Transcriber’s Note
When italics were used in the original book, the corresponding text has
been surrounded by _underscores_.
Some corrections have been made to the printed text. These are listed in
a second transcriber’s note at the end of the text.
PHYSICAL GEOGRAPHY.
BY
MARY SOMERVILLE,
AUTHOR OF THE “CONNECTION OF THE PHYSICAL SCIENCES,”
“MECHANISM OF THE HEAVENS,” ETC., ETC.
SECOND AMERICAN,
FROM THE NEW AND REVISED LONDON EDITION.
WITH ADDITIONS AND
A GLOSSARY
PREPARED FOR THIS EDITION.
----------------------------
PHILADELPHIA:
LEA & BLANCHARD.
1850.
ENTERED, according to Act of Congress, in the year 1849, by LEA &
BLANCHARD, in the Clerk’s Office, of the District Court of the Eastern
District of Pennsylvania.
TO
SIR JOHN F. W. HERSCHEL, BART., K.H.,
&c., &c.
DEAR SIR JOHN,
I avail myself with pleasure of your permission to dedicate my book to
you, as it gives me an opportunity of expressing my admiration of your
talents, and my sincere estimation of your friendship.
I remain, with great regard,
Yours truly,
MARY SOMERVILLE.
_London, 29th February, 1848._
ADVERTISEMENT.
THE improvements and additions embodied in the new London edition are
fully enumerated in the Author’s Preface. The American publishers have
supplied what was much needed, a complete Glossary of Scientific and
Technical Terms; which, with some few additions scattered through the
pages, and enclosed in brackets [ ], were prepared by Dr. Ruschenberger.
These additions, while they have improved the work, have added
materially to the number and size of the pages. The publishers,
consequently, trust that it will be found more effectually suited to the
wants of the private reader as well as to those of the higher classes in
schools.
_Philadelphia, December, 1849._
PREFACE
TO THE NEW EDITION.
SINCE the publication of the first edition of this work, the Author has
been able to correct many inaccuracies that had crept into it, and to
collect much new matter from works since published, which is embodied in
the present edition, and has considerably added to its size. The
recently-published Second Volume of Baron Humboldt’s invaluable
“Cosmos,”[1] with Colonel Sabine’s learned notes, and sundry papers that
have appeared in the scientific periodicals of Europe, America, and
India, bearing on questions of Physical Geography, have yielded
profitable information.
It was the Author’s wish, and her Publisher’s intention, that the
present edition should be accompanied by a series of Maps to illustrate
the more important questions of Physical Geography treated of in it; but
Mr. A. Keith Johnston having announced the publication of a new edition
of his “Physical Atlas” in a reduced size, at a low price,[2] the first
two Nos. of which have already appeared, the project was relinquished,
in the belief that Mr. A. K. Johnston’s smaller Atlas will furnish
suitable illustrations to this work.
The reader will find this edition enriched with some of the results of
the recent researches of Messrs. Campbell, Thomson, Strachey, and Dr.
Hooker in the Himalaya, which tend largely to elucidate the Physical
History of that gigantic chain. The book is also indebted to Mr.
Pentland for some new matter, hitherto unpublished, on the countries of
South America visited by him during his two public missions to Peru and
Bolivia.
The absence of the Author from England during the printing of the
following sheets has obliged her to have recourse to a friend conversant
with her subject to revise the press; to him she begs to express her
acknowledgments, as well as to Sir William Jackson Hooker, who very
kindly undertook to correct the portion of this new edition connected
with Geographical Botany and Vegetable Physiology—subjects respecting
which he has so much contributed to extend our knowledge. Captain
Beechey has been good enough to render similar service, in revising
certain passages bearing upon Hydrography.
_May, 1849._
CONTENTS.
CHAPTER I.
Of Physical Geography—Position of the Earth in the Solar System—
Distance from the Sun—Civil Year—Inclination of Terrestrial
Orbit—Mass of the Sun—Distance of the Moon—Figure and Density of
the Earth from the Motions of the Moon—Figure of the Earth from
Arcs of the Meridian—From Oscillations of Pendulum—Local
Disturbances—Mean Density of the Earth—Known Depth below its
Surface—Outline of Geology PAGE 13
CHAPTER II.
Direction of the Forces that raised the Continents—Proportion of
Land and Water—Size of the Continents and Islands—Outline of the
Land—Extent of Coasts, and proportion they bear to the Areas of
the Continents—Elevation of the Continents—Forms of Mountains—
Forms of Rocks—Connection between Physical Geography of Countries
and their Geological Structure—Contemporaneous Upheaval of
parallel Mountain Chains—Parallelism of Mineral Veins or
Fissures—Mr. Hopkins’s Theory of Fissures—Parallel Chains similar
in Structure—Interruptions in Continents and Mountain Chains—Form
of the Great Continent—The High Lands of the Great Continent—The
Atlas, Spanish, French, and German Mountains—The Alps, Balkan,
and Apennines—Glaciers—Geological Notice 37
CHAPTER III.
The High Lands of the Great Continent (_continued_)—The Caucasus—
The Western Asiatic Table-Land and its Mountains 54
CHAPTER IV.
The High Lands of the Great Continent (_continued_)—The Oriental
Table-Land and its Mountains 58
CHAPTER V.
Secondary Mountain Systems of the Great Continent—That of
Scandinavia—Great Britain and Ireland—The Ural Mountains—The
Great Northern Plain 69
CHAPTER VI.
The Southern Low Lands of the Great Continent, with their
Secondary Table-Lands and Mountains 77
CHAPTER VII.
Africa—Table-Land—Cape of Good Hope and Eastern Coast—Western
Coast—Abyssinia—Senegambia—Low Lands and Deserts 85
CHAPTER VIII.
American Continent—The Mountains of South America—The Andes—The
Mountains of the Parima and Brazil 93
CHAPTER IX.
The Low Lands of South America—Desert of Patagonia—The Pampas of
Buenos Ayres—The Silvas of the Amazons—The Llanos of the Orinoco
and Venezuela—Geological Notice 105
CHAPTER X.
Central America—West Indian Islands—Geological Notice 114
CHAPTER XI.
North America—Table-Land and Mountains of Mexico—The Rocky
Mountains—The Maritime Chain and Mountains of Russian America 119
CHAPTER XII.
North America (_continued_)—The Great Central Plains, or Valley
of the Mississippi—The Alleghany Mountains—The Atlantic Slope—The
Atlantic Plain—Geological Notice—The Mean Height of the
Continents 123
CHAPTER XIII.
The Continent of Australia—Tasmania, or Van Diemen’s Land—
Islands—Continental Islands—Pelasgic islands—New Zealand—New
Guinea—Borneo—Atolls—Encircling Reefs—Coral Reefs—Barrier Reefs—
Volcanic Islands—Areas of Subsidence and Elevation in the Bed of
the Pacific—Active Volcanos—Earthquakes—Secular Changes in the
Level of the Land 136
CHAPTER XIV.
Arctic Lands—Greenland—Spitzbergen—Iceland—Its Volcanic Phenomena
and Geysers—Jan Mayen’s Land—New Siberian Islands—Antarctic
Lands—Victoria Continent 159
CHAPTER XV.
Nature and Character of Mineral Veins—Metalliferous Deposits—
Mines—Their Drainage and Ventilation—Their Depth—Diffusion of the
Metals—Gold—Silver—Lead—British Mines—Quicksilver—Copper—Tin—
Cornish Mines—Coal—Iron—Most abundant in the Temperate Zones,
especially in the Northern—European and British Iron and Coal—
American Iron and Coal—Arsenic and other Metals—Salt—Sulphur—
Diffusion of the Gems 168
CHAPTER XVI.
The Ocean—Its Size, Colour, Pressure, and Saltness—Tides—Waves—
their Height and Force—Currents—their Effect on Voyages—
Temperature—The Stratum of Constant Temperature—Line of Maximum
Temperature—North and South Polar Ice—Inland Seas 188
CHAPTER XVII.
Springs—Basins of the Ocean—Origin, Course, and Heads of Rivers—
Hydraulic Systems of Europe—African Rivers—the Nile, Niger, &c. 209
CHAPTER XVIII.
Asiatic Rivers—Euphrates and Tigris—River Systems South of the
Himalaya—Chinese Rivers—Siberian Rivers 224
CHAPTER XIX.
River Systems of North America—Rivers of Central America—Rivers
of South America and of Australia 234
CHAPTER XX.
Lakes—Northern System of the Great Continent—Mountain System of
the same—American Lakes 245
CHAPTER XXI.
Temperature of the earth—Temperature of the Air—Radiation—Foci of
Maximum Cold—Thermal Equator—Its Temperature, mean and absolute—
Isothermal Lines—Continental and Insular Climates—Extreme
Climates—Stability of Climate—Decrease of Heat in Altitude—Line
of Perpetual Snow—Density of the Atmosphere—The Barometer—
Measurement of Heights—Variations in Density and their Causes—
Horary Variations—Independent Effect of the dry and aqueous
Atmospheres—Mean height of Barometer in different Latitudes—
Depression in the Antarctic Ocean and in Eastern Siberia—
Barometric Storms—Polar and Equatorial Currents of Air—
Trade-Winds—Monsoons—Land and Sea Breezes—Gyration of the Winds
in the Extra-Tropical Zones—Winds in Middle European Latitudes—
Hurricanes—The Laws of their Motion—Their Effect on the
Barometer—How to steer clear of them—The Storm-Wave—
Storm-Currents—Arched Squalls—Tornadoes—Whirlwinds—Water Spouts 254
CHAPTER XXII.
Evaporation—Distribution of Vapour—Dew—Hoar-Frost—Fog—Region of
Clouds—Forms of Clouds—Rain—Distribution of Rain—Quantity—Number
of rainy Days in different Latitudes—Rainless Districts—Snow
Crystals—Line of perpetual Snow—Limit of Winter Snow on the
Plains—Sleet—Hail—Minuteness of the ultimate Particles of Matter—
Their Densities and Forms—Their Action on Light—Colour of Bodies—
Colour of the Atmosphere—Its Absorption and Reflection of Light—
Mirage—Fog Images—Coronæ and Halos—The Rainbow—Iris in Dewdrops—
The Polarization of the Atmosphere—Atmospheric Electricity—Its
Variations—Electricity of Fogs and Rain—Inductive Action of the
Earth—Lightning—Thunder—Distribution of Thunder-Storms—Back
Stroke—St. Elmo’s Fire—Phosphorescence—Aurora—Magnetism—
Terrestrial Magnetism—The Dip—Magnetic Poles and Equator—Magnetic
Intensity—Dynamic Equator—Declination—Magnetic Meridian—Lines of
equal Variation—Horary Variations—Line of Alternate Horary
Phenomena—Magnetic Storms—Coincidence of the Lines of equal
Magnetic Intensity with Mountain Chains—Diamagnetism 272
CHAPTER XXIII.
Vegetation—Nourishment and Growth of Plants—Effects of the
different Rays of the Solar Spectrum—Classes—Botanical Districts 298
CHAPTER XXIV.
Vegetation of the Great Continent—Of the Arctic Islands—And of
the Arctic and North Temperate Regions of Europe and Asia 312
CHAPTER XXV.
Flora of Tropical Asia—Of the Indian Archipelago, India, and
Arabia 323
CHAPTER XXVI.
African Flora—Flora of Australia, New Zealand, Norfolk Island,
and of Polynesia 330
CHAPTER XXVII.
American Vegetation—Flora of North, Central, and South America—
Antarctic Flora—Origin and Distribution of the Cerealia—Ages of
Trees—Marine Vegetation 341
CHAPTER XXVIII.
Distribution of Insects 363
CHAPTER XXIX.
Distribution of Marine Animals in general—Fishes—The Marine
Mammalia—Phocæ, Dolphins, and Whales 368
CHAPTER XXX.
Distribution of Reptiles—Frogs and Toads—Snakes, Saurians, and
Tortoises 383
CHAPTER XXXI.
Distribution of Birds in the Arctic Regions—In Europe, Asia,
Africa, America, and the Antarctic Regions 392
CHAPTER XXXII.
Distribution of Mammalia throughout the Earth 412
CHAPTER XXXIII.
The Distribution, Condition, and future Prospects of the Human
Race 436
APPENDIX.
Table of Heights above the Sea of some of the Principal Mountain
Chains 475
GLOSSARY 487
INDEX 527
PHYSICAL GEOGRAPHY.
CHAPTER I.
GEOLOGY.
Of Physical Geography—Position of the Earth in the Solar System—Distance
from the Sun—Civil Year—Inclination of Terrestrial Orbit—Mass of the
Sun—Distance of the Moon—Figure and Density of the Earth from the
Motions of the Moon—Figure of the Earth from Arcs of the Meridian—from
Oscillations of Pendulum—Local Disturbances—Mean Density of the Earth—
Known Depth below its Surface—Outlines of Geology.
PHYSICAL Geography is a description of the earth, the sea, and the air,
with their inhabitants animal and vegetable, of the distribution of
these organized beings, and the causes of that distribution. Political
and arbitrary divisions are disregarded, the sea and the land are
considered only with respect to those great features that have been
stamped upon them by the hand of the Almighty, and man himself is viewed
but as a fellow-inhabitant of the globe with other created things, yet
influencing them to a certain extent by his actions, and influenced in
return. The effects of his intellectual superiority on the inferior
animals, and even on his own condition by the subjection of some of the
most powerful agents in nature to his will, together with the other
causes which have had the greatest influence on his physical and moral
state, are among the most important subjects of this science.
The former state of our terrestrial habitation, the successive
convulsions which have ultimately led to its present geographical
arrangement, and to the actual distribution of land and water, so
powerfully influential on the destinies of mankind, are circumstances of
primary importance.
The position of the earth with regard to the sun, and its connection
with the bodies of the solar system, have been noticed by the author
elsewhere. It was there shown that our globe forms but an atom in the
immensity of space, utterly invisible from the nearest fixed star, and
scarcely a telescopic object to the remote planets of our system. The
increase of temperature with the depth below the surface of the earth,
and the tremendous desolation hurled over wide regions by numerous
fire-breathing mountains, show that man is removed but a few miles from
immense lakes or seas of liquid fire. The very shell on which he stands
is unstable under his feet, not only from those temporary convulsions
that seem to shake the globe to its centre, but from a slow almost
imperceptible elevation in some places, and an equally gentle subsidence
in others, as if the internal molten matter were subject to secular
tides, now heaving and now ebbing, or that the subjacent rocks were in
one place expanded and in another contracted by changes of temperature.
The earthquake and the torrent, the august and terrible ministers of
Almighty Power, have torn the solid earth and opened the seals of the
most ancient records of creation, written in indelible characters on the
“perpetual hills and the everlasting mountains.” There we read of the
changes that have brought the rude mass to its present fair state, and
of the myriads of beings that have appeared on this mortal stage, have
fulfilled their destinies, and have been swept from existence to make
way for new races, which, in their turn, have vanished from the scene,
till the creation of man completed the glorious work. Who shall define
the periods of those mornings and evenings when God saw that his work
was good? and who shall declare the time allotted to the human race,
when the generations of the most insignificant insect existed for
unnumbered ages? Yet man is also to vanish in the ever-changing course
of events. The earth is to be burnt up, and the elements are to melt
with fervent heat—to be again reduced to chaos—possibly to be renovated
and adorned for other races of beings. These stupendous changes may be
but cycles in those great laws of the universe, where all is variable
but the laws themselves, and He who has ordained them.
The earth is one of seventeen planets which revolve about the sun in
elliptical orbits: of these, twelve have been discovered since the year
1787.[3] Mercury and Venus are nearer the sun than the earth, the others
are more remote. The earth revolves at a mean distance of 95,000,000
miles from the sun’s centre, in a civil year of 365 days 5 hours 48
minutes 49·7 seconds, at the same time that it rotates in 24 hours about
an axis which always remains parallel to itself, and inclined at an
angle of 23° 27ʹ 34ʺ·69 to the plane of the ecliptic; consequently, the
days and nights are of equal length at the equator, from whence their
length progressively differs more and more as the latitude increases,
till at each pole alternately there is perpetual day for six months, and
a night of the same duration: thus the light and heat are very unequally
distributed, and both are modified by the atmosphere by which the earth
is encompassed to the height of about forty miles.
With regard to magnitude, Mars, Jupiter, Saturn, Uranus, and Neptune are
larger than the earth, the rest are smaller, but even the largest is
incomparably inferior to the sun in size: his mass is 354,936 times
greater than that of the earth, but the earth is nearly four times as
dense.
Though the planets disturb the earth in its motion, their form has no
effect on account of their great distance; but it is otherwise with
regard to the moon, which revolves about the earth at a mean distance of
240,000 miles, and is therefore so near that the form of both bodies
causes mutual disturbances in their respective motions. The
perturbations in the moon’s motions from that cause, compared with the
same computed from theory, show that the earth is not a perfect sphere,
but that it bulges at the equator, and is flattened at the poles: it
even gives a value of the compression or flattening.[4] Again, theory
shows that, if the earth were throughout of the same density, it would
be much less flat at the poles than the moon’s motions show it to be,
but that it would be very nearly the same were the earth to increase
regularly in density from the surface to its centre; and thus the lunar
motions not only make known the form, but reveal the internal structure
of the globe. Actual measurement has proved the truth of these results.
The courses of the great rivers, which are generally navigable to a
considerable extent, show that the curvature of the land differs but
little from that of the ocean; and as the heights of the mountains and
continents are inconsiderable when compared with the magnitude of the
earth, its figure is understood to be determined by a surface at every
point perpendicular to the direction of gravitation, or of the
plumb-line, and is the same which the sea would have if it were
continued all round the earth beneath the continents. Such is the figure
that has been measured in various parts of the globe.
A terrestrial meridian is a line passing through both poles, all the
points of which have their noon contemporaneously, and a degree of a
meridian is its 360th part. Now, if the earth were a sphere, all degrees
would be of the same length; but, as it is flattened at the poles, the
degrees are longest there, and decrease in length to the equator, where
they are least. The form and size of the earth may therefore be
determined by comparing the length of degrees in different latitudes.[5]
Eleven arcs have been measured in Europe, one in Peru, and two in the
East Indies; but a comparison of no two gives the same result, which
shows that the earth has a slightly irregular form. From a mean of ten
of these arcs, M. Bessel found that the equatorial radius of the earth
is 3963·025 miles, and the polar radius 3949·8 miles nearly. Whence,
assuming the earth to be a sphere, the length of a mean degree of the
meridian is 69·05 British statute miles; therefore 360 degrees, or the
whole circumference of the globe, is 24,858 miles; the diameter, which
is something less than a third of the circumference, is about 8286, or
8000 statute miles; and the length of a geographical mile of 60 to a
degree is 6086·76 feet. The breadth of the torrid zone is 705
geographical miles, the breadth of each of the temperate zones is 645
miles, and that of each of the spaces within the arctic and antarctic
circles 11,431 miles nearly. The Astronomer Royal Mr. Airy’s results,
obtained ten years afterwards, only differ from those of M. Bessel by
127 feet in the equatorial, and 138 feet in the polar radius, quantities
not greater than the length of a ball-room. In consequence of the round
form of the earth, the dip or depression of the horizon is a fathom for
every three miles of distance; that is to say, an object a fathom or six
feet high would be hid by the curvature of the earth at the distance of
three miles. Since the dip increases as the square, a hill 100 fathoms
high, would be hid at the distance of ten miles, and the top of
Dhawalagori, the culminating point of the Himalaya, 28,000 feet high,
would be seen to sink beneath the horizon by a person about 167 miles
off; thus, when the height is known, an estimate can be formed of the
distance of a mountain.
The oscillations of the pendulum have afforded another method of
ascertaining the form of the earth. Like all heavy bodies, its descent,
and consequently its oscillations, are accelerated in proportion to the
force of gravitation, which increases from the equator to the poles. In
order, therefore, that the oscillations may be everywhere performed in
the same time, the length of the pendulum must be increased
progressively in going from the equator to the poles, according to a
known law,[6] from whence the compression or flattening at the poles may
be deduced. Experiments for that purpose have been made in a great
number of places, but, as in the measurement of the arcs, no two sets
give exactly the same results; the mean of the whole, however, differs
very little from that given by the degrees of the meridian and the
perturbations of the moon; and as the three methods are so entirely
independent of each other, the figure and dimensions of the earth may be
considered to be known. The sea has little effect on these experiments,
both because its mean density is less than that of the earth, and that
its mean depth of perhaps four miles is inconsiderable when compared
with 4000 miles, the mean terrestrial radius.[7]
The discrepancies in the results, from the comparison of the different
sets of pendulum experiments, and also of degrees of the meridian, arise
from local attraction, as well as from irregularities in the form of the
earth. These attractions, arising from dense masses of rock in
mountains, cause the plumb-line to deviate from the vertical, and when
under ground they alter the oscillations of the pendulum. Colonel
Sabine, who made experiments with the pendulum from the equator to
within ten degrees of the north pole, discovered that the intensity is
greatly augmented by volcanic islands. A variation to the amount of a
tenth of a second in twenty-fours can be perfectly ascertained in the
rate of the pendulum, but from some of these local attractions a
variation of nearly ten seconds has occurred during the same period. The
islands of St. Helena, Ascension, St. Thomas, the Isle of France, are
some of those noted by Colonel Sabine.
There are other remarkable instances of local disturbance, arising from
the geological nature of the soil; for example, the intensity of
gravitation is very small at Bordeaux, from whence it increases rapidly
to Clermont-Ferrand, Milan, and Padua, where it attains a maximum (owing
probably to dense masses of rock under ground), and from thence it
extends to Parma. In consequence of this local attraction, the degrees
of the meridian in that part of Italy seem to increase towards the
equator through a small space, instead of decreasing, as if the earth
were drawn out instead of flattened at the poles.
It appears from this that the effect of the whole earth on a pendulum or
torsion balance may be compared with the effect of a small part of it,
and thus a comparison maybe instituted between the mass of the earth and
the mass of that part of it. Now, a leaden ball was weighed against the
earth by comparing the effects of each upon a balance of torsion; the
nearness of the smaller mass making it produce a sensible effect as
compared with that of the larger, for by the laws of attraction the
whole earth must be considered as collected in its centre; in this
manner a value of the mass of the earth was obtained, and, as its volume
was known, its mean density was found to be 5·675 times greater than
that of water at the temperature of 62° of Fahrenheit’s thermometer.
Now, as that mean density is double that of basalt, and more than double
that of granite, rocks which undoubtedly emanate from very great depths
beneath the surface of the earth, it affords another proof of the
increase in density towards the earth’s centre. These experiments were
first made by Mr. Cavendish and Mitchell, and latterly with much greater
accuracy by M. Baily, who devoted four years of unremitted attention to
the accomplishment of this important and difficult object.[8]
Although the earth increases in density regularly from the surface to
the centre, as might naturally be expected from the increasing pressure,
yet the surface consists of a great variety of substances of different
densities, some of which occur in amorphous masses; others are disposed
in regular layers or strata, either horizontal or inclined at all angles
to the horizon. By mining, man has penetrated only a very little way;
but by reasoning from the dip or inclination of the strata at or near
the surface, and from other circumstances, he has obtained a pretty
accurate idea of the structure of our globe to the depth of about ten
miles. All the substances of which we have any information are divided
into four classes, distinguished by the manner in which they have been
formed: namely,—plutonic and volcanic rocks, both of igneous origin,
though produced under different circumstances; aqueous or stratified
rocks, entirely due to the action of water, as the name implies; and
metamorphic rocks, deposited by water, according to the opinion of many
eminent geologists, and consequently stratified, but subsequently
altered and crystallized by heat. The aqueous and volcanic rocks are
formed at the surface of the earth, the plutonic and metamorphic at
great depths; but all of them have originated simultaneously during
every geological period, and are now in a state of slow and constant
progress. The antagonist principles of fire and water have ever been and
still are the cause of the perpetual vicissitudes to which the crust of
the earth is liable.
It has been ascertained by observation that the plutonic rocks,
consisting of the granites and some of the porphyries, were formed in
the deep and fiery caverns of the earth, of melted matter, which
crystallized as it slowly cooled under enormous pressure, and was then
heaved up in unstratified masses by the elastic force of the internal
heat even to the tops of highest mountains, or forced in a semi-fluid
state into fissures of the superincumbent strata, sometimes into the
cracks of the previously formed granite: for that rock, which
constitutes the base of so large a portion of the earth’s crust, has not
been all formed at once; some portions had been solid, while others were
yet in a liquid state. This class of rocks is completely destitute of
fossil remains.
Although granite and the volcanic rocks are both due to the action of
fire, their nature and position are very different; granite, fused in
the interior of the earth, has been cooled and consolidated before
coming to the surface; besides, it generally consists of few
ingredients, so that it has nearly the same character in all countries.
But as the volcanic fire rises to the very surface of the earth, fusing
whatever it meets with, volcanic rocks take various forms, not only from
the different kinds of strata which are melted, but from the different
conditions under which the liquid matter has been cooled, though most
frequently on the surface—a circumstance that seems to have had the
greatest effect on its appearance and structure. Sometimes it approaches
so nearly to granite that it is difficult to perceive a distinction; at
other times it becomes glass; in short, all those massive, unstratified,
and occasionally columnar rocks, as basalt, greenstone, porphyry, and
serpentine, are due to volcanic fires, and are devoid of fossil remains.
There seems scarcely to have been any age of the world in which volcanic
eruptions have not taken place in some part of the globe. Lava has
pierced through every description of rocks, spread over the surface of
those existing at the time, filled their crevices, and flowed between
their strata. Ever changing its place of action, it has burst out at the
bottom of the sea as well as on dry land. Enormous quantities of scoriæ
and ashes have been ejected from numberless craters, and have formed
extensive deposits in the sea, in lakes, and on the land, in which are
embedded the remains of the animals and vegetables of the epoch. Some of
these deposits have become hard rock, others remain in a crumbling
state; and as they alternate with the aqueous strata of almost every
period, they contain the fossils of all the geological epochs, chiefly
fresh and salt water testaceæ.
According to a theory now generally adopted, which originated with Sir
Charles Lyell, whose works are models of philosophical investigation,
the metamorphic rocks, which consist of gneiss, micaschist, clay-slate,
statuary marble, &c., were formed of the sediment of water in regular
layers, differing in kind and colour, but, having been deposited near
the place where plutonic rocks were generated, they have been changed by
the heat transmitted from the fused matter, and, in cooling under heavy
pressure and at great depths, they have become as highly crystallized as
the granite itself, without losing their stratified form. An earthy
stratum has sometimes been changed into a highly crystallized rock, to
the distance of a quarter of a mile from the point of contact, by
transmitted heat; and there are instances of dark-coloured limestone,
full of fossil shells, that has been changed into statuary marble from
that cause. Such alterations may frequently be seen to a small extent on
rocks adjacent to a stream of lava. There is seldom a trace of organic
remains in the metamorphic rocks; their strata are sometimes horizontal,
but they are usually tilted at all angles to the horizon, and form some
of the highest mountains and most extensive table-lands on the face of
the globe. Although there is the greatest similarity in the plutonic
rocks in all parts of the world, they are by no means identical; they
differ in colour, and even in ingredients, though these are few.
Aqueous rocks are all stratified, being the sedimentary deposits of
water. They originate in the wear of the land by rain, streams, or the
ocean. The débris carried by running water is deposited at the bottom of
the seas and lakes, where it is consolidated, and then raised up by
subterraneous forces, again to undergo the same process after a lapse of
time. By the wasting away of the land the lower rocks are laid bare,
and, as the materials are deposited in different places according to
their weight, the strata are exceedingly varied, but consist chiefly of
arenaceous or sandstone rocks, composed of sand, clay, and carbonate of
lime. They constitute three great classes, which, in an ascending order,
are the primary and secondary fossiliferous strata and the tertiary
formations.
The primary fossiliferous strata, the most ancient of all the
sedimentary rocks, consisting of limestone, sandstones, and shales, are
entirely of marine origin, having been formed far from land at the
bottom of a very deep ocean; consequently, they contain the exuviæ of
marine animals only, and after the lapse of unnumbered ages the
ripple-marks of the waves are still distinctly visible on some of their
strata. This series of rocks is subdivided into the Cambrian and the
upper and lower Silurian systems, on account of differences in their
fossil remains.
The Cambrian rocks, sometimes many thousand yards thick, are, for the
most part, destitute of organic remains, but the Silurian rocks abound
in them more and more as the strata lie higher in the series. In the
lower Silurian group are the remains of shell-fish, almost all of
extinct genera, and the few that have any affinity to those alive are of
extinct species; crinoidea, or stone lilies, which had been fixed to the
rocks like tulips on their stems, are coëval with the earliest
inhabitants of the deep; and the trilobite, a jointed creature of the
crab kind, with prominent eyes, are almost exclusively confined to the
Silurian strata, but the last traces of them are found in the
coal-measures above. In the upper Silurian group are abundance of marine
shells of almost every order, together with crinoidea, vast quantities
of corals, and some sea-weeds: several fossil sauroid fishes, of extinct
genera, but high organization, have been found in the highest beds—the
only vertebrated animals that have yet been discovered among the
countless profusion of the lower orders of creatures that are entombed
in the primary fossiliferous strata. The remains of one or more land
plants, in a very imperfect state, have been found in the Silurian rocks
of North America, which shows that there had been land with vegetation
at that early period. The type of these plants, as well as the size of
the shells and the quantity of the coral, indicate that a uniformly warm
temperature had then prevailed over the globe. During the Silurian
period an ocean covered the northern hemisphere, islands and lands of
moderate size had just begun to rise, and earthquakes, with volcanic
eruptions from insular and submarine volcanos, were frequent towards its
close.
The secondary fossiliferous strata, which comprise a great geological
period, and constitute the principal part of the high land of Europe,
were deposited at the bottom of an ocean, like the primary, from the
débris of all the others, carried down by water, and still bear
innumerable tokens of their marine origin, although they have for ages
formed a part of the dry land. Calcareous rocks are more abundant in
these strata than in the crystalline, probably because the carbonic acid
was then, as it still is, driven off from the lower strata by the
internal heat, and came to the surface as gas or in calcareous springs,
which either rose in the sea and furnished materials for shell-fish and
coral insects to build their habitations and form coral-reefs, or
deposited their calcareous matter on the land in the form of rocks.
The Devonian or old red sandstone group, in many places 10,000 feet
thick, consisting of strata of dark red and other sandstones, marls,
coralline limestones, conglomerates, &c., is the lowest of the secondary
fossiliferous strata, and forms a link between them and the Silurian
rocks, by an analogy in their fossil remains. It has fossils peculiarly
its own, but it has also some shells and corals common to the strata
both above and below it. There are various families of extinct sauroid
fishes in this group, some of which were gigantic, others had strong
bony shields on their heads, and one genus, covered with enamelled
scales, had appendages like wings. The shark approaches nearer to some
of these ancient fish than any other now living.
During the long period of perfect tranquility that prevailed after the
Devonian group was deposited, a very warm, moist, and extremely equable
climate, which extended all over the globe, had clothed the islands and
lands in the ocean then covering the northern hemisphere with exuberant
tropical forests and jungles. Subsequent inroads of fresh water, or of
the sea, or rather partial sinkings of the land, had submerged these
forests and jungles, which, being mixed with layers of sand and mud, had
in time been consolidated into one mass, and were then either left dry
by the retreat of the waters or gently raised above their surface.
These constitute the remarkable group of the carboniferous strata, which
consists of numberless layers of various substances filled with a
prodigious quantity of the remains of fossil land-plants, intermixed
with beds of coal, which is entirely composed of vegetable matter. In
some cases, the plants appear to have been carried down by floods, and
deposited in estuaries; but in most instances the beauty, delicacy, and
sharpness of the impressions show that they had grown on the spot where
the coal was formed. More than 300 fossil plants have been collected
from the strata where they abound, frequently with their seeds and
fruits, so that enough remains to show the peculiar nature of this
flora, whose distinguishing feature is the preponderance of ferns; among
these there were tree-ferns which must have been 40 or 50 feet high.
There were also plants resembling the horse-tail tribe, of gigantic
size, others like the tropical club mosses; an aquatic plant of an
extinct family was very abundant, beside many others to which we have
nothing analogous. Forest-trees of great magnitude, of the pine and fir
tribes, flourished at that period. The remains of an extinct araucaria,
one of the largest of the pine family, have been found in the British
coal-fields; the existing species now grow in very warm countries; a few
rare instances occur of grasses, palms, and liliaceous plants. The
botanical districts were very extensive when the coal-plants were
growing, for the species are nearly identical throughout the coal-fields
of Europe and America. From the extent of the ocean, the insular
structure of the land, the profusion of ferns and fir-trees, and the
warm, moist, and equable climate, the northern hemisphere during the
formation of the coal strata is thought to have borne a strong
resemblance to the South Pacific, with its fern and fir clothed lands of
New Zealand, Kerguelen islands, and others.
The animal remains of this period are in the mountain limestone, a rock
occasionally 900 feet thick, which in some instances lies beneath the
coal-measures, and sometimes alternates with the shale and sandstone.
They consist of crinoidea and marine testaceæ, among which the size of
the chambered shells, as well as that of the corals, shows that the
ocean was very warm at that time, even in the high northern latitudes.
The footsteps of a very large reptile of the frog tribe have been found
on some of the carboniferous strata of North America.
The coal strata have been very much broken and deranged in many places
by earthquakes, which frequently occurred during the secondary
fossiliferous period, and from time to time raised islands and land from
the deep. However, these and all other changes that have taken place on
the earth have been gradual and partial, whether brought about by fire
or water. The older rocks are more shattered by earthquakes than the
newer, because the movement came from below; but these convulsions have
never extended all over the earth at the same time—they have always been
local: for example, the Silurian strata have been dislocated and tossed
in Britain, while a vast area in the south of Sweden and Russia still
retains a horizontal position. There is no proof that any mountain-chain
has ever been raised at once; on the contrary, the elevation has always
been produced by a long-continued and reiterated succession of internal
convulsions with intervals of repose. In many instances the land has
risen up or sunk down by an imperceptible equable motion continued for
ages, while in other places the surface of the earth has remained
stationary for long geological periods.
The magnesian limestone, or permian formation, comes immediately above
the coal-measures, and consists of breccias or conglomerates, gypsum,
sandstone, marl, &c.; but its distinguishing feature is a yellow
limestone rock, containing carbonate of magnesia, which often takes a
granular texture, and is then known as dolomite. The permian formation
has a fossil flora and fauna peculiar to itself, mingled with those of
the coal strata. Here the remnant of an earlier creation gradually tends
to its final extinction, and a new one begins to appear. The flora is,
in many instances, specifically the same with that in the coal strata
below. Certain fish are also common to the two, which never appear
again. They belong to a race universal in the early geological periods,
and bear a strong resemblance to saurian reptiles. A small number of
existing genera only, such as the shark and sturgeon, make some approach
to the structure of these ancient inhabitants of the waters. The new
creation is marked by the introduction of two species of saurian
reptiles; the fossil remains of one have been found in the magnesian
limestone in England, and those of the other in a corresponding
formation in Germany. They are the earliest members of a family which
was to have dominion on the land and water for ages.
A series of red marls, rock-salt, and sandstones, which have arisen from
the disintegration of metamorphic slates and porphyritic trap,
containing oxide of iron, and known as the trias or new red sandstone
system, lies above the magnesian limestone. In England this formation is
particularly rich in rocksalt, which, with layers of gypsum and marl, is
sometimes 600 feet thick; but in this country the muschelkalk, a
peculiar kind of shell limestone, is wanting, which in Germany is so
remarkable for the quantity of organic remains. At this time creatures
like frogs, of enormous dimensions, had been frequent, as they have left
their footsteps on what must then have been a soft shore. Forty-seven
genera of fossil remains have been found in the trias in Germany,
consisting of shells, cartilaginous fish, encrinites, &c., all distinct
in species, and many distinct in genera, from the organic fossils of the
magnesian limestone below, and also from those entombed in the strata
above.
During a long period of tranquility the oolite or Jurassic group was
next deposited in a sea of variable depth, and consists of sands,
sandstones, marls, clays, and limestone. At this time there was a
complete change in the aqueous deposits all over Europe. The red
iron-stained arenaceous rocks, the black coal, and dark strata, were
succeeded by light-blue clays, pale-yellow limestones, and, lastly,
white chalk. The water that deposited the strata must have been highly
charged with carbonate of lime, since few of the formations of that
period are without calcareous matter, and calcareous rocks were formed
to a prodigious extent throughout Europe; the Pyrenees, Alps, Apennines,
and Balkan abound in them; and the Jura mountains, which have given
their name to the series, are formed of them. The European ocean then
teemed with animal life; whole beds consist almost entirely of marine
shells and corals. Belemnites and ammonites, from an inch in diameter to
the size of a cart-wheel, are entombed by myriads in the strata: whole
forests of that beautiful zoophyte the stone-lily flourished on the
surface of the oolite, then under the waters; and the encrinite, one of
the same genus, is embedded in millions in the enchorial shell-marble,
which occupies such extensive tracts in Europe. Fossil fish are numerous
in these strata, but different from those of the coal series, the
permian formation, and trias: not one genus of the fish of this period
is now in existence. The newly-raised islands and lands were clothed
with vegetation like that of the large islands of the intertropical
archipelagos of the present day, which, though less rich than during the
carboniferous period, still indicates a very moist and warm climate.
Ferns were less abundant, and they were associated with various genera
and species of the cicadeæ, which had grown on the southern coast of
England, and in other parts of northern Europe, congeners of the present
cycas and zamia of the tropics. These plants had been very numerous, and
the pandanæ, or screw-pine, the first tenant of the new lands in ancient
and modern times, is a family found in a fossil state in the inferior
oolite of England, which was but just rising from the deep at that time.
The species now flourishing grows only on the coasts of such coral
islands in the Pacific as have recently emerged from the waves. In the
upper strata of this group, however, the confervæ and monocotyledonous
plants become more rare—an indication of a change of climate.
The new lands that were scattered on the ocean of the oolite period were
drained by rivers, and inhabited by huge crocodiles and saurian reptiles
of gigantic size, mostly of extinct genera. The crocodiles come nearest
to modern reptiles; but the others, though bearing a remote similitude
in general structure to living forms, were quite anomalous, combining in
one the structure of various distinct creatures, and so monstrous that
they must have been more like the visions of a troubled dream than
things of real existence; yet in organization a few of them came nearer
to the type of living mammalia than any existing reptiles do. Some of
these had lived in rivers, others in the ocean—some were inhabitants of
the land, others were amphibious; and the various species of one genus
even had wings like a bat, and fed on insects. There were both
herbivorous and predaceous saurians; and from their size and strength
they must have been formidable enemies. Besides, the numbers deposited
are so great, that they must have swarmed for ages in the estuaries and
shallow seas of the period, especially in the lias, a marine stratum of
clay, the lowest of the oolite series. They gradually declined towards
the end of the secondary fossiliferous epoch; but as a class they lived
in all subsequent eras, and still exist in tropical countries, although
the species are very different from their ancient congeners. Tortoises
of various kinds were contemporary with the saurians, also a family that
still exists. In the Stonefield slate, a stratum of the lower oolite
group, there are the remains of insects, and the bones of two small
quadrupeds have been found there belonging to the marsupial tribe, such
as the opossum—a very remarkable circumstance, because that family of
animals at the present time is confined to New Holland, South America,
and as far north as Pennsylvania at least. The great changes in animal
life during this period were indications of the successive alterations
that had taken place on the earth’s surface.
The cretaceous strata follow the oolite in ascending order, consisting
of clay, green and iron sands, blue limestone, and chalk, probably
formed of the decay of coral and shells, which predominates so much in
England and other parts of Europe, that it has given the name and its
peculiar feature to the whole group. It is, however, by no means
universal; the chalk is wanting in many parts of the world where the
other strata of this series prevail, and then their connection with the
group can only be ascertained by the identity of their fossil remains.
With the exception of some beds of coal among the oolitic series, the
Wealden clay, the lowest of the cretaceous group in England, is the only
fresh-water formation, and the tropical character of its flora shows
that the climate was still very warm. Plants allied to the zamias and
cycades of our tropical regions, many ferns and pines of the genus
araucaria, characterized its vegetation, and the upright stems of a
fossil forest at Portland show that it had been covered with trees. It
was inhabited by tortoises approaching to families now living in warm
countries, and saurian reptiles of five different genera swarmed in the
lakes and estuaries. This clay contains fresh-water shells and fish of
the carp kind. The Wealden clay is one of the various instances of the
subsidence of land which took place during this period.
The cretaceous strata above our Wealden clay are full of marine exuviæ.
There are vast tracts of sand in Northern Europe, and many very
extensive tracts of chalk; but in the southern part of the Continent the
cretaceous rocks assume a different character. There and elsewhere
extensive limestone rocks, filled with very peculiar shells, show that,
when the cretaceous strata were forming, an ocean extended from the
Atlantic into Asia, which covered the south of France, all Southern
Europe, part of Syria, the isles of the Ægean Sea, the coasts of Thrace
and the Troad. The remains of turtles have been found in the cretaceous
group, quantities of coral, and abundance of shells of extinct species;
some of the older kinds still existed, new ones were introduced, and
some of the most minute species of microscopic shells, which constitute
a large portion of the chalk, are supposed to be the same with creatures
now alive, the first instance of identity of species in the ancient and
modern creation. An approximation to recent times is to be observed also
in the arrangement of organized nature, since at this early period, and
even in the Silurian and oolitic epochs, the marine fauna was divided,
as now, into distinct geographical provinces. The great saurians were on
the decline, and many of them were found no more, but a gigantic
creature, intermediate between the monitor and iguana, lived at this
period. From the permian group to the chalk inclusive, only two
instances of fossil birds occur, one in a chalk deposit in the Swiss
Alps, and the other a kind of albatross in the chalk in England; in
North America, however, the foot-marks of a variety of birds have been
found in the strata between the coal and lias, some of which are larger
than those of the ostrich.
An immense geological cycle elapsed between the termination of the
secondary fossiliferous strata and the beginning of the tertiary. With
the latter a new order of things commenced, approaching more closely to
the actual state of the globe. During the tertiary formation the same
causes under new circumstances produced an infinite variety in the order
and kind of the strata, accompanied by a corresponding change in the
animal and vegetable life. The old creation, which had nothing in common
with the existing order of things, had passed away and given place to
one more nearly approaching to that which now prevails. Among the
myriads of beings that inhabited the earth and the ocean during the
secondary fossiliferous epoch, scarcely one species is to be found in
the tertiary. Two planets could hardly differ more in their natural
productions. This break in the law of continuity is the more remarkable,
as hitherto some of the newly-created animals were always introduced
before the older were extinguished. The circumstances and climate suited
to the one became more and more unfit for the other, which consequently
perished gradually, while their successors increased. It is possible
that, as observations become more extended, this hiatus maybe filled up.
The series of rocks, from the granite to the end of the secondary
fossiliferous strata, taken as a whole, constitute the solid crust of
the globe, and in that sense are universally diffused over the earth’s
surface. The tertiary strata occupy the hollows formed in this crust,
whether by subterraneous movements, by lakes, or denudation by water, as
in the estuaries of rivers, and consequently occur in irregular tracts,
often, however, of prodigious thickness and extent. Indeed, they seem to
have been as widely developed as any other formation, though time has
been wanting to bring them into view.
The innumerable basins and hollows with which the continents and larger
islands had been indented for ages after the termination of the
secondary fossiliferous series had sometimes been fresh-water lakes, and
at other times inundated by the sea; consequently, the deposits which
took place during these changes alternately contain the spoils of
terrestrial and marine animals. The frequent intrusion of volcanic
strata among the tertiary formations shows that, in Europe, the earth
had been in a very disturbed state, and that these repeated vicissitudes
had been occasioned by elevations and depressions of the soil, as well
as by the action of water.
There are three distinct groups in these strata: the lowest tertiary or
Eocene group, so called by Sir Charles Lyell, because, among the myriads
of fossil shell-fish it contains, very few are identical with those now
living; the Miocene, or middle group, has a greater number of the exuviæ
of existing species of shells; and the Pleiocene, or upper tertiary
group, still more. Though frequently heaved up to great elevations on
the flanks of the mountain-chains, as, for example, on the Alps and
Apennines, by far the greater part of the tertiary strata maintain their
original horizontal position in the very places where they were formed.
Immense insulated deposits of this kind are to be met with all over the
world; Europe abounds with them, London, Paris, and Vienna stand on such
basins, and they cover immense tracts both in North and South America.
The monstrous reptiles had mostly disappeared, and the mammalia now took
possession of the earth, of forms scarcely less anomalous than their
predecessors, though approaching more nearly to those now living.
Numerous species of extinct animals that lived during the earliest or
Eocene period have been found in various parts of the world, especially
in the Paris basin, of the order of Pachydermata, to the greater number
of which we have nothing analogous; they were mostly herbivorous
quadrupeds, which had frequented the borders of the rivers and lakes
that covered the greater part of Europe at that time. This is the more
extraordinary, as existing animals most similar to these, the tapirs for
instance, are confined to the torrid zone. These creatures were widely
diffused, and some of them were associated with genera still existing,
though of totally different species; such as animals allied to the
racoon and dormouse, the ox, bear, deer, the fox, the dog, and others.
Although these quadrupeds differ so widely from those of the present
day, the same proportion existed then as now between the carnivorous and
herbivorous genera. The spoils of marine mammalia of this period have
also been found, sometimes of great elevations above the sea, all of
extinct species, and some of these cetacea were of huge size. This
marvellous change of the creative power was not confined to the earth
and the ocean; the air also was now occupied by many extinct races of
birds allied to the owl, buzzard, quail, curlew, &c. The climate must
still have been warmer than at present, from the remains of land and sea
plants found in high latitudes. Even in England, bones of the opossum,
monkey, and boa have been discovered, all animals of warm countries,
besides fossil sword and saw fish, both of genera foreign to the British
seas.
During the Miocene period, new amphibious quadrupeds were associated
with the old, of which the deinotherium is the most characteristic and
much the largest of the mammalia yet found, far surpassing the largest
elephant in size, of a singular form, and unknown nature.
The palæotherium was of this period, and also the mastodon, both of
large dimensions. Various families, and even genera, of quadrupeds now
existing were associated with these extraordinary creatures, though of
extinct species, such as the elephant, rhinoceros, hippopotamus, tapir,
horse, bear, wolf, hyæna, weasel, beaver, ox, buffalo, deer, &c.; and
also marine mammalia, as dolphins, sea-calves, walruses, and lamantines.
Indeed, in the constant increase of animal life manifested throughout
the whole of the tertiary strata, the forms approach nearer to living
species as their remains lie high in the series.
In the older Pleiocene period some of the large amphibious quadrupeds,
and other genera of mammalia of the earlier tertiary periods, appear no
more; but there were the mastodon, and the Elephas primigenius or
mammoth, some species of which, of prodigious size, were associated with
numerous quadrupeds of existing genera, but lost species. Extinct
species of almost all the quadrupeds now alive seem to have inhabited
the earth at that time; their bones have been discovered in caverns;
they were embedded in the breccias and in most of the strata of that
epoch—as the hippopotamus, rhinoceros, elephant, horse, bear, wolf,
water-rat, hyæna, and various birds. It is remarkable that in the
caverns of Australia the fossil bones all belong to extinct species of
gigantic kangaroos and wombats, animals belonging to the marsupial
family, which are so peculiarly the inhabitants of that country at the
present day, but of diminished size. The newer Pleiocene strata show
that the same analogy existed between the extinct and recent mammalia of
South America, which, like their living congeners, as far as we know,
belonged to that continent alone; for the fossil remains, quite
different from those in the old world, are of animals of the same
families with the sloths, anteaters, and armadilloes which now inhabit
that country, but of vastly superior size and different species. In
fact, there were giants in the land in those days. Were change of
species possible, one might almost fancy that these countries had
escaped the wreck of time, and that their inhabitants had pined and
dwindled under the change of circumstances. The megatherium and Equus
curvidens, or extinct horse, had so vast a range in America, that, while
Sir Charles Lyell collected their bones in Georgia in 33° N. latitude,
Mr. Darwin brought them from the corresponding latitude in South
America. The Equus curvidens differed as much from the living horse as
the quagga or zebra does, and the European fossil horse is also probably
a distinct and lost species.
A comparison of the fossil remains with the living forms has shown the
analogy between these beings of the ancient world and those that now
people the earth; and the greatest triumph of the geologist is the
certainty with which he can decide upon the nature of animals that have
been extinct for thousands of years, from a few bones entombed on the
earth’s surface. Baron Cuvier will ever be celebrated as the founder of
this branch of comparative anatomy, and which Professor Owen has brought
to the highest perfection. Among many discoveries, he has found, by the
most minute microscopic observation, that the structure of the tissue of
which teeth are formed is different in different classes of animals, and
that the species can in many instances be determined from the fragment
of a tooth. A small portion of a bone enabled him to decide on the
nature of an extinct race of birds, and the subsequent discovery of the
whole skeleton confirmed the accuracy of this determination.
The greater part of the land in the northern hemisphere was elevated
above the deep during the tertiary period, and such lands as already
existed acquired additional height; consequently the climate, which had
previously been tropical, became gradually colder, for an increase of
land, which raises the temperature between the tropics, has exactly the
contrary effects in higher latitudes. Hence, excessive cold prevailed
during the latter part of the Pleiocene period, and a great part of the
European continent was covered by an ocean full of floating ice, not
unlike that seen at this day off the north-eastern coast of America.[9]
During the latter part of the Pleiocene period, however, the bed of that
glacial ocean rose partially, and after many vicissitudes the European
continent assumed nearly the form it now bears. There is every reason to
believe that the glacial sea extended also over great portions of the
arctic lands of Asia and America. Old forms of animal and vegetable life
were destroyed by these alterations in the surface of the earth, and the
consequent change of temperature; and when, in the progress of the
Pleiocene period, the mountain-tops appeared as islands above the water,
they were clothed with the flora and peopled by the animals they still
retain; and new forms were added as the land rose and became dry and
fitted to receive and maintain the races of animals now alive, all of
which had possession of the earth for ages prior to the historical or
human period. Some of the extinct animals had long resisted the great
vicissitudes of the times; of these the mammoth or Elephas primigenius,
whose fossil remains are found all over Europe, Asia, and America, but
especially in the gelid soil of Siberia, alone outlived its associates,
the last remnant of a former world. In two or three instances this
animal has been discovered entire, entombed in frozen mud, with its hair
and its flesh so fresh that wolves and dogs fed upon it. The globe of
the eye of one found by M. Middendorf at Tas, between the rivers Oby and
Jenesei, was so perfect that it is now preserved in the museum at
Moscow. It has been supposed that, as the Siberian rivers flow for
hundreds of miles from the southern part of the country to the Arctic
Ocean, these elephants might have been drowned by floods while browsing
in the milder regions, and that their bodies were carried down by the
rivers and embedded in mud, and frozen before they had time to decay.
Mr. Darwin has suggested that, if the climate of Siberia has at any time
been similar to that of the high latitudes of South America, where the
line of perpetual snow in the Andes, and its sudden flexure in Southern
Chile, come close to a nearly tropical vegetation, such a vegetation may
have prevailed south of the frozen regions in Siberia, and,
consequently, the bodies of animals entombed a few feet below the icy
surface might be preserved for ages. On the other hand, although the
congeners of this animal are now inhabitants of the torrid zone, they
may have been able to endure the cold of a Siberian winter, for Baron
Cuvier found that this animal differed as much from the living elephant
as the horse does from the ass. Mr. Darwin has shown that the supply of
food in summer was probably sufficient, since the quantity requisite for
the maintenance of the larger animals is by no means in proportion to
their bulk, or it may have migrated to a more genial climate in the
colder months.
Shell-fish seem to have been more able to endure all the great
geological changes than any of their organic associates, but they show a
constant approximation to modern species during the progress of the
tertiary period. The whole of these strata contain enormous quantities
of shells of extinct species; in the oldest, three and a half per cent.
of the shells are identical with species now existing, while on the
uppermost strata of this geological period there are not less than from
ninety to ninety-five in a hundred identical with those now alive.
Of all the fossil fishes, from the Silurian strata to the end of the
tertiary, scarcely one is specifically the same with living forms: the
Mallotus villosus, or captan, of the salmon family, is an exception, and
perhaps a few others of the most recent of these periods. In the Eocene
strata one-third belong to extinct genera.
Under the vegetable mould in every country there is a stratum of loose
sand, gravel, and mud, lying upon the subjacent rocks, often of great
thickness, called alluvium, which in the high latitudes of North America
and Europe is mixed with enormous fragments of rock, sometimes angular
and sometimes rounded and water-worn, which have been transported
hundreds of miles from their origin. It is there known as the Boulder
formation, or Northern Drift, because, from the identity of the boulders
with the rocks of the northern mountains, they evidently have come from
them, and their size becomes less as the distance increases. In Russia
there are blocks of great magnitude that have been carried 800 and even
1000 miles south-east from their origin in the Scandinavian range. There
is much reason to believe that such masses, enormous as they are, have
been transported by ice-bergs, and deposited when the northern parts of
the continents were covered by the glacial sea, by which part of Russia
was submerged to the depth of at least 1000 feet. The same process is
now in progress in the high southern latitudes, where icebergs have been
met with covered with fragments of rock and boulders.[10]
The last manifestation of creative power, with few exceptions, differs
specifically from all that preceded it; the recent strata contain only
the exuviæ of animals now living, often mixed with the works of man.
The solid earth thus tells us of mountains washed down into the sea with
their forests and inhabitants; of lands raised from the bottom of the
ocean loaded with the accumulated spoils of centuries; of torrents of
water and torrents of fire. In the ordinances of the heavens no voice
declares a beginning, no sign points to an end; in the bosom of the
earth, however the dawn of life appears, the time is obscurely marked
when first living things moved in the waters, when the first plants
clothed the land. There we see that during ages of tranquillity the
solid rock was forming at the bottom of the ocean, that during ages it
was tossed and riven by fire and earthquake. What years must have gone
by since that ocean flowed which has left its ripple-marks on the sand,
now a solid mass on the mountain—since those unknown creatures left
their foot-prints on the shore, now fixed by time on the rock for ever!
time, which man measures by days and years, nature measures by thousands
of centuries.
The thickness of the fossiliferous strata up to the end of the tertiary
formation has been estimated at about seven or eight miles; so that the
time requisite for their deposition must have been immense. Every river
carries down mud, sand, or gravel, to the sea: the Ganges brings more
than 700,000 cubic feet of mud every hour, the Yellow River in China
2,000,000,[11] and the Mississippi still more; yet, notwithstanding
these great deposits, the Italian hydrographer Manfredi has estimated
that, if the sediment of all the rivers on the globe were spread equally
over the bottom of the ocean, it would require 1000 years to raise its
bed one foot, so that at that rate it would require 3,960,000 years to
raise the bed of the ocean alone to a height nearly equal to the
thickness of the fossiliferous strata, or seven miles and a half, not
taking account of the waste of the coasts by the sea itself: but if the
whole globe be considered, instead of the bottom of the sea only, the
time would be nearly four times as great, even supposing as much
alluvium to be deposited uniformly both with regard to time and place,
which it never is. Besides, in various places the strata have been more
than once carried to the bottom of the ocean, and again raised above its
surface by subterranean fires after many ages, so that the whole period
from the beginning of the primary fossiliferous strata to the present
day must be great beyond calculation, and only bears comparison with the
astronomical cycles, as might naturally be expected, the earth being
without doubt of the same antiquity with the other bodies of the solar
system. What then shall we say if the time be included which the
granitic, metamorphic, and recent series occupied in forming? These
great periods of time correspond wonderfully with the gradual increase
of animal life and the successive creation and extinction of numberless
orders of being, and with the incredible quantity of organic remains
buried in the crust of the earth in every country on the face of the
globe.
Every great geological change in the nature of the strata was
accompanied by the introduction of a new race of beings, and the gradual
extinction of those that had previously existed, their structure and
habits being no longer fitted for the new circumstances in which these
changes had placed them. The change, however, never was abrupt; and it
may be observed that there is no proof of progressive development of
species by generation from a low to a high organization, for animals and
plants of high organization appeared among the earliest of their kind,
yet throughout the whole, the gradual approach to living and more
perfect forms is undoubted, not by change of species, but by increasing
similarity of type.
The geographical distribution of animated beings was much more extensive
in the ancient seas and land than in latter times. In very remote ages
the same animal inhabited the most distant parts of the sea: the
corallines built from the equator to within ten or fifteen degrees of
the pole; and previous to the formation of the carboniferous strata
there appears to have been even a greater uniformity in the vegetable
than in the animal world, though New Holland had formed even then a
peculiar district, supposing the coal in that country to be of the same
epoch as in Europe and America; but as the strata became more varied,
species were less widely diffused. Some of the saurians were inhabitants
of both the Old and New World, while others lived in the latter only.
During the tertiary period, the animals of Australia and America
differed nearly as much from those of Europe as they do at the present
day. The world was then, as now, divided into great physical regions,
each inhabited by a peculiar race of animals; and even the different
species of mollusca of the same sea were confined to certain shores. Of
405 species of shell-fish which inhabited the Atlantic Ocean during the
early and middle parts of the tertiary period, only 12 were common to
the American and European coasts. In fact, the divisions of the animal
and vegetable creation into geographical districts had been in the
latter periods contemporaneous with the rise of the land, each portion
of which, as it rose above the deep, had been clothed with a vegetation
and peopled with creatures suited to its position with regard to the
equator, and to the existing circumstances of the globe; and the marine
creatures had, no doubt, been divided into districts at the same
periods, because the bed of the ocean had been subject to similar
changes.
The quantity of fossil remains is so great that, with the exception of
the metals and some of the primary rocks, probably not a particle of
matter exists on the surface of the earth that has not at some time
formed part of a living creature. Since the commencement of animated
existence, zoophytes have built coral reefs extending hundreds of miles,
and mountains of limestone are full of their remains all over the globe.
Mines of shells are worked to make lime; ranges of hills and rocks, many
hundred feet thick, are almost entirely composed of them, and they
abound in every mountain-chain throughout the earth. The prodigious
quantity of microscopic shells discovered by M. Ehrenberg is still more
astonishing; shells not larger than a grain of sand form entire
mountains; a great portion of the hills of San Casciano, in Tuscany,
consist of chambered shells so minute that Signor Soldani collected
10,454 of them from one ounce of stone. Chalk is often almost entirely
composed of them. Tripoli, a fine powder long in use for polishing
metals, is also almost wholly composed of shells which owe their
polishing property to their siliceous coats; and there are even hills of
great extent consisting of this substance, the débris of an infinite
variety of microscopic insects.
The facility with which many clays and slates are split is owing, in
some instances, to layers of minute shells. Fossil fish are found in all
parts of the world, and in all the fossiliferous strata, with the
exception of some of the lowest, but each great geological period had
species of fish peculiar to itself.
The remains of the great saurians are innumerable; those of extinct
quadrupeds are very numerous; but there is no circumstance in the whole
science of fossil geology more remarkable than the inexhaustible
multitudes of fossil elephants that are found in Siberia. Their tusks
have been an object of traffic in ivory for centuries, and in some
places they have been in such prodigious quantities, that the ground is
tainted with the smell of animal matter. Their huge skeletons are found
from the frontier of Europe through all Northern Asia to its extreme
eastern point, and from the foot of the Altaï mountains to the shores of
the Frozen Ocean, a surface equal in extent to the whole of Europe. Some
islands in the Arctic Sea, as, for instance, the first of the Lächow
group, are chiefly composed of their remains, mixed with the bones of
various other animals of living genera, but extinct species.[12]
Equally wonderful is the quantity of fossil plants that still remain, if
it be considered that, from the frail nature of many vegetable
substances, multitudes must have perished without leaving a trace
behind. The vegetation that covered the terrestrial part of the globe
previous to the formation of the carboniferous strata had far surpassed
in exuberance the rankest tropical jungles. There are many coal-fields
of great extent in various parts of the earth, especially in North
America, where that of Pittsburg occupies an area of about 14,000 square
miles, and that in the Illinois is not much inferior to the area of all
England.[13]
As coal is entirely a vegetable substance, some idea may be formed of
the richness of the ancient flora: in latter times it was less
exuberant, and never has again been so luxuriant, probably on account of
the decrease of temperature during the deposition of the tertiary
strata, and in the glacial period which immediately preceded the
creation of the present tribes of plants and animals. Even after their
introduction the temperature must have been very low, but by subsequent
changes in the distribution of the sea and land the cold was gradually
mitigated, till at last the climate of the northern hemisphere became
what it now is.
Such is the marvellous history laid open to us on the earth’s surface.
Surely it is not the heavens only that declare the glory of God—the
earth also proclaims His handiwork![14]