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[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.