The story of the universe. Volume 2 (of 4)

Chapter I: Part 1

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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

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:

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.

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The story of the universe. Volume 2 (of 4)Chapter I: Part 1

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