and separated by +channels either unfathomable or very deep (the channel between Ross +and Ari atolls is 150 fathoms, and that between the north and south +Nillandoo atolls is 200 fathoms in depth), that it is impossible to +look at a map of them without believing that they were once more +intimately related. And in this same archipelago, Mahlos-Mahdoo atoll +is divided by a bifurcating channel from 100 to 132 fathoms in depth, +in such a manner that it is scarcely possible to say whether it ought +strictly to be called three separate atolls or one great atoll not +yet finally divided. + +I will not enter on many more details; but I must remark that the +curious structure of the northern Maldive atolls receives (taking +into consideration the free entrance of the sea through their broken +margins) a simple explanation in the upward and outward growth of +the corals, originally based both on small detached reefs in their +lagoons, such as occur in common atolls, and on broken portions of +the linear marginal reef, such as bounds every atoll of the ordinary +form. I can not refrain from once again remarking on the singularity +of these complex structures--a great sandy and generally concave disk +rises abruptly from the unfathomable ocean, with its central expanse +studded, and its edge symmetrically bordered with oval basins of +coral-rock just lipping the surface of the sea, sometimes clothed +with vegetation, and each containing a lake of clear water! + +One more point in detail: as in two neighboring archipelagoes +corals flourish in one and not in the other, and as so many +conditions before enumerated must affect their existence, it would +be an inexplicable fact if, during the changes to which earth, +air, and water are subjected, the reef-building corals were to +keep alive for perpetuity on any one spot or area. And as by our +theory the areas including atolls and barrier-reefs are subsiding, +we ought occasionally to find reefs both dead and submerged. In +all reefs, owing to the sediment being washed out of the lagoon +or lagoon-channel to leeward, that side is least favorable to the +long-continued vigorous growth of the corals; hence dead portions of +reef not infrequently occur on the leeward side; and these, though +still retaining their proper wall-like form, are now in several +instances sunk several fathoms beneath the surface. The Chagos group +appears from some cause, possibly from the subsidence having been +too rapid, at present to be much less favorably circumstanced for +the growth of reefs than formerly: one atoll has a portion of its +marginal reef, nine miles in length, dead and submerged; a second +has only a few quite small living points which rise to the surface; +a third and fourth are entirely dead and submerged; a fifth is a +mere wreck, with its structure almost obliterated. It is remarkable +that in all these cases the dead reefs and portions of reefs lie at +nearly the same depth; namely, from six to eight fathoms beneath the +surface, as if they had been carried down by one uniform movement. +One of these “half-drowned atolls,” so called by Captain Scoresby (to +whom I am indebted for much invaluable information), is of vast size; +namely, ninety nautical miles across in one direction and seventy +miles in another line; and is in many respects eminently curious. As +by our theory it follows that new atolls will generally be formed +in each new area of subsidence, two weighty objections might have +been raised; namely, that atolls must be increasing indefinitely in +number; and, secondly, that in old areas of subsidence each separate +atoll must be increasing indefinitely in thickness, if proofs of +their occasional destruction could not have been adduced. Thus have +we traced the history of these great rings of coral-rock, from their +first origin through their normal changes, and through the occasional +accidents of their existence, to their death and final obliteration. + +Authors have noticed with surprise that, although atolls are the +commonest coral structures throughout some enormous oceanic tracts, +they are entirely absent in other seas, as in the West Indies: we +can now at once perceive the cause, for where there has not been +subsidence, atolls can not have been formed; and in the case of the +West Indies and parts of the East Indies, these tracts are known +to have been rising within the recent period. The larger areas are +all elongated; and there is a degree of rude alternation, as if +the rising of one had balanced the sinking of the other. Taking +into consideration the proofs of recent elevation both on the +fringed coasts and on some others (for instance, in South America) +where there are no reefs, we are led to conclude that the great +continents are for the most part rising areas; and from the nature +of the coral-reefs, that the central parts of the great oceans are +sinking areas. The East Indian archipelago, the most broken land in +the world, is in most parts an area of elevation, but surrounded +and penetrated, probably in more lines than one, by narrow areas of +subsidence. + +Bearing in mind the statements made with respect to the upraised +organic remains, we must feel astonished at the vastness of the areas +which have suffered changes in level either downward or upward, +within a period not geologically remote. It would appear, also, +that the elevatory and subsiding movements follow nearly the same +laws. Throughout the spaces interspersed with atolls, where not a +single peak of high land has been left above the level of the sea, +the sinking must have been immense in amount. The sinking, moreover, +whether continuous, or recurrent with intervals sufficiently long for +the corals again to bring up their living edifices to the surface, +must necessarily have been extremely slow. This conclusion is +probably the most important one which can be deduced from the study +of coral formations; and it is one which it is difficult to imagine +how otherwise could ever have been arrived at. Nor can I quite pass +over the probability of the former existence of large archipelagoes +of lofty islands, where now only rings of coral-rock scarcely break +the open expanse of the sea, throwing some light on the distribution +of the inhabitants of the other high islands, now left standing so +immensely remote from each other in the midst of the great oceans. +The reef-constructing corals have indeed reared and preserved +wonderful memorials of the subterranean oscillations of level; we see +in each barrier-reef a proof that the land has there subsided, and in +each atoll a monument over an island now lost. We may thus, like unto +a geologist who had lived his ten thousand years and kept a record of +the passing changes, gain some insight into the great system by which +the surface of this globe has been broken up, and land and water +interchanged. + + + + + MAGNITUDE AND COLOR OF THE SEA + --G. HARTWIG + + +Of all the gods that divide the empire of the earth, Neptune rules +over the widest realms. If a giant hand were to uproot the Andes and +cast them into the sea, they would be engulfed in the abyss, and +scarcely raise the general level of the waters. The South American +Pampas, bounded on the north by tropical palm-trees, and on the south +by wintry firs, are no doubt of magnificent dimensions, yet these +vast deserts seem insignificant when compared with the boundless +plains of earth-encircling ocean. Nay! a whole continent, even +America or Asia, appears small against the immensity of the sea, +which covers with its rolling waves nearly three-fourths of the +entire surface of the globe. + +The length of all the coasts which form the boundary between sea and +land can only be roughly estimated, for who has accurately measured +the numberless windings of so many shores? The entire coast-line of +deeply indented Europe and her larger isles measures about 21,600 +miles, equal to the circumference of the earth; while the shores of +compact Africa extend to a length of only 14,000 miles. The coasts of +America measure about 45,000 miles, those of Asia 40,000, while those +of Australia and Polynesia may safely be estimated at 16,000. Thus +the entire coast-line of the globe amounts to about 136,000 miles, +which it would take the best pedestrian to traverse from end to end. + +How different is the aspect of these shores, along which the +ever-restless sea continually rises or falls! Here steep rock-walls +tower up from the deep, while there a low sandy beach extends its +flat profile as far as the eye can reach. While some coasts are +scorched by the vertical sunbeam, others are perpetually blocked up +with ice. Here the safe harbor bids welcome to the weather-beaten +sailor, the lighthouse greets him from afar with friendly ray; the +experienced pilot hastens to guide him to the port, and all along the +smiling margin of the land rise the peaceful dwellings of civilized +man. There, on the contrary, the roaring breakers burst upon the +shore of some dreary wilderness, the domain of the savage or the +brute. What a wonderful variety of scenes unrolls itself before +our fancy as it roams along the coasts of ocean from zone to zone! +What changes, as it wanders from the palm-girt coral island of the +tropical seas to the melancholy strands where, verging toward the +poles, all vegetable life expires! And how magnificently grand does +the idea of ocean swell out in our imagination, when we consider that +its various shores witness at one and the same time the rising and +setting of the sun, the darkness of night and the full blaze of day, +the rigor of winter and the smiling cheerfulness of spring! + +The sea is not colorless; its crystal mirror not only reflects the +bright sky or the passing cloud, but naturally possesses a pure +bluish tint, which is only rendered visible to the eye when the light +penetrates through a stratum of water of considerable depth. In the +Gulf of Naples, we find the inherent color of the water exhibited to +us by Nature on a most magnificent scale. The splendid “Azure Cave,” +at Capri, might almost be said to have been created for the purpose. + +All profound and clear seas are more or less of a deep blue color, +while, according to seamen, a green color indicates soundings. The +bright blue of the Mediterranean, so often vaunted by poets, is found +over all the deep pure ocean, not only in tropical and temperate +zones, but also in the regions of eternal frost. Scoresby speaks with +enthusiasm of the splendid blue of the Greenland seas, and all along +the great ice-barrier which under 77° S. lat. obstructed the progress +of Sir James Ross toward the pole, that illustrious navigator found +the waters of as deep a blue as in the classical Mediterranean. The +North Sea is green, partly from its water not being so clear, and +partly from the reflection of its sandy bottom mixing with the +essentially blue tint of the water. In the Bay of Loanga the sea has +the color of blood, and Captain Tuckey discovered that this results +from the reflection of the red ground-soil. + +But the essential color of the sea undergoes much more frequent +changes over large spaces, from enormous masses of minute _algæ_, +and countless hosts of small sea-worms, floating or swimming on its +surface. + +“A few days after leaving Bahia,” says Mr. Darwin, “not far from +the Abrolhos islets, the whole surface of the water, as it appeared +under a weak lens, seemed as if covered by bits of hay with their +ends jagged. Each bundle consisted of from twenty to sixty filaments, +divided at regular intervals by transverse septa, containing a +brownish-green flocculent matter. The ship passed several bands +of them, one of which was about ten yards wide, and, judging from +the mud-like color of the water, at least two and a half miles +long. Similar masses of floating vegetable matter are a very common +appearance near Australia. During two days preceding our arrival at +the Keeling Islands, I saw in many parts masses of flocculent matter +of a brownish-green color floating in the ocean. They were from half +to three inches square, and consisted of two kinds of microscopical +confervæ. Minute cylindrical bodies, conical at each extremity, were +involved in large numbers in a mass of fine threads.” + +“On the coast of Chili,” says the same author, “a few leagues north +of Concepcion, the _Beagle_ one day passed through great bands of +muddy water; and again a degree south of Valparaiso, the same +appearance was still more extensive. Mr. Sullivan, having drawn up +some water in a glass, distinguished by the aid of a lens moving +points. The water was slightly stained, as if by red dust, and after +leaving it for some time quiet a cloud collected at the bottom. +With a slightly magnifying lens, small hyaline points could be +seen darting about with great rapidity and frequently exploding. +Examined with a much higher power, their shape was found to be oval, +and contracted by a ring round the middle, from which line curved +little setæ proceeded on all sides, and these were the organs of +motion. Their minuteness was such that they were individually quite +invisible to the naked eye, each covering a space equal only to the +one-thousandth of an inch, and their number was infinite, for the +smallest drop of water contained very many. In one day we passed +through two spaces of water thus stained, one of which alone must +have extended over several square miles. The color of the water was +like that of a river which has flowed through a red clay district, +and a strictly defined line separated the red stream from the blue +water.” + +In the neighborhood of Callao, the Pacific has an olive-green color, +owing to a greenish matter which is also found at the bottom of the +sea in a depth of 800 feet. In its natural state it has no smell, but +when cast on the fire it emits the odor of burned animal substances. + +Near Cape Palmas, on the coast of Guinea, Captain Tuckey’s ship +seemed to sail through milk, a phenomenon which was owing to an +immense number of little white animals swimming on the surface and +concealing the natural tint of the water. + +The peculiar coloring of the Red Sea, from which it has derived its +name, is owing to the presence of a microscopic alga, _sui generis_, +floating at the surface of the sea and even less remarkable for its +beautiful red color than for its prodigious fecundity. + +I could add many more examples, where, either from minute _algæ_, or +from small animals, the deep blue sea suddenly appeared in stripes +of white, yellow, blue, brown, orange, or red. For fear, however, +of tiring the reader’s patience, I shall merely mention the _olive +green_ water which covers a considerable part of the Greenland seas. +It is found between 74° and 80° N. lat., but its position varies with +the currents, often forming isolated stripes, and sometimes spreading +over two or three degrees of latitude. Small yellowish Medusæ, of +from one-thirtieth to one-twentieth of an inch in diameter, are the +principal agents that change the pure ultramarine of the Arctic Ocean +into a muddy green. + +When the sea is perfectly clear and transparent, it allows the +eye to distinguish objects at a very great depth. Near Mindora, +in the Indian Ocean, the spotted corals are plainly visible under +twenty-five fathoms of water. + +The crystalline clearness of the Caribbean Sea excited the admiration +of Columbus. “In passing over these splendidly adorned grounds,” says +Schöpf, “where marine life shows itself in an endless variety of +forms, the boat, suspended over the purest crystal, seems to float in +the air, so that a person unaccustomed to the scene easily becomes +giddy. On the clear sandy bottom appear thousands of sea-stars, +sea-urchins, mollusks, and fishes of a brilliancy of color unknown +in our temperate seas. Fiery red, intense blue, lively green, and +golden yellow perpetually vary; the spectator floats over groves of +sea-plants, gorgonias, corals, alcyoniums, flabellums, and sponges +that afford no less delight to the eye, and are no less gently +agitated by the heaving waters, than the most beautiful garden on +earth when a gentle breeze passes through the waving boughs.” + + + + + TIDAL ACTION + --SIR ROBERT S. BALL + + +Every one is familiar with the fact that the moon raises tides on the +earth; these tides ebb and flow along our coasts, and in virtue of +them the satellite exercises a certain control on the movements of +our globe. If the moon had liquid oceans on its surface there can not +be a doubt that the attraction of the earth would generate tides in +the oceans on the moon just as the attraction of the moon generates +tides in the oceans of the earth. But there would be a fundamental +difference between the two cases; the shores of the lunar seas would +be periodically inundated by tides far vaster than any tides which +the moon can create on the earth. But it may be said that as the +moon contains no water it seems idle to talk of the tides that might +have been produced in oceans if they had existed. It is no doubt +true that the moon contains no visible liquid water on its surface +at the present time; it is, however, by no means certain that our +satellite was always void of water; it is not at all impossible +that spreading oceans may have once occupied a large part of that +surface now an arid wilderness. The waters from those oceans have +vanished, but the basins they presumably filled are still left as +characteristic features on our satellite. For our present argument, +however, it is really not material that the moon should ever have had +oceans as we understand them. The water at those remote periods must +have been suspended in the form of vapor around the more solid parts +of the glowing globe. But tides can be manifested in other liquids +besides that which forms our seas. In fact if the basins of our great +oceans were filled with oil or with mercury, or even with molten iron +instead of water, the moon would still cause tides to ebb and flow, +no matter what the material might be, so long as it possessed to some +extent the properties of a liquid. It need not be a perfect liquid, +for any material which is in some degree viscous, like honey or +treacle, would still respond to tidal influence, though not, it may +be well believed, with the same alacrity and freedom of movement as +would a fluid of a more perfect character. In the molten moon itself, +throughout the very body of our satellite, the tidal influence of the +earth must have been experienced in these primitive ages. + +There can not be a doubt that in ancient days when the moon was +sufficiently fluid, the action of the tides tended without ceasing to +the establishment of such an adjustment between the rotation of the +moon around its axis and the revolution of the moon around the earth, +that the two should be brought to have equal periods. Friction would +incessantly operate until this adjustment had been effected, and +owing to the preponderating mass of the earth such strenuous tides +must have been evoked in the moon that our satellite was brought +under tidal control with comparative facility. Hence it arose that +in those early days the habit of bending the same face incessantly +toward the earth around which it revolved was established on our +satellite. + +Time passed on, the moon gradually dispensed its excessive heat by +radiation into space, and it gradually became transformed from a +molten globe to a globe with a solid crust. It may be that the water +was condensed from vapor and then collected together into oceans on +the newly formed surface; if so, these oceans would not have any +ebbing tide or flowing tide, for it would be constant high tide at +some places and constant low tide at others. Such a state of things +would at all events endure so long as the adjustment of equality +between the moon’s rotation and its revolution continued. In fact, +should any departure from this adjustment have manifested itself, +corresponding tides would have begun to throb in the lunar oceans, +and their tendency would be to restore the adjustment which was +disturbed. This arrangement between the two movements was necessarily +stable when tidal control was always at hand to check any tendency to +depart from it. + +It may be that the moon has now cooled so thoroughly that not only +is it hard and congealed on the exterior as we see, but it seems +highly probable that the heat may have so entirely forsaken even +the interior that there is no longer any fluid in the globe of our +satellite to respond to tidal impulse. There is, therefore, in all +probability, no longer any actual tidal control. On the other hand, +however, there is nothing to disturb the adjustment. It was, as +we have seen, caused by the tides which have done their work; the +consequences of that work are still exhibited in the constant face of +the moon, which, now that it has been brought about, seems likely to +exist permanently as a stable adaptation of the movement. + +The tendency of the tides on a tide-disturbed globe is to adjust the +movements of that globe in such a way that the tides shall no longer +ebb or flow, but that permanent high tide shall be established in +some places and permanent low tide in others. If the rotation of the +body be not fast enough the tide will pull the body round in order to +effect this object. If the rotation of the body be too rapid, then +the influence of the tide will tend to check the movement and bring +down the speed of rotation until the desired adjustment is obtained. +At present the earth is spinning too fast to permit the high tides to +remain at permanent localities, and consequently tides are applied +with the effect of checking the rotation. The earth is, however, so +vast, and the tides generated by so small a body as the moon are +relatively so impotent, that their effects in reducing the speed of +the earth’s rotation are insignificant. Nevertheless, small though +they are, they unquestionably exist, and there can not be a doubt +that to some extent the earth is affected by the unremitting action +of the tides; the consequence is that the rapidity with which the +earth rotates upon its axis is gradually declining. + +One result of this can be stated in a very simple manner. The +length of the day must be increasing. It is true that this gradual +stretching of the day is very slow; it is indeed quite inappreciable +in so far as our ordinary use of the day as a measure of time is +concerned. The alteration almost eludes any means of measurement +at our disposal. Even in a thousand years the change is so small +that the increase in the length of the day is only a fraction of a +second. We can doubtless afford to disregard so trifling a variation +in our standard of time so far as the period contemplated in mere +human affairs is concerned. In fact the change is absolutely devoid +of significance within such periods as are contemplated since the +erection of the Pyramids, or indeed since any other human monument +has been reared. We must not, however, conclude that the change in +the length of the day has no significance in earth history. + +The significance of the gradual elongation of the day by the tides +arises from the circumstance that the change always takes place in +one direction. In this form of effect the tide differs from other +more familiar astronomical phenomena which sometimes advance in one +direction and then after the lapse of suitable periods return in +the opposite direction, and thus restore again the initial state of +things. But the alteration of the length of the day is not of this +character, it is not periodic, its motion is never reversed, is never +even arrested. Only one condition is therefore necessary to enable +it to obtain tremendous dimensions, and that is sufficient time in +which it can operate. + +There are many lines of reasoning which show the extreme antiquity +of our globe: the disclosures of geology are specially instructive +on this head. Think, for instance, of that mighty reptile the +Atlantosaurus, which once roamed over the regions now known as +Colorado. The bones of this vast creature indicate an animal +surpassing in proportions the greatest elephant ever known. No one +can count the æons of years that have elapsed since the Atlantosaurus +whose bones are now to be seen in the museum at Yale University +breathed its last. A still more striking conception of time than +even the antiquity of this creature affords is derived from the +consideration that his mighty form was itself the product of a long +and immeasurable line of ancestry, extending to a depth in the remote +past far beyond the limits of computation. I have mentioned this +illustration of the antiquity of the earth for the purpose of showing +the ample allowance of time that is available for tides to accomplish +great work in earlier stages of our globe’s history. + +As the evidence of the earth’s crust proves that our globe has +lasted for incalculable ages, it becomes of interest to think how +far the gradual elongation of the day may have attained significant +proportions since very early time. It may be that even in a thousand +years the effect of the tides is not sufficient to alter the length +of the day by so much as a single second. But the effect may be +very appreciable or even large in a million years, or ten million +years. We have the best reasons for knowing that in intervals of time +comparable with those I have mentioned, the change in the length of +the day may have amounted not merely to seconds or minutes, but even +to hours. Looking into the remote past, there was a time at which +this globe spun round in twenty-three hours instead of twenty-four; +at a still earlier period the rate must have been twenty hours, and +the further we look back the more and more rapidly does the earth +appear to be spinning. At last, as we strain our gaze to some epoch +so excessively remote that it must have been long anterior to those +changes which geology recognizes, we see that our globe was spinning +round in a period of six hours or five hours, or possibly even less. +Here then is a lesson which the tides have taught us: they have shown +that if the causes at present in operation have subsisted without +interruption for a sufficiently long period in the past, the day must +have gradually grown to its present length from an initial condition +in which the earth seems to have spun round about four times as +quickly as it does at present. + +We should, however, receive a very inadequate impression of what +tides are able to accomplish if we merely contemplated this change in +the length of the day, striking and significant though it doubtless +is. The student of natural philosophy is well aware that there is +no action without a corresponding reaction, and it is instructive +to examine in this case the form which the reaction assumes. Our +reasoning has been founded on the supposition that it is the +attraction of the moon on the waters of our globe that gives rise to +the tides. It is, therefore, the influence of the moon which checks +the speed of the earth’s rotation and adds to the length of the day. + +As the moon acts in this fashion on the earth, so, by the general +law that I have mentioned, the earth reacts upon the moon. The form +which this reaction assumes expresses itself in a tendency to allow +the moon gradually to move further and further away from the earth +than the earth’s attraction would permit if our globe were a solid +mass void of all liquid capable of being distracted by tides. It +is, therefore, certain that the distance of the moon, which is at +present about two hundred and forty thousand miles, must be gradually +increasing; but we need not look for any appreciable change in the +moon’s distance arising from this cause when only an interval of +a few centuries is considered. We need not expect to measure the +difference due to tides between the size of the moon’s orbit this +month and the size of the orbit last month. In fact, there are so +many periodic causes of change in the dimensions of the moon’s orbit +that it becomes impossible to detect the tidal influence even in +the course of centuries. Here, again, we have to remember that in +dealing with the history of our earth we are to consider not merely +the thousands of years that include the human period, not merely the +millions of years that are required by the necessities of geology, +but also those unknown periods anterior to geological phenomena to +which we have already referred. + +In the course of such vast ages the reaction of the earth on +the moon’s orbit has not only become perceptible, it has become +conspicuous; it has not only become conspicuous, but it has become +the chief determining agent in making the moon’s orbit as we find it +at the present day. We have seen that as we look into the past the +length of the day seems ever shorter and shorter; and concurrent with +this decline in the day is the diminution in the moon’s distance from +the earth. There was a time when the moon, instead of revolving at +a distance of two hundred and forty thousand miles, as it does at +present, revolved at a distance of only two hundred thousand miles. +As we think of epochs still earlier we discern the moon ever closer +and closer to the earth, until at last, at that critical time in +the history of the earth-moon system, when the earth was quickly +revolving in a period of a few hours, our satellite seems to have +been quite close to the earth; in fact, the two bodies were almost in +contact + +The study of the tides has therefore conducted us to the knowledge +of a remarkable configuration exhibited in the primitive earth-moon +system. The earth was then spinning round rapidly in a day which +was only a few hours long, while close to the earth, or almost in +contact with it, the moon coursed around our globe, the period of +its revolution being shorter to such an extent that the satellite +completed its circuit in the same time as the earth required for one +turn round its axis. + +We must remember that the materials destined to form the pair of +allied planets did not then form two solid bodies as they do at +present; they were both, in all probability, incandescent masses +glowing with fervor, and soft, if not actually molten, or incoherent, +or even gaseous. These aggregations were close together, and one of +them was whirling around the other in a period of a few hours, the +duration of that period being equal to the time in which the larger +mass revolved on its axis. In fact, the two objects, even though +distinct, seem to have revolved the one around the other as if they +had been bound together by rigid bonds. The rapid rotation with which +they were animated suggests a cause for this state of things. It is +well known that a fly-wheel, when driven at an unduly high speed, +is liable to break asunder in consequence of its rapid motion. If a +grindstone be urged around with excessive velocity the force tending +to rend the stone into fragments may overcome its cohesion, and it +will fly into pieces, often projected with such violence that fearful +accidents have been the consequence. + +Viewing the earth as a rotating body, it must be subject to the law +that there is a speed which can not be exceeded with safety. With +the present period of rotation of once in every twenty-four hours +the tendency to disruption is but small and consequently the earth +retains its integrity, though no doubt the protuberance at the +equator is the result of the accommodation of the shape of the globe +to the circumstances attending its revolution. But let us suppose +that the length of the day was greatly diminished, or, what comes to +the same thing, that the speed with which the earth rotates on its +axis was greatly increased; it is then conceivable that the tension +thus arising might be too great for the coherence of the material +to withstand. We believe that the earth could turn round with double +the speed that it has at present before this tension approached the +point at which disruption would ensue. But supposing the day were to +be so much shortened that the period of rotation was only a very few +hours instead of twenty-four, there is then good reason to know that +the tension in the earth arising from this rapid rotation would be so +great that a rupture of the globe would be imminent. + +Provided with this conception, let us think of the initial stage +when the moon was quite close to the earth. Our globe was then, as +we know, spinning round so rapidly that its materials were almost on +the point of breaking up in consequence of the strain produced by +the rotation. It is interesting to note that the tidal action of the +sun would also conduce to the rupture of our globe in the critical +circumstances we have supposed. It seems hardly possible to doubt +that such a separation of the glowing mass did actually take place, a +small fragment was discarded, and gradually drew itself by the mutual +attraction of its particles into a globular form and thus became the +moon. + +We have seen that at the present moment the day is becoming gradually +longer and the moon is steadily receding further and further from the +earth. At present these changes take place with extreme slowness, +but in the primitive periods of which we have already spoken, the +changes in the length of the day, and the changes in the distance +of the moon, proceeded at a rate far more rapid than at present. +As the moon has receded further from the earth its efficiency as a +tide-producer has declined, and consequently the rate at which the +consequences of tidal action have proceeded is continually lessening. +It must therefore be expected that the progress of tidal evolution +in the future will be ever getting slower and slower, so that the +periods of time required for the further development of the phenomena +far exceed those which have elapsed in the course of the history +already given. We can, however, foreshadow what is to happen in the +following manner. The length of the day will slowly increase; and +we can indicate a state of things in the excessively remote future +toward which it may be said the system is tending. The day will grow +until it becomes not merely twenty-five or twenty-six hours, but +until it becomes as long as two or three of our present days. In +fact, as we stretch our imagination through ages so inconceivable +that I forbear to specify any figures which might characterize them, +we seem to discern that the length of the day may go on ever getting +longer and longer until at last a stage is reached when the day is +about fifty or sixty times as long as our present day. + +All this time, in accordance with the general law of action and +reaction, the moon must be gradually retreating. As the orbit of +the moon is gradually enlarging, the time that the moon takes to +revolve around the earth must be continually on the increase; from +the present month of twenty-seven days the length of the month will +gradually augment as the ages roll by until at last when the moon +has reached a certain distance the period of its rotation will have +become double what it is at present, or indeed rather more than +double, and we shall have the day and the month equal, each being +about fourteen hundred hours long. When this state of things is +reached, the earth will always turn the same face toward the moon, +just as the moon at present always turns the same face toward the +earth. + +We have already explained how the constant face of the moon can +be accounted for by the action of tides raised in the moon by the +attraction of the earth. Owing to the small size of the moon the +tides have already wrought all that they were capable of doing, and +have compelled the moon to succumb to the conditions they imposed. +Owing to the great mass of the earth and the comparatively small +mass of the moon the tides on the earth raised by the moon have +required a much longer period wherein to accomplish their effects +than was the case when the earth raised tides on the moon. But small +though our satellite may be, yet the tides raised on the earth have +incessantly tended to wear down the speed of our globe and reduce it +to conformity with the law that the two bodies shall bear the same +face toward each other. At present the earth turns round twenty-seven +times while the moon goes round once, so the tides have still a +gigantic task to accomplish. With unflagging energy, however, they +are incessantly engaged at the work, and they are constantly tending +to bring down the speed of the earth; constantly tending toward that +ultimate condition of things in which the earth and moon are destined +to revolve in a period of fourteen hundred hours as if they were +connected with invisible bonds. + +If such a state of things as this were established then it is plain +that tides would no longer ebb and flow, that is, at least, if we +exclude from our consideration the intervention of any other body. +High tides must prevail at some parts of the earth, and low tides at +other parts, but the position of these tides will remain fixed. Where +it is high tide it will always be high tide; where it is low tide +it will always be low tide. When this state of things is reached, +the moon will be constantly visible in the same part of the sky from +one half of our globe, while the other half of our globe will never +be turned toward the moon. In fact, the moon would always appear +to us in a fixed position as the earth would always appear to be +if viewed by an observer stationed on the moon. If there were any +Lunarians whose residence was confined to the opposite side of the +moon, they could never see this earth at all, while those who lived +on this side of our satellite would always be able to see the earth +apparently fixed in the same part of the sky. An observatory located +at the middle of the moon’s disk, say near the crater Ptolemy, would +always have the earth in its zenith or very near thereto, while the +astronomer, let us say, in the Mare Crisium, would always find the +earth low down near his horizon. + +In order to facilitate our reasoning I have assumed that the moon +is the only tide-producing agent; this is, however, not the case. +No doubt the ebb and the flow around our coasts is generated mainly +by the attraction of the moon. It must not, however, be forgotten +that a portion of the tide is originated by the attraction of the +sun. These solar tides will still continue to ebb and flow quite +independently of the lunar tides, so that even if the accommodation +between the earth and the moon had been completed some further tidal +disturbance would not be wanting. The effect of the solar tides will +be to abate still further the velocity with which the earth turns +round on its axis, and consequently a time must ultimately arrive +when the length of the day will be longer than the time which the +moon takes to revolve around our earth. + + + + + THE GULF STREAM + --LORD KELVIN + + +I mean by the Gulf Stream that mass of heated water which pours from +the Strait of Florida across the North Atlantic, and likewise a wider +but less definite warm current, evidently forming part of the same +great movement of water, which curves northward to the eastward of +the West Indian Islands. I am myself inclined, without hesitation, +to regard this stream as simply the reflux of the equatorial +current, added to no doubt during its northeasterly course by the +surface-drift of the anti-trades which follows in the main the same +direction. + +The scope and limit of the Gulf Stream will be better understood if +we inquire in the first place into its origin and cause. As is well +known--in two bands, one to the north and the other to the south +of the equator--the northeast and southeast trade-winds, reduced +to meridional directions by the eastward frictional impulse of the +earth’s rotation, drive before them a magnificent surface current of +hot water 4,000 miles long by 450 miles broad at an average rate of +thirty miles a day. Off the coast of Africa, near its starting-point +to the south of the Islands of St. Thomas and Anna Bon this +“equatorial current” has a speed of forty miles in the twenty-four +hours, and a temperature of 23° C. + +Increasing quickly in bulk, and spreading out more and more on both +sides of the equator, it flows rapidly due west toward the coast +of South America. At the eastern point of South America, Cape St. +Roque, the equatorial current splits into two, and one portion +trends southward to deflect the isotherms of 21°, 15°.5, 10°, and +4°.5 C. into loops upon our maps, thus carrying a scrap of comfort +to the Falkland Islands and Cape Horn; while the northern portion +follows the northeast coast of South America, gaining continually +in temperature under the influence of the tropical sun. Its speed +has now increased to sixty-eight miles in twenty-four hours, and +by the union with it of the waters of the river Amazon, it rises +to one hundred miles (6.5 feet in a second), but it soon falls off +again when it gets into the Caribbean Sea. Flowing slowly through +the whole length of this sea, it reaches the Gulf of Mexico through +the Strait of Yucatan, when a part of it sweeps immediately round +Cuba; but the main stream, “having made the circuit of the Gulf of +Mexico, passes through the Strait of Florida; thence it issues +as the ‘Gulf Stream’ in a majestic current upward of thirty miles +broad, two thousand two hundred feet deep, with an average velocity +of four miles an hour, and a temperature of 86° Fahr. (30° C.).” +The hot water pours from the strait with a decided though slight +northeasterly impulse on account of its great initial velocity. Mr. +Croll calculates the Gulf Stream as equal to a stream of water fifty +miles broad and a thousand feet deep flowing at a rate of four miles +an hour; consequently conveying 5,575,680,000,000 cubic feet of water +per hour, or 133,816,320,000,000 cubic feet per day. This mass of +water has a mean temperature of 18° C. as it passes out of the gulf, +and on its northern journey it is cooled down to 4°.5, thus losing +heat to the amount of 13°.5 C. The total quantity of heat therefore +transferred from the equatorial regions per day amounts to something +like 154,959,300,000,000,000,000 foot-pounds. + +This is nearly equal to the whole of the heat received from the +sun by the Arctic regions, and, reduced by a half to avoid all +possibility of exaggeration, it is still equal to one-fifth of the +whole amount received from the sun by the entire area of the North +Atlantic. The Gulf Stream, as it issues from the Strait of Florida +and expands into the ocean on its northward course, is probably the +most glorious natural phenomenon on the face of the earth. The water +is of a clear crystalline transparency and an intense blue, and long +after it has passed into the open sea it keeps itself apart, easily +distinguished by its warmth, its color, and its clearness; and with +its edges so sharply defined that a ship may have her stem in the +clear blue stream while her stern is still in the common water of the +ocean. + +Setting aside the wider question of the possibility of a general +oceanic circulation arising from heat, cold, and evaporation, I +believe that Captain Maury and Dr. Carpenter are the only authorities +who of late years have disputed this source of the current which +we see and can gauge and measure as it passes out of the Strait +of Florida; for it is scarcely necessary to refer to the earlier +speculations that it is caused by the Mississippi River, or that it +flows downward by gravitation from a “head” of water produced by the +trade-winds in the Caribbean Sea. + +Captain Maury writes that “the dynamical force that calls forth the +Gulf Stream is to be found in the difference as to specific gravity +of intertropical and polar waters.” “The dynamical forces which are +expressed by the Gulf Stream may with as much propriety be said to +reside in those northern waters as in the West India seas: for on +one side we have the Caribbean Sea and Gulf of Mexico with their +waters of brine; on the other the great polar basin, the Baltic and +the North Sea, the two latter with waters which are little more than +brackish. In one set of these sea-basins the water is heavy; in the +other it is light. Between them the ocean intervenes; but water is +bound to seek and to maintain its level; and here, therefore, we +unmask one of those agents concerned in causing the Gulf Stream. What +is the power of this agent? Is it greater than that of other agents? +and how much? We can not say how much; we only know it is one of the +chief agents concerned. Moreover, speculate as we may as to all the +agencies concerned in collecting these waters, that have supplied the +trade-winds with vapor, into the Caribbean Sea, and then in driving +them across the Atlantic, we are forced to conclude that the salt +which the trade-wind vapor leaves behind it in the tropics has to be +conveyed away from the trade-wind region, to be mixed up again in +due proportion with the other water of the sea--the Baltic Sea and +the Arctic Ocean included--and that these are some of the waters, at +least, which we see running off through the Gulf Stream. To convey +them away is doubtless one of the offices which in the economy of the +ocean has been assigned to it.” + +Dr. Carpenter attributes all the great movements of ocean water to a +general convective circulation, and of this general circulation he +regards the Gulf Stream as a peculiarly modified case. Dr. Carpenter +states that “the Gulf Stream constitutes a peculiar case, modified by +local conditions,” of “a great general movement of equatorial water +toward the polar area.” I confess I feel myself compelled to take a +totally different view. It seems to me that the Gulf Stream is the +one natural physical phenomenon on the surface of the earth whose +origin and principal cause, the drift of the trade-winds, can be most +clearly and easily traced. + +The further progress and extension of the Gulf Stream through the +North Atlantic in relation to influence upon climate has been, +however, a fruitful source of controversy. The first part of its +course, after leaving the strait, is sufficiently evident, for its +water long remains conspicuously different in color and temperature +from that of the ocean, and a current having a marked effect on +navigation is long perceptible in the peculiar Gulf Stream water. +“Narrow at first, it flows round the peninsula of Florida, and, with +a speed of about 70 or 80 miles, follows the coast at first in a +due north, afterward in a northeast direction. At the latitude of +Washington it leaves the North American coast altogether, keeping +its northeastward course; and to the south of the St. George’s and +Newfoundland banks it spreads its waters more and more over the +Atlantic Ocean, as far as the Azores. At these islands a part of it +turns southward again toward the African coast. The Gulf Stream has, +so long as its waters are kept together along the American coast, a +temperature of 26°.6 C.; but, even under north latitude 36°, Sabine +found it 23°.3 C. at the beginning of December, while the sea-water +beyond the stream showed only 16°.9 C. Under north latitude 40-41° +the water is, according to Humboldt, at 22°.5 C. within, and 17°.5 C. +without the stream.” + +Opposite Tortugas, passing along the Cuban coast, the stream is +unbroken and the current feeble; the temperature at the surface is +about 26°.7 C. Issuing from the Strait of Bemini the current is +turned nearly directly northward by the form of the land; a little +to the north of the strait, the rate is from three to five miles +an hour. The depth is only 325 fathoms, and the bottom, which in +the Strait of Florida was a simple slope and counter-slope, is now +corrugated. The surface temperature is about 26°.5 C., while the +bottom temperature is 4°.5; so that in the moderate depth of 325 +fathoms the equatorial current above and the polar counter-current +beneath have room to pass one another, the current from the north +being evidently tempered considerably by mixture. North of Mosquito +inlet the stream trends to the eastward of north, and off St. +Augustine it has a decided set to the eastward. Between St. Augustine +and Cape Hatteras the set of the stream and the trend of the coast +differ but little, making 5° of easting in 5° of northing. At +Hatteras it curves to the northward, and then runs easterly. In the +latitude of Cape Charles it turns quite to the eastward, having a +velocity of from a mile to a mile and a half in the hour. + +A brief account of one of the sections will best explain the general +phenomena of the stream off the coast of America. I will take the +section following a line at right angles to the coast off Sandy Hook. +From the shore out, for a distance of about 250 miles, the surface +temperature gradually rises from 21° to 24° C.; at 10 fathoms it +rises from 19° to 22° C.; and at 20 fathoms it maintains, with a few +irregularities, a temperature of 19° C. throughout the whole space; +while at 100, 200, 300, and 400 fathoms it maintains in like manner +the respective temperatures of 8°.8, 5°.7, 4°.5, and 2°.5 C. This +space is, therefore, occupied by cold water, and observation has +sufficiently proved that the low temperature is due to a branch of +the Labrador current creeping down along the coast in a direction +opposite to that of the Gulf Stream. In the Strait of Florida this +cold stream divides--one portion of it passing under the hot Gulf +Stream water into the Gulf of Mexico, while the remainder courses +round the western end of Cuba. Two hundred and forty miles from the +shore the whole mass of water takes a sudden rise of about 10° C. +within 25 miles, a rise affecting nearly equally the water at all +depths, and thus producing the singular phenomenon of two masses of +water in contact--one passing slowly southward and the other more +rapidly northward, at widely different temperatures at the same +levels. This abutting of the side of the cold current against that +of the Gulf Stream is so abrupt that it has been aptly called by +Lieutenant George M. Bache the “cold wall.” Passing the cold wall, +we reach the Gulf Stream, presenting all its special characters of +color and transparency and of temperature. In the section which we +have chosen as an example, upward of 300 miles in length, the surface +temperature is about 26°.5 C., but the heat is not uniform across +the stream, for we find that throughout its entire length, as far +south as the Cape Canaveral section, the stream is broken up into +longitudinal alternating bands of warmer and cooler water. Off Sandy +Hook, beyond the cold wall, the stream rises to a maximum of 27°.8 +C., and this warm band extends for about 60 miles. The temperature +then falls to a minimum of 26°.5 C., which it retains for about +30 miles, when a second maximum of 27°.4 succeeds, which includes +the axis of the Gulf Stream, and is about 170 miles wide. This +is followed by a second minimum of 25°.5 C., and this by a third +maximum, when the bands become indistinct. It is singular that the +minimum bands correspond with valley-like depressions in the bottom, +which follow in succession the outline of the coast and lodge deep +southward extensions of the polar indraught. + +The last section of the Gulf Stream surveyed by the American +hydrographers extends in a southeasterly direction from Cape Cod, +lat. 41° N., and traces the Gulf Stream, still broken up by its +bands of unequal temperature, spreading directly eastward across the +Atlantic; its velocity has, however, now become inconsiderable, and +its limits are best traced by the thermometer. + +The course of the Gulf Stream beyond this point has given rise +to much discussion. I again quote Professor Buff for what may +be regarded as the view most generally received among physical +geographers: + +“A great part of the warm water is carried partly by its own motion, +but chiefly by the prevailing west and northwest winds, toward the +coast of Europe and even beyond Spitzbergen and Nova Zembla; and +thus a part of the heat of the south reaches far into the Arctic +Ocean. Hence, on the north coast of the Old Continent, we always +find driftwood from the southern regions, and on this side the +Arctic Ocean remains free from ice during a great part of the year, +even as far up as 80° north latitude; while on the opposite coast +(of Greenland) the ice is not quite thawed even in summer.” The two +forces invoked by Professor Buff to perform the work are thus the +_vis à tergo_ of the trade-wind drift and the direct driving power +of the anti-trades, producing what has been called the anti-trade +drift. This is quite in accordance with the views here advocated. +The proportion in which these two forces act, it is undoubtedly +impossible in the present state of our knowledge to determine. + +Mr. A. G. Findlay, a high authority on all hydrographic matters, read +a paper on the Gulf Stream before the Royal Geographical Society, +reported in the 13th volume of the Proceedings of the Society. Mr. +Findlay, while admitting that the temperature of Northern Europe is +abnormally ameliorated by a surface-current of the warm water of the +Atlantic which reaches it, contends that the Gulf Stream proper--that +is to say, the water injected, as it were, into the Atlantic through +the Strait of Florida by the impulse of the trade-winds--becomes +entirely thinned out, dissipated, and lost opposite the Newfoundland +banks about lat. 45° N. The warm water of the southern portion of +the North Atlantic basin is still carried northward; but Mr. Findlay +attributes this movement solely to the anti-trades--the southwest +winds--which by their prevalence keep up a balance of progress in a +northeasterly direction in the surface layer of the water. + +Dr. Carpenter entertains a very strong opinion that the dispersion of +the Gulf Stream may be affirmed to be complete in about lat. 45° N. +and long. 35° W. Dr. Carpenter admits the accuracy of the projection +of the isotherms on the maps of Berghaus, Dove, Petermann, and Keith +Johnston, and he admits likewise the conclusion that the abnormal +mildness of the climate on the northwestern coast of Europe is due to +a movement of equatorial water in a northeasterly direction. “What I +question is the correctness of the doctrine that the northeast flow +is an extension or prolongation of the Gulf Stream, still driven on +by the _vis à tergo_ of the trade-winds--a doctrine which (greatly to +my surprise) has been adopted and defended by my colleague, Professor +Wyville Thomson. But while these authorities attribute the whole or +nearly the whole of this flow to the true Gulf Stream, _I_ regard a +large part, if not the whole, of that which takes place along our +own western coast, and passes north and northeast between Iceland +and Norway toward Spitzbergen, as quite independent of that agency; +so that it would continue if the North and South American Continents +were so completely disunited that the equatorial currents would be +driven straight onward by the trade-winds into the Pacific Ocean, +instead of being embayed in the Gulf of Mexico and driven out in a +northeast direction through the ‘narrows’ off Cape Florida.” Dr. +Carpenter does not mean by this to indorse Mr. Findlay’s opinion +that the movement beyond the 54th parallel of latitude is due solely +to the drift of the anti-trades; he says, “On the view I advocate, +the northeasterly flow is regarded as due to the _vis à fronte_ +originating in the action of cold upon the water of the polar area, +whereby its level is always tending to depression.” The amelioration +of the climate of northwestern Europe is thus caused by a “modified +case” of the general oceanic circulation, and neither by the Gulf +Stream nor by the anti-trade drift. + +Although there are, up to the present time, very few trustworthy +observations of deep-sea temperatures, the surface temperature of +the North Atlantic has been investigated with considerable care. +The general character of the isothermal lines, with their singular +loop-like northern deflections, has long been familiar through the +temperature charts of the geographers already quoted, and of late +years a prodigious amount of data have been accumulated. + +In 1870, Dr. Petermann, of Gotha, published an extremely valuable +series of temperature charts, embodying the results of the reduction +of upward of 100,000 observations. + +Dr. Petermann has devoted the special attention of a great part of +his life to the distribution of heat on the surface of the ocean, and +the accuracy and conscientiousness of his work in every detail are +beyond the shadow of a doubt. + +In the North Atlantic every curve of equal temperature, whether for +the summer, for the winter, for a single month, or for the whole +year, instantly declares itself as one of a system of curves which +are referred to the Strait of Florida as a source of heat, and the +flow of warm water may be traced in a continuous stream--indicated +when its movement can no longer be observed by its form--fanning out +from the neighborhood of the Strait across the Atlantic, skirting +the coasts of France, Britain, and Scandinavia, rounding the North +Cape, and passing the White Sea and the Sea of Kari, bathing the +western shores of Nova Zembla and Spitzbergen, and finally coursing +round the coast of Siberia, a trace of it still remaining to find its +way through the narrow and shallow Behring’s Strait into the North +Pacific. + +Now, it seems to me that if we had only these curves upon the +chart, deduced from an almost infinite number of observations which +are themselves merely laboriously multiplied corroborations of +many previous ones, without having any clew to their rationale, +we should be compelled to admit that whatever might be the +amount and distribution of heat derived from a general oceanic +circulation--whether produced by the prevailing winds of the region, +by convection, by unequal barometric pressure, by tropical heat, or +by arctic cold--the Gulf Stream, the majestic stream of warm water +whose course is indicated by the deflections of the isothermal lines, +is sufficiently powerful to mask all the rest, and, broadly speaking, +to produce of itself all the abnormal thermal phenomena. + +The deep-sea temperatures taken in the _Porcupine_ have an important +bearing upon this question, since they give us the depth and volume +of the mass of water which is heated above its normal temperature, +and which we must regard as the softener of the winds blowing on +the coasts of Europe. In the Bay of Biscay, after passing through a +shallow band superheated by direct radiation, a zone of warm water +extends to the depth of 800 fathoms, succeeded by cold water to a +depth of nearly two miles. In the Rockall channel the warm layer +has nearly the same thickness, and the cold underlying water is 500 +fathoms deep. Off the Butt of Lewis the bottom temperature is 5°.2 +C. at 767 fathoms, so that there the warm layer evidently reaches +to the bottom. In the Faroe channel the warm water forms a surface +layer, and the cold water underlies it, commencing at a depth of 200 +fathoms--567 fathoms above the level of the bottom of the warm water +off the Butt of Lewis. The cold water abuts against the warm--there +is no barrier between them. Part of the warm water flows over the +cold indraught, and forms the upper layer in the Faroe channel. What +prevents the cold water from slipping, by virtue of its greater +weight, under the warm water of the Butt of Lewis? It is quite +evident that there must be some force at work keeping the warm water +in that particular position, or, if it be moving, compelling it to +follow that particular course. The comparatively high temperature +from 100 fathoms to 900 fathoms I have always attributed to the +northern accumulation of the water of the Gulf Stream. The amount of +heat derived directly from the sun by the water as it passes through +any particular region, must be regarded, as I have already said, as +depending almost entirely upon latitude. Taking this into account, +the surface temperatures in what we were in the habit of calling the +“warm area” coincided precisely with Petermann’s curves indicating +the northward path of the Gulf Stream. + +[Illustration: Typical Forms of Snowflakes + +Showing the Tendency to take the Form of Six-Pointed Figures] + +The North Atlantic and Arctic seas form together a _cul de sac_ +closed to the northward, for there is practically no passage for +a body of water through Behring’s Strait. While, therefore, a large +portion of the water, finding no free outlet toward the northeast, +turns southward at the Azores, the remainder, instead of thinning +off, has rather a tendency to accumulate against the coasts bounding +the northern portions of the trough. We accordingly find that it +has a depth off the west coast of Iceland of at least 4,800 feet, +with an unknown lateral extension. Dr. Carpenter, discussing this +opinion, says: “It is to me physically inconceivable that this +surface film of _lighter_ (because warmer) water should collect +itself together again--even supposing it still to retain any excess +of temperature--and should burrow downward into the ‘trough,’ +_displacing colder and heavier water_, to a depth much greater than +that which it possesses at the point of its greatest ‘glory’--its +passage through the Florida Narrows. The upholders of this hypothesis +have to explain how such a recollection and dipping-down of the +Gulf Stream water is to be accounted for on physical principles.” I +believe that, as a rule, experimental imitations on a small scale are +of little use in the illustration of natural phenomena; a very simple +experiment will, however, show that such a process is possible. If we +put a tablespoonful of cochineal into a can of hot water, so as to +give it a red tint, and then run it through a piece of India-rubber +tube with a considerable impulse along the surface of a quantity of +cold water in a bath, we see the red stream widening out and becoming +paler over the general surface of the water till it reaches the +opposite edge, and very shortly the rapidly heightening color of a +band along the opposite wall indicates an accumulation of the colored +water where its current is arrested. If we now dip the hand into the +water of the centre of the bath, a warm bracelet merely encircles the +wrist; while at the end of the bath opposite the warm influx, the hot +water, though considerably mixed, envelops the whole hand. + +The North Atlantic forms a basin closed to the northward. Into the +corner of this basin, as into a bath--with a northeasterly direction +given to it by its initial velocity, as if the supply pipe of the +bath were turned so as to give the hot water a definite impulse--this +enormous flood is poured, day and night, winter and summer. When the +basin is full--and not till then--overcoming its northern impulse, +the surplus water turns southward in a southern eddy, so that there +is a certain tendency for the hot water to accumulate in the northern +basin, to “bank down” along the northeastern coasts. + +It is scarcely necessary to say that for every unit of water which +enters the basin of the North Atlantic, and which is not evaporated, +an equivalent must return. As cold water can gravitate into the +deeper parts of the ocean from all directions, it is only under +peculiar circumstances that any movement having the character of +a current is induced; these circumstances occur, however, in the +confined and contracted communication between the North Atlantic and +the Arctic Sea. Between Cape Farewell and North Cape there are only +two channels of any considerable depth, the one very narrow along +the east coast of Iceland, and the other along the east coast of +Greenland. The shallow part of the sea is entirely occupied, at all +events during summer, by the warm water of the Gulf Stream, except +at one point, where a rapid current of cold water, very restricted +and very shallow, sweeps round the south of Spitzbergen and then dips +under the Gulf Stream water at the northern entrance of the German +Ocean. + +This cold flow, at first a current, finally a mere indraught, affects +greatly the temperature of the German Ocean; but it is entirely +lost, for the slight current which is again produced by the great +contraction at the Strait of Dover has a summer temperature of 7°.5 +C. The path of the cold indraught from Spitzbergen may be readily +traced by the depressions in the surface isothermal lines, and in +dredging by the abundance of gigantic amphipodous and isopodous +crustaceans, and other well-known Arctic animal forms. + +From its low initial velocity the Arctic return current, or +indraught, must doubtless tend slightly in a westerly direction, and +the higher specific gravity of the cold water may probably even more +powerfully lead it into the deepest channels; or possibly the two +causes may combine, and in the course of ages the currents may hollow +out deep southwesterly grooves. The most marked is the Labrador +current, which passes down inside the Gulf Stream along the coasts +of Carolina and New Jersey, meeting it in the strange abrupt “cold +wall,” dipping under it as it issues from the Gulf, coming to the +surface again on the other side, and a portion of it actually passing +under the Gulf Stream, as a cold counter-current, into the Gulf of +Mexico. + +Fifty or sixty miles out from the west coast of Scotland, I believe +the Gulf Stream forms another, though a very mitigated, “cold wall.” +In 1868, after our first investigation of the very remarkable cold +indraught into the channel between Shetland and Faroe, I stated my +belief that the current was entirely banked up in the Faroe Channel +by the Gulf Stream passing its gorge. Since that time I have been led +to suspect that a part of the Arctic water oozes down the Scottish +coast, much mixed, and sufficiently shallow to be affected throughout +by solar radiation. About sixty or seventy miles from shore the +isothermal lines have a slight but uniform deflection. Within that +line types characteristic of the Scandinavian fauna are numerous in +shallow water, and in the course of many years’ use of the towing net +I have never met with any of the Gulf Stream pteropods, or of the +lovely Polycystina and Acanthometrina which absolutely swarm beyond +that limit. The difference in mean temperature between the east and +west coasts of Scotland, amounting to about 1° C., is almost somewhat +less than might be expected if the Gulf Stream came close to the +western shore. + +While the communication between the North Atlantic and the Arctic +Sea--itself a second _cul de sac_--is thus restricted, limiting the +interchange of warm and cold water in the normal direction of the +flow of the Gulf Stream, and causing the diversion of a large part +of the stream to the southward, the communication with the Antarctic +basin is as open as the day; a continuous and wide valley upward of +2,000 fathoms in depth stretching northward along the western coasts +of Africa and Europe. + +That the southern water wells up into this valley there could be +little doubt from the form of the ground; but here again we have +curious corroborative evidence in the remarkable reversal of the +curves of the isotherms. The temperature of the bottom water at 1,230 +fathoms off Rockall is 3°.22 C., exactly the same as that of water +at the same depth in the serial sounding, lat. 47° 38′ N., long. +12° 08′ W. in the Bay of Biscay, which affords a strong presumption +that the water in both cases is derived from the same source; and +the bottom water off Rockall is warmer than the bottom water in the +Bay of Biscay (2°.5 C.), while a cordon of temperature soundings +drawn from the northwest of Scotland to a point on the Iceland +shallow gives no temperature lower than 6°.5 C. This makes it very +improbable that the low temperature of the Bay of Biscay is due to +any considerable portion of the Spitzbergen current passing down +the west coast of Scotland; and as the cold current to the east of +Iceland passes southward considerably to the westward, as indicated +by the successive depressions in the surface isotherms, the balance +of probability seems to be in favor of the view that the conditions +of temperature and the slow movement of this vast mass of moderately +cold water, nearly two statute miles in depth, are to be referred to +an Antarctic rather than to an Arctic origin. + +The North Atlantic Ocean seems to consist first of a great sheet of +warm water, the general northerly reflux of the equatorial current. +Of this the greater part passes through the Strait of Florida, and +its northeasterly flow is aided and maintained by the anti-trades, +the whole being generally called the Gulf Stream. This layer is of +varying depths, apparently from the observations of Captain Chimmo +and others, thinning to a hundred fathoms or so in the mid-Atlantic, +but attaining a depth of 700 to 800 fathoms off the west coasts of +Ireland and Spain. Secondly, of a “stratum of intermixture” which +extends to about 200 fathoms in the Bay of Biscay, through which the +temperature falls rather rapidly; and, thirdly, of an underlying mass +of cold water, in the Bay of Biscay 1,500 fathoms deep, derived as +an indraught falling in by gravitation from the deepest available +source, whether Arctic or Antarctic. It seems at first sight a +startling suggestion, that the cold water filling deep ocean valleys +in the Northern Hemisphere may be partly derived from the southern; +but this difficulty, I believe, arises from the idea that there is a +kind of diaphragm at the equator between the northern and southern +ocean basins, one of the many misconceptions which follow in the +train of a notion of a convective circulation in the sea similar to +that in the atmosphere. There is undoubtedly a gradual elevation +of an intertropical belt of the underlying cold water, which is +being raised by the subsiding of still colder water into its bed +to supply the place of the water removed by the equatorial current +and by excessive evaporation; but such a movement must be widely +and irregularly diffused and excessively slow, not in any sense +comparable with the diaphragm produced in the atmosphere by the +rushing upward of the northeast and southeast trade-winds in the zone +of calms. Perhaps one of the most conclusive proofs of the extreme +slowness of the movement of the deep indraught is the nature of the +bottom. Over a great part of the floor of the Atlantic a deposit +is being formed of microscopic shells. These with their living +inhabitants differ little in specific weight from the water itself, +and form a creamy flocculent layer, which must be at once removed +wherever there is a perceptible movement. In water of moderate depth, +in the course of any of the currents, this deposit is entirely +absent, and is replaced by coarser or finer gravel. + +It is only on the surface of the sea that a line is drawn between the +two hemispheres by the equatorial current, whose effect in shedding a +vast intertropical drift of water on either side as it breaks against +the eastern shores of equatorial land may be seen at a glance on the +most elementary physical chart. + +The Gulf Stream loses an enormous amount of heat in its northern +tour. At a point 200 miles west of Ushant, where observations at the +greatest depths were made on board the _Porcupine_, a section of +the water of the Atlantic shows three surfaces at which interchange +of temperature is taking place. First, the surface of the sea--that +is to say, the upper surface of the Gulf Stream layer--is losing +heat rapidly by radiation, by contact with a layer of air which is +in constant motion and being perpetually cooled by convection, and +by the conversion of water into vapor. As this cooling of the Gulf +Stream layer takes place principally at the surface, the temperature +of the mass is kept pretty uniform by convection. Secondly, the +band of contact of the lower surface of the Gulf Stream water with +the upper surface of the cold indraught. Here the interchange of +temperature must be very slow, though that it does take place is +shown by the slight depression of the surface isotherms over the +principal paths of the indraught. But there is a good deal of +intermixture extending through a considerable layer. The cold water +being beneath, convection in the ordinary sense can not occur, +and interchange of temperature must depend mainly upon conduction +and diffusion, causes which in the case of masses of water must +be almost secular in their action, and probably to a much greater +extent upon mixture produced by local currents and by the tides. The +third surface is that of contact between the cold indraught and the +bottom of the sea. The temperature of the crust of the earth has been +variously calculated at from 4° to 11° C., but it must be completely +cooled down by anything like a movement and constant renewal of cold +water. All we can say, therefore, is that contact with the bottom can +never be a source of depression of temperature. As a general result +the Gulf Stream water is nearly uniform in temperature throughout the +greater part of its depth; there is a marked zone of intermixture at +the junction between the warm water and the cold, and the water of +the cold indraught is regularly stratified by gravitation; so that +in deep water the contour lines of the sea-bottom are, speaking +generally, lines of equal temperature. Keeping in view the enormous +influence which ocean currents exercise in the distribution of +climates at the present time, I think it is scarcely going too far to +suppose that such currents--movements communicated to the water by +constant winds--existed at all geological periods as the great means, +I had almost said the sole means, of producing a general oceanic +circulation, and thus distributing heat in the ocean. They must have +existed, in fact, wherever equatorial land interrupted the path of +the drift of the trade-winds. Wherever a warm current was deflected +to north or south from the equatorial belt a polar indraught crept in +beneath to supply its place; and the ocean consequently consisted, +as in the Atlantic and doubtless in the Pacific at the present day, +of an upper warm stratum and a lower layer of cold water becoming +gradually colder with increasing depth. + +I must repeat that I have seen as yet no reason to modify the opinion +which I have consistently held from the first, that the remarkable +conditions of climate on the coasts of Northern Europe are due in a +broad sense solely to the Gulf Stream. That is to say that, although +movements, some of them possibly of considerable importance, must be +produced by differences of specific gravity, yet the influence of the +great current which we call the Gulf Stream, the reflux of the great +equatorial current, is so paramount as to reduce all other causes to +utter insignificance. + + + + + THE PHOSPHORESCENCE OF THE SEA + --G. HARTWIG + + +He who still lingers on the shore after the shades of evening have +descended not seldom enjoys a most magnificent spectacle; for lucid +flashes burst from the bosom of the waters, as if the sea were +anxious to restore to the darkened heavens the light it had received +from them during the day. On approaching the margin of the rising +flood to examine more closely the sparkling of the breaking wave, the +spreading waters seem to cover the beach with a sheet of fire. + +Each footstep over the moist sands elicits luminous star-like points +and a splash in the water resembles the awakening of slumbering +flames. The same wonderful and beauteous aspect frequently gladdens +the eye of the navigator who plows his way through the wide deserts +of ocean, particularly if his course leads him through the tropical +seas. + +“When a vessel,” says Humboldt, “driven along by a fresh wind, +divides the foaming waters, one never wearies of the lovely spectacle +their agitation affords; for, whenever a wave makes the ship incline +sidewise, bluish or reddish flames seem to shoot upward from the +keel. Beautiful beyond description is the sight of a troop of +dolphins gamboling in the phosphorescent sea. Every furrow they draw +through the waters is marked by streaks of intense light. In the Gulf +of Cariaco, between Cumana and the peninsula of Maniquarez, this +scene has often delighted me for hours.” + +But even in the colder oceanic regions the brilliant phenomenon +appears from time to time in its full glory. During a dark and stormy +September night, on the way from the Sealion Island, Saint George, +to Unalashka, Chamisso admired as beautiful a phosphorescence of +the ocean as he had ever witnessed in the tropical seas. Sparks of +light, remaining attached to the sails that had been wetted by the +spray, continued to glow in another element. Near the south point +of Kamtchatka, at a water temperature hardly above freezing point, +Ermann saw the sea no less luminous than during a seven months’ +sojourn in the tropical ocean. This distinguished traveler positively +denies that warmth decidedly favors the luminosity of the sea. + +At Cape Colborn, one of the desolate promontories of the desolate +Victoria Land, the phosphoric gleaming of the waves, when darkness +closed in, was so intense that Simpson assures us he had seldom +seen anything more brilliant. The boats seemed to cleave a flood of +molten silver, and the spray, dashed from their bows before the fresh +breeze, fell back in glittering showers into the deep. + +Mr. Charles Darwin paints in vivid colors the magnificent spectacle +presented by the sea while sailing in the latitude of Cape Horn on +a very dark night. There was a fresh breeze, and every part of the +surface, which during the day is seen as foam, now glowed with a +pale light. The vessel drove before her bows two billows of liquid +phosphorus, and in her wake she was followed by a milky train. As +far as the eye reached, the crest of every wave was bright, and +the sky above the horizon, from the reflected glare of these livid +flames, was not so utterly obscure as over the rest of the heavens. + +While _La Venus_ was at anchor before Simon’s Town, the breaking +of the waves produced so strong a light that the room in which the +naturalists of the expedition were seated was illumined as by sudden +flashes of lightning. Although more than fifty paces from the beach +when the phenomenon took place, they tried to read by this wondrous +oceanic light, but the successive glimpses were of too short duration +to gratify their wishes. + +Thus we see the same nocturnal splendor which shines forth in the +tropical seas and gleams along our shores burst forth from the Arctic +waters, and from the waves that bathe the southern promontories of +the Old and the New Worlds. + +But what is the cause of the beautiful phenomenon so widely spread +over the face of the ocean? How comes it that at certain times flames +issue from the bosom of an element generally so hostile to their +appearance? + +Without troubling the reader with the groundless surmises of ancient +naturalists, or repeating the useless tales of the past, I shall at +once place myself with him on the stage of our actual knowledge of +this interesting and mysterious subject. + +It is now no longer a matter of doubt that many of the inferior +marine animals possess the faculty of secreting a luminous matter, +and thus adding their mite to the grand phenomenon. When we +consider their countless multitudes, we shall no longer wonder at +such magnificent effects being produced by creatures individually so +insignificant. + +In our seas it is chiefly a minute gelatinous animal, the _Noctiluca +miliaris_, most probably an aberrant member of the infusorial group, +which, as it were, repeats the splendid spectacle of the starry +heavens on the surface of the ocean. In form it is nearly globular, +presenting on one side a groove, from the anterior extremity of which +issues a peculiar curved stalk or appendage marked by transverse +lines, which might seem to be made use of as an organ of locomotion. +Near the base of this tentacle is placed the mouth, which passes into +a dilatable digestive cavity, leading, according to Mr. Huxley, to +a distinct anal orifice. From the rather firm external coat proceed +thread-like prolongations through the softer mass of the body, so as +to divide it into irregular chambers. This little creature, which is +just large enough to be discerned by the naked eye when the water +in which it may be swimming is contained in a glass jar exposed to +the light, seems to feed on diatoms, as this loricæ may frequently +be detected in its interior. It multiplies by spontaneous fission, +and the rapidity of this process may be inferred from the immensity +of its numbers. A single bucket of luminous sea-water will often +contain thousands, while for miles and miles every wave breaking on +the shore expands in a sheet of living flame. It was first described +by Forster in the Pacific Ocean; it occurs on all the shores of the +Atlantic; and the Polar Seas are illumined by its fairy light. +“The nature of its luminosity,” says Dr. Carpenter, “is found by +microscopic examination to be very peculiar; for what appears to the +eye to be a uniform glow is resolvable under a sufficient power into +a multitude of evanescent scintillations, and these are given forth +with increased intensity whenever the body of the animal receives any +mechanical shock.” + +The power of emitting a phosphorescent light is widely diffused, +both among the free-swimming and the sessile _Cœlenterata_. Many +of the _Physophoridæ_ are remarkable for its manifestation, and a +great number of the jelly-fishes are luminous. Our own _Thaumantias +lucifera_, a small and by no means rare medusid, displays the +phenomenon in a very beautiful manner, for, when irritated by +contact of fresh water, it marks its position by a vivid circlet of +tiny stars, each shining from the base of a tentacle. A remarkable +greenish light, like that of burning silver, may also be seen to glow +from many of our Sertularians, becoming much brighter under various +modes of excitation. + +Among the _Ctenophora_ the large _Cestum Veneris_ of the +Mediterranean is specially distinguished for its luminosity, and +while moving beneath the surface of the water gleams at night like a +brilliant band of flame. + +The Sea-pens are eminently phosphorescent, shining at night with a +golden-green light of a most wonderful softness. When touched, every +branchlet above the shock emits a phosphoric glow, while all the +polyps beneath remain in darkness. When thrown into fresh water +or alcohol, they scatter sparks about in all directions, a most +beautiful sight; dying, as it were, in a halo of glory. + +But of all the marine animals the Pyrosomas, doing full justice to +their name (fire bodies), seem to emit the most vivid coruscations. +Bibra relates in his _Travels to Chili_ that he once caught half a +dozen of these remarkable light-bearers, by whose phosphorescence +he could distinctly read their own description in a naturalist’s +vade-mecum. Although completely dark when at rest, the slightest +touch sufficed to elicit their clear blue-green light. During a +voyage to India, Mr. Bennett had occasion to admire the magnificent +spectacle afforded by whole shoals of Pyrosomas. The ship, proceeding +at a rapid rate, continued during an entire night to pass through +distinct but extensive fields of these mollusks, floating and +glowing as they floated on all sides of her course. Enveloped in a +flame of bright phosphorescent light, and gleaming with a greenish +lustre, the Pyrosomas, in vast sheets, upward of a mile in breadth, +and stretching out till lost in the distance, presented a sight the +glory of which may be easily imagined. The vessel, as it chased +the gleaming mass, threw up strong flashes of light, as if plowing +through liquid fire, which illuminated the hull, the sails, and the +ropes with a strange, unearthly radiance. + +In his memoir on the Pyrosoma, M. Péron describes with lively +colors the circumstances under which he first made its discovery, +during a dark and stormy night, in the tropical Atlantic. “The +sky,” says this distinguished naturalist, “was on all sides loaded +with heavy clouds; all around the obscurity was profound; the wind +blew violently; and the ship cut her way with rapidity. Suddenly we +discovered at some distance a great phosphorescent band stretched +across the waves, and occupying an immense tract in advance of the +ship. Heightened by the surrounding circumstances, the effect of this +spectacle was romantic, imposing, sublime, riveting the attention of +all on board. Soon we reached the illuminated tract, and perceived +that the prodigious brightness was certainly and only attributable to +the presence of an innumerable multitude of largish animals floating +with the waves. From their swimming at different depths they took +apparently different forms--those at the greatest depths were very +indefinite, presenting much the appearance of great masses of fire, +or rather enormous, red-hot cannon-balls; while those more distinctly +seen near the surface perfectly resembled incandescent cylinders of +iron. + +“Taken from the water, these animals entirely resembled each other +in form, color, substance, and the property of phosphorescence, +differing only in their sizes, which varied from three to seven +inches. The large, longish tubercles with which the exterior +of the Pyrosomas was bristled were of a firmer substance, and +more transparent than the rest of the body, and were brilliant +and polished like diamonds. These were the principal scene of +phosphorescence. Between these large tubercles, smaller ones, +shorter and more obtuse, could be distinguished; these also were +phosphorescent. Lastly, in the interior of the substance of the +animal, could be seen, by the aid of the transparency, a number of +little, elongated, narrow bodies (viscera), which also participated +in a high degree in the possession of phosphoric light.” + +In the Pholades or Lithodomes, that bore their dwellings in hard +stone, as other shell-fish do in the loose sands, the whole mass of +the body is permeated with light. Pliny gives us a short but animated +description of the phenomenon in the edible date-shell of the +Mediterranean (_Pholas dactylus_): + +“It is in the nature of the pholades to shine in the darkness with +their own light, which is the more intense as the animal is more +juicy. While eating them, they shine in the mouth and on the hands, +nay, even the drops falling from them upon the ground continue to +emit light, a sure proof that the luminosity we admire in them is +associated with their juice.” + +Milne-Edwards found this observation perfectly correct, for, wishing +to place some living pholades in alcohol, he saw a luminous matter +exude from their bodies, which, on account of its weight, sank in +the liquid, covering the bottom of the vessel, and there forming a +deposit as shining as when it was in contact with the air. + +Several kinds of fishes likewise possess the luminous faculty. The +sunfish, that strange deformity emits a phosphoric gleam; and a +species of Gunard (_Trigla lucerna_) is said to sparkle in the night, +so as to form fiery streams through the water. + +With regard to the luminosity of the larger marine animals, Ermann, +however, remarks that he so often saw small luminous crustacea in +the abdominal cavity of the transparent _Salpa pinnata_ that it may +well be asked whether the phosphorescence of the larger creatures is +not in reality owing to that of their smaller companions. + +According to Mr. Bennett--_Whaling Voyage Round the Globe_--a +species of shark first discovered by himself is distinguished by +an uncommonly strong emission of light. When the specimen, taken +at night, was removed into a dark apartment, it afforded a very +interesting spectacle. The entire inferior surface of the body and +head emitted a vivid and greenish phosphorescent gleam, imparting +to the creature by its own light a truly ghastly and terrific +appearance. The luminous effect was constant, and not perceptibly +increased by agitation or friction. When the shark expired (which +was not until it had been out of the water more than three hours), +the luminous appearance faded entirely from the abdomen, and more +gradually from other parts, lingering longest around the jaws and on +the fins. + +The only part of the under surface of the animal which was free from +luminosity was the black collar round the throat; and while the +inferior surface of the pectoral, anal, and caudal fins shone with +splendor, their superior surface (including the upper lobe of the +tail fin) was in darkness, as were also the dorsal fins and the back +and summit of the head. + +Mr. Bennett is inclined to believe that the luminous power of this +shark resides in a peculiar secretion from the skin. It was his +first impression that the fish had accidentally contracted some +phosphorescent matter from the sea, or from the net in which it +was captured; but the most rigid investigation did not confirm +this suspicion, while the uniformity with which the luminous gleam +occupied certain portions of the body and fins, its permanence during +life, and decline and cessation upon the approach and occurrence +of death, did not leave a doubt in his mind but that it was a +vital principle essential to the economy of the animal. The small +size of the fins would seem to denote that this fish is not active +in swimming; and, since it is highly predaceous and evidently of +nocturnal habits, we may perhaps indulge in the hypothesis that the +phosphorescent power it possesses is of use to attract its prey, upon +the same principle as the Polynesian islanders and others employ +torches in night-fishing. + +Some of the lower sea-plants also appear to be luminous. Thus, over +a space of more than 600 miles (between lat. 8° N. and 2° S.), Meyen +saw the ocean covered with phosphorescent _Oscillatoria_, grouped +together into small balls or globules, from the size of a poppy-seed +to that of a lentil. + +But if the luminosity of the ocean generally proceeds from living +creatures, it sometimes also arises from putrefying organic fibres +and membranes, resulting from the decomposition of these living light +bearers. “Sometimes,” says Humboldt, “even a high magnifying power is +unable to discover any animals in the phosphorescent water, and yet +light gleams forth wherever a wave strikes against a hard body and +dissolves in foam. The cause of this phenomenon lies then most likely +in the putrefying fibres of dead mollusks, which are mixed with the +waters in countless numbers.” + +Summing up the foregoing in a few words, it is thus an indisputable +fact that the phosphorescence of the sea is by no means an electrical +or magnetic property of the water, but exclusively bound to organic +matter, living or dead. But although thus much has been ascertained, +we have as yet only advanced one step toward the unraveling of +the mystery, and its prominent cause remains an open question. +Unfortunately, science is still unable to give a positive answer, and +we are obliged to be content with a more or less plausible hypothesis. + +We know as little of what utility marine phosphorescence may be. Why +do the countless myriads of Mammariæ gleam and sparkle along our +coasts? Is it to signify their presence to other animals, and direct +them to the spot where they may find abundance of food? So much is +certain, that so grand and widespread a phenomenon must necessarily +serve some end equally grand and important. + +As the phosphorescence of the sea is owing to living creatures, it +must naturally show itself in its greatest brilliancy when the ocean +is at rest; for during the daytime we find the surface of the waters +most peopled with various animals when only a slight zephyr glides +over the sea. In stormy weather, the fragile or gelatinous world of +the lower marine creatures generally seek a greater depth, until the +elementary strife has ceased, when it again loves to sport in the +warmer or more cheerful superficial waters. + +In the tropical zone, Humboldt saw the sea most brilliantly luminous +before a storm, when the air was sultry and the sky covered with +clouds. In the North Sea we observe the phenomenon most commonly +during fine, tranquil autumnal nights; but it may be seen at +every season of the year, even when the cold is most intense. Its +appearance is, however, extremely capricious; for, under seemingly +unaltered circumstances, the sea may one night be very luminous +and the next quite dark. Often months, even years, pass by without +witnessing it in full perfection. Does this result from a peculiar +state of the atmosphere, or do the little animals love to migrate +from one part of the coast to another? + +It is remarkable that the ancients should have taken so little +notice of oceanic phosphorescence. The _Periplus_ of Hanno contains, +perhaps, the only passage in which the phenomenon is described. + +To the south of Cerne the Carthaginian navigator saw the sea burn, as +it were, with streams of fire. Pliny, in whom the miracle (miraculum, +as he calls it) of the date-shell excited so lively an admiration, +and who must often have seen the sea gleam with phosphoric light, +as the passage proves where he mentions in a few dry words the +luminous gurnard (_lucerna_) stretching out a fiery tongue, has +no exclamation of delight for one of the most beautiful sights of +nature. Homer also, who has given us so many charming descriptions of +the sea in its ever-changing aspects, and who so often leads us with +long-suffering Ulysses through the nocturnal floods, never once makes +them blaze or sparkle in his immortal hexameters. Even modern poets +mention the phenomenon but rarely. Camoens himself, whom Humboldt, +on account of his beautiful oceanic descriptions, calls, above all +others, the “poet of the sea,” forgets to sing it in his _Lusiad_. +Byron in his _Corsair_ has a few lines on the subject: + + “Flash’d the dipt oars, and, sparkling with the stroke, + Around the waves phosphoric brightness broke;” + +but contents himself, as we see, with coldly mentioning a phenomenon +so worthy of all a poet’s enthusiasm. In Coleridge’s wondrous ballad +of _The Ancient Mariner_ we find a warmer description: + + “Beyond the shadow of the ship + I watch’d the water-snakes: + They moved in tracks of shining white, + And, when they rear’d, the elfin light + Fell off in hoary flakes. + + “Within the shadow of the ship + I watch’d their rich attire-- + Blue, glossy green, and velvet black: + They coiled and swam, and every track + Was a flash of golden fire.” + +These, indeed, are lines whose brilliancy emulates the splendor of +the phenomenon they depict, but, even they are hardly more beautiful +than Crabbe’s admirable description: + + “And now your view upon the ocean turn, + And there the splendor of the waves discern; + Cast but a stone, or strike them with an oar, + And you shall flames within the deep explore; + Or scoop the stream phosphoric as you stand, + And the cold flames shall flash along your hand; + When, lost in wonder, you shall walk and gaze + On weeds that sparkle, and on waves that blaze.” + +Or the graphic numbers of Sir Walter Scott: + + “Awak’d before the rushing prow, + The mimic fires of ocean glow, + Those lightnings of the wave; + Wild sparkles crest the broken tides, + And dashing round, the vessel’s sides + With elfish lustre lave; + While, far behind, their vivid light + To the dark billows of the night + A blooming splendor gave.” + + + + + THE SEASHORE + --P. MARTIN DUNCAN + + +The seashore is the debatable ground where the sea is constantly +striving to wear away the land. It is the present limit to the ocean +and sea, and a little beyond, for it reaches inland further than the +wildest waves and the highest tides can attain. + +Where the seashore begins and ends is a matter of opinion; but all +of it is influenced in some way or other by the sea. In some places, +high cliffs or rocks keep the sea from driving in upon the land; +they are lofty, and may reach for miles along the coast. The high +tide comes up their steep faces for many yards, and when it retires +a rocky strip is seen at their feet, and thence a breadth of rock, +shingle, or sand leads down with a greater or less slope to the +water’s edge. Here there can be no doubt how far the shore reaches +inland, for the cliffs limit it. In other parts of our own and other +maritime countries, there may be no high land on the coast; but +marshes and low lands, with, or without sand-hills, form barriers to +the incursion of the sea. The highest tides have their limit in those +places, but the wash of the sea and the spray, together with the +drainage of the sea into the land, make the water saltish for some +distance inland, and the earth close by is sodden with salt. Then, +long stretches of mud or of sand form the slope, over which the sea +rolls up to the land, and which is exposed and remains more or less +wet at low tide. + +In these low-lying parts of the coast the shore is not very +distinctly separated from the land, and often miles of swamp, marsh, +and sand-banks are invaded by the sea during storms and very high +tides. The ditches near the sea contain salt or brackish water, and +the whole of this kind of coast-line has a peculiar and desolate +appearance. If these two kinds of coast are taken as the extremes, +all the varieties of seashores will fit in between them; but still it +will appear that while in some the limit between the land and the sea +is very decided, in others it is not so. + +Seaward the shore is very variable in its extent. In some places +it may barely exist, or may only be a ledge of rock, between the +cliff, the high land, and the water; and in others, miles of sand, +shingle, and mud may be between the furthest reach of the waves and +the limit of the low tide. The commonest examples of shores are +those which are between these extremes. Some seashores slope very +gradually to the sea and their extent is then usually great; and +others, which are limited in their breadth, are more precipitous. +Perhaps it is best to say that a seashore is the part of a coast +which, at some time or other, is covered or uncovered by the sea; +and that it has an extension inland, where the spray and wind are +felt and act on the land, and also seaward, where some shore is only +uncovered during excessively low tides. According to this view, it +is possible to portion out a seashore into a greater or less number +of breadths, which may be placed, side by side, from the land to the +sea. First, a breadth will pass along the coast, and will contain +the marshy, swampy land, or the hard rock down to the edge of the +highest tide-mark. It may be miles across, or only a few feet in +extent. Secondly, a breadth will be found between this last and the +sea, where it is highest during common tides and storms. Thirdly, a +breadth will exist four times in the twenty-four hours as dry land, +and for the rest of the time it will be beneath the waves, and this +is situated between ordinary high and low-tide marks. Finally, a +breadth will be between this last and the everlasting sea; it is +narrow, and is only uncovered for a few hours, in the months of the +year when there are what are called “low spring tides.” These four +breadths are termed zones, or belts; and in common language the first +is the beach and coast-line, the second is the shore, the third is +the tide-shore, and the fourth is “low spring shore.” + +Differing in their extent, and in the nature of their surface, +in every few miles of the coast of a maritime country like Great +Britain, the zones have their peculiar animals and plants, and +waifs and strays--the wreckage of the sea, of its floor, and of the +coast-line. When the whole of the shore slopes very rapidly to the +sea, the third and fourth zones are small in extent, but when the +slope is gradual, they are large. And when the tide rises much and +falls correspondingly, the third zone is usually uncovered but for +a short time. The tide usually moves along the shore, and does not +simply come in on to the land and recede; for one tide moves in one +direction and the next in the opposite. Thus floating substances are +carried along the coast for miles by the rising tide, and come back +again, more or less, with the falling tide. + +Tide, wind, and wave forever act on the surface of the zones, but +their action is the greatest on those which are landward. There are +other wreckers of the coast; for the heat of the sun, the winter’s +frost, the rain, and the chemical action of the air, one and all +crumble and break off pieces of rock or earth. These fall on to the +tidal shore, and are rolled here and there, and up and down, to be +turned into mud, sand, and pebbles. The cliffs and bold headlands are +worn year by year, and during centuries they lose much, and retire +landward. Needles and “no man’s lands” stand out on the shore, or out +at sea, testifying to the former extension of the land; and shore +exists where there was once high solid rock. The shore consists +of the worn surface of the old land, rock, or earth, and this is +usually hidden by stone or stuff which has fallen from the cliffs, +and by sand, or mud, or pebble and stone, which the tide has swept +along. But often the jagged or rounded remains of the former rock +project out of the sand, mud, and stone on the shore, and they may +be bare, or covered with sea-weed. In other spots, the hard rock is +hollowed out into places which let the water stand in them like so +many puddles, pools, and ponds, when the tide has gone down. These +are often crowded with marine plants and animals of the shore. The +rolling stones, the wash of the tide, and the rush and drawback of +the waves, are ever wearing off the surface of the shore and grooving +it, or planing it flat, and in some places where the stones do not +collect, this is very evident; but where they form great masses of +pebbles or shingle, it can not be readily seen. + +There are many shores around Great Britain, where the rock is hard, +which are rarely covered with pebbles, bowlders, and sand; and the +sea-weed grows on them and protects them against the sea. But usually +the rock is only exposed here and there, and the stones which collect +and cover much of it come from a distance, and are on the move at +every tide. In some places, where the coast is composed of clay or +soft sandstone, the shore is muddy, soft, and may be uncovered or +covered by stones. + +The wear of the sea is but little seen in such places as this, and +still less so where the coast is low and flat, and the shore is very +extensive and the water is shallow for a long distance. In fact, +on many of these flat shores, instead of erosion taking place, the +sea is adding to the land by depositing. This is particularly the +case at the entrance of great, and of many small rivers. Their mud +collects in the shallows at their mouths, and is added to by sand +and shingle, so that land grows seaward, instead of the reverse. The +seashore is then, usually, uninviting and often consists of large +mud flats. Again, in some localities, where much sand collects on +the surface of the rock forming the seashore, it may be “quick” in +many places. The rising tide gets under the sand, which suddenly +becomes like so much sand and water, and the falling tide leaves it +hard for a while. The ordinary condition of a sandy shore is either +that of a number of very slightly rounded stretches of sand, with +drainage-streams between them, or it is pretty hard, readily dug +into, and marked on the surface by ripples. The ripple-mark on sand +always strikes the observer; it represents little ripple-like waves, +wonderfully regular, and each has a ridge and a valley. They are very +lasting, but disappear on the slightest movement of the wet sand as +the tide comes in. These little ridges and valleys are not found +when the water covers the sand at a considerable depth, but they are +especially seen between high and low spring-tide limit. Such marks +can be made, artificially, with sand, for instance, on the bottom +of a large basin. If some sand is placed on the bottom, and water +be poured in, and the edge of the basin be pushed, a to-and-fro +movement of the water will occur, and it will be continued down to +the sand. As the motion ceases, the sand will be seen to collect +in ridges, side by side, and they will be perfect when the motion +stops. Motion of the sea-water in one direction over soft sand will +not produce ripple-mark well, but a slight to-and-fro movement will +do it to perfection. Infinitely more wonderful than these ripples +are the pebble beaches, for they often extend for many miles, and +have a very considerable thickness. Worn, in the first instance, +from distant rocks, born of huge bowlders, which the mighty waves +laden with rolling stones have broken down, the pebble is formed +by rolling against others, and the result of its wear and tear is +carried off in the form of sand. They travel miles and miles along +the coast with the tide, and therefore it is very common to find one +kind of rock forming the coast-line, and the shore close by having +pebbles made up of stone which is not known to be near at hand. Thus, +on the coast of South Devon, the red rocks form the coast-line; +they are sandy, and are covered in some places by a beautiful green +vegetation. The sea is often of the brightest blue, or gray, when the +sky is not much tinted with color. But the sea covering the shore at +high tide looks whitish, and this is produced by the white and light +slate-colored pebbles which reach up close to the red rocks. They are +not made up of red sand; on the contrary, they are of gray and bluish +limestone, and come from rocks which are situated miles to the west. +Further east, the Chesil Bank is seen, and it is an enormous shore +of pebbles, which have been carried along the coast and have found +an uncertain resting-place there. Every tide makes more sand out of +the hardest pebbles, as they knock one against the other and wear +away, and the sand already made scrubs them as it is hurried hither +and thither by the waves. In some places where the sea is giving up +rather than taking off land, the sand which is cast up may be the +result of the wear of distant pebble-making, or it may be composed of +myriads of broken tiny shells which once lived in shallow water. + +It has been already stated that the sea is encroaching on the land +in some parts of England, and that it does not do so in others, +while it appears to be giving place to land elsewhere. In the first +instance, the seashore must grow, as it were, must increase landward, +and it really does so at different rates, in different parts of the +country. In some parts of the coast a yard is lost every year and the +sea comes in on the land so much the more. But all the space once +occupied by cliff and rock is soon worn by the sea and is covered +gradually by the tide, and after years have elapsed this _fore-shore_ +is deepened seaward by the rolling stone and rushing waves, so that +the visible beach or shore diminishes in size, unless a corresponding +landward extension takes place. Although the cliffs and rocks fall, +and their remains are swept away from the level of the shore, by +currents, tides, and waves; yet, as has already been noticed, much +of the ruined surface, leveled down as it has been, is covered up by +relics of their wear and tear or by stone brought from a distance. It +is only after some severe gale of wind, accompanied by a very high +tide, that these stones and covering-up relics are swept away and the +old rock-surface comes in view. All these matters are of importance, +for the living creatures of the seashore depend upon the state of +things, in each of the zones, for their ability to exist and flourish. + +Where the coast has been low and the sea has gradually encroached, +the remains of stumps of trees are often exposed after a gale. Then +what is called part of a submarine forest is opened to the sight. +There are many of them around England and especially on the coast of +Norfolk and Essex, on the east; in many places on the south coast +as far as Torbay; and on the west they are found in the Bristol +Channel, and about Holyhead and the river Mersey. Sometimes it +appears that the sunken forest has not been altogether produced by +the encroachment of the sea on the land, and that sinking of the +coast, or slipping of part of it, has caused the event. When the sea +comes in on the land, it wears everything before it, and any forest +land would in most instances be completely wrecked and the roots of +the great trees would be worn and torn out of the soft earth and +carried off to sea by the waves, tides, and currents. On looking at +some smaller forests which are laid bare at very low tides, it is +found that they consist of stumps of trees of great size, whose roots +are still in the clay in which they grew, and a quantity of mud and +sand is between the stumps and protects them from the usual action of +water on submerged land. It appears that some movement of the earth’s +crust had caused the coast to sink down, and then the sea invaded +without wearing off the land. The trees were ruined by the sea-water, +and broken off, and the mud, sand, and stone collected around the +stumps. + +It is not uncommon to see collections of stone and shells high up on +the face of a rock or cliff, and when they are carefully examined +they are found to resemble a bit of a shore or a piece of the beach, +hoisted up many feet above the present line of the waves and tides. + +They are called raised beaches, and they were formed by an upheaval +of part of the coast with its shore during movements in the crust of +the globe. There was a shore and a cliff, as there may be now, and +the whole was pushed up some twenty, thirty, or more than a hundred +feet beyond the reach of the highest tides and waves. In years past +the waves broke upon the cliff beneath the upraised portion, and wore +it away bit by bit; and then the air and sun acted with the rain in +wearing it, and now only a portion remains. + +Every coast-line is subject to these sinkings-down and upheavals, +and of course a seashore is produced rapidly, and is made broad and +shallow during the first kind of occurrence, and is stopped and has +to be formed afresh during the last. As these remarkable movements +of the outside of the globe are not universal, and affect some parts +of a coast more than others, they will tend to give great variety +to the seashores of a country. Together with the varying action of +the tides, waves, and currents upon cliffs and rocks of different +stones and earths, and of many hardnesses, these movements have made +the shores of Great Britain very curiously varied in their size and +character. + +It must be remembered that as new shores are formed, or old ones +are extended, the zones are kept within their bounds, and that as +one zone creeps in on the land, those to the seaward move up also; +so that where there was once a between-tide zone there may now be +deep water. This change in the position of zones is very important; +for certain animals and plants of the shore only live in certain +zones, and their increase or decrease in numbers depends upon the +corresponding state of their special locality. + + + + + III.--THE ATMOSPHERE + + + + + THE OCEAN OF AIR + --AGNES GIBERNE + + +Our earth has many robes. Closely-fitting garments come first, of +brown soil or gray rock and green grass, with wide liquid underskirts +of deep blue filling up the spaces between. Outside these are +coverings more wonderful still; fragile, yet strong, transparent, +almost invisible, folded around layer upon layer, or, as one +might say, veil upon veil, each more gossamer-like than the last. +These form earth’s surrounding atmosphere--a substance pervading +everything, found everywhere. One may travel from the equator to the +poles, one may journey by sea or by land, one may soar high in a +balloon or descend deep into a mine, but one can never in this world +go to a place where the atmosphere is not. + +A substance--for air can be felt; air has weight; air occupies space; +air, like any other body, can be made hot or cold; air is composed of +particles of substantial matter. Air has a faint bluish tint, which +on a sunshiny day becomes in the sky a very pure and deep blue. This +tint is not believed to be the natural color of the atmosphere. Were +it so, the air would merely act the part of a blue pane of glass, +rendering the white light of the sun blue as it reaches our eyes; +but the blue of the atmosphere is known to be a reflected blue. + +If reflected, there must be something in the atmosphere to reflect +it; and such indeed is the case. Perfectly pure air would doubtless +be without color, but perfectly pure air we do not find. The whole +atmosphere is full of multitudinous minute specks, so small as to be +in themselves invisible, so light as to remain aloft. To the presence +of these the blue tint is believed to be due. They scatter the light +of the sun, and produce the blue effect. + +A beam of strong white light, caused to pass through a liquid which +contains a large supply of minute floating particles, is affected +by them in a like manner. The short blue waves are more abundantly +reflected than the long red waves; and so the water seems to be blue. +This explanation serves for the deep-blue color of the ocean, as well +as for the blue of the atmosphere. + +The whole earth is surrounded by this marvelous air-ocean; an +ocean of gaseous matter, at least one hundred times as deep as the +water-ocean. At the bottom of the gaseous ocean we small human +creatures crawl about, commonly on flat lower levels--the ocean +bottom, in fact. Sometimes, with much toil and trouble, we climb the +little ridges and mounds called “mountains”; little compared with the +depth of the atmosphere, though not little compared with ourselves. +The highest mountain-peaks of even the vast Himalayas lie low down +near the bottom of the ocean of air. + +But the very extent of the ocean of air adds to our difficulty +in studying its nature. All observations that we can make must be +limited by the state of the atmosphere just around ourselves. We +can never get out of and beyond the atmosphere, so as to see it as +a whole. At any time a slight local fog is enough to put a stop +altogether to such observations, beyond the unpleasant experience of +the fog itself. + +It used to be supposed that the atmosphere reached only to a height +of about fifty miles above earth’s surface. Of late years the opinion +has gained ground that the atmosphere reaches to a height certainly +of two or three hundred miles, probably of four or five hundred, +possibly a good deal more. But the condition of the air far above is +different from that of the air in lower levels, where we live and +breathe. The higher we ascend, the more thin or “rare” becomes the +air. A less quantity fills a certain space up there than down here. +The particles float further apart one from another. + +This difference in the density of the air is chiefly due to +attraction. Each separate air-particle is drawn steadily earthward by +the force of gravitation, and that force is stronger on the surface +of earth than at a distance. The closer to earth, the heavier the +pull; the further from earth, the less the pull. Besides the actual +attraction of the earth drawing the air-particles downward, there is +the great weight of the whole atmosphere above, caused by the same +attraction. Miles and miles of air overhead press mightily downward, +packing tightly together the lower layers of air near to earth’s +surface. + +Without this pressure of the overlying atmosphere, the air down +here would not be nearly so dense as it is; and, indeed, would not +be fitted to support life. A man ascending a mountain or rising in a +balloon leaves heavy layers of air below, and has an ever-lightening +weight above, so that the atmosphere around him becomes constantly +more thin, more difficult to breathe. + +In the beginning of the last century Humboldt made a vigorous attempt +to scale Chimborazo, one of the loftiest of the Andes. He and his +party suffered severely from sickness, giddiness, and difficulty in +breathing, and the attempt proved a failure. Not till over seventy +years later was the ascent actually accomplished by Mr. Whymper. + +Carried upward passively in a balloon, without effort, men have risen +higher than the highest mountains. Mr. Coxwell and Mr. Glaisher in +their celebrated aerial voyage of 1862 are believed to have mounted +seven miles above the sea. No little peril and suffering were +involved, alike from the extreme thinness of the air, and from the +bitter cold. + +The voyagers suffered from severe “sea-sickness,” though not from +bleeding of the nose or singing in the ears, popularly expected on +such occasions. They had enough to bear without these additions. +Mr. Glaisher held manfully to his task, observing and noting down +the state of the atmosphere minute by minute, despite sickness, +brain-pressure, violent headache, and a pulse at 108 per minute, all +due to the rarity of the air. + +In those lofty regions of the air-ocean no living creatures exist. +The voyagers passed through boundless silent solitudes--silent +except for the hurried beating of their own hearts, the sound of +their own panting breath, the sharp ticking of their watches, and the +“clang of the valve door.” + +On leaving earth the thermometer stood at 59°. Soon afterward the +balloon passed through masses of cloud, thousands of feet in depth, +then came out into dazzling sunshine, with deep-blue sky above and +countless mountain masses of billowy cloud below. + +As they rose, they released at intervals a captive pigeon. One set +free at a height of nearly five miles “fell downward like a stone.” +Of two others taken higher, one died of the cold and the other +was stupefied. When they reached five miles above the sea, the +temperature was below zero. + +Still upward, further upward, rose the resolute pair. Then blinding +darkness and insensibility seized Mr. Glaisher. Had he been alone, he +could never have revived. With no one to open the valve, the balloon +must have carried him onward into yet higher and deathlier regions, +where for lack of air he would have perished. Even then Mr. Coxwell +did not at once give in; but he was strictly on the watch. At the +seven miles’ level, a tremendous height, he too felt signs of failing +consciousness. In a few minutes more all would have been over with +them both, and at last he yielded. It was indeed time that he should. +His hands were powerless to act, but he seized the valve rope in his +teeth and pulled. The gas rushed out; the balloon steadily sank. Both +lives were saved, and a mighty feat had been accomplished. + +Yes, a mighty feat, and a tremendous height--in consideration of +human powers! Seven miles high would seem to be the outside limit at +which animals generally can exist for even a short time. Birds may +be to some extent an exception. Certain birds are believed to soar +occasionally two or three miles higher still. + +But what are seven miles--what are even ten miles--compared with the +four or five hundred miles of atmosphere-depth? With all our utmost +efforts, we and the birds still find ourselves only able to creep and +flutter on or near the floor of the ocean of air. + +What earth would be without her surrounding ocean of air, we can +scarcely imagine. The atmosphere plays so extraordinary and essential +a part in all around, that to picture its entire absence is not easy. +We see faintly on the moon something of what an airless world must +be. Yet since we only “see” from a distance of two hundred and forty +thousand miles, that does not mean much. Imagination has to come in, +and imagination is apt to play us curious tricks when running after +affairs which lie outside the range of human experience. + +Without air, man and beast can not breathe. Without air, plants and +trees can not grow. Without air, life as we know it--the lower animal +life common to man and beast--is a thing impossible. Without air, our +world would be, as we suppose the moon to be, a world of lifelessness. + +Air is earth’s outer robe, “for use and for beauty”--for use in modes +uncountable; for beauty, not so much in itself as in the softening, +the diffusing, the controlling effects of its presence. Air is a +mighty ocean, in which all things living must dwell. Even the living +things of the sea are not exceptions to this rule, for water itself +is pervaded by air. A man, going into and under water, does not get +beyond the touch of air; only, not being provided, like fishes, +with breathing gills, he can not make use of what is there--he can +not separate the air from the water, and so keep himself alive by +breathing it. + +Some animals living in the water-ocean are as dependent upon the +air-ocean as man himself for “the breath of life.” Whales are a +remarkable example of this. They are not fishes, though often +mistakenly called so, but belong to the same “family” of creatures as +men and land-quadrupeds generally. A whale is warm-blooded, has no +gills, and breathes atmospheric air, coming to the surface for it. +A whale kept forcibly for a long while under water would be drowned +exactly as a man would be. If a whale is thrown upon the shore, it +does not die of suffocation, but of inanition. A fish’s gills are +no more fitted to breathe air in bulk than a man’s lungs are fitted +to breathe air diffused in minute particles through water. The fish +out of water is suffocated by getting air too rapidly: the man under +water by exactly the reverse. A whale breathes like a man, and on +land it simply starves fast from lack of the incessant food required +by such a huge carcass. + +There is a difference certainly between man and whale in the matter +of breathing. A man has to take in fresh supplies of air constantly, +and if he is beyond reach of air for more than a few minutes he dies. +A whale comes to the surface for about ten minutes, spouting out +enormous supplies of used-up air and taking in enormous supplies of +fresh air, after which it can remain under water for half an hour or +more: some say an hour. Then a fresh bout of noisy breathing becomes +an absolute necessity. This, however, is merely a matter of internal +arrangement. The whale has an immense reservoir of blood, which, +being thoroughly purified by the air during ten minutes of vigorous +breathing, serves slowly to supply the creature’s requirements while +below. But the need for air, and the effect of that air upon the +blood, are much the same in man and whale. + +Small creatures, as well as big ones, spending much time under water, +and yet breathing air, have to come regularly to the surface. + +If our world had no ocean of air, there could be on earth no men, no +quadrupeds, no whales or fishes, no birds or insects, no forms of +life. + +Like the ocean of water, the ocean of air knows no repose or +stagnation. What we call stillness on the most sultry of summer days +does not mean absolute stillness. Though not enough wind may stir +to lift a feather, yet the air is in ceaseless motion, to and fro, +hither and thither. The whole atmosphere is a vast and complicated +system of air-currents, and each lesser portion of air has its own +lesser circulation. You can not lift your hand without causing a tiny +breeze; you can not turn a wheel without making a minute whirlwind; +and every separate air-movement draws other movements in its train. + +There is water enough on earth for all needed purposes; but we +should find ourselves in direful straits if the whole water-carrying +from lakes and rivers for men and animals had to be performed by +human agencies. + +Far from this, a mighty apparatus is provided. The scanty aid that +man can give only shows how little he is capable of. The entire +atmosphere is a tremendous pumping engine, an enormous watering +machine, always at work; always receiving supplies of liquid from the +ocean, from seas, lakes, rivers; always showering this water down +again upon the land, as needful drink for plants and animals, as +needful cleansing for all things. + +Air, the great carrier of water, in its wonderful strength and +restlessness, bears vast layers of cloud to and fro, wafts away +superfluous damp, drenches the dry and thirsty earth, fills ponds and +lakes, feeds--nay, actually makes--the rivers, never flags in its +ceaseless energy. If clouds hang low or fogs arise, we are glad of +the moving air which sweeps them elsewhere. If the soil is caked and +plants droop, we are glad of the moving air which brings rain. Thus +our wants are supplied, and the wide water circulation of earth is +carried on. Without circulation, without motion, stir, change, there +can not be life. Stagnation must mean death. Our earth, without her +ocean of moving air, would be a world of death. + +Without air, earth would be in great measure a soundless world. +Silence would reign here, as probably it does reign on the moon. +Sound, as it commonly reaches our ears, depends for its very +existence upon air. Let the concussion of two bodies be ever so +mighty, if there were no air to bear away the vibrations of that +concussion, there could be no crash of sound. True, sound-waves can +be conveyed through a liquid or through a solid as well as through +air; and we might be conscious of the ground’s vibrations, but our +ears would hear no noise. + +So an airless world would be a silent world. Without air, supposing +we could ourselves exist, we should hear no trickling brooks, no rush +of waterfalls, no breaking ocean waves, no sighing of the wind, no +whisper of leaves, no singing of birds, no voices of men, no music, +no thunder, no one of the thousand concomitant sound-waves which +together make up the babble and murmur of country and town. Those +only who are perfectly deaf can know what such silence means. + +Without air our world would not be in darkness; for light does +not, like sound, depend mainly upon air for its transmission. +Light travels through regions where air is not; and if light is +communicated by waves, they are not waves of air. But though the +absence of air would not deprive the earth of light, it would make a +very great difference in the kind and degree of light received. + +Without air the blue sky would be black as ink; stars would glitter +coldly in the daytime beside a glaring sun; deep shadows would +alternate with blinding dazzle, and all the soft tints of sunrise +and sunset would be wanting. Earth would be like the almost airless +moon--all fierce whiteness and utter blackness--with no gray shades, +no rosy gleams, no golden evening clouds; nay, without air there +could be no clouds. On the moon is no twilight; for no air-particles +float about, reflecting the sunlight from one to another, and forming +a soft veil of brightness, to reach further than the direct sunlight +alone can reach. + +Sunbeams travel straight to earth, unbending as arrows in their +flight, and unaided they can not creep any distance round a solid +body, though they may be reflected or turned back from it. But the +air breaks up the sunbeams, bends them, diffuses them, spreads them +about, surrounds us with a delicate lacework of woven light. A +sunbeam traveling through space is invisible till it strikes upon +some object. If that object is solid, the light of the sunbeam is +partly absorbed, partly reflected; if the object is transparent, +the sunbeam passes through and onward. Few substances, if any, are +perfectly transparent. We call air transparent, yet it is so only in +a measure. Each sunbeam passing through the atmosphere loses part +of its brightness by the way, and so the great glare of the sun is +softened before it reaches the lower depths of the air-ocean. + +The sun’s rays are rays of heat as well as of light. While the +atmosphere softens the glare, giving us shade and twilight, it also +modifies the extremes of temperature, from which, without air, we +should suffer. + +When the sun goes down, although we are often conscious of a chill, +it is not the instant and overwhelming chill which we should feel but +for the atmosphere. All day long the sun has been warming the earth +and air. When his direct rays are withdrawn, the warm air for a while +keeps its warmth, and gives over of that warmth to us. + + + + + WEATHER + --SIR RALPH ABERCROMBY + + +The earliest records of weather among every nation are to be found in +those myths, or popular tales, which, while describing rain, cloud, +wind, and other natural phenomena in highly figurative language, +refer them to some supernatural or personal agency by way of +explanation. + +The most interesting thing about these mythical stories is the +remarkable fidelity with which they reflect the climate of the +country that gave them birth. For example, from the mythologies of +Greece and Scandinavia we can almost construct an account of the +climate of those two countries by simply translating the figurative +phraseology of their legends into the language of modern meteorology. + +Many survivals of mystic speech are still found among popular +prognostics, and especially in cloud names. + +In England and Sweden “Noah’s Ark” is still seen in the sky, while +in Germany the “Sea-Ship” still turns its head to the wind before +rain. In Scotland the “Wind-Dog” and the “Boar’s Head” are still the +dread of the fisherman, while such names as “Goat’s Hair” and “Mare’s +Tails” recall some of the shaggy monsters of antiquity. + +At a rather later period of intellectual development, the premonitory +signs of good or bad weather become formulated into short sayings, +or popular prognostics. A large number of these are still current in +every part of the world, but their quality and value are very varied. +Some represent the astrological attitude of mind, by referring +weather changes to the influence of the stars or phases of the moon; +others, on the contrary, are very valuable, and, in conjunction with +other aids to weather forecasting, prognostics will never be entirely +superseded, especially for use on board ship. Till within a very +recent period, their science and explanation had hardly advanced +since they were first recorded. In many cases the prognostics came +true; when they failed, no explanation could be suggested why they +did so; neither could any reason be given why the same weather was +not always preceded by the same signs. A halo sometimes precedes a +storm; why does it not always do so? Why is rain sometimes preceded +by a soft sky and sometimes by hard clouds? + +About one hundred and fifty years ago the barometer was invented. +Very soon after that discovery, observation showed that, in a general +way, the mercury fell before rain and wind, and rose for finer +weather. Also that bad weather was more common when the whole level +of the barometer was low, independent of its motion one way or the +other, than when the level was high. But as with prognostics, so +with these indications, many failures occurred. Sometimes rain would +fall with a high or rising barometer, and sometimes there would be a +fine day with a very low or falling glass. No reason could be given +for these apparent exceptions, and the whole science of barometric +readings seemed to be shrouded in mystery. + +The science of probabilities came into existence about the +commencement of the Nineteenth Century, and developed the science of +statistics. By this method the average readings of meteorological +instruments, such as the height of the barometer or thermometer, or +the mean direction and force of the wind, at any number of places +were calculated, and the results were sometimes plotted on charts so +as to show the distribution of mean pressure, temperature, etc., over +the world. + +By this means a great advance was made. Besides giving a numerical +value to many abstract quantities, the plotting of such lines as the +isothermals of Dove conclusively showed that many meteorological +elements hitherto considered capricious were really controlled by +general causes, such as the distribution of land and sea. + +Still more fruitful were these charts as the parents of the more +modern methods of plotting the readings of the barometer over large +areas at a given moment, instead of the mean value for a month +or year. Then by tabulating statistics the relative frequency of +different winds at sea, many ocean voyages--notably those across +the “doldrums,” or belt of calms near the equator--were materially +shortened. + +Statistics also of the annual amount of rainfall became of commercial +value as bearing on questions of the economic supply of water for +large towns, and much valuable information was acquired as to +the dependence of mortality on different kinds of weather. Of +more purely scientific interest were the variations of pressure, +temperature, wind, etc., depending on the time of day, or what are +technically known as diurnal variations, which were brought to light +by these comparisons. + +This branch of the subject is known as “Statistical Meteorology,” and +has advanced very little since it was first developed by Dove and +Kaemtz. + +When the attempt was made to apply statistics to weather changes +from day to day, it was found that average results were useless. +The mean temperature for any particular day of the year might be +50°, if deduced from the returns of a great many years, but in any +particular year it might be as low as 40°, or as high as 60°. The +first application of the method was made by the great Napoleon, who +requested Laplace to calculate when the cold set in severely over +Russia. The latter found that on an average it did not set in hard +till January. The emperor made his plans accordingly; a sharp spell +of cold came in December, and the army was lost. + +It has now been thoroughly recognized that statistics give a +numerical representation of climate, but little or none of weather, +and that large masses of figures have been accumulated, to which +it is difficult to attach any physical significance. The misuse of +statistics has done much to bring the science of meteorology into +disrepute. + +But within the last thirty years a new treatment of weather problems +has been introduced, known as the synoptic method, by which the +whole aspect of meteorology has been changed. By this method, a +chart of a large area of the earth’s surface is taken, and after +marking on the map the height of the barometer at each place, +lines are drawn through all stations at which the barometer marks +a particular height. Thus a line would be drawn through all places +where the pressure was 30.0 inches, another through all where it +was 29.8 inches, and so on at any intervals which were considered +necessary. These lines are called “isobars,” because they mark out +lines of equal pressure. When these charts were first introduced, +the estimation of the value of the mean pressure was so great that, +instead of drawing lines where pressure was equal at the moment, +they were drawn through those places where the pressure was equally +distant from the mean of the day for each place. These lines were +called “is-abnormals”; that is, equal from the mean. After the +isobars have been put in, lines are usually drawn through all places +where the temperature is equal at the moment. These are called +“isotherms,” or lines of equal temperature. Then arrows to mark +the velocity and direction of the wind are inserted; and finally +letters, or other symbols, to denote the appearance of the sky, the +amount of cloud, or the occurrence of rain or snow. Such a chart is +called a “synoptic chart,” because it enables the meteorologist to +take a general view, as it were, over a large area. Sometimes they +are called “synchronous charts,” because they are compiled from +observations taken at the same moment of time. + +[Illustration: Typical Forms of Clouds + +1, Squall Cumulus; 2, Pillar Cumulus; 3, Cirrus; 4, High Stratus and +Cumulus] + +When these came to be examined, the following important +generalizations were discovered: + +1. That in general the configuration of the isobars assumed one of +seven well-defined forms. + +2. That, independent of the shape of the isobars, the wind always +took a definite direction relative to the trend of those lines, and +the position of the nearest area of low pressure. + +3. That the velocity of the wind was always nearly proportional to +the closeness of the isobars. + +4. That the weather--that is to say, the kind of cloud, rain, fog, +etc.--at any moment was related to the shape, and not the closeness, +of the isobars, some shapes inclosing areas of fine, others of bad, +weather. + +5. That the regions thus mapped out by isobars were constantly +shifting their position, so that changes of weather were caused by +the drifting past of these areas of good or bad weather, just as on +a small scale rain falls as a squall drives by. The motion of these +areas was found to follow certain laws, so that forecasting weather +changes in advance became possible. + +6. That sometimes in the temperate zone, and habitually in the +tropics, rain fell without any appreciable change in the isobars, +though the wind conformed to the general law of these lines. + +Observation also showed that, though the same shapes of isobars +appear all over the world, the details of weather within them, and +the nature of their motion, are modified by numerous local, diurnal, +and annual variations. Hence modern weather science consists in +working out for each country the details of the character and motion +of the isobars which are usually found over it; just as the geologist +finds crumplings and denudation all over the world, and works out the +history of the physical appearance of his own scenery by studying the +local development of these agencies. + +So far the science rests on pure observation--that such and such +wind or weather comes with such and such a shape of isobars. But it +has been found, still further, that the seven fundamental shapes +of isobars are, as it were, the product of so many various ways in +which an atmosphere circulating from the equator to the poles may +move. Just as the motion of a river sometimes forms descending eddies +or whirlpools, sometimes back-waters in which the water is rising +upward, or yet at other times ripples in which the circulation is +very complex, so it now appears that the general movement of the +atmosphere from the equator to the pole sometimes breaks up into a +rotating and descending movement round that configuration of isobars +known as an anticyclone, sometimes into a rotating and ascending +movement round that known as a cyclone, or at other times quite in a +different way during certain kinds of squalls and thunderstorms. + +_Isobars, therefore, represent the effect on our barometers of the +movements of the air above us, so that by means of isobars we trace +the circulation and eddies of the atmosphere._ + +By carrying the general laws of physics into the conception of a +circulating gas, we find that a cold mixed atmosphere of air and +vapor descending into a warmer soil would remain clear and bright; +while a similar atmosphere rising into cooler strata would condense +some of its vapor into rain or cloud. It is by reasoning of this +nature that the origin of some of the most beautiful and complex +forms of clouds has been discovered. + +Following out these lines of research, a new science of meteorology +has grown up, which entirely alters the attitude of mind with which +we regard weather changes, and gives rise to an entirely new method +of weather forecasting that far surpasses all previous efforts, and +which explains and develops all that was known before. + +On the one hand, the new method not only explains why certain +prognostics are usually signs of good or bad weather, and the reason +why the indications sometimes fail; but also the reason why rain, for +instance, is sometimes foretold by one prognostic and sometimes by a +totally different one. + +On the other hand, it not only gives a more extended meaning to all +the statistics which partially represent the climate of a place, and +to the relation of the diurnal to the general changes of weather; but +it also enables new inferences to be drawn, which had hitherto been +impossible from some observations, and explains why other sets of +figures must always remain without any physical significance. + +We may notice here an attempt which has been made by one school of +meteorologists to deduce all weather _à priori_ from changes in the +radiative energy of the sun; that is to say, that from a knowledge of +greater or less heat being emitted by the sun, they would treat the +consequent alteration of weather as a direct hydrodynamical problem. +Given an earth surrounded by fifty miles of damp air, and a sun at +varying altitude, and of varying radiative energy, deduce from that +all the diverse changes of weather. This is doubtless a very tempting +ideal, for there is no doubt that the sun’s heat is the prime mover +of all atmospheric circulation; but when we have explained what the +nature of weather changes is, we see that there is little hope that +this method will ever lead to satisfactory results. + +Other meteorologists, who lay less stress on the varying power of +the sun, have taken up the indications of synoptic charts, and +endeavored to construct a mathematical theory of cyclones and the +general circulation of the atmosphere. Ferrel, Mohn, Gulberg, Sprung, +and others have all started with the analysis of the motion of a +free mass of air on the earth’s surface, first given by Professor +Ferrel, and worked out, from that and other general principles, +schemes of the nature and propagation of cyclones, and of the general +distribution of pressure over the world. + + + + + THE ROMANCE OF A RAINDROP + --ARTHUR H. BELL + + +Depth of rainfall is, of course, ascertained by means of a +rain-gauge, which measures the amount of water precipitated from +the atmosphere during certain definite periods--usually twenty-four +hours. Sir Christopher Wren has the credit of constructing the +first rain-gauge; but they have been made in various shapes and +sizes since his time; and perhaps none of the instruments in the +meteorologist’s armory is so familiar to the general public as the +rain-gauge. The methods of using the instrument and the meaning of +rainfall statistics are also thoroughly understood nowadays. However, +behind these statistics and the methods of obtaining them, there +are questions of great interest that obtrude themselves when we are +watching the falling rain, and we desire to learn about the history +of the raindrop--for example: Why is a raindrop round? How are +raindrops formed? At what particular time does vapor become visible +as mist? And what are the causes which change this mist into cloud +and subsequently into rain? + +The two prime causes of rain are, of course, the sun and the ocean; +and since these two factors do not appreciably vary from year to +year, it follows that the annual rainfall on the earth as a whole, +if it could be measured, would also be found to be invariable. It +is obvious, however, that the rainfall at all places is not equal. +In London, for instance, the average yearly rainfall is twenty-two +inches; but on the Khasi Hills in India it is no less than six +hundred inches. Similar contrasts are observable in other parts of +the world, the differences being due to local geographical conditions. + +The starting points in the history of rain are, therefore, heat +and moisture. From the surface of land and water tiny globules or +vesicles of moisture are continually rising into the atmosphere by +the force of the sun’s heat; and the warmer the air the greater the +number of these globules of water the atmosphere is able to absorb. +In this respect the atmosphere may be likened to a sponge, for it +is from the moisture thus retained that the subsequent raindrops +are formed. Most persons are well acquainted with the very familiar +phenomenon which is to be noticed when a glass of very cold water is +brought into a warm room: the drops of moisture which form on the +outside of the glass being among the commonest phenomena in what may +be termed domestic meteorology. There is a similar transformation in +the outside atmosphere; so that when the warm, moist currents of air +flow against the sides of a cold mountain, or it may be against a +body of cold air, there is a reduction in temperature, the atmosphere +is squeezed like a sponge, and the particles of moisture are forced +out of it. The particles then assume the form of cloud, fog, mist, +rain, snow, and hail, as the case may be. + +Now, as regards the globules of moisture, the most recent experiments +and observations point to the conclusion that before the drops of +vapor can form, there must be a tiny nucleus of dust upon which the +condensed water may settle. At the centre of every drop of vapor in +a cloud there is probably a little core of dust; and without these +little atoms there could be no rain. These atoms of dust are visible +only under the strongest microscopes; and so minute are they that in +a cubic foot of saturated air it has been calculated that they number +one thousand millions, their total weight being only three grains. + +It is commonly considered that the particles of moisture within +a cloud are quite motionless; and when looking at a huge cloud +floating serenely in a summer sky it is difficult not to think of its +constituent parts as being quite at rest. The apparently stationary +cloud is all commotion and movement, the particles within it being +always on the move, some going up and others down. The particles of +moisture, moreover, being probably only about the four-thousandth +part of an inch in diameter, the resistance offered by the air to +their movement is very slight; indeed, as soon as they are condensed +they immediately begin to fall downward, and were it not for the +atoms of dust waiting to catch them the particles would at once +fall to the ground. It is often asked why the vapor, if so readily +condensed in the atmosphere, does not continually fall to the earth. +The answer to this question, it will be seen, is that the moisture, +instead of always pouring down on the earth, settles on the surface +of the atoms of dust. Thus the first downward movement of the +incipient raindrop is arrested by the dust-nuclei which swarm in all +parts of the atmosphere; so that instead of being destroyed as soon +as it is formed, the particle of moisture is preserved and stored for +future use. In realizing the fact that a cloud is always in motion, +the first step has been taken in discovering how a raindrop is formed. + +It might be supposed that the raindrops would evaporate as quickly as +they were condensed; but observation of the drops of moisture running +down a window-pane and forming larger drops gives a good idea of +what occurs in the clouds; as also does the fact that in a bottle of +soda-water the bubbles of air overtake one another and, colliding, +make larger bubbles. + +One of the principal causes of the manufacture of a raindrop is +to be found in the circumstance that there is a similar process +of amalgamation at work in every part of the atmosphere. It often +happens that a drop of moisture falls downward through a cloud for a +distance of a mile or more; and although it may pass through strata +of very warm air, thus running a great risk of being evaporated +and destroyed, it has also many collisions by which its bulk is +considerably increased, and eventually becomes so heavy that its rate +of progress is very much accelerated. Then, no longer able to float +in the air, it plumps down to the earth as a full-grown raindrop. + +Supposing it were possible for an observer to occupy a position +immediately below a cloud, and close enough to see all that was +taking place, he would notice raindrops of all sizes leaping from the +under side of the cloud and plunging toward the earth. The simplest +experiment to get some idea concerning the variation in raindrops +is to expose an ordinary slate for a few minutes during a shower of +rain, and it will be seen by the different-shaped blotches on the +slate that, although the raindrops have all made a similar journey, +they have, nevertheless, contrived to acquire an individuality during +their downward passage. That the raindrops are round admits of a +very simple explanation. They are this shape owing to the action of +capillarity, which in the case of the raindrop acts equally in all +directions. + +In many parts of the world the very curious phenomenon of colored +rain sometimes occurs, and in many instances it is due to very simple +causes. In some cases the coloring matter is found to be nothing but +the pollen-dust shaken out of the flowers on certain trees at such +times as a strong wind happened to be blowing over them. Fir trees +and cypress trees, when grouped together in large forests, at certain +seasons of the year give off enormous quantities of pollen, and this +vegetable dust is often carried many miles through the atmosphere by +the wind, and frequently falls to earth during a shower of rain. The +microscope clearly reveals the origin of such colored rain, which has +on more than one occasion puzzled and mystified the inexperienced. +Pollen is, therefore, very largely responsible for the reports sent +from different parts of the world of golden, black, and red rain. +Fish and insects also descend to earth during showers of rain; but +since it is probable that these and other unwonted visitors to the +atmosphere were originally drawn up into the air during the passage +across the country of a whirling storm, with powerfully ascending +currents of air, there is no need to look for any far-fetched +explanation of what, after all, is a very simple occurrence. + +The history of a raindrop, then, has some very romantic and +interesting episodes connected with it; but, wonderful as are the +incidents in what is really a very remarkable career, it is not until +the raindrops fall on the earth that the full purport of the work +they do is wholly realized. Contemplated by itself, a raindrop seems +a very insignificant thing; but when the drops combine in a heavy +downpour of rain the result is truly wonderful. The information +that one inch of rain has fallen over a certain area is not very +impressive; the amount does not seem very great. A fall of one inch +of rain means, however, that no less than one hundred tons of water +have fallen on each acre of surface, or no less than sixty thousand +tons on each square mile. Instead of expressing the amount of water +in tons, it may be thus stated in gallons, taking the Thames basin +as a convenient area for reference: a rainfall of three inches over +that area means that one hundred and sixty thousand million gallons +of water have been precipitated from the atmosphere. At times, too, +when the rainfall is still heavier, rivers overflow their banks and +floods occur, and still further evidence is then forthcoming of the +power and the might of the raindrops working toward one common end. +Sooner or later the raindrop, whether it runs off the surface of the +earth in a river or in a disastrous flood, finds its way, under the +influence of evaporation, back into the atmosphere, and is then ready +to start on another journey, which, like all its predecessors, will +be full of incident from start to finish. + + + + + THE RAINBOW + --JOHN TYNDALL + + +The oldest historic reference to the rainbow is known to all: “I do +set my bow in the clouds, and it shall be for a token of a covenant +between me and the earth.... And the bow shall be in the cloud; and +I will look upon it, that I may remember the everlasting covenant +between God and every living creature of all flesh that is upon the +earth.” + +To the sublime conceptions of the theologian succeeded the desire for +exact knowledge characteristic of the man of science. Whatever its +ultimate cause might have been, the proximate cause of the rainbow +was physical, and the aim of science was to account for the bow on +physical principles. Progress toward this consummation was very slow. +Slowly the ancients mastered the principles of reflection. Still more +slowly were the laws of refraction dug from the quarries in which +Nature had imbedded them. I use this language because the laws were +incorporate in Nature before they were discovered by man. Until the +time of Alhazan, an Arabian mathematician, who lived at the beginning +of the Twelfth Century, the views entertained regarding refraction +were utterly vague and incorrect. After Alhazan came Roger Bacon and +Vitellio, who made and recorded many observations and measurements on +the subject of refraction. To them succeeded Kepler, who, taking the +results tabulated by his predecessors, applied his amazing industry +to extract from them their meaning--that is to say, to discover +the physical principles which lay at their root. In this attempt +he was less successful than in his astronomical labors. In 1604, +Kepler published his _Supplement to Vitellio_, in which he virtually +acknowledged his defeat by enunciating an approximate rule, instead +of an all-satisfying natural law. The discovery of such a law, which +constitutes one of the chief corner-stones of optical science, was +made by Willebrod Snell, about 1621. + +A ray of light may, for our purposes, be presented to the mind as a +luminous straight line. Let such a ray be supposed to fall vertically +upon a perfectly calm water-surface. The incidence, as it is called, +is then perpendicular, and the ray goes through the water without +deviation to the right or left. In other words, the ray in the air +and the ray in the water form one continuous straight line. But the +least deviation from the perpendicular causes the ray to be broken, +or “refracted,” at the point of incidence. What, then, is the law +of refraction discovered by Snell? It is this, that no matter how +the angle of incidence and with it the angle of refraction may vary, +the relative magnitude of two lines, dependent on these angles, +and called their sines, remains, for the same medium, perfectly +unchanged. Measure, in other words, for various angles, each of these +two lines with a scale, and divide the length of the longer one by +that of the shorter; then, however the lines individually vary in +length, the quotient yielded by this division remains absolutely the +same. It is, in fact, what is called “the index of refraction” of the +medium. + +Science is an organic growth, and accurate measurements give +coherence to the scientific organism. Were it not for the antecedent +discovery of the law of sines, founded as it was on exact +measurements, the rainbow could not have been explained. Again and +again, moreover, the angular distance of the rainbow from the sun +had been determined and found constant. In this divine remembrancer +there was no variableness. A line drawn from the sun to the rainbow, +and another drawn from the rainbow to the observer’s eye, always +inclosed an angle of 41°. Whence this steadfastness of position--this +inflexible adherence to a particular angle? Newton gave to De +Dominis[4] the credit of the answer; but we really owe it to the +genius of Descartes. He followed with his mind’s eye the rays of +light impinging on a raindrop. He saw them in part reflected from +the outside surface of the drop. He saw them refracted on entering +the drop, reflected from its back, and again refracted on their +emergence. Descartes was acquainted with the law of Snell, and taking +up his pen, he calculated, by means of that law, the whole course +of the rays. He proved that the vast majority of them escaped from +the drop as _divergent_ rays, and, on this account, soon became +so enfeebled as to produce no sensible effect upon the eye of an +observer. At one particular angle, however--namely, the angle 41° +aforesaid--they emerged in a practically _parallel sheaf_. In their +union was strength, for it was this particular sheaf which carried +the light of the “primary” rainbow to the eye. + +There is a certain form of emotion called intellectual pleasure which +may be excited by poetry, literature, nature, or art. But I doubt +whether among the pleasures of the intellect there is any more pure +and concentrated than that experienced by the scientific man when a +difficulty which has challenged the human mind for ages melts before +his eyes, and re-crystallizes as an illustration of natural law. This +pleasure was doubtless experienced by Descartes when he succeeded in +placing upon its true physical basis the most splendid meteor of our +atmosphere. Descartes showed, moreover, that the “secondary bow” was +produced when the rays of light underwent two reflections within the +drop, and two refractions at the points of incidence and emergence. + +Descartes proved that, according to the principles of refraction, a +circular band of light must appear in the heavens exactly where the +rainbow is seen. But how are the colors of the bow to be accounted +for? Here his penetrative mind came to the very verge of the +solution, but the limits of knowledge at the time barred his further +progress. He connected the colors of the rainbow with those produced +by a prism; but then these latter needed explanation just as much as +the colors of the bow itself. The solution, indeed, was not possible +until the composite nature of white light had been demonstrated by +Newton. Applying the law of Snell to the different colors of the +spectrum, Newton proved that the primary bow must consist of a series +of concentric circular bands, the largest of which is red and the +smallest violet; while in the secondary bow these colors must be +reversed. The main secret of the rainbow, if I may use such language, +was thus revealed. + +I have said that each color of the rainbow is carried to the eye by +a sheaf of approximately parallel rays. But what determines this +parallelism? Here our real difficulties begin. Let us endeavor to +follow the course of the solar rays before and after they impinge +upon a spherical drop of water. Take, first of all, the ray that +passes through the centre of the drop. This particular ray strikes +the back of the drop as a perpendicular, its reflected portion +returning along its own course. Take another ray close to this +central one and parallel to it--for the sun’s rays when they reach +the earth are parallel. When this second ray enters the drop it is +refracted; on reaching the back of the drop it is there reflected, +being a second time refracted on its emergence from the drop. Here +the incident and the emergent rays inclose a small angle with each +other. Take, again, a third ray a little further from the central +one than the last. The drop will act upon it as it acted upon its +neighbor, the incident and the emergent rays inclosing in this +instance a larger angle than before. As we retreat further from the +central ray the enlargement of this angle continues up to a certain +point, where it reaches a maximum, after which further retreat +from the central ray diminishes the angle. Now, a maximum resembles +the ridge of a hill, or a watershed, from which the land falls in a +slope at each side. In the case before us the divergence of the rays +when they quit the raindrop would be represented by the steepness +of the slope. On the top of the watershed--that is to say, in the +neighborhood of our maximum--is a kind of summit-level, where the +slope for some distance almost disappears. But the disappearance of +the slope indicates, as in the case of our raindrop, the absence of +divergence. Hence we find that at our maximum, and close to it, there +issues from the drop a sheaf of rays which are nearly, if not quite, +parallel to each other. They are the so-called “effective rays” of +the rainbow. + +But though the step here taken by Descartes and Newton was a great +one, it left the theory of the bow incomplete. Within the rainbow +proper, in certain conditions of the atmosphere, are seen a series of +richly colored zones, which were not explained by either Descartes +or Newton. They are said to have been first described by Mariotte, +and they long challenged explanation. At this point our difficulties +thicken, but, as before, they are to be overcome by attention. It +belongs to the very essence of a maximum, approached continuously +on both sides, that on the two sides of it pairs of equal value may +be found. The maximum density of water, for example, is 39° Fahr. +Its density, when 5° colder and when 5° warmer than this maximum, is +the same. So also with regard to the slopes of a watershed. A series +of pairs of points of the same elevation can be found upon the two +sides of the ridge; and, in the case of the rainbow, on the two +sides of the maximum deviation we have a succession of pairs of rays +having the same deflection. Such rays travel along the same line, +and add their forces together after they quit the drop. But light, +thus reinforced by the coalescence of non-divergent rays, ought to +reach the eye. It does so; and were light what it was once supposed +to be--a flight of minute particles sent by luminous bodies through +space--then these pairs of equally deflected rays would diffuse +brightness over a large portion of the area within the primary bow. +But inasmuch as light consists of _waves_, and not of particles, the +principle of interference comes into play, in virtue of which waves +alternately reinforce and destroy each other. Were the distance +passed over by the two corresponding rays within the drop the same, +they would emerge as they entered. But in no case are the distances +the same. The consequence is that when the rays emerge from the drop +they are in a condition either to support or to destroy each other. +By such alternate reinforcement and destruction, which occur at +different places for different colors, the colored zones are produced +within the primary bow. They are called “supernumerary bows,” and +are seen, not only within the primary, but sometimes also outside +the secondary bow. The condition requisite for their production is +that the drops which constitute the shower shall all be of nearly the +same size. When the drops are of different sizes, we have a confused +superposition of the different colors, an approximation to white +light being the consequence. This second step in the explanation of +the rainbow was taken by a man the quality of whose genius resembled +that of Descartes or Newton, and who in 1801 was appointed Professor +of Natural Philosophy in the Royal Institution. I refer, of course, +to the illustrious Thomas Young. + +But our task is not, even now, complete. The finishing touch to +the explanation of the rainbow was given by the eminent Astronomer +Royal, Sir George Airy. Bringing the knowledge possessed by the +founders of the undulatory theory, and that gained by subsequent +workers, to bear upon the question, Sir George Airy showed that, +though Young’s general principles were unassailable, his calculations +were sometimes wide of the mark. It was proved by Airy that the +curve of maximum illumination in the rainbow does not quite +coincide with the geometric curve of Descartes and Newton. He also +extended our knowledge of the supernumerary bows, and corrected +the positions which Young had assigned to them. Finally, Professor +Miller of Cambridge and Dr. Galle of Berlin illustrated with careful +measurements with the theodolite the agreement which exists between +the theory of Airy and the facts of observation. Thus, from Descartes +to Airy, the intellectual force expended in the elucidation of the +rainbow, though broken up into distinct personalities, might be +regarded as that of an individual artist, engaged throughout this +time in lovingly contemplating, revising, and perfecting his work. + +The white rainbow (_l’arc-en-ciel blanc_) was first described by the +Spanish Don Antonio de Ulloa, lieutenant of the Company of Gentleman +Guards of the Marine. By order of the King of Spain, Don Jorge Juan +and Ulloa made an expedition to South America, an account of which +is given in two amply illustrated quarto volumes to be found in the +library of the Royal Institution. The bow was observed from the +summit of the mountain Pambamarca, in Peru. The angle subtended by +its radius was 33° 30′, which is considerably less than the angle +subtended by the radius of the ordinary bow. + +The white rainbow has been explained in various ways. The genius of +Thomas Young throws light upon this subject, as upon so many others. +He showed that the whiteness of the bow was a direct consequence of +the smallness of the drops which produce it. The smaller the drops, +the broader are the zones of the supernumerary bows, and Young proved +by calculation that when the drops have a diameter of 1-3000th or +1-4000th of an inch, the bands overlap each other, and produce white +light by their mixture. + + + + + SNOW, HAIL, AND DEW + --ALEXANDER BUCHAN + + +Snow is the frozen moisture which falls from the atmosphere when the +temperature is 32° or lower. It is composed of crystals, usually in +the form of six-pointed stars, of which about 1,000 different kinds +have been already observed, and many of them figured, by Scoresby, +Glaisher, and others. These numerous forms have been reduced to +five principal varieties: Thin plates, the most numerous class, +containing several hundred forms of the rarest and most exquisite +beauty; spherical nucleus or plane figure studded with needle-shaped +crystals; six or more rarely three-sided prismatic crystals; pyramids +of six sides; prismatic crystals, having at the ends and middle thin +plates perpendicular to their length. The forms of the crystals +in the same fall of snow are generally similar to each other. The +crystals of hoar-frost being formed on leaves and other bodies +disturbing the temperature are often irregular and opaque; and it has +been observed that each tree or shrub has its own peculiar crystals. + +Snowflakes vary from an inch to 7-100ths of an inch in diameter, the +largest occurring when the temperature is near 32°, and the smallest +at very low temperatures. As air has a smaller capacity for retaining +its vapor as the temperature sinks, it follows that the aqueous +precipitation, snow or rain, is much less in polar than in temperate +regions. The white color of snow is the result of the combination of +the different prismatic rays issuing from the _minute_ snow-crystals. +Pounded glass and foam are analogous cases of the prismatic colors +blending together and forming the white light out of which they had +been originally formed. It may be added that the air contained in +the crystals intensifies the whiteness of the snow. The limit of +the fall of snow coincides nearly with 30° N. lat., which includes +nearly the whole of Europe; on traversing the Atlantic, it rises to +45°, but on nearing America descends to near Charleston; rises on +the west of America to 47°, and again falls to 40° in the Pacific. +It corresponds nearly with the winter isothermal of 52° Fahr. Snow +is unknown at Gibraltar; at Paris, it falls 12 days on an average +annually, and at St. Petersburg 170 days. It is from 10 to 12 times +lighter than an equal bulk of water. From its loose texture, and its +containing about 10 times its bulk of air, it is a very bad conductor +of heat, and thus forms an admirable covering for the earth from +the effects of radiation--it not infrequently happening, in times +of great cold, that the soil is 40° warmer than the surface of the +overlying snow. The flooding of rivers from the melting of the snow +on mountains in summer carries fertility into regions which would +otherwise remain barren wastes. + +The word hail in English is unfortunately used to denote two +phenomena of apparently different origin. In French, we have the +terms _grèle_ and _grésil_--the former of which is hail proper; the +latter denotes the fine grains, like small shot, which often fall +in winter, much more rarely in summer, and generally precede snow. +The cause of the latter seems to be simply the freezing of raindrops +as they pass in their fall through a colder region of air than that +where they originated. We know by balloon ascents and various other +methods of observation that even in calm weather different strata +of the atmosphere have extremely different temperatures, a stratum +far under the freezing point being often observed between two others +comparatively warm. + +But that true hail, though the process of its formation is not yet +perfectly understood, depends mainly upon the meeting of two nearly +opposite currents of air--one hot and saturated with vapor, the +other very cold--is rendered pretty certain by such facts as the +following. A hailstorm is generally a merely local phenomenon, or at +most, ravages a belt of land of no great breadth, though it may be +of considerable length. Hailstorms occur in the greatest perfection +in the warmest season, and at the warmest period of the day, and +generally are most severe in the most tropical climates. A fall of +hail generally _precedes_, sometimes accompanies, and rarely, if +ever, follows a thunder-shower. + +When a mass of air, saturated with vapor, rising to a higher level, +meets a cold one, there is, of course, instant condensation of vapor +into ice by the cold due to expansion; at the same time, there is +generally a rapid production of electricity, the effect of which upon +such light masses as small hailstones is to give them in general +rapid motion in various directions successively. These motions are in +addition to the vortex motions or eddies, caused in the air by the +meeting of the rising and descending currents. The small ice-masses +then moving in all directions impinge upon each other, sometimes with +great force, producing that peculiar rattling sound which almost +invariably precedes a hail-shower. At the same time, by a well-known +property of ice, the impinging masses are frozen together; and this +process continues until the weight of the accumulated mass enables +it to overcome the vortices and the electrical attractions, when it +falls as a larger or smaller hailstone. On examining such hailstones, +which may have any size from that of a pea to that of a walnut, or +even an orange, we at once recognize the composite character which +might be expected from such a mode of aggregation. + +A curious instance of the fall of large hail, or rather ice-masses, +occurred on one of her Majesty’s ships off the Cape in January, 1860. +Here the stones were the size of half-bricks, and beat several of +the crew off the rigging, doing serious injury. We may conclude by +a description (taken from _Mem. de l’Acad. des Sciences_, 1790) of +one of the most disastrous hailstorms that has occurred in Europe +for many years back. This storm passed over Holland and France in +July, 1788. It traveled _simultaneously_ along two lines nearly +parallel--the eastern one had a breadth of from half a league to +five leagues, the western of from three to five leagues. The space +between was visited only by heavy rain; its breadth varied from three +to five and a half leagues. At the outer border of each, there was +also heavy rain, but we are not told how far it extended. The length +was at least a hundred leagues; but from other reports it may be +gathered that it really extended to nearly two hundred. It seems to +have originated near the Pyrenees, and to have traveled at a mean +rate of about sixteen and a half leagues per hour toward the Baltic, +where it was lost sight of. The hail only fell for about seven and +a half minutes at any one place. The hailstones were generally of +irregular form, the heaviest weighing about eight French ounces. This +storm devastated 1,039 parishes in France alone, and the damage was +officially placed at 24,690,000 francs. + +For any assigned temperature of the atmosphere, there is a certain +quantity of aqueous vapor which it is capable of holding in +suspension at a given pressure. Conversely, for any assigned quantity +of aqueous vapor held in suspension in the atmosphere, there is +a minimum temperature at which it can remain so suspended. This +minimum temperature is called the dew-point. During the daytime, +especially if there has been sunshine, a good deal of aqueous vapor +is taken into suspension in the atmosphere. If the temperature in the +evening now falls below the dew-point, which after a hot and calm +day generally takes place about sunset, the vapor which can be no +longer held in suspension is deposited on the surface of the earth, +sometimes to be seen visibly falling in a fine mist. This is one +form of the phenomenon of dew, but there is another. The surface of +the earth, and all things on it, and especially the smooth surfaces +of vegetable productions, are constantly parting with their heat by +radiation. If the sky is covered with clouds, the radiation sent +back from the clouds nearly supplies an equivalent for the heat thus +parted with; but if the sky be clear, no equivalent is supplied, and +the surface of the earth and things growing on it become colder than +the atmosphere. + +If the night also be calm, the small portion of air contiguous to +any of these surfaces will become cooled below the dew-point, and +its moisture deposited on the surface in the form of dew. If this +chilled temperature be below 32° Fahr., the dew becomes frozen and is +called _hoar-frost_. The above two phenomena, though both expressed +in our language by the word dew, which perhaps helps to give rise to +a confusion of ideas on the subject, are not necessarily expressed +by the same word. For instance, in French, the first phenomenon--the +falling evening-dew--is expressed by the word _serein_; while the +latter--the dew seen in the morning gathered in drops by the leaves +of plants, or other cool surfaces--is expressed by the word _rosée_. + +The merit of the discovery of the “Theory of Dew” has been commonly +ascribed to Dr. William Charles Wells, who published in 1814 his +_Essay on Dew_, which obtained great popularity. The merit should, +however, be divided between him and several others. M. Le Roi of +Montpellier, M. Pictet of Geneva, and especially Professor Alexander +Wilson of Glasgow, largely contributed by experiment and inducement +to its formation. + + + + + THE AURORA BOREALIS + --RICHARD A. PROCTOR + + +The aurora is one of those phenomena of nature which are +characterized by exceeding beauty, and sometimes by an imposing +grandeur, but are unaccompanied by any danger, and indeed, so far +as can be determined, by any influence whatever upon the conditions +which affect our well-being. Comparing the aurora with a phenomenon +akin to it in origin--lightning--we find in this respect the most +marked contrast. Both phenomena are caused by electrical discharges; +both are exceedingly beautiful. It is doubtful which is the more +imposing so far as visible effects are concerned. When the auroral +crown is fully formed, and the vault of heaven is covered with the +auroral banners, waving hither and thither silently, now fading from +view, anon glowing with more intense splendor, the mind is not less +impressed with a sense of the wondrous powers which surround us than +when, as the forked lightnings leap from the thundercloud, the whole +heavens glow with violet light, and then sink suddenly into darkness. +The solemn stillness of the auroral display is as impressive in its +kind as the crashing peal of the thunderbolt. + +The reader is no doubt aware that auroras or polar streamers, as they +are sometimes called, are appearances seen not around the true poles +of the earth, but around the magnetic poles which lie very far away +from those geographical poles which our Arctic and Antarctic seamen +have in vain attempted to reach. The formation of auroral streamers +around the magnetic poles of the earth shows that these lights are +due to electrical discharges of electricity, which, though only +visible at night, take place in reality in the daytime also. + +Remembering that the aurora is due to electrical discharges in the +upper regions of the air, it is interesting to learn what are the +appearances presented by the aurora at places where the auroral +arch is high above the horizon--these being, in fact, places nearly +_under_ the auroral arch. M. Ch. Martins, who observed a great number +of auroras in Spitzbergen in 1839, thus writes: “At times they are +simple diffused gleams or luminous patches; at others, quivering rays +of pure white which run across the sky, starting from the horizon +as if an invisible pencil were being drawn over the celestial vault; +at times it stops in its course, the incomplete rays do not reach +the zenith, but the aurora continues at some other point; a bouquet +of rays darts forth, spreads into a fan, then becomes pale, and +dies out. At other times long golden draperies float above the head +of the spectator, and take a thousand folds and undulations as if +agitated by the wind. They appear to be but at a slight elevation +in the atmosphere, and it seems strange that the rustling of the +folds as they double back on each other is not audible. Generally, a +luminous bow is seen in the north; a black segment separates it from +the horizon, the dark color forming a contrast with the pure white +or bright red of the bow, which darts forth rays, extends, becomes +divided, and soon presents the appearance of a luminous fan, which +fills the northern sky, and mounts nearly to the zenith, where the +rays, uniting, form a crown, which in its turn darts forth luminous +jets in all directions. The sky then looks like a cupola of fire; the +blue, the green, the yellow, the red, and the white vibrate in the +palpitating rays of the aurora. But this brilliant spectacle lasts +only a few minutes; the crown first ceases to emit luminous jets, +and then gradually dies out; a diffused light fills the sky; here +and there a few luminous patches, resembling light clouds, open and +close with incredible rapidity, like a heart that is beating fast. +They soon get pale in their turn, everything fades away and becomes +confused, the aurora seems to be in its death-throes; the stars, +which its light had obscured, shine with a renewed brightness; and +the long polar night, sombre and profound, again assumes its sway +over the icy solitudes of earth and ocean.” + +The association between auroral phenomena and those of terrestrial +magnetism has long been placed beyond a doubt. Wargentin in 1750 +first established the fact, which had been previously noted, however, +by Halley and Celsius. But the extension of the relation to phenomena +occurring outside the earth--very far away from the earth--belongs +to recent times. The first point to be noticed, as showing that the +aurora depends partly on extra-terrestrial circumstance, is the +fact that the frequency of its appearance varies greatly from time +to time. It is said that the aurora was hardly ever seen in England +during the Seventeenth Century, although the northern magnetic pole +was then much nearer to England than it is at present Halley states +that before the great aurora of 1716 none had been seen (or at least +recorded) in England for more than eighty years, and no remarkable +aurora since 1574. In the records of the Paris Academy of Sciences no +aurora is mentioned between 1666 and 1716. At Berlin one was recorded +in 1707 as a very unusual phenomenon; and the one seen at Bologna +in 1723 was described as the first which had ever been seen there. +Celsius, who described in 1733 no less than three hundred and sixteen +observations of the aurora in Sweden between 1706 and 1732, states +that the oldest inhabitants of Upsala considered the phenomenon as a +great rarity before 1716. Anderson of Hamburg states that in Iceland +the frequent occurrence of auroras between 1716 and 1732 was regarded +with great astonishment. In the Sixteenth Century, however, they had +been frequent. + +Here then we seem to find the evidence of some cause external to +the earth as producing auroras, or at least as tending to make +their occurrence more or less frequent. The earth has remained +to all appearance unchanged in general respects during the last +three centuries, yet in the Sixteenth her magnetic poles have been +frequently surrounded by auroral streamers; during the Seventeenth +these streamers have been seldom seen; during the last two-thirds of +the Seventeenth Century auroras have again been frequent; and during +the Eighteenth Century they have occurred sometimes frequently during +several years in succession, at others very seldom. + +Connected as auroras are with the phenomena of terrestrial magnetism, +we may expect to find some help in our inquiry from the study of +these phenomena. Now it appears certain that magnetic phenomena are +partly influenced by changes in the sun’s condition. We may well +believe that they are in the main due to the sun’s ordinary action, +but the peculiarities which affect them seem to depend on _changes_ +in the sun’s action. + +Many of my readers will doubtless remember the auroras of May 13, +1869, and October 24, 1870, both of which occurred when the sun’s +surface was marked by many spots, and both of which were accompanied +by remarkable disturbance of the earth’s magnetism. + +It may, then, fairly be assumed that the occurrence of auroras +depends in some way, directly or indirectly, on the condition of the +sun. But what the real nature of that connection may be is not easily +determined. + +Angström was the first to observe the spectrum of the aurora +borealis. He found that the greater part of the auroral light, as +observed in 1867, was of one color, yellow, but three faint bands of +green and greenish blue color were also seen. The aurora of April 15, +1869, was seen under very favorable conditions in America. Professor +Winlock, observing it at New York, found its spectrum to consist of +five bright lines, of which the brightest was the yellow line just +mentioned. One of the others seems to agree very nearly, if not +exactly, in position with a green line, which is the most conspicuous +feature of the spectrum of the solar corona. During the aurora of +October 6, 1869, Flögel noticed the strong yellow line and a faint +green band. Schmidt, on April 5, 1870, made a similar observation. +He saw the strong yellow line, and from it there extended toward the +violet end of the spectrum a faint greenish band, which, however, at +times showed three defined lines, fainter than the yellow line. + +It was not till the magnificent aurora of October 24-25, 1870, +that any red lines were seen in the spectrum of an aurora. On that +occasion the background of the auroral light was ruddy, and on the +ruddy background there were seen three deep red streamers very well +defined. The ruddy streamers, on the night of October 25, converged +toward the auroral crown, which was on that occasion singularly well +seen. Förster of Berlin failed to see any red line or band despite +the marked ruddiness of the auroral light. But Capron at Guildford +saw a faint line in the red part of the spectrum; and Elger at +Bedford observed a red band in the light of the red streamers, the +band disappearing, however, when the spectroscope was directed on the +white rays of the aurora. + +As yet the auroral spectrum has not been interpreted. The reason +probably is, that the conditions under which the light of the aurora +as of the corona is formed are not such as have been or perhaps can +be attained or even approached in laboratory experiments. + + + + + CLOUDS + --D. WILSON BARKER + + +Those who are professionally engaged in the scientific work of +weather bureaus recognize the importance of accurate observations +of cloud forms and nature, and much good work has been done in this +connection in recent years by scientific observers in England, +Australia, and the United States; but as a popular study, nephology +is almost entirely overlooked, and this notwithstanding the fact +that, perhaps, no branch of knowledge offers greater facility and +ease of acquisition. Each cloud has its history fraught with meaning; +its open secret is writ on its face, and may be read by any one who +will, give himself a little trouble, nor need he go deeply into the +study in order to make observations interesting to himself, and +perhaps of great use in the furthering and perfecting of weather +lore. To the ancients, the sky was doubtless an object of constant +remark and interest, and possibly their intuitive knowledge of +weather forecasting was much more accurate than ours. The dwellers +in our modern cities see little of the sky, clouds have no interest +for them beyond the personal consideration as to the advisability +of taking out an umbrella or not. But farmers, fishermen, sailors, +and others following open-air avocations are dependent on the +weather, and to be wise in its forecast is of importance to them. +To these, especially, cloud study should appeal; it can not fail to +be profitable to them in their personal work, and they have all the +opportunity, if the will be there, to forward the general knowledge +of the subject by careful painstaking observations, which they may +transmit to those scientifically engaged in dealing with weather +laws, and thus assist in the elucidation of questions on which we are +at present but very imperfectly informed. + +In this article the broad distinctions of clouds will be dealt with. +There are two well-defined types--Stratus and Cumulus--so distinct +in actual appearance and in physical formation that they may be +taken as the basis of classification. Sometimes both types appear to +merge into each other, in which case no variety of classification +suffices to describe them satisfactorily, as any one who has studied +cloud-forms must allow. “Stratus” is a sheet-like formation of +cloud. “Cumulus” is recognizable by its heaped-up appearance and +vertical thickness. Numerous varieties of cloud-forms may be observed +graduating from one of these types to the other, but when an +observer can clearly distinguish Stratus from Cumulus he has already +acquired valuable knowledge. + +The presence of either type of cloud alone indicates a more or +less set condition of the atmosphere, and generally foretells a +continuance of the existing weather. The simultaneous presence of +both types indicates a coming change, the gradation of Stratus +into Cumulus foreboding worse weather, and of Cumulus into Stratus +heralding good. Again, as we shall show later on, the vertical +thickening of the stratiform clouds is a distinctly bad indication. + +Up to quite recently, Luke Howard’s division of clouds, formulated +in 1802, held first place; even now it is in constant use, for +though attempts have been made at a more scientific classification, +all of them, with the single exception of that proposed by the +late Rev. Clement Ley, can only be termed make-shifts. Mr. Ley’s +classification, unfortunately, is long, and not well adapted to the +use of any but professional investigators, or enthusiasts with ample +time on their hands. There exists a so-called “international” system +of cloud nomenclature, but, for all that, each country has its own +especial system, with the result that vast collections of cloud +statistics are of little value as helps to a classification, and are +useful only as records of clouds present at certain times. + +Clouds owe their existence to two causes: + +1. Through the passing of warm, moist air into colder, when, owing to +condensation, a certain proportion of the moisture becomes visible in +the form of a cloud. + +2. Through changes occurring in the atmosphere as it rises +into higher regions of atmosphere, where decrease in pressure +and expansion and consequent loss of heat take place and cause +condensation of moisture. + +The first process may be described as the condensation formation of +clouds, and the second as the adiabatic formation of clouds. As a +matter of fact, no hard and fast line separates these two operations; +they act in unison, and the combination of vertical and horizontal +currents goes to make up the diversity of forms which clouds assume. + +In settled states of the atmosphere, Stratus clouds are common, or +the sky may be clear. In unsettled conditions, Cumulus or Heap clouds +are formed. + +We shall now describe a few familiar forms of cloud, giving them +simple names and endeavoring to compare them with other nomenclatures. + +Of Cumulus clouds there are five well-defined varieties. + +_Rain Cumulus_, of which there are two sub-varieties: + +(_a_) Shower-cumulus, when rain falls from the cloud without +increment of wind. The edges of this cloud are not cirrus-topped. + +(_b_) Squall-cumulus, when the rain is accompanied by wind, or by +wind with hail and snow falling from this cloud. + +In these cases the Cumulus cloud is generally much serrated, having +a cirriform edging. In some cases this cirriform edging extends far +over the sky and forms halos, particularly at the rear. + +Two rarer varieties of Cumulus are: + +_Pillar-cumulus_, generally noticed over the calm belts of the +ocean, and distinguishable by its slender forms, which rise to great +altitudes. + +_Roll-cumulus_ generally accompanies strong winds, particularly polar +west winds, which succeed cyclonic disturbances. Here we have the +ordinary Cumulus cloud so blown along by the wind as to assume the +roll formation from which it is named. + +A still rarer form of Cumulus appears in scattered patches over the +sky, and is indicative of an electrical state of the atmosphere. + +Cumulus clouds form at a low altitude, but they frequently tower +upward to great heights. + +It should be noticed that in these clouds the fine weather form is of +soft, smooth outline, and has a quiet appearance. + +_Stratus Clouds_ may be divided into four varieties as follows: + +1. _Fog_, so well known as not to need description. It is, in fact, a +Stratus cloud resting on the earth’s surface. + +2. _Stratus_, a cloud sheet which covers the whole sky at a +moderate elevation. Here and there the cloud is thin, and under +surfaces appear as parallel lines all round the horizon. This is +the characteristic cloud of anti-cyclonic, or dry, fine weather +conditions. It may continue to cover the sky for several days in +succession. + +3. _High Stratus_, including all the varying forms of Cirro-cumulus +from the mackerel skies to the Cirro-macula of Clement Ley. Many +beautiful varieties of this cloud of minute cumuliform appearance +are caused by the changes taking place in the atmosphere. We notice +waves, wavelets, stipplings, and flecks. To it are due the coronas +sometimes seen round the sun, as also iridescent clouds occasionally +noticed in the same vicinity. The wave-like appearance of the clouds +is due to the passage of a more rapidly moving air current over a +slower one, or of a wave current crossing a motionless portion of the +air. When two air currents pass over one another at an angle, the +particles of clouds tend to fall into different shapes, hence our +mackerel skies. But this cloud, although beautiful, is essentially +one of warning, more especially when the flecks are of a thin, scaly +appearance (resembling the scales of certain fishes so closely that +I have called it the scale cloud). Sometimes these detached flecks +appear in lines, and very striking is the effect produced. + +4. _Cirrus._--The highest form of cloud and the most important as a +factor in the science of weather forecasting. Cirrus, ordinarily, +appears as wisps and feather pieces scattered over the sky, and its +significance is then of no import. + +When, however, this cloud takes the form of lines parallel to the +horizon, or of lines appearing to radiate as wheel-spokes from any +one part of the horizon, it should be carefully noted as indicative +of approaching weather. Its movement and propagating transition +should be observed. This cloud is composed of ice-dust or crystals. + +When a cyclonic disturbance is about to pass over an observer, Cirrus +generally appears first in parallel lines, or at a radiant point; +the threads gradually increase and interlace until a complete sheet +of Cirro-stratus covers the sky, causing a halo. The cloud further +thickens, the halo disappears, all becomes overcast, and rain comes +on. The cloud is now known as Nimbus, and after it has endured some +time, the wind shifts, the Nimbus clears off, and it is succeeded by +a polar west wind. + +In addition to these forms of clouds, we may often notice, +particularly during high winds, fragments of clouds hurrying across +the sky. These are known as “scud”; they are generally pieces carried +off by the winds from the main bodies of clouds. + +Occasionally two forms of cloud are present at the same time. This +is ordinarily taken as a case of Cumulus and Stratus, and has become +known as Cumulo-stratus; but, if observed in the zenith, it may +readily be noted that the two forms of cloud are distinct, and they +had better be dealt with separately. The appearance of Cumulo-stratus +is an effect of perspective. + +Clouds float at varying altitudes, according to the latitude and +elevation of the ground; the vertical temperature and adiabatic +gradients determining the level at which the vapor becomes visible as +cloud. It is desirable in all cloud observations, that note should +be made of the approximate relative altitudes of clouds and of their +velocity of motion. This is particularly desirable when dealing with +the stratiform clouds, whether as ordinary Cirro-cumulus or as very +high Cirro-macula. + +The beautiful coloring of clouds results from the breaking up of +light beams in passing through them or along their edges. This +phenomenon is caused by diffraction, and to it is due our lovely +sunrises and sunsets. When the sun is high in the heavens, the light +is white, but as the orb nears the horizon, and its rays pass through +thicker layers of atmosphere, the smaller light waves get gradually +cut off, until the sun sinks as a red ball below the horizon. The +largest waves of light produce the red rays and the after glow +which are so beautiful. Sunrise and sunset effects are matters of +much interest, but are of too complicated a nature to be fully gone +into here; we must, however, notice them briefly, because of their +importance in weather forecasts. Soft sunset colors indicate fine +settled weather; fiery brilliant hues denote change to stormy or wet +weather. + +Other color effects in clouds are due to phenomena, known as halos +and coronas. Halos appear as rings round the sun and moon; they +are caused by the shining of the orb through very high Stratus or +Cirrus clouds, and have a diameter of 42°. Sometimes shades of +color, resembling those of a rainbow, are visible--red appears on +the inside and blue on the outside. These rings of color are due +to the reflection and refraction of light passing through the fine +ice crystals of which high Stratus or Cirrus clouds are composed. +Occasionally a complicated series of beautifully colored rings is +noticeable. Generally speaking, these rings are due to the thinness +of the high cloud through which the light is passing. Still more +curious arrangements of halos sometimes occur. + +Coronæ are broader rings seen quite close to the sun or moon, and are +due to the shining of light through the edges of loose Cumulus or +Stratus clouds. They have red on the outside and blue on the inside +of the ring; the colors are, generally, easily distinguishable. +The more brilliant hues occasionally seen, as has been said, in +the vicinity of the sun and moon, would appear to be incomplete +sections of circles intermediate in size between coronæ and halos. An +interested observer will be well repaid if he chooses to study more +closely the many curious optical phenomena connected with clouds, but +it would be beyond the scope and object of this paper to go into them +more fully here. + +Whoever wishes to be weatherwise, and who has time to study the +weather charts published daily, may easily acquire such knowledge +of local characteristics as will enable him to forecast fairly +accurately. Cirrus clouds, as a rule, are reliable guides; they +form, as we have said, in parallel threads, from the position and +movements of which forecasts may be made. Should the threads appear +on, and parallel to, the west horizon, and moving from a northerly +point, a depression is approaching from the west, but, although +causing some bad weather, it will probably pass to the north of +the observer. Should the lines appear parallel to the southwest or +south-southwest horizon, and be moving from a northwesterly point, +the depression will very likely pass over the observer and occasion +very bad weather. These are two of many possible prognostics. Weather +forecasting is much helped by a study of the daily weather charts. +Again, weather is often very local, and to predict with fair accuracy +a knowledge of local conditions is necessary. + + + + + WINDS + --WILLIAM HUGHES + + +Among the secondary causes affecting climate, probably none is of +greater importance than the direction of prevailing winds. The +currents of air are warm or cold, wet or dry, according as they +have had their origin in warm or cold latitudes, and have traversed +inland tracts, or the expanse of ocean, in their advancing course. +With us, and in the northern half of the globe in general, north +and east winds are cold and dry, while south and west winds are +warm, and often accompanied by moisture. Within the Southern +Hemisphere these conditions are reversed, and the hottest currents +of air come from a northwardly direction. The prevailing winds of +western Europe are from the west and southwest; and it is to this +fact that we must mainly ascribe the high winter temperature, as +well as the comparative freedom from extremes of heat and cold +which distinguishes the countries of western Europe. The same cause +explains the abundant moisture which belongs to those regions in +general, and which distinguishes the western shores of our own +islands in a remarkable degree. Such winds have traversed the immense +expanse of the Atlantic, and come to the western seaboard of Europe +laden with the moist vapors gathered on their course. These vapors, +condensed upon the high grounds which line the western side of the +British Islands, or, further to the northward, upon the long chain of +the Scandinavian Mountains, fall to the earth in copious torrents of +rain. In the process of condensation, a vast quantity of latent heat +is disengaged, and the temperature is correspondingly raised. Warmth +and moisture are, indeed, speaking generally, concomitant conditions +of European climate, and are especially so in the case of western +Europe. + +Even in the case of lands which nearly approach the tropic, the +influence of prevailing winds in raising or lowering the temperature +is strikingly seen. At New Orleans, bordering on the Mexican Gulf, +and throughout the adjacent portions of the United States, the +winters are often of excessive severity. Cold winds, generated in +the higher latitudes of the New World, and blowing for weeks in +succession from the northern quarter of the sky, are the cause of +this. The generally level interior of the North American Continent--a +vast lowland plain, bounded only to the east and west by the +Alleghanies and the Rocky Mountains--presents no obstacle to the +advance of these cold northerly blasts. The middle and eastwardly +parts of North America are subject to like influences, in this +regard, to the plains of eastern Europe. To the westward of the +Rocky Mountains, on the other hand, the conditions affecting climate +present greater analogy to those that belong to western Europe. + +In the case of many countries, some local wind, of occasional +prevalence, forms a marked characteristic of climate. The most +remarkable of these local winds are the simoon, the sirocco, the +föhn, the harmattan, and the mistral. + +The often-described _simoon_ of the desert is an intensely heated +and dry wind, which raises the temperature like the blast of a +furnace, and fills the air with particles of sand, of suffocating +quality. The same wind is known in the deserts of Turkestan as the +_tebbad_ (fever-wind), the terrible conditions of which are thus +described by the pen of a traveler. “The kervanbashi (_leader of the +caravan_) and his people drew our attention to a cloud of dust that +was approaching, and told us to lose no time in dismounting from the +camels. These poor brutes knew well enough that it was the _tebbad_ +that was hurrying on; uttering a loud cry they fell on their knees, +stretched their long necks along the ground, and strove to bury their +heads in the sand. We intrenched ourselves behind them, lying there +as behind a wall; and scarcely had we, in our turn, knelt under their +cover, than the wind rushed over us with a dull, clattering sound, +leaving us, in its passage, covered with a crust of sand two fingers +thick. The first particles that touched me seemed to burn like a +rain of flakes of fire. Had we encountered it when we were deeper +in the desert we should all have perished. I had not time to make +observations upon the disposition to fever and vomiting caused by +the wind itself, but the air became heavier and more oppressive than +before.” + +The _sirocco_ of the Mediterranean coasts is the hot wind of the +African desert, tempered, before reaching the coasts of southern +Europe, by its passage across the great expanse of inland waters. +The enervating influences of this wind are well known to the resident +on the shores of Sicily, the Italian mainland, or the islands of the +Archipelago. The same wind, when it reaches the high mountain regions +of the Apennines and the Alps, is known as the föhn. + +The _föhn_, or warm south wind, is an important agent in modifying +the climate of the higher Alpine region, where its prevalence for a +few days in succession causes the snow-line to recede, and is often +accompanied by inundations occasioned by the suddenly melted snows. +Its absence during a longer period than usual is attended, on the +other hand, by a prolongation of the glaciers into a lower region +of the mountain valleys. The Swiss peasants have a saying, when +they talk of the melting of the snow, that the sun could do nothing +without the föhn. + +The _harmattan_ of Senegambia and Guinea is a cold and intensely dry +wind, which blows from the northeast during the months of December +and January. + +The _mistral_ of southern France possesses similar qualities to the +last-named wind, and blows, for days together, down the valley of the +Rhone. + +Winds transport particles of dust, and, with them, the minuter forms +of vegetable and animal life, to vast distances. The phenomena known +to sailors as _red fogs_ and _sea-dust_ are evidence of this. In +the Mediterranean, and also in the neighborhood of the Cape Verde +Islands, showers of dust, of brick-red or cinnamon color, are +sometimes experienced in such quantity as to cover the sails and +rigging hundreds of miles away from land. Among this sea-dust, +examination with the microscope has detected infusoria and other +organisms native to the tropical regions of South America. + +The prevailing currents of the atmosphere, or _winds_, constitute an +important feature in the climate of any country, and it belongs to +Physical Geography to explain the prevalent winds which distinguish +great regions of the globe. Such explanation is more easily made +in regard to the warmer latitudes of the earth, where alone the +direction of the wind is constant, than might be at first supposed +by those whose personal experience is limited to such countries as +Britain, and other temperate lands, where the variable condition +of the atmosphere is the well-known subject of common observation +and remark. But within those parts of the globe which experience +a vertical sun, and for a few degrees beyond the exact line which +marks the limit of the sun’s vertical influence on either side of +the equator, the conditions either of perennial calm, or of currents +of air that constantly blow in one given direction, are the uniform +characteristics of climate. + +Throughout a zone of a few degrees in breadth, which extends round +the globe in the neighborhood of the equator, and the limits of which +undergo a certain amount of variation, dependent on the sun’s passage +of the equinox, the variation of temperature throughout the year +is confined within very narrow limits, and the result is a general +prevalence of calms--that is, of undisturbed atmosphere. Wind is air +set in motion, mainly by the existence of different conditions of +temperature between adjacent bodies of air--of colder and denser air +pressing against warmer and lighter air, and taking the place which +is left vacant by the latter, as it rises into the higher regions +of the entire aerial sea. Between the heated air of the tropics in +general, and the comparatively cooler air of the regions lying some +distance north and south of the tropics, for example, there is a very +manifest difference as to temperature, as well as in regard to other +conditions; but for a few degrees in the immediate neighborhood of +the equator there is no such obvious difference, and, consequently, +nothing to occasion disturbance (temperature alone being considered) +in the general equilibrium of the atmosphere. Hence the prevalence +of calms in that region. Within the parallels of 8° or 10° on either +side of the line, the angle at which the solar rays reach the earth +is at no time more than a few degrees from the perpendicular, for +the equator divides the total amount of angular difference which is +involved in the entire yearly path of the sun. + +The average breadth of the calm latitudes--or the _Zone of Calms_, +as it is the custom, in books and maps, to term it--may be stated +at about six or seven degrees. The mid-line of this zone does not +coincide with the equator, for the reason that the equator does +not represent the line of the earth’s highest temperature, owing +to the preponderance of land in the Northern Hemisphere. Hence +the Zone of Calms is, for the most part, to the northward of the +equator--extending, with varying seasonal limits, from about the +first to the seventh or eighth parallel of north latitude. But the +limits oscillate with the sun’s passage of the equinox and consequent +place in the heavens vertically over either side of the equator. + +The calm latitudes are the dread of the mariner, whose ship is often +delayed for weeks together within their limits. The wearisome and +tantalizing nature of this delay can, perhaps, only be adequately +appreciated by those who have experienced the monotony attendant on +a calm in mid-ocean, when, with a still and glassy sea around, a +glittering atmosphere, and a burning sun overhead, the sails hang +idly by the yards, and the vessel makes no appreciable progress. + +Between the oscillating limit of the Zone of Calms and the parallel +of 28° in the Northern Hemisphere, on one side of the globe, and +between the correspondent limit and the parallel of 25° south +latitude, on the opposite hemisphere, there prevail through above +two-thirds of the earth’s circumference, steady winds, blowing +with almost undeviating uniformity from the eastward. These are +the trade-winds. More precisely, _the trade-wind of the Northern +Hemisphere is a wind blowing from the northeastward_--that is, a +_northeast wind_. _The trade-wind of the Southern Hemisphere blows +from the southeastward_, and is _a southeast wind_. + +The trade-wind belts stretch round more than two-thirds of the +earth’s surface. They comprehend (within the latitudinal limits +already defined) the Atlantic and Pacific Oceans, with the countries +that lie adjacent to those vast areas of water. In the Pacific, +however, their limits are less distinctly marked, and their influence +less powerful, to the southward of the equator than to the north of +that line. Over the Indian Ocean and its shores, the atmospheric +currents follow, during portions of the year, an opposite course. + +The trade-winds of the Atlantic and Pacific--blowing constantly, +and with almost undeviating steadiness, from the eastward--regulate +the course of the mariner across those oceans. They, of course, +facilitate the passage of either ocean in a westerly direction--that +is, from the shores of the Old World to the eastern seaboard of +America, or from the western coast of the New World to the eastern +shores of the Asiatic and Australian Continents. It was the +trade-wind of the Northern Atlantic that carried Columbus to the +westward, on the adventurous voyage which resulted in the discovery +of the New World, inspiring terror in the breasts of his companions, +while in the mind of the great navigator himself it strengthened the +assurance of reaching land by pursuing the direction in which his +vessels’ prows were turned. On a like great occasion, the trade-wind +of the Pacific carried Magellan’s ship steadily forward through the +ocean which he was the first to cross, and facilitated the earliest +circumnavigation of the globe. On the other hand, the same winds +compel the return voyage across either ocean to be made in higher +latitudes, where westerly winds prevail. + +The explanation of the trade-winds is found in the different measure +in which the sun’s heat is experienced by regions within or nearly +adjacent to the tropics, and by those of higher latitudes. They +are currents of air set in motion by the differences of density +consequent upon such various conditions of temperature--conditions +which are of uniform prevalence, and the result of which is also +constant. + +To sum up, we may say that the trade-winds, like the currents of +the ocean, are due, _first_, to the sun,--that is to the different +measure in which the solar heat is distributed on the globe’s +surface; and, _secondly_, to the earth’s axial rotation, which +affects the direction of currents in the aerial ocean in manner +precisely analogous to that in which it affects the like currents +in the aqueous ocean. In truth, the ocean of water, and the ocean +of air--in contact with one another, and possessing many properties +in common--act and react upon one another, mutually imparting their +respective temperatures, movements, and other conditions. This is +only one among the instances of mutual harmony--one of the many mute +sympathies--which abound in the natural world. + +The monsoons are winds which blow over the Indian Ocean, and the +countries adjacent to its waters. In general terms, it may be said +that they prevail within the same latitudes as those over which the +trade-winds of the Atlantic and Pacific blow. But the monsoons differ +from the trade-winds of the two greater oceans in the fact that they +are _periodical winds_, not perennial. The monsoon blows for half the +year from one quarter of the heavens, and for the other half from an +opposite quarter. + +[Illustration: Charts Showing the General Directions of Wind and Tide +Currents] + +Over the northerly portion of the Indian Ocean--from the +neighborhood of the equator to the shores of the Asiatic Continent, +including the Malay Archipelago and the adjacent China Sea--a +northeast monsoon blows during the winter months of the Northern +Hemisphere; that is, from October to March, inclusive. During the +summer months--April to September--and within the same limits, +the southwest monsoon blows. Southward from the equator to the +neighborhood of the tropic of Capricorn, the southeast monsoon +blows during the winter of those latitudes (April to September): +this is exchanged, during the other half of the year, for a +northwest monsoon in the neighborhood of the Australian coasts, +and for a northeast monsoon along the line of the African shores. +The term _monsoon_--derived from a Malay word which signifies +“season”--expresses the periodical nature of these winds, and +indicates to how large an extent the climate of Indian seas and lands +is dependent upon their periodical recurrence. + +The change from the one monsoon to that from an opposite quarter +is not accomplished at once. The breaking-up of the monsoon, as it +is termed, is attended by thunderstorms and other meteorological +phenomena, which prevail during some weeks, until the setting-in +of the coming monsoon is fairly accomplished. The nature of these +changes, and the general characteristics of the monsoon itself, are +admirably depicted in the following passage, by a master hand: + +“Meanwhile the air becomes loaded to saturation with aqueous vapor +drawn up by the augmented force of evaporation acting vigorously +over land and sea; the sky, instead of its brilliant blue, assumes +the sullen tint of lead, and not a breath disturbs the motionless +rest of the clouds that hang on the lower range of hills. At length, +generally about the middle of the month, but frequently earlier, the +sultry suspense is broken by the arrival of the wished-for change. +The sun has by this time nearly attained his greatest northern +declination, and created a torrid heat throughout the lands of +southern Asia and the peninsula of India. The air, lightened by its +high temperature and such watery vapor as it may contain, rises into +loftier regions, and is replaced by indraughts from the neighboring +sea, and thus a tendency is gradually given to the formation of a +current bringing up from the south the warm humid air of the equator. +The wind, therefore, which reaches Ceylon comes laden with moisture, +taken up in its passage across the great Indian Ocean. As the monsoon +draws near, the days become more overcast and hot, banks of clouds +rise over the ocean to the west, and in the peculiar twilight the eye +is attracted by the unusual whiteness of the sea-birds that sweep +along the strand to seize the objects flung on shore by the rising +surf. + +“At last sudden lightnings flash among the hills and shoot through +the clouds that overhang the sea, and with a crash of thunder the +monsoon bursts over the thirsty land, not in showers or partial +torrents, but in a wide deluge, that in the course of a few hours +overtops the river banks and spreads in inundations over every level +plain. + +“All the phenomena of this explosion are stupendous: thunder, as we +are accustomed to be awed by it, affords but the faintest idea of +its overpowering grandeur in Ceylon, and its sublimity is infinitely +increased as it is faintly heard from the shore, resounding through +night and darkness over the gloomy sea. The lightning, when it +touches the earth where it is covered with the descending torrent, +flashes into it and disappears instantaneously; but when it strikes a +drier surface, in seeking better conductors, it often opens a hollow +like that formed by the explosion of a shell, and frequently leaves +behind it traces of vitrification. In Ceylon, however, occurrences +of this kind are rare, and accidents are seldom recorded from +lightning, probably owing to the profusion of trees, and especially +of cocoanut palms, which, when drenched with rain, intercept the +discharge, and conduct the electric matter to the earth. The rain at +these periods excites the astonishment of a European; it descends +in almost continuous streams, so close and so dense that the level +ground, unable to absorb it sufficiently fast, is covered with one +uniform sheet of water, and down the sides of acclivities it rushes +in a volume that wears channels in the surface. For hours together, +the noises of the torrent as it beats upon the trees and bursts upon +the roofs, flowing thence in rivulets along the ground, occasions an +uproar that drowns the ordinary voice and renders sleep impossible.” + +The monsoons of the Indian Ocean are not divided by any such +distinctly defined belt of calms as separates the opposite +trade-winds of the northern and southern Pacific and Atlantic. The +southeast monsoon of the southern Indian Ocean passes gradually +into the southwest monsoon, which prevails at the same time in the +northern half of that ocean. Nor is the season of change from the +one monsoon to the other precisely the same over all parts of that +ocean. Indeed, the Indian Ocean, from the geographical conditions +already adverted to, is exposed in much higher measure than either +of the other oceans to the disturbing influences consequent upon +proximity to land, and its winds are hence affected in a vastly +greater degree by local conditions. Thus the Indian monsoon, the +Arabian and East African monsoon, and the monsoon of northwestern +Australia, assume in each case a direction which is dependent upon +the geographical position and contour of the lands whence they derive +their distinguishing names. In the Red Sea, the monsoons follow +the direction of its shores, and blow, for six months of the year, +alternately, up and down its long and trough-like valley, confined +and guided in their passage by the mountain-chains which bound it +upon either side. + +We have hitherto spoken of the monsoons only in connection with the +Indian Ocean. But, in truth, a monsoon, or season-wind--which is what +the word monsoon means--is experienced upon a large portion of the +West African coasts, and thence far out into the mid-Atlantic, within +the proper region of the Atlantic trades. The evidence of this is one +among the many valuable results due to the Wind and Current Charts +of Maury, and the cause of it is precisely the same as that which +occasions the monsoon of the Indian coasts. Between the equator +and the parallel of 13° north, the intense heat of a vertical sun, +acting upon the western coasts and adjacent interior of the African +Continent, occasions a reversal of the ordinary wind of that region. +The intensely heated atmosphere of the land, owing to superior +rarity, ascends, and the cooler air of the neighboring sea sets in to +fill its place. The monsoon thus generated lasts as long as the sun +remains to the northward of the equator. Further to the south a like +phenomenon accompanies, in those localities, the passage of the sun +into south declination. The influence of these monsoons extends to +a distance of a thousand miles or more from land, the entire space +within which they prevail forming a cuneiform (or wedge-shaped) +region in the midst of the Atlantic, the base of which rests upon the +African Continent, while its apex is within ten or fifteen degrees of +the mouth of the Amazon. + +A similar reversal of the trade-winds of the North Pacific occurs +off the western shores of Central America, capable of explanation +in precisely like manner--due, that is, to the excess of heat which +the summer sun brings to the adjacent lands, and the consequent +rarefaction and rising of the currents of air over those lands. +This, and the like instance of the West African monsoons, show in +the most striking manner how powerfully the land is affected by the +sun’s heat, and to how wide a distance the atmospheric movements +which are generated by such influences extend over the adjacent +seas. Even such limited tracts of land as the Society and Sandwich +Islands have a marked influence upon the winds experienced over the +surrounding waters. They interfere, says Maury, with the trade-winds +of the Pacific very often, and even turn them back, for westerly and +equatorial winds are common at both groups, in their winter time. + +Upon the coasts of most countries that are within the warmer +latitudes of the globe, there occur daily, at or shortly before the +hour of early dawn, and toward the approach of sunset, breezes that +blow respectively _off the shore_ or from _off the adjacent waters_. +The former is known as the land-breeze; the latter as the sea-breeze. + +These refreshing movements of the air are not confined to countries +within, or even very near to, the tropics, though they are more +powerful in the case of countries that are within the torrid zone +than in the case of other lands. But they are felt upon the coasts +of the Mediterranean, and in even much higher latitudes than those +of the Mediterranean, during the warmer portions of the year. The +hour at which they begin to be perceptible is not the same in all +localities; but, speaking generally, the land-breeze begins to be +felt about an hour before sunrise, and the sea-breeze toward the +early evening, as the time of sunset approaches. During the midday +hours the intense heat of the atmosphere, accompanied by general calm +and almost perfect repose of the animal world, is painfully felt by +all residents in warm countries, and the cooling sea-breeze which +sets in as the sun approaches the horizon is welcomed with intense +delight. To the sojourner in Indian lands, it is the signal for +outdoor exercise, and is accompanied by a general reawakening of +the outer world of nature. The dweller on the African or Australian +coasts equally rejoices in its refreshing power. The mariner within +Indian seas, frequently becalmed during the stillness of the +night-watch, finds like relief in the breeze which blows off the land +with the approach of early morning. + +The land and sea-breezes are due to a cause strictly analogous +to that which produces the monsoon of eastern seas--that is, the +influence of the sun heating in various measures the lands and +seas, and with them the superincumbent air. Successive movements +are generated in the atmosphere according as different portions of +the whole acquire, with difference of temperature, various degrees +of density. During the hours of midday heat, the air over the land +becomes relatively hotter, by many degrees, than the air which is +above the adjacent water, for it is the well-known attribute of land +to experience much greater extremes of temperature than water does. +As afternoon, with its sultry temperature, advances, this continued +heat occasions the land-air to form an ascending current, while the +cooler (and relatively denser) air from the neighboring waters flows +in to take its place. This cooling breeze is an effort of nature to +restore equilibrium in the atmosphere, the heavier portions of the +whole body of air assuming the place of lower strata, and the higher +portions spreading over the superior regions. This effort continues +until the desired balance is attained, and, with the approach of +midnight, the air is again calm and settled. But during the night, +while the water retains a nearly uniform temperature, the land +rapidly parts with the heat, so that the air over the land becomes +at length colder than that over the water. This latter, therefore, +relatively the warmer of the two, tends to rise, while the cooler air +of the land fills its place. A wind blowing from off the land is thus +generated. In some localities this blows during great part of the +night. But the period of its commencement varies in different places, +and the intervals of calm between both land and sea-breezes are often +of uncertain duration. + +The land and sea-breezes repeat, on a scale of diurnal variation, the +phenomena shown by the monsoons on a scale of yearly change. They +show how readily the atmosphere yields to the slightest pressure, +and how powerful an influence on the laws of climate, and, with +them, on the condition of mankind, is exercised by every change, +of temperature, or otherwise, to which it is subject. Similar +winds--alternating from opposite quarters of the heavens--are +experienced in inland districts, as on the banks of the Tapajos +River, in South America. + +The rotary storms which occur, at uncertain intervals, in particular +latitudes, are to be included among the exceptional phenomena of +atmospheric change. They prevail, however, over larger areas than +was formerly supposed, and perhaps belong to a general system of +atmospheric movements in which electric and magnetic influences fill +an important place. The hurricanes of the West Indies, the tornadoes +and cyclones of the Indian Ocean, and the typhoons of the China Sea, +are winds of this description. Within the Southern Hemisphere, the +direction of the rotating circle is always found to correspond to the +movement of the hands of a watch (_i. e._, from west to north, east, +and south): to the north of the equator, the circle of wind follows +an opposite direction (or west to south, east, and north). By a +knowledge of this law, combined with careful observation of the track +usually taken by such storms, mariners are enabled to avoid some of +the dangers incident to their occurrence. The destruction which they +occasion, however, within maritime tracts exposed to their influence, +as well as upon the high seas, is at times fearfully great. + +Waterspouts are another form in which the rotary movements of the +air are manifested. In the case of these phenomena, a taper column +of cloud, descending from above, is joined by a spiral column of +water which winds upward from the agitated surface of the sea, the +two together forming, by their union, a continuous column which moves +over the sea. Waterspouts seldom last longer than half an hour. They +are more frequent near the coast than on the high seas, and more +commonly seen in warm climates. + + + + + SQUALLS, WHIRLWINDS, AND TORNADOES + --SIR RALPH ABERCROMBY + + +If we watch the stages of gradually increasing wind, we find that as +the strength rises the tendency is more and more to blow in gusts. +Gradually these gusts get still more violent, and in their highest +development come with a boom like the discharge of a piece of heavy +ordnance. This is what sailors call “blowing in great guns,” and +these are the gusts which blow sails into ribbons, and dismast ships +more than any amount of steady wind. These gusts only last a few +minutes, but they seem to be very closely allied to the simplest form +of squalls. In a true, simple squall the wind generally need not be +of the exceptional violence which causes “guns”; but after it has +rather fallen a little, the blast comes on suddenly with a burst, +and rain or hail, according to intensity, or other circumstances, +while the whole rarely lasts more than five or ten minutes. At sea +one often sees two or three squalls flying about at a time. Then we +readily observe that over the squall there is firm, hard, cumulus +cloud; that the disturbance only reaches a short distance above the +earth’s surface; that the squall moves nearly in the same direction +as the wind; and that there is little or no shift of the wind before +or during the squall. We also see that the shape of the squall is +merely that of an irregular patch, with a tendency rather to be +longer in the direction of the wind than in any other quarter; and +that the motion of the squall as a whole is much slower than that +of the wind which accompanies the first blasts. If, at the same +time, we watch our barometer closely, we find that if the squall +is sufficiently strong, the mercury invariably rises--sometimes as +much as one-tenth of an inch--and returns to its former level after +the squall is over. No difference is observed in this sudden rise, +whether the squall is accompanied with rain, hail, or thunder and +lightning; and though we are unable exactly to explain why the wind +sometimes takes this irregular method of blowing, we have still to do +with a comparatively simple phenomenon. + +The simplest kind of thunderstorm may more properly be described as +a squall accompanied by thunder and lightning, instead of only with +wind and rain. On a wild, stormy day, with common squalls, one or two +of these, which are exceptionally violent, will be accompanied by one +or two claps of thunder with lightning. The principal interest which +attaches to this type of thunderstorm consists in the proof which +is afforded that there is no essential difference between a common +squall and another which may be associated with electrical discharge, +except intensity. The look and motion of the clouds, and the sudden +rise of the barometer, are identical in both cases. In western Europe +this class of thunderstorm is much more common in winter than in +summer, which is the reverse of what takes place with all other kinds +of thunderstorm. So much is this the case that in Iceland there are +no summer thunderstorms, but only winter ones, of this simple squall +type. In Norway both types occur; and the winter ones are there +found to be the most destructive, because they are lower down, and +therefore the lightning is the more likely to strike buildings. In +that country, however, the summer thunderstorms are not nearly so +violent as in more southern latitudes. + +We must now just mention a class of thunderstorms which are more +complicated than a simple squall, and yet differ in many ways from +line-thunderstorms. They are associated with secondary cyclones, +and are much commoner in England than line-thunderstorms, but none +have been tracked over a sufficiently long area to allow us to say +anything about their shape or motion. All we know is, that as surely +as we see a secondary on the charts in summer, so certainly will +thunderstorms occur during the day, though we can not say in what +portion of the small depression. + +The special features of this class of thunderstorm are the calm +sultry weather with which they are associated, so different from +the squall of a line-thunderstorm, and the limited rotation of +the surface-wind during the progress of the storm. Another very +remarkable feature is that this surface circling of the wind extends +only a very short distance upward, and whenever a glimpse can be +caught of the drift of the upper clouds, they are found to move in +the same direction throughout the whole period of the disturbance. +This is the familiar class of thunderstorm which we associate with +sultry weather, and with the thunder coming against the wind. + +One of the first things which must strike everybody is, that even +in the temperate zone some countries are far more ravaged by +thunderstorms than others. For instance, France suffers more than any +other part of Europe, and England the least. We may probably find at +least two causes which modify the development of thunderstorms. In +the first place, the geographical position of the country relative to +the great seasonal areas of high and low pressure. From this point +of view we can readily see that France is far more exposed to the +influence of small secondaries, which come in from the Atlantic, +and which die out before they reach central Europe, than any other +portion of that continent. + +In Great Britain, though the bulk of winter rain is cyclonic, a +great deal of summer rainfall is non-isobaric; in Continental Europe +a still larger proportion is of the latter character; so are most +tropical rains, except the downpour of hurricanes; while the whole of +the heavy rain on the equator, and all that falls in the doldrums, is +also absolutely non-isobaric. + +A moderate whirlwind may be two hundred feet high, and not above ten +feet in diameter. The dimensions, however, are very variable, for a +whirlwind may vary in intensity from a harmless eddy in a dusty road +to the destructive tornado of the United States. + +But by far the most striking non-isobaric rain in the world is the +burst of the southwest monsoon in the Indian Ocean. The quality of +the rain, if nothing else, distinguishes the monsoon from cyclonic +precipitation. The rain in front of a Bengal cyclone seems to grow +out of the air, while that of the monsoon falls in thunderstorms and +from heavy cumulo-form clouds. The only rational suggestion which has +been made to account for this burst of rain would look to a sudden +inrush of damp air from the region of the doldrums as the source +of the change in weather, but not of the direction of the wind, or +of the shape of the isobars; for the burst is apparently almost +coincident with the disappearance of the belt of high pressure to +the south of the Bay of Bengal. + +The word “pampero” is, unfortunately, used in a very vague manner +in the Argentine Republic and neighboring states. Every southwest +wind which blows from off the pampas is sometimes called a pampero; +and there is a still further confusion caused by calling certain dry +dust-storms _pamperos sucios_, or dry pamperos. The true pampero +may be described as a southwest wind, ushered in by a sudden short +squall, usually accompanied by rain and thunder, with a very peculiar +form of cloud-wreath. + +The barometer always falls pretty steadily for from two to four days +before the pampero, and always rises for some days after the squall. +Temperature is always very high before the squall, and then the +sudden change of wind sends the thermometer rapidly down, sometimes +as much as 33° in six hours. Thunder accompanies about three out of +four pamperos; but more or less rain always falls, except in the +rarest cases. The wind before this class of pampero almost invariably +blows moderately or gently for some days from easterly points, and +then with a sudden burst the southwest wind comes down with its full +strength, and, after blowing thus from ten to thirty minutes, either +ceases entirely or continues with diminished force for a certain +number of hours. In all cases but one the upper wind-currents have +been seen to come from the northwest before, during and after the +pampero. + +The general appearance of a pampero will be best understood by a +description of an actual squall. “In the early morning of a day +in November, the wind blew rather strongly from the northeast. The +sky was cloudy, but not overcast, save in the southwest horizon. +The clouds were moving very slowly from the west, or a little +south of it, throwing out long streamers eastward. About 8 A. M. +the threatening masses in the southwest had advanced near enough +to show that at their head marched two dense and perfectly regular +battalions of cloud, one behind the other, in close contact, yet +not intermingling, and completely distinguished by their striking +difference of color, the first being of a uniform leaden gray, while +the second was as black as the smoke of a steamer. On arriving +overhead, it was seen that the front, although slightly sinuous, was +perfectly straight in its general direction, and that the bands were +of uniform breadth. As they rushed at a great speed under the other +clouds without uniting with them, preserving their own formation +unbroken, their force seemed irresistible, as if they were formed of +some solid material rather than vapor. The length of these wonderful +clouds could not be conjectured, as they disappeared beneath the +horizon at both ends, but probably at least fifty miles of them +must have been visible, as the ‘Cerro’ commands a view of twenty +miles of country. Their breadth was not great, as they only took +a few minutes to pass overhead, and appeared to diminish from the +effects of perspective to mere lines on the horizon. At the instant +when the first band arrived, the wind--which was still blowing, and +something more than gently, from the northeast--went round by north +to southwest; at the same time a strong, cold blast fell from the +leaden cloud, and continued to blow till both bands had passed.” + +A whirlwind may be described as a mass of air whose height is +enormously greater than its width, rotating rapidly round a more or +less vertical axis. + +A tornado is simply a whirlwind of exceptional violence; if it were +to encounter a lake or the sea, it would be called a waterspout. +Its most characteristic feature is a funnel, or spout, which is the +visible manifestation of a cylinder of air that is revolving rapidly +round a nearly vertical axis. This spout is propagated throughout the +northern temperate zone in a northeasterly direction at a rate of +about thirty miles an hour, and tears everything to pieces along its +narrow path. + +The diameter of the actual spout often does not exceed a few yards, +and the total area of destructive wind is rarely more than three +or four hundred yards across. The height of the spout is that of +the lowest layer of clouds, which are then never high; and, as in +thunderstorms, the upper currents are unaffected by the violent +commotion below. + +The spout as a whole has four distinct motions: + +1. A motion of translation generally toward the northeast at a +variable rate, but which may be taken to average thirty miles an hour. + +2. A complex gyration. The horizontal portion of this rotation is +always in a direction opposite to that of the hands of a watch--that +is to say, in the same manner as an ordinary cyclone. But in addition +to this there is a violent upward current in the centre of the +cylinder of vapor or dust which constitutes the spout, and sometimes +small clouds seem to dart down the outer sides of the funnel whenever +these float in close proximity. There are, however, no authentic +instances of any object being thrown to the ground by the individual +effort of a downward current. The slight downward motion of a few +small clouds is probably only a slight eddying of a violent uprush. + +3. A swaying motion to and fro like a dangling whip, or an elephant’s +trunk, though the general direction of the spout is always vertical. + +4. A rising and falling motion, that is to say, that sometimes the +end of the funnel rises from the surface of the ground and then +descends again, and so on. Owing to this rise and fall, the general +appearance of the tornado changes a good deal. When the bottom of +the spout is some distance above the ground, the whole is somewhat +pointed, and does comparatively little harm as it passes over any +place. As the spout descends, a commotion commences on the surface of +the ground. This latter gradually rises so as to meet the descending +part of the spout, and then the whole takes the shape of an +hour-glass. This is the most dangerous and destructive form, because +the ground gets the whole force of the tornado. + +The general appearance of the cloud over a tornado or whirlwind is +always described as peculiarly smoky, or like the fumes of a burning +haystack. The tornado is also never an isolated phenomenon; it is +always associated with rain and electrical disturbance. + +The destructive effects of the tornado are very curious, from the +sharp and narrow belt to which the injury is confined. It appears +that in the passage of some tornadoes wind-pressures of various +amounts, from eighteen to a hundred and twelve pounds per square +foot, have been demonstrated by destruction of bridges, brick +buildings, etc. The upward pressures are sometimes as great as the +horizontal, and even greater. Downward pressures or movements of +wind have not been clearly proved. Upward velocities of 135 miles +per hour seem not to be unusual, and horizontal velocities of eighty +miles have been recorded with the anemometer. The destructive +wind-velocities are confined to very small areas. A destruction of +fences, trees, etc., is often visible over a path many miles long and +a few hundred yards wide, but the path of greatest violence is very +much narrower. The excessive cases above referred to are observed +only in small isolated spots, less than a hundred feet square, +unequally distributed along the middle of the track. Thus, in very +large buildings, only a small part is subject to destructive winds. +In different parts of this area of _maximum_ severity, the winds are +simultaneously blowing in different, perhaps opposite, directions, +the resultant tending not to overturn or carry off or crush in, but +rather to twist round a vertical axis. Buildings are generally lifted +and turned round before being torn to pieces. As the chances are very +small that a building will be exposed to the violent twisting action, +it is evidently the average velocity of rectilinear winds within the +path of moderate destruction that it is most necessary to provide +against in ordinary structures. These winds may attain a velocity +of eighty miles an hour over an area of a thousand feet broad, and +generally blow from the southwest; the next in frequency blow from +the northwest. The time during which an object is exposed to the +more destructive winds varies from six to sixty seconds. An exposed +building experiences but one stroke, like the blow of a hammer, and +the destruction is done. Hence, in a suspension-bridge, chimney, +or other structure liable to be set into destructive rhythmic +vibrations, the _maximum_ winds do not produce such vibrations. The +duration of the heavy southwest or northwest winds over the area +of moderate destruction is rarely over two minutes. The motion of +translation of the central spout of a tornado, in which there is a +strong vertical current, is, on ah average, at the rate of thirty +miles an hour. + +Tornadoes mostly occur on sultry days and either in the southeast or +right front of cyclones, or in front of the trough of V-depressions. + +The general character of all tornadoes is so similar that the +description of one will do for all. We shall therefore give some of +the description furnished by an eye-witness to the United States +Signal Office, which is described in the reports as the “Delphos +tornado”: + +“On Friday morning, May 30, 1879, the weather was very pleasant, +but warm, with the wind from the southeast, from which direction it +had blown for several days. The ground was very dry, and no rain +had fallen for a number of weeks. About 2 P. M. threatening clouds +appeared very suddenly in the west (against the wind), attended in a +few minutes by light rain, the wind still in the southeast It stopped +in about five minutes, and then commenced again, wind still the same, +accompanied by hail, which was thick and small at first, but rapidly +grew less in quantity and larger in size, some stones measuring +three and a half inches in diameter, and one was found weighing +one-fourth of a pound. This last precipitation continued for about +thirty minutes, after which a cloud in the shape of a waterspout +was seen forming in the southwest, and moving rapidly forward to +the northeast. The cloud from which the funnel depended, seen at a +distance of eight miles, appeared to be in terrible commotion; in +fact, while the hail was falling, a sort of tumbling in the clouds +was noticed as they came up from the northwest and southwest, and +about where they appeared to meet was the point from which the funnel +was seen to descend. There was but one funnel at first, which was +soon accompanied by several smaller ones, dangling down from the +overhanging clouds like whiplashes, and for some minutes they were +appearing and disappearing like fairies at a play. Finally one of +them seemed to expand and extend downward more steadily than the +others, resulting at length in what appeared to be their complete +absorption. This funnel-shaped cloud now moved onward, growing in +power and size, whirling rapidly from right to left, rising and +descending, and swaying from side to side. When within a distance +of three or four miles, its terrible roar could be heard, striking +terror into the hearts of the bravest.” The eye-witness judged that +the funnel itself would reach a height of about five hundred feet +from the ground. As the storm crossed a river, a cone-shaped mass +came up from the earth to meet it, carrying mud, débris, and a large +volume of water. The cloud then passed the observer’s house very +near to 4 P. M. The progressive velocity at the time was considered +to be about thirty miles per hour, although at Delphos, three and a +half miles distant, it had slackened down to near twenty miles. A few +minutes previous to and during the passage of the funnel, the air was +very oppressive; but ten minutes after the wind was so cold from the +northwest that it became necessary to wear an overcoat when outside. + +The actual diameter of this storm appears to have been only +forty-three yards. On the right of the track, destructive winds +extended to a further distance of from one to two miles, sensibly +deflected winds for another mile and a half, beyond which only the +usual wind of the day was experienced. On the left or northern side +of the tornado path, the damage did not extend quite so far, for the +width of the belt of destructive winds was not more than twenty-eight +yards across and that of sensibly deflected winds one mile and a +quarter. + +As a specimen of the damage done a large two-horse sulky plow, +weighing about seven hundred pounds, was carried a distance of twenty +yards, breaking off one of the iron wheels attached to an iron axle +one and three-quarter inches in diameter. A woman was carried to the +northwest two hundred yards, lodged against a barbed-wire fence, and +instantly killed. Her clothing was entirely stripped from her body, +which was found covered with black mud, and her hair matted with it. +A cat was found half a mile to the northwest of the house, in which +she had been seen just before the storm, with every bone broken. +Chickens were stripped of their feathers, and one was found three +miles to the northwest. + +A few miles further on, another eye-witness says, “the dark, inky, +funnel-shaped cloud rapidly descended to the earth, which reaching, +it destroyed everything within its grasp. Everything was taken up and +carried round and round in the mighty whirl of the terrible monster. +The surrounding clouds seemed to roll and tumble toward the vortex. + +“The funnel, now extending from the earth upward to a great height, +was black as ink, excepting the cloud near the top, which resembled +smoke of a light color. Immediately after passing the town, there +came a wave of hot air, like the wind blowing from a burning +building. It lasted but a short time. Following this peculiar +feature, there came a stiff gale from the northwest, cold and bleak, +so much so that during the night frost occurred, and water in some +low places was frozen.” + + +END OF VOLUME TWO + + +FOOTNOTES: + +[1] The lakes of Sweden, which cover one-twelfth of the surface of +the country, exercise an important influence on climate according as +they are frozen or open. + +[2] Another variety or species of seal inhabits Lake Baikal. + +[3] Count von Helmersen, however, has stated his belief that for this +extreme northern prolongation of the Aralo-Caspian Sea there is no +evidence. The shells, on the presence of which over the Tundras the +opinion was chiefly based, are, according to him, all fresh-water +species, and there are no marine shells of living species to be met +with in the plains at the foot of the Ural Mountains. + +[4] Archbishop of Spalato and Primate of Dalmatia. + + + + + TRANSCRIBER’S NOTE + + Obvious typographical errors and punctuation errors have been + corrected after careful comparison with other occurrences within + the text and consultation of external sources. + + Some hyphens in words have been silently removed, some added, + when a predominant preference was found in the original book. + + Except for those changes noted below, all misspellings in the text, + and inconsistent or archaic usage, have been retained. + + Pg 470: ‘chiefly Brachipods of’ replaced by ‘chiefly Brachiopods of’. + Pg 472: ‘these same familes’ replaced by ‘these same families’. + Pg 483: ‘constituing links’ replaced by ‘constituting links’. + Pg 563: ‘Camaroons Mountains’ replaced by ‘Cameroon Mountains’. + Pg 563: ‘with Teneriffe in’ replaced by ‘with Tenerife in’. + Pg 569: ‘existing, denundation’ replaced by ‘existing, denudation’. + Pg 650: ‘their relativ size’ replaced by ‘their relative size’. + Pg 718: ‘incalulable ages’ replaced by ‘incalculable ages’. + Pg 722: ‘greatly diminshed’ replaced by ‘greatly diminished’. + + +*** END OF THE PROJECT GUTENBERG EBOOK 77792 *** |