THE GEOLOGICAL RECORD
WHAT A FORMATION RECORDS. We have already learned that each
individual body of stratified rock, or formation, constitutes a
record of the time when it was laid. The structure and the
character of the sediments of each formation tell whether the area
was land or sea at the time when they were spread; and if the
former, whether the land was river plain, or lake bed, or was
covered with wind-blown sands, or by the deposits of an ice sheet.
If the sediments are marine, we may know also whether they were
laid in shoal water near the shore or in deeper water out at sea,
and whether during a period of emergence, or during a period of
subsidence when the sea transgressed the land. By the same means
each formation records the stage in the cycle of erosion of the
land mass from which its sediments were derived. An unconformity
between two marine formations records the fact that between the
periods when they were deposited in the sea the area emerged as
land and suffered erosion. The attitude and structure of the
strata tell also of the foldings and fractures, the deformation
and the metamorphism, which they have suffered; and the igneous
rocks associated with them as lava flows and igneous intrusions
add other details to the story. Each formation is thus a separate
local chapter in the geological history of the earth, and its
strata are its leaves. It contains an authentic record of the
physical conditions--the geography--of the time and place when and
where its sediments were laid.
PAST CYCLES OF EROSION. These chapters in the history of the
planet are very numerous, although much of the record has been
destroyed in various ways. A succession of different formations is
usually seen in any considerable section of the crust, such as a
deep canyon or where the edges of upturned strata are exposed to
view on the flanks of mountain ranges; and in any extensive area,
such as a state of the Union or a province of Canada, the number
of formations outcropping on the surface is large.
It is thus learned that our present continent is made up for. the
most part of old continental deltas. Some, recently emerged as the
strata of young coastal plains, are the records of recent cycles
of erosion; while others were deposited in the early history of
the earth, and in many instances have been crumpled into
mountains, which afterwards were leveled to their bases and
lowered beneath the sea to receive a cover of later sediments
before they were again uplifted to form land.
The cycle of erosion now in progress and recorded in the layers of
stratified rock being spread beneath the sea in continental deltas
has therefore been preceded by many similar cycles. Again and
again movements of the crust have brought to an end one cycle--
sometimes when only well under way, and sometimes when drawing
toward its close--and have begun another. Again and again they
have added to the land areas which before were sea, with all their
deposition records of earlier cycles, or have lowered areas of
land beneath the sea to receive new sediments.
THE AGE OF THE EARTH. The thickness of the stratified rocks now
exposed upon the eroded surface of the continents is very great.
In the Appalachian region the strata are seven or eight miles
thick, and still greater thicknesses have been measured in several
other mountain ranges. The aggregate thickness of all the
formations of the stratified rocks of the earth's crust, giving to
each formation its maximum thickness wherever found, amounts to
not less than forty miles. Knowing how slowly sediments accumulate
upon the sea floor, we must believe that the successive cycles
which the earth has seen stretch back into a past almost
inconceivably remote, and measure tens of millions and perhaps
even hundreds of millions of years.
HOW THE FORMATIONS ARE CORRELATED AND THE GEOLOGICAL RECORD MADE
UP. Arranged in the order of their succession, the formations of
the earth's crust would constitute a connected record in which the
geological history of the planet may be read, and therefore known
as the GEOLOGICAL RECORD. But to arrange the formations in their
natural order is not an easy task. A complete set of the volumes
of the record is to be found in no single region. Their leaves and
chapters are scattered over the land surface of the globe. In one
area certain chapters may be found, though perhaps with many
missing leaves, and with intervening chapters wanting, and these
absent parts perhaps can be supplied only after long search
through many other regions.
Adjacent strata in any region are arranged according to the LAW OF
SUPERPOSITION, i.e. any stratum is younger than that on which it
was deposited, just as in a pile of paper, any sheet was laid
later than that on which it rests. Where rocks have been
disturbed, their original attitude must be determined before the
law can be applied. Nor can the law of superposition be used in
identifying and comparing the strata of different regions where
the formations cannot be traced continuously from one region to
the other.
The formations of different regions are arranged in their true
order by the LAW OF INCLUDED ORGANISMS; i.e. formations, however
widely separated, which contain a similar assemblage of fossils
are equivalent and belong to the same division of geological time.
The correlation of formations by means of fossils may be explained
by the formations now being deposited about the north Atlantic.
Lithologically they are extremely various. On the continental
shelf of North America limestones of different kinds are forming
off Florida, and sandstones and shales from Georgia northward.
Separated from them by the deep Atlantic oozes are other
sedimentary deposits now accumulating along the west coast of
Europe. If now all these offshore formations were raised to open
air, how could they be correlated? Surely not by lithological
likeness, for in this respect they would be quite diverse. All
would be similar, however, in the fossils which they contain. Some
fossil species would be identical in all these formations and
others would be closely allied. Making all due allowance for
differences in species due to local differences in climate and
other physical causes, it would still be plain that plants and
animals so similar lived at the same period of time, and that the
formations in which their remains were imbedded were
contemporaneous in a broad way. The presence of the bones of
whales and other marine mammals would prove that the strata were
laid after the appearance of mammals upon earth, and imbedded
relics of man would give a still closer approximation to their
age. In the same way we correlate the earlier geological
formations.
For example, in 1902 there were collected the first fossils ever
found on the antarctic continent. Among the dozen specimens
obtained were some fossil ammonites (a family of chambered shells)
of genera which are found on other continents in certain
formations classified as the Cretaceous system, and which occur
neither above these formations nor below them. On the basis of
these few fossils we may be confident that the strata in which
they were found in the antarctic region were laid in the same
period of geologic time as were the Cretaceous rocks of the United
States and Canada.
THE RECORD AS A TIME SCALE. By means of the law of included
organisms and the law of superposition the formations of different
countries and continents are correlated and arranged in their
natural order. When the geological record is thus obtained it may
be used as a universal time scale for geological history.
Geological time is separated into divisions corresponding to the
times during which the successive formations were laid. The
largest assemblages of formations are known as groups, while the
corresponding divisions of time are known as eras. Groups are
subdivided into systems, and systems into series. Series are
divided into stages and substages,--subdivisions which do not
concern us in this brief treatise. The corresponding divisions of
time are given in the following table.
STRATA TIME
Group Era
System Period
Series Epoch
The geologist is now prepared to read the physical history--the
geographical development--of any country or of any continent by
means of its formations, when he has given each formation its true
place in the geological record as a time scale.
The following chart exhibits the main divisions of the record, the
name given to each being given also to the corresponding time
division. Thus we speak of the CAMBRIAN SYSTEM, meaning a certain
succession of formations which are classified together because of
broad resemblances in their included organisms; and of the
CAMBRIAN PERIOD, meaning the time during which these rocks were
deposited.
Group and Era System and Period Series and Epoch
|Quaternary-----|Recent
Cenozoic------| |Pleistocene
|
|Tertiary-------|Pliocene
|Miocene
|Eocene
|Cretaceous
Mesozoic------|Jurassic
|Triassic
|Permian
|Carboniferous--|Pennsylvanian
| |Mississippian
Paleozoic-----|Devonian
|Silurian
|Ordovician
|Cambrian
Algonkian
Archean
FOSSILS AND WHAT THEY TEACH
The geological formations contain a record still more important
than that of the geographical development of the continents; the
fossils imbedded in the rocks of each formation tell of the kinds
of animals and plants which inhabited the earth at that time, and
from these fossils we are therefore able to construct the history
of life upon the earth.
FOSSILS. These remains of organisms are found in the strata in all
degrees of perfection, from trails and tracks and fragmentary
impressions, to perfectly preserved shells, wood, bones, and
complete skeletons. As a rule, it is only the hard parts of
animals and plants which have left any traces in the rocks.
Sometimes the original hard substance is preserved, but more often
it has been replaced by some less soluble material. Petrifaction,
as this process of slow replacement is called, is often carried on
in the most exquisite detail. When wood, for example, is
undergoing petrifaction, the woody tissue may be replaced,
particle by particle, by silica in solution through the action of
underground waters, even the microscopic structures of the wood
being perfectly reproduced. In shells originally made of
ARAGONITE, a crystalline form of carbonate of lime, that mineral
is usually replaced by CALCITE, a more stable form of the same
substance. The most common petrifying materials are calcite,
silica, and pyrite.
Often the organic substance has neither been preserved nor
replaced, but the FORM has been retained by means of molds and
casts. Permanent impressions, or molds, may be made in sediments
not only by the hard parts of organisms, but also by such soft and
perishable parts as the leaves of plants, and, in the rarest
instances, by the skin of animals and the feathers of birds. In
fine-grained limestones even the imprints of jellyfish have been
retained.
The different kinds of molds and casts may be illustrated by means
of a clam shell and some moist clay, the latter representing the
sediments in which the remains of animals and plants are entombed.
Imbedding the shell in the clay and allowing the clay to harden,
we have a MOLD OF THE EXTERIOR of the shell, as is seen on cutting
the clay matrix in two and removing the shell from it. Filling
this mold with clay of different color, we obtain a CAST OF THE
EXTERIOR, which represents accurately the original form and
surface markings of the shell. In nature, shells and other relics
of animals or plants are often removed by being dissolved by
percolating waters, and the molds are either filled with sediments
or with minerals deposited from solution.
Where the fossil is hollow, a CAST OF THE INTERIOR is made in the
same way. Interior casts of shells reproduce any markings on the
inside of the valves, and casts of the interior of the skulls of
ancient vertebrates show the form and size of their brains.
IMPERFECTION OF THE LIFE RECORD. At the present time only the
smallest fraction of the life on earth ever gets entombed in rocks
now forming. In the forest great fallen tree trunks, as well as
dead leaves, decay, and only add a little to the layer of dark
vegetable mold from which they grew. The bones of land animals
are, for the most part, left unburied on the surface and are soon
destroyed by chemical agencies. Even where, as in the swamps of
river, flood plains and in other bogs, there are preserved the
remains of plants, and sometimes insects, together with the bones
of some animal drowned or mired, in most cases these swamp and bog
deposits are sooner or later destroyed by the shifting channels of
the stream or by the general erosion of the land.
In the sea the conditions for preservation are more favorable than
on land; yet even here the proportion of animals and plants whose
hard parts are fossilized is very small compared with those which
either totally decay before they are buried in slowly accumulating
sediments or are ground to powder by waves and currents.
We may infer that during each period of the past, as at the
present, only a very insignificant fraction of the innumerable
organisms of sea and land escaped destruction and left in
continental and oceanic deposits permanent records of their
existence. Scanty as these original life records must have been,
they have been largely destroyed by metamorphism of the rocks in
which they were imbedded, by solution in underground waters, and
by the vast denudation under which the sediments of earlier
periods have been eroded to furnish materials for the sedimentary
records of later times. Moreover, very much of what has escaped
destruction still remains undiscovered. The immense bulk of the
stratified rocks is buried and inaccessible, and the records of
the past which it contains can never be known. Comparatively few
outcrops have been thoroughly searched for fossils. Although new
species are constantly being discovered, each discovery may be
considered as the outcome of a series of happy accidents,--that
the remains of individuals of this particular species happened to
be imbedded and fossilized, that they happened to escape
destruction during long ages, and that they happened to be exposed
and found.
SOME INFERENCES FROM THE RECORDS OF THE HISTORY OF LIFE UPON THE
PLANET. Meager as are these records, they set forth plainly some
important truths which we will now briefly mention.
1. Each series of the stratified rocks, except the very deepest,
contains vestiges of life. Hence THE EARTH WAS TENANTED BY LIVING
CREATURES FOR AN UNCALCULATED LENGTH OF TIME BEFORE HUMAN HISTORY
BEGAN.
2. LIFE ON THE EARTH HAS BEEN EVERCHANGING. The youngest strata
hold the remains of existing species of animals and plants and
those of species and varieties closely allied to them. Strata
somewhat older contain fewer existing species, and in strata of a
still earlier, but by no means an ancient epoch, no existing
species are to be found; the species of that epoch and of previous
epochs have vanished from the living world. During all geological
time since life began on earth old species have constantly become
extinct and with them the genera and families to which they
belong, and other species, genera, and families have replaced
them. The fossils of each formation differ on the whole from those
of every other. The assemblage of animals and plants (the FAUNA-
FLORA) of each epoch differs from that of every other epoch.
In many cases the extinction of a type has been gradual; in other
instances apparently abrupt. There is no evidence that any
organism once become extinct has ever reappeared. The duration of
a species in time, or its "vertical range" through the strata,
varies greatly. Some species are limited to a stratum a few feet
in thickness; some may range through an entire formation and be
found but little modified in still higher beds. A formation may
thus often be divided into zones, each characterized by its own
peculiar species. As a rule, the simpler organisms have a longer
duration as species, though not as individuals, than the more
complex.
3. THE LARGER ZOOLOGICAL AND BOTANICAL GROUPINGS SURVIVE LONGER
THAN THE SMALLER. Species are so short-lived that a single
geological epoch may be marked by several more or less complete
extinctions of the species of its fauna-flora and their
replacement by other species. A genus continues with new species
after all the species with which it began have become extinct.
Families survive genera, and orders families. Classes are so long-
lived that most of those which are known from the earliest
formations are represented by living forms, and no sub-kingdom has
ever become extinct.
Thus, to take an example from the stony corals,--the ZOANTHARIA,--
the particular characters--which constituted a certain SPECIES--
Facosites niagarensis--of the order are confined to the Niagara
series. Its GENERIC characters appeared in other species earlier
in the Silurian and continued through the Devonian. Its FAMILY
characters, represented in different genera and species, range
from the Ordovician to the close of the Paleozoic; while the
characters which it shares with all its order, the Zoantharia,
began in the Cambrian and are found in living species.
4. THE CHANGE IN ORGANISMS HAS BEEN GRADUAL. The fossils of each
life zone and of each formation of a conformable series closely
resemble, with some explainable exceptions, those of the beds
immediately above and below. The animals and plants which tenanted
the earth during any geological epoch are so closely related to
those of the preceding and the succeeding epochs that we may
consider them to be the descendants of the one and the ancestors
of the other, thus accounting for the resemblance by heredity. It
is therefore believed that the species of animals and plants now
living on the earth are the descendants of the species whose
remains we find entombed in the rocks, and that the chain of life
has been unbroken since its beginning.
5. THE CHANGE IN SPECIES HAS BEEN A GRADUAL DIFFERENTIATION.
Tracing the lines of descent of various animals and plants of the
present backward through the divisions of geologic time, we find
that these lines of descent converge and unite in simpler and
still simpler types. The development of life may be represented by
a tree whose trunk is found in the earliest ages and whose
branches spread and subdivide to the growing twigs of present
species.
6. THE CHANGE IN ORGANISMS THROUGHOUT GEOLOGIC TIME HAS BEEN A
PROGRESSIVE CHANGE. In the earliest ages the only animals and
plants on the earth were lowly forms, simple and generalized in
structure; while succeeding ages have been characterized by the
introduction of types more and more specialized and complex, and
therefore of higher rank in the scale of being. Thus the Algonkian
contains the remains of only the humblest forms of the
invertebrates. In the Cambrian, Ordovician, and Silurian the
invertebrates were represented in all their subkingdoms by a
varied fauna. In the Devonian, fishes--the lowest of the
vertebrates--became abundant. Amphibians made their entry on the
stage in the Carboniferous, and reptiles came to rule the world in
the Mesozoic. Mammals culminated in the Tertiary in strange forms
which became more and more like those of the present as the long
ages of that era rolled on; and latest of all appeared the noblest
product of the creative process, man.
Just as growth is characteristic of the individual life, so
gradual, progressive change, or evolution, has characterized the
history of life upon the planet. The evolution of the organic
kingdom from its primitive germinal forms to the complex and
highly organized fauna-flora of to-day may be compared to the
growth of some noble oak as it rises from the acorn, spreading
loftier and more widely extended branches as it grows.
7. While higher and still higher types have continually been
evolved, until man, the highest of all, appeared, THE LOWER AND
EARLIER TYPES HAVE GENERALLY PERSISTED. Some which reached their
culmination early in the history of the earth have since changed
only in slight adjustments to a changing environment. Thus the
brachiopods, a type of shellfish, have made no progress since the
Paleozoic, and some of their earliest known genera are represented
by living forms hardly to be distinguished from their ancient
ancestors. The lowest and earliest branches of the tree of life
have risen to no higher levels since they reached their climax of
development long ago.
8. A strange parallel has been found to exist between the
evolution of organisms and the development of the individual. In
the embryonic stages of its growth the individual passes swiftly
through the successive stages through which its ancestors evolved
during the millions of years of geologic time. THE DEVELOPMENT OF
THE INDIVIDUAL RECAPITULATES THE EVOLUTION OF THE RACE.
The frog is a typical amphibian. As a tadpole it passes through a
stage identical in several well-known features with the maturity
of fishes; as, for example, its aquatic life, the tail by which it
swims, and the gills through which it breathes. It is a fair
inference that the tadpole stage in the life history of the frog
represents a stage in the evolution of its kind,--that the
Amphibia are derived from fishlike ancestral forms. This inference
is amply confirmed in the geological record; fishes appeared
before Amphibia and were connected with them by transitional
forms.
THE GREAT LENGTH OF GEOLOGIC TIME INFERRED FROM THE SLOW CHANGE OF
SPECIES. Life forms, like land forms, are thus subject to change
under the influence of their changing environment and of forces
acting from within. How slowly they change may be seen in the
apparent stability of existing species. In the lifetime of the
observer and even in the recorded history of man, species seem as
stable as the mountain and the river. But life forms and land
forms are alike variable, both in nature and still more under the
shaping hand of man. As man has modified the face of the earth
with his great engineering works, so he has produced widely
different varieties of many kinds of domesticated plants and
animals, such as the varieties of the dog and the horse, the apple
and the rose, which may be regarded in some respects as new
species in the making. We have assumed that land forms have
changed in the past under the influence of forces now in
operation. Assuming also that life forms have always changed as
they are changing at present, we come to realize something of the
immensity of geologic time required for the evolution of life from
its earliest lowly forms up to man.
It is because the onward march of life has taken the same general
course the world over that we are able to use it as a UNIVERSAL
TIME SCALE and divide geologic time into ages and minor
subdivisions according to the ruling or characteristic organisms
then living on the earth. Thus, since vertebrates appeared, we
have in succession the Age of Fishes, the Age of Amphibians, the
Age of Reptiles, and the Age of Mammals.
The chart given on page 295 is thus based on the law of
superposition and the law of the evolution of organisms. The first
law gives the succession of the formations in local areas. The
fossils which they contain demonstrate the law of the progressive
appearance of organisms, and by means of this law the formations
of different countries are correlated and set each in its place in
a universal time scale and grouped together according to the
affinities of their imbedded organic remains.
GEOLOGIC TIME DIVISIONS COMPARED WITH THOSE OF HUMAN HISTORY. We
may compare the division of geologic time into eras, periods, and
other divisions according to the dominant life of the time, to the
ill-defined ages into which human history is divided according to
the dominance of some nation, ruler, or other characteristic
feature. Thus we speak of the DARK AGES, the AGE OF ELIZABETH, and
the AGE OF ELECTRICITY. These crude divisions would be of much
value if, as in the case of geologic time, we had no exact
reckoning of human history by years.
And as the course of human history has flowed in an unbroken
stream along quiet reaches of slow change and through periods of
rapid change and revolution, so with the course of geologic
history. Periods of quiescence, in which revolutionary forces are
perhaps gathering head, alternate with periods of comparatively
rapid change in physical geography and in organisms, when new and
higher forms appear which serve to draw the boundary line of new
epochs. Nevertheless, geological history is a continuous progress;
its periods and epochs shade into one another by imperceptible
gradations, and all our subdivisions must needs be vague and more
or less arbitrary.
HOW FOSSILS TELL OF THE GEOGRAPHY OF THE PAST. Fossils are used
not only as a record of the development of life upon the earth,
but also in testimony to the physical geography of past epochs.
They indicate whether in any region the climate was tropical,
temperate, or arctic. Since species spread slowly from some center
of dispersion where they originate until some barrier limits their
migration farther, the occurrence of the same species in rocks of
the same system in different countries implies the absence of such
barriers at the period. Thus in the collection of antarctic
fossils referred to on page 294 there were shallow-water marine
shells identical in species with Mesozoic shells found in India
and in the southern extremity of South America. Since such
organisms are not distributed by the currents of the deep sea and
cannot migrate along its bottom, we infer a shallow-water
connection in Mesozoic times between India, South America, and the
antarctic region. Such a shallow-water connection would be offered
along the marginal shelf of a continent uniting these now widely
separated countries.